From c9c152e6c239707c0d86a8f44c4fbcaf564b694e Mon Sep 17 00:00:00 2001
From: Christopher Carson <chris@nowzoo.com>
Date: Tue, 28 Jun 2022 18:00:51 -0500
Subject: [PATCH 01/29] fix and almost working tests

---
 packages/kit/src/runtime/client/client.js     |  6 +++-
 packages/kit/src/runtime/server/index.js      | 25 +++++++++++++
 .../errors/endpoint-throws/fetch-throws.js    |  3 ++
 .../endpoint-throws/fetch-throws.svelte       | 17 +++++++++
 .../errors/endpoint-throws/index.svelte       |  1 +
 .../page-endpoint-get-throws.js               |  3 ++
 .../page-endpoint-get-throws.svelte           |  3 ++
 .../page-endpoint-http-post-throws.js         |  3 ++
 .../page-endpoint-http-post-throws.svelte     |  3 ++
 packages/kit/test/apps/basics/test/test.js    | 36 +++++++++++++++++++
 10 files changed, 99 insertions(+), 1 deletion(-)
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.js
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.svelte
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.js
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.svelte
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.js
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.svelte

diff --git a/packages/kit/src/runtime/client/client.js b/packages/kit/src/runtime/client/client.js
index b844f637f464..97746a582116 100644
--- a/packages/kit/src/runtime/client/client.js
+++ b/packages/kit/src/runtime/client/client.js
@@ -719,7 +719,11 @@ export function create_client({ target, session, base, trailing_slash }) {
 							props = res.status === 204 ? {} : await res.json();
 						} else {
 							status = res.status;
-							error = new Error('Failed to load data');
+							try {
+								error = await res.json();
+							} catch (e) {
+								error = new Error('Failed to load data');
+							}
 						}
 					}
 
diff --git a/packages/kit/src/runtime/server/index.js b/packages/kit/src/runtime/server/index.js
index c10bbacdd937..3f14a6ae3109 100644
--- a/packages/kit/src/runtime/server/index.js
+++ b/packages/kit/src/runtime/server/index.js
@@ -309,6 +309,31 @@ export async function respond(request, options, state) {
 		const error = coalesce_to_error(e);
 
 		options.handle_error(error, event);
+		// start change
+    /**
+     *  Note: error is guaranteed to be an Error at this point, because coalesce_to_error
+     *
+     * 	We need to check whether the request is a client side fetch. There are two cases:
+     * - It's a navigation, in which case the const is_data_request,
+     *   defined further up in this function, is true
+     * - It's some other client side fetch initiated from userland (as opposed to SvelteKit) code.
+     *   In this case we check whether the Accept header starts with application/json.
+     */
+		 if (
+      is_data_request ||
+      (typeof event.request.headers.get('accept') === 'string' &&
+        event.request.headers.get('accept')?.startsWith('application/json'))
+    ) {
+      /**
+       * Return the error as JSON. For simplicity's sake, boil it down to the
+       * message.
+       */
+      return new Response(JSON.stringify({ message: error.message }), {
+        status: 500,
+        headers: { 'content-type': 'application/json; charset=utf-8' }
+      });
+    }
+    // end change
 
 		try {
 			const $session = await options.hooks.getSession(event);
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.js b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.js
new file mode 100644
index 000000000000..af490a514cd8
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.js
@@ -0,0 +1,3 @@
+export const post = () => {
+	throw new Error('Some userland error.');
+};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.svelte
new file mode 100644
index 000000000000..83520e08cc58
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.svelte
@@ -0,0 +1,17 @@
+<script>
+	import { onMount } from 'svelte';
+
+	export let error;
+	onMount(async () => {
+		const response = await fetch('/errors/endpoint-throws/fetch-throws', {
+			method: 'post',
+			headers: { Accept: 'application/json' }
+		});
+		const out = await response.json();
+		error = out.message;
+	});
+</script>
+
+<div id="error">
+	{error}
+</div>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte
new file mode 100644
index 000000000000..11f1f89c2333
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte
@@ -0,0 +1 @@
+<a href="/errors/endpoint-throws/page-endpoint-get-throws">Page endpoint get throws</a>
\ No newline at end of file
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.js b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.js
new file mode 100644
index 000000000000..a8f318161969
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.js
@@ -0,0 +1,3 @@
+export const get = () => {
+	throw new Error('Some userland error.');
+};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.svelte
new file mode 100644
index 000000000000..887e173a773a
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.svelte
@@ -0,0 +1,3 @@
+<p>
+  Should not show up.
+</p>
\ No newline at end of file
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.js b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.js
new file mode 100644
index 000000000000..af490a514cd8
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.js
@@ -0,0 +1,3 @@
+export const post = () => {
+	throw new Error('Some userland error.');
+};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.svelte
new file mode 100644
index 000000000000..92405a067d50
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.svelte
@@ -0,0 +1,3 @@
+<form method="post">
+  <button type="submit" id="submit">Submit</button>
+</form>
\ No newline at end of file
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index a726480ac740..202baf013fdd 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1132,6 +1132,42 @@ test.describe.parallel('Errors', () => {
 			'500: Cannot prerender pages that have endpoints with mutative methods'
 		);
 	});
+	test('Case: "page endpoint" GET method throws', async ({browser}) => {
+		// The case where we're navigating to a page with the client side router.
+		// The GET "page endpoint" throws a userland error.
+		// It should show the __error template with our message.
+		const page = await browser.newPage();
+		await page.goto('/errors/endpoint-throws');
+		await page.click('a');
+		await page.waitForSelector('#message');
+		expect(await page.textContent('#message')).toContain(
+			'Some userland error.'
+		);
+		await page.goto('/errors/endpoint-throws');
+		await page.click('a');
+		await page.waitForSelector('#message');
+		expect(await page.textContent('#message')).toContain(
+			'Some userland error.'
+		);
+		await page.close();
+	});
+	test('Case: submitting a form w/o fetch', async ({page}) => {
+		// The case where we're submitting a POST request via a form.
+		// It should show the __error template with our message.
+		await page.goto('/errors/endpoint-throws/page-endpoint-http-post-throws');
+		await Promise.all([page.waitForNavigation(), page.click('#submit')]);
+		expect(await page.textContent('#message')).toContain(
+			'Some userland error.'
+		);
+	});
+	test('Case: fetch POST with Accept: application/json', async ({page}) => {
+		// The case where we're fetching a POST with Accept: application/json from javascript.
+		// It should return JSON with {error: string}, not the __error template HTML.
+		await page.goto('/errors/endpoint-throws/fetch-throws');
+		expect(await page.textContent('#error')).toBe(
+			'Some userland error.'
+		);
+	});
 });
 
 test.describe.parallel('ETags', () => {

From 48ddd8c709c35e5998655756938f538468ec3de3 Mon Sep 17 00:00:00 2001
From: Christopher Carson <chris@nowzoo.com>
Date: Tue, 28 Jun 2022 18:05:36 -0500
Subject: [PATCH 02/29] todo note in failing test

---
 packages/kit/test/apps/basics/test/test.js | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 202baf013fdd..bd7b8579f3f5 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1160,9 +1160,11 @@ test.describe.parallel('Errors', () => {
 			'Some userland error.'
 		);
 	});
-	test('Case: fetch POST with Accept: application/json', async ({page}) => {
+	test('Case: client side fetch POST with Accept: application/json', async ({page}) => {
 		// The case where we're fetching a POST with Accept: application/json from javascript.
 		// It should return JSON with {error: string}, not the __error template HTML.
+		// @TODO Note this test fails for the config [dev-js] (which I think disables javascript.) Figure out how to fix.
+		
 		await page.goto('/errors/endpoint-throws/fetch-throws');
 		expect(await page.textContent('#error')).toBe(
 			'Some userland error.'

From 5ecfc932c6ee2674bfc805c9ad1ac9407a3067a3 Mon Sep 17 00:00:00 2001
From: Christopher Carson <chris@nowzoo.com>
Date: Tue, 28 Jun 2022 18:25:09 -0500
Subject: [PATCH 03/29] tests working

---
 packages/kit/test/apps/basics/test/test.js | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index bd7b8579f3f5..31b45dce71b6 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1132,7 +1132,7 @@ test.describe.parallel('Errors', () => {
 			'500: Cannot prerender pages that have endpoints with mutative methods'
 		);
 	});
-	test('Case: "page endpoint" GET method throws', async ({browser}) => {
+	test('Case: a "page endpoint" GET method throws', async ({browser}) => {
 		// The case where we're navigating to a page with the client side router.
 		// The GET "page endpoint" throws a userland error.
 		// It should show the __error template with our message.
@@ -1143,6 +1143,7 @@ test.describe.parallel('Errors', () => {
 		expect(await page.textContent('#message')).toContain(
 			'Some userland error.'
 		);
+		// do it all over again just to make sure...
 		await page.goto('/errors/endpoint-throws');
 		await page.click('a');
 		await page.waitForSelector('#message');
@@ -1160,15 +1161,17 @@ test.describe.parallel('Errors', () => {
 			'Some userland error.'
 		);
 	});
-	test('Case: client side fetch POST with Accept: application/json', async ({page}) => {
+	test('Case: client side fetch POST with Accept: application/json', async ({page, javaScriptEnabled}) => {
 		// The case where we're fetching a POST with Accept: application/json from javascript.
 		// It should return JSON with {error: string}, not the __error template HTML.
-		// @TODO Note this test fails for the config [dev-js] (which I think disables javascript.) Figure out how to fix.
-		
 		await page.goto('/errors/endpoint-throws/fetch-throws');
-		expect(await page.textContent('#error')).toBe(
-			'Some userland error.'
-		);
+		// this case is only relevant if javascript is enabled...
+		if (javaScriptEnabled) {
+			expect(await page.textContent('#error')).toBe(
+				'Some userland error.'
+			);
+		}
+		
 	});
 });
 

From bfa3dec127bffbeb0a6c00afa9b5f046004e28dd Mon Sep 17 00:00:00 2001
From: Christopher Carson <chris@nowzoo.com>
Date: Tue, 28 Jun 2022 18:29:56 -0500
Subject: [PATCH 04/29] formatting

---
 packages/kit/src/runtime/server/index.js   | 48 +++++++++++-----------
 packages/kit/test/apps/basics/test/test.js | 26 +++++-------
 2 files changed, 34 insertions(+), 40 deletions(-)

diff --git a/packages/kit/src/runtime/server/index.js b/packages/kit/src/runtime/server/index.js
index 3f14a6ae3109..01ef9eb029d8 100644
--- a/packages/kit/src/runtime/server/index.js
+++ b/packages/kit/src/runtime/server/index.js
@@ -310,30 +310,30 @@ export async function respond(request, options, state) {
 
 		options.handle_error(error, event);
 		// start change
-    /**
-     *  Note: error is guaranteed to be an Error at this point, because coalesce_to_error
-     *
-     * 	We need to check whether the request is a client side fetch. There are two cases:
-     * - It's a navigation, in which case the const is_data_request,
-     *   defined further up in this function, is true
-     * - It's some other client side fetch initiated from userland (as opposed to SvelteKit) code.
-     *   In this case we check whether the Accept header starts with application/json.
-     */
-		 if (
-      is_data_request ||
-      (typeof event.request.headers.get('accept') === 'string' &&
-        event.request.headers.get('accept')?.startsWith('application/json'))
-    ) {
-      /**
-       * Return the error as JSON. For simplicity's sake, boil it down to the
-       * message.
-       */
-      return new Response(JSON.stringify({ message: error.message }), {
-        status: 500,
-        headers: { 'content-type': 'application/json; charset=utf-8' }
-      });
-    }
-    // end change
+		/**
+		 *  Note: error is guaranteed to be an Error at this point, because coalesce_to_error
+		 *
+		 * 	We need to check whether the request is a client side fetch. There are two cases:
+		 * - It's a navigation, in which case the const is_data_request,
+		 *   defined further up in this function, is true
+		 * - It's some other client side fetch initiated from userland (as opposed to SvelteKit) code.
+		 *   In this case we check whether the Accept header starts with application/json.
+		 */
+		if (
+			is_data_request ||
+			(typeof event.request.headers.get('accept') === 'string' &&
+				event.request.headers.get('accept')?.startsWith('application/json'))
+		) {
+			/**
+			 * Return the error as JSON. For simplicity's sake, boil it down to the
+			 * message.
+			 */
+			return new Response(JSON.stringify({ message: error.message }), {
+				status: 500,
+				headers: { 'content-type': 'application/json; charset=utf-8' }
+			});
+		}
+		// end change
 
 		try {
 			const $session = await options.hooks.getSession(event);
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 31b45dce71b6..fa04074f7d5c 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1132,7 +1132,7 @@ test.describe.parallel('Errors', () => {
 			'500: Cannot prerender pages that have endpoints with mutative methods'
 		);
 	});
-	test('Case: a "page endpoint" GET method throws', async ({browser}) => {
+	test('Case: a "page endpoint" GET method throws', async ({ browser }) => {
 		// The case where we're navigating to a page with the client side router.
 		// The GET "page endpoint" throws a userland error.
 		// It should show the __error template with our message.
@@ -1140,38 +1140,32 @@ test.describe.parallel('Errors', () => {
 		await page.goto('/errors/endpoint-throws');
 		await page.click('a');
 		await page.waitForSelector('#message');
-		expect(await page.textContent('#message')).toContain(
-			'Some userland error.'
-		);
+		expect(await page.textContent('#message')).toContain('Some userland error.');
 		// do it all over again just to make sure...
 		await page.goto('/errors/endpoint-throws');
 		await page.click('a');
 		await page.waitForSelector('#message');
-		expect(await page.textContent('#message')).toContain(
-			'Some userland error.'
-		);
+		expect(await page.textContent('#message')).toContain('Some userland error.');
 		await page.close();
 	});
-	test('Case: submitting a form w/o fetch', async ({page}) => {
+	test('Case: submitting a form w/o fetch', async ({ page }) => {
 		// The case where we're submitting a POST request via a form.
 		// It should show the __error template with our message.
 		await page.goto('/errors/endpoint-throws/page-endpoint-http-post-throws');
 		await Promise.all([page.waitForNavigation(), page.click('#submit')]);
-		expect(await page.textContent('#message')).toContain(
-			'Some userland error.'
-		);
+		expect(await page.textContent('#message')).toContain('Some userland error.');
 	});
-	test('Case: client side fetch POST with Accept: application/json', async ({page, javaScriptEnabled}) => {
+	test('Case: client side fetch POST with Accept: application/json', async ({
+		page,
+		javaScriptEnabled
+	}) => {
 		// The case where we're fetching a POST with Accept: application/json from javascript.
 		// It should return JSON with {error: string}, not the __error template HTML.
 		await page.goto('/errors/endpoint-throws/fetch-throws');
 		// this case is only relevant if javascript is enabled...
 		if (javaScriptEnabled) {
-			expect(await page.textContent('#error')).toBe(
-				'Some userland error.'
-			);
+			expect(await page.textContent('#error')).toBe('Some userland error.');
 		}
-		
 	});
 });
 

From f72ddd19a669f9cbf2ff3b530af37cb4ebf3ddf9 Mon Sep 17 00:00:00 2001
From: Christopher Carson <chris@nowzoo.com>
Date: Wed, 29 Jun 2022 09:48:56 -0500
Subject: [PATCH 05/29] added changeset

---
 .changeset/swift-dots-cry.md | 7 +++++++
 1 file changed, 7 insertions(+)
 create mode 100644 .changeset/swift-dots-cry.md

diff --git a/.changeset/swift-dots-cry.md b/.changeset/swift-dots-cry.md
new file mode 100644
index 000000000000..ec4ccbe063d1
--- /dev/null
+++ b/.changeset/swift-dots-cry.md
@@ -0,0 +1,7 @@
+---
+'@sveltejs/kit': patch
+'test-basics': patch
+---
+
+Fixes the error handling issue where an error thrown from an endpoint is returned as HTML (\_\_error.svelte)
+regardless of whether or not the request was a client side fetch.

From 2e41c78d8013df56a1ace44453710b6b48220a38 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Wed, 6 Jul 2022 15:56:13 -0400
Subject: [PATCH 06/29] make tests more consistent with the rest of the
 codebase

---
 packages/kit/test/apps/basics/test/test.js | 18 +++++-------------
 1 file changed, 5 insertions(+), 13 deletions(-)

diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index cc847ea9caec..8a577748f880 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1133,30 +1133,22 @@ test.describe('Errors', () => {
 		);
 	});
 
-	test('Case: a "page endpoint" GET method throws', async ({ browser }) => {
-		// The case where we're navigating to a page with the client side router.
-		// The GET "page endpoint" throws a userland error.
-		// It should show the __error template with our message.
-		const page = await browser.newPage();
+	test('page endpoint GET method throws', async ({ page }) => {
 		await page.goto('/errors/endpoint-throws');
 		await page.click('a');
 		await page.waitForSelector('#message');
 		expect(await page.textContent('#message')).toContain('Some userland error.');
-		// do it all over again just to make sure...
-		await page.goto('/errors/endpoint-throws');
-		await page.click('a');
-		await page.waitForSelector('#message');
-		expect(await page.textContent('#message')).toContain('Some userland error.');
-		await page.close();
 	});
-	test('Case: submitting a form w/o fetch', async ({ page }) => {
+
+	test('submitting a form w/o fetch', async ({ page }) => {
 		// The case where we're submitting a POST request via a form.
 		// It should show the __error template with our message.
 		await page.goto('/errors/endpoint-throws/page-endpoint-http-post-throws');
 		await Promise.all([page.waitForNavigation(), page.click('#submit')]);
 		expect(await page.textContent('#message')).toContain('Some userland error.');
 	});
-	test('Case: client side fetch POST with Accept: application/json', async ({
+
+	test('client side fetch POST with Accept: application/json', async ({
 		page,
 		javaScriptEnabled
 	}) => {

From 9d0fbcf7eb3d17e9c37be1f69c277b3043807656 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Wed, 6 Jul 2022 16:12:39 -0400
Subject: [PATCH 07/29] simplify tests

---
 .../errors/endpoint-throws/fetch-throws.js    |  3 --
 .../endpoint-throws/fetch-throws.svelte       | 17 ---------
 .../src/routes/errors/endpoint-throws/get.js  |  3 ++
 .../routes/errors/endpoint-throws/get.svelte  |  1 +
 .../errors/endpoint-throws/index.svelte       |  6 +++-
 .../page-endpoint-get-throws.js               |  3 --
 .../page-endpoint-get-throws.svelte           |  3 --
 .../page-endpoint-http-post-throws.js         |  3 --
 .../page-endpoint-http-post-throws.svelte     |  3 --
 .../src/routes/errors/endpoint-throws/post.js |  3 ++
 .../routes/errors/endpoint-throws/post.svelte |  3 ++
 packages/kit/test/apps/basics/test/test.js    | 35 +++++++++----------
 12 files changed, 32 insertions(+), 51 deletions(-)
 delete mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.js
 delete mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.svelte
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.js
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.svelte
 delete mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.js
 delete mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.svelte
 delete mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.js
 delete mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.svelte
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.js
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.svelte

diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.js b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.js
deleted file mode 100644
index af490a514cd8..000000000000
--- a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.js
+++ /dev/null
@@ -1,3 +0,0 @@
-export const post = () => {
-	throw new Error('Some userland error.');
-};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.svelte
deleted file mode 100644
index 83520e08cc58..000000000000
--- a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/fetch-throws.svelte
+++ /dev/null
@@ -1,17 +0,0 @@
-<script>
-	import { onMount } from 'svelte';
-
-	export let error;
-	onMount(async () => {
-		const response = await fetch('/errors/endpoint-throws/fetch-throws', {
-			method: 'post',
-			headers: { Accept: 'application/json' }
-		});
-		const out = await response.json();
-		error = out.message;
-	});
-</script>
-
-<div id="error">
-	{error}
-</div>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.js b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.js
new file mode 100644
index 000000000000..a3f81411957c
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.js
@@ -0,0 +1,3 @@
+export const get = () => {
+	throw new Error('oops');
+};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.svelte
new file mode 100644
index 000000000000..d5e02b47db24
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.svelte
@@ -0,0 +1 @@
+<p>Should not show up.</p>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte
index 11f1f89c2333..390fba3cb82f 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte
@@ -1 +1,5 @@
-<a href="/errors/endpoint-throws/page-endpoint-get-throws">Page endpoint get throws</a>
\ No newline at end of file
+<a id="get" href="/errors/endpoint-throws/get">GET</a>
+
+<form action="/errors/endpoint-throws/post" method="post">
+	<button type="submit" id="post">POST</button>
+</form>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.js b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.js
deleted file mode 100644
index a8f318161969..000000000000
--- a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.js
+++ /dev/null
@@ -1,3 +0,0 @@
-export const get = () => {
-	throw new Error('Some userland error.');
-};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.svelte
deleted file mode 100644
index 887e173a773a..000000000000
--- a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-get-throws.svelte
+++ /dev/null
@@ -1,3 +0,0 @@
-<p>
-  Should not show up.
-</p>
\ No newline at end of file
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.js b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.js
deleted file mode 100644
index af490a514cd8..000000000000
--- a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.js
+++ /dev/null
@@ -1,3 +0,0 @@
-export const post = () => {
-	throw new Error('Some userland error.');
-};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.svelte
deleted file mode 100644
index 92405a067d50..000000000000
--- a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/page-endpoint-http-post-throws.svelte
+++ /dev/null
@@ -1,3 +0,0 @@
-<form method="post">
-  <button type="submit" id="submit">Submit</button>
-</form>
\ No newline at end of file
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.js b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.js
new file mode 100644
index 000000000000..4cdd25946513
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.js
@@ -0,0 +1,3 @@
+export const post = () => {
+	throw new Error('oops');
+};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.svelte
new file mode 100644
index 000000000000..ad00bb6bf699
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.svelte
@@ -0,0 +1,3 @@
+<form method="post">
+	<button type="submit" id="submit">Submit</button>
+</form>
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 8a577748f880..c79aba44facc 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1133,32 +1133,31 @@ test.describe('Errors', () => {
 		);
 	});
 
-	test('page endpoint GET method throws', async ({ page }) => {
+	test('page endpoint GET error message is preserved', async ({ page }) => {
 		await page.goto('/errors/endpoint-throws');
-		await page.click('a');
+		await page.click('#get');
 		await page.waitForSelector('#message');
-		expect(await page.textContent('#message')).toContain('Some userland error.');
+		expect(await page.textContent('#message')).toContain('oops');
 	});
 
-	test('submitting a form w/o fetch', async ({ page }) => {
+	test('page endpoint POST error message is preserved', async ({ page }) => {
 		// The case where we're submitting a POST request via a form.
 		// It should show the __error template with our message.
-		await page.goto('/errors/endpoint-throws/page-endpoint-http-post-throws');
-		await Promise.all([page.waitForNavigation(), page.click('#submit')]);
-		expect(await page.textContent('#message')).toContain('Some userland error.');
+		await page.goto('/errors/endpoint-throws');
+		await Promise.all([page.waitForNavigation(), page.click('#post')]);
+		expect(await page.textContent('#message')).toContain('oops');
 	});
 
-	test('client side fetch POST with Accept: application/json', async ({
-		page,
-		javaScriptEnabled
-	}) => {
-		// The case where we're fetching a POST with Accept: application/json from javascript.
-		// It should return JSON with {error: string}, not the __error template HTML.
-		await page.goto('/errors/endpoint-throws/fetch-throws');
-		// this case is only relevant if javascript is enabled...
-		if (javaScriptEnabled) {
-			expect(await page.textContent('#error')).toBe('Some userland error.');
-		}
+	test('page endpoint error respects `accept: application/json`', async ({ request }) => {
+		const response = await request.get('/errors/endpoint-throws/get', {
+			headers: {
+				accept: 'application/json'
+			}
+		});
+
+		expect(await response.json()).toEqual({
+			message: 'oops'
+		});
 	});
 
 	test('returns 400 when accessing a malformed URI', async ({ page, javaScriptEnabled }) => {

From 5216d42c0525efa8b441ff40d09fb099c43660d0 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Wed, 6 Jul 2022 16:26:21 -0400
Subject: [PATCH 08/29] include stack trace, tweak tests a bit

---
 packages/kit/src/runtime/server/index.js      | 42 +++++++--------
 packages/kit/src/runtime/server/page/index.js | 53 +------------------
 packages/kit/src/utils/http.js                | 52 ++++++++++++++++++
 packages/kit/test/apps/basics/test/test.js    | 22 ++++++--
 4 files changed, 90 insertions(+), 79 deletions(-)

diff --git a/packages/kit/src/runtime/server/index.js b/packages/kit/src/runtime/server/index.js
index 947c65fef61f..f560783a2eed 100644
--- a/packages/kit/src/runtime/server/index.js
+++ b/packages/kit/src/runtime/server/index.js
@@ -6,6 +6,7 @@ import { coalesce_to_error } from '../../utils/error.js';
 import { decode_params } from './utils.js';
 import { normalize_path } from '../../utils/url.js';
 import { exec } from '../../utils/routing.js';
+import { negotiate } from '../../utils/http.js';
 
 const DATA_SUFFIX = '/__data.json';
 
@@ -314,31 +315,24 @@ export async function respond(request, options, state) {
 		const error = coalesce_to_error(e);
 
 		options.handle_error(error, event);
-		// start change
-		/**
-		 *  Note: error is guaranteed to be an Error at this point, because coalesce_to_error
-		 *
-		 * 	We need to check whether the request is a client side fetch. There are two cases:
-		 * - It's a navigation, in which case the const is_data_request,
-		 *   defined further up in this function, is true
-		 * - It's some other client side fetch initiated from userland (as opposed to SvelteKit) code.
-		 *   In this case we check whether the Accept header starts with application/json.
-		 */
-		if (
-			is_data_request ||
-			(typeof event.request.headers.get('accept') === 'string' &&
-				event.request.headers.get('accept')?.startsWith('application/json'))
-		) {
-			/**
-			 * Return the error as JSON. For simplicity's sake, boil it down to the
-			 * message.
-			 */
-			return new Response(JSON.stringify({ message: error.message }), {
-				status: 500,
-				headers: { 'content-type': 'application/json; charset=utf-8' }
-			});
+
+		const type = negotiate(event.request.headers.get('accept') || 'text/html', [
+			'text/html',
+			'application/json'
+		]);
+
+		if (is_data_request || type === 'application/json') {
+			return new Response(
+				JSON.stringify({
+					message: error.message,
+					stack: options.get_stack(error)
+				}),
+				{
+					status: 500,
+					headers: { 'content-type': 'application/json; charset=utf-8' }
+				}
+			);
 		}
-		// end change
 
 		try {
 			const $session = await options.hooks.getSession(event);
diff --git a/packages/kit/src/runtime/server/page/index.js b/packages/kit/src/runtime/server/page/index.js
index 1900b5ef0f99..df62270b1724 100644
--- a/packages/kit/src/runtime/server/page/index.js
+++ b/packages/kit/src/runtime/server/page/index.js
@@ -1,3 +1,4 @@
+import { negotiate } from '../../../utils/http.js';
 import { render_endpoint } from '../endpoint.js';
 import { respond } from './respond.js';
 
@@ -39,55 +40,3 @@ export async function render_page(event, route, options, state, resolve_opts) {
 		route
 	});
 }
-
-/**
- * @param {string} accept
- * @param {string[]} types
- */
-function negotiate(accept, types) {
-	const parts = accept
-		.split(',')
-		.map((str, i) => {
-			const match = /([^/]+)\/([^;]+)(?:;q=([0-9.]+))?/.exec(str);
-			if (match) {
-				const [, type, subtype, q = '1'] = match;
-				return { type, subtype, q: +q, i };
-			}
-
-			throw new Error(`Invalid Accept header: ${accept}`);
-		})
-		.sort((a, b) => {
-			if (a.q !== b.q) {
-				return b.q - a.q;
-			}
-
-			if ((a.subtype === '*') !== (b.subtype === '*')) {
-				return a.subtype === '*' ? 1 : -1;
-			}
-
-			if ((a.type === '*') !== (b.type === '*')) {
-				return a.type === '*' ? 1 : -1;
-			}
-
-			return a.i - b.i;
-		});
-
-	let accepted;
-	let min_priority = Infinity;
-
-	for (const mimetype of types) {
-		const [type, subtype] = mimetype.split('/');
-		const priority = parts.findIndex(
-			(part) =>
-				(part.type === type || part.type === '*') &&
-				(part.subtype === subtype || part.subtype === '*')
-		);
-
-		if (priority !== -1 && priority < min_priority) {
-			accepted = mimetype;
-			min_priority = priority;
-		}
-	}
-
-	return accepted;
-}
diff --git a/packages/kit/src/utils/http.js b/packages/kit/src/utils/http.js
index 8329de9fa392..8ddd7b07a1eb 100644
--- a/packages/kit/src/utils/http.js
+++ b/packages/kit/src/utils/http.js
@@ -19,3 +19,55 @@ export function to_headers(object) {
 
 	return headers;
 }
+
+/**
+ * @param {string} accept
+ * @param {string[]} types
+ */
+export function negotiate(accept, types) {
+	const parts = accept
+		.split(',')
+		.map((str, i) => {
+			const match = /([^/]+)\/([^;]+)(?:;q=([0-9.]+))?/.exec(str);
+			if (match) {
+				const [, type, subtype, q = '1'] = match;
+				return { type, subtype, q: +q, i };
+			}
+
+			throw new Error(`Invalid Accept header: ${accept}`);
+		})
+		.sort((a, b) => {
+			if (a.q !== b.q) {
+				return b.q - a.q;
+			}
+
+			if ((a.subtype === '*') !== (b.subtype === '*')) {
+				return a.subtype === '*' ? 1 : -1;
+			}
+
+			if ((a.type === '*') !== (b.type === '*')) {
+				return a.type === '*' ? 1 : -1;
+			}
+
+			return a.i - b.i;
+		});
+
+	let accepted;
+	let min_priority = Infinity;
+
+	for (const mimetype of types) {
+		const [type, subtype] = mimetype.split('/');
+		const priority = parts.findIndex(
+			(part) =>
+				(part.type === type || part.type === '*') &&
+				(part.subtype === subtype || part.subtype === '*')
+		);
+
+		if (priority !== -1 && priority < min_priority) {
+			accepted = mimetype;
+			min_priority = priority;
+		}
+	}
+
+	return accepted;
+}
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index c79aba44facc..c0d6a212a362 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1138,6 +1138,11 @@ test.describe('Errors', () => {
 		await page.click('#get');
 		await page.waitForSelector('#message');
 		expect(await page.textContent('#message')).toContain('oops');
+
+		if (process.env.DEV) {
+			const lines = (await page.textContent('pre')).split('\n');
+			expect(lines[1]).toContain('src/routes/errors/endpoint-throws/get.js:2:8');
+		}
 	});
 
 	test('page endpoint POST error message is preserved', async ({ page }) => {
@@ -1146,6 +1151,11 @@ test.describe('Errors', () => {
 		await page.goto('/errors/endpoint-throws');
 		await Promise.all([page.waitForNavigation(), page.click('#post')]);
 		expect(await page.textContent('#message')).toContain('oops');
+
+		if (process.env.DEV) {
+			const lines = (await page.textContent('pre')).split('\n');
+			expect(lines[1]).toContain('src/routes/errors/endpoint-throws/post.js:2:8');
+		}
 	});
 
 	test('page endpoint error respects `accept: application/json`', async ({ request }) => {
@@ -1155,9 +1165,15 @@ test.describe('Errors', () => {
 			}
 		});
 
-		expect(await response.json()).toEqual({
-			message: 'oops'
-		});
+		const { message, stack } = await response.json();
+
+		expect(message).toBe('oops');
+
+		if (process.env.DEV) {
+			expect(stack.split('\n').length).toBeGreaterThan(1);
+		} else {
+			expect(stack.split('\n').length).toBe(1);
+		}
 	});
 
 	test('returns 400 when accessing a malformed URI', async ({ page, javaScriptEnabled }) => {

From f7f9fc254cbdf198ba5723483c590af289c4f191 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Wed, 6 Jul 2022 16:34:59 -0400
Subject: [PATCH 09/29] update changeset

---
 .changeset/swift-dots-cry.md | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/.changeset/swift-dots-cry.md b/.changeset/swift-dots-cry.md
index ec4ccbe063d1..428204437a82 100644
--- a/.changeset/swift-dots-cry.md
+++ b/.changeset/swift-dots-cry.md
@@ -3,5 +3,4 @@
 'test-basics': patch
 ---
 
-Fixes the error handling issue where an error thrown from an endpoint is returned as HTML (\_\_error.svelte)
-regardless of whether or not the request was a client side fetch.
+Returns errors from page endpoints as JSON where appropriate

From 1852bbeb24c25bd603edf2c8ae6108d8fc87d8fe Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Wed, 6 Jul 2022 16:50:29 -0400
Subject: [PATCH 10/29] oh do fuck off windows

---
 packages/kit/test/apps/basics/test/test.js | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index c0d6a212a362..71d87a426a0b 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1141,7 +1141,7 @@ test.describe('Errors', () => {
 
 		if (process.env.DEV) {
 			const lines = (await page.textContent('pre')).split('\n');
-			expect(lines[1]).toContain('src/routes/errors/endpoint-throws/get.js:2:8');
+			expect(lines[1]).toContain('get.js:2:8');
 		}
 	});
 
@@ -1154,7 +1154,7 @@ test.describe('Errors', () => {
 
 		if (process.env.DEV) {
 			const lines = (await page.textContent('pre')).split('\n');
-			expect(lines[1]).toContain('src/routes/errors/endpoint-throws/post.js:2:8');
+			expect(lines[1]).toContain('post.js:2:8');
 		}
 	});
 

From 618e16e1f0d6c5e5dc9f1a1aea05b00396a82190 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Wed, 6 Jul 2022 17:51:27 -0400
Subject: [PATCH 11/29] shuffle things around a bit, ensure handleError is
 called

---
 .../errors/endpoint-throws/index.svelte       |  5 ----
 .../get.js => page-endpoint/get-implicit.js}  |  0
 .../get-implicit.svelte}                      |  0
 .../routes/errors/page-endpoint/index.svelte  |  5 ++++
 .../post-implicit.js}                         |  0
 .../post-implicit.svelte}                     |  0
 packages/kit/test/apps/basics/test/test.js    | 26 ++++++++++++-------
 7 files changed, 21 insertions(+), 15 deletions(-)
 delete mode 100644 packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte
 rename packages/kit/test/apps/basics/src/routes/errors/{endpoint-throws/get.js => page-endpoint/get-implicit.js} (100%)
 rename packages/kit/test/apps/basics/src/routes/errors/{endpoint-throws/get.svelte => page-endpoint/get-implicit.svelte} (100%)
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/page-endpoint/index.svelte
 rename packages/kit/test/apps/basics/src/routes/errors/{endpoint-throws/post.js => page-endpoint/post-implicit.js} (100%)
 rename packages/kit/test/apps/basics/src/routes/errors/{endpoint-throws/post.svelte => page-endpoint/post-implicit.svelte} (100%)

diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte b/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte
deleted file mode 100644
index 390fba3cb82f..000000000000
--- a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/index.svelte
+++ /dev/null
@@ -1,5 +0,0 @@
-<a id="get" href="/errors/endpoint-throws/get">GET</a>
-
-<form action="/errors/endpoint-throws/post" method="post">
-	<button type="submit" id="post">POST</button>
-</form>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.js
similarity index 100%
rename from packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.js
rename to packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.js
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.svelte
similarity index 100%
rename from packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/get.svelte
rename to packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.svelte
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/index.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/index.svelte
new file mode 100644
index 000000000000..4c220bc7c85c
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/index.svelte
@@ -0,0 +1,5 @@
+<a id="get-implicit" href="/errors/page-endpoint/get-implicit">GET (implicit)</a>
+
+<form action="/errors/page-endpoint/post-implicit" method="post">
+	<button type="submit" id="post-implicit">POST (implicit)</button>
+</form>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.js
similarity index 100%
rename from packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.js
rename to packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.js
diff --git a/packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.svelte
similarity index 100%
rename from packages/kit/test/apps/basics/src/routes/errors/endpoint-throws/post.svelte
rename to packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.svelte
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 71d87a426a0b..4aed569b3d32 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1133,33 +1133,39 @@ test.describe('Errors', () => {
 		);
 	});
 
-	test('page endpoint GET error message is preserved', async ({ page }) => {
-		await page.goto('/errors/endpoint-throws');
-		await page.click('#get');
+	test('page endpoint GET thrown error message is preserved', async ({ page, read_errors }) => {
+		await page.goto('/errors/page-endpoint');
+		await page.click('#get-implicit');
 		await page.waitForSelector('#message');
 		expect(await page.textContent('#message')).toContain('oops');
 
 		if (process.env.DEV) {
 			const lines = (await page.textContent('pre')).split('\n');
-			expect(lines[1]).toContain('get.js:2:8');
+			expect(lines[1]).toContain('get-implicit.js:2:8');
 		}
+
+		const error = read_errors('/errors/page-endpoint/get-implicit');
+		expect(error).toContain('oops');
 	});
 
-	test('page endpoint POST error message is preserved', async ({ page }) => {
+	test('page endpoint POST thrown error message is preserved', async ({ page, read_errors }) => {
 		// The case where we're submitting a POST request via a form.
 		// It should show the __error template with our message.
-		await page.goto('/errors/endpoint-throws');
-		await Promise.all([page.waitForNavigation(), page.click('#post')]);
+		await page.goto('/errors/page-endpoint');
+		await Promise.all([page.waitForNavigation(), page.click('#post-implicit')]);
 		expect(await page.textContent('#message')).toContain('oops');
 
 		if (process.env.DEV) {
 			const lines = (await page.textContent('pre')).split('\n');
-			expect(lines[1]).toContain('post.js:2:8');
+			expect(lines[1]).toContain('post-implicit.js:2:8');
 		}
+
+		const error = read_errors('/errors/page-endpoint/post-implicit');
+		expect(error).toContain('oops');
 	});
 
-	test('page endpoint error respects `accept: application/json`', async ({ request }) => {
-		const response = await request.get('/errors/endpoint-throws/get', {
+	test('page endpoint thrown error respects `accept: application/json`', async ({ request }) => {
+		const response = await request.get('/errors/page-endpoint/get-implicit', {
 			headers: {
 				accept: 'application/json'
 			}

From e4f886b22cbcd88e1624aca24e954d8770550a7a Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 07:35:53 -0400
Subject: [PATCH 12/29] add (failing) tests for explicit errors

---
 .../errors/page-endpoint/__error.svelte       |  13 +++
 .../errors/page-endpoint/get-explicit.js      |   6 ++
 .../errors/page-endpoint/get-explicit.svelte  |   1 +
 .../errors/page-endpoint/get-implicit.svelte  |   2 +-
 .../routes/errors/page-endpoint/index.svelte  |   5 +
 .../errors/page-endpoint/post-explicit.js     |   6 ++
 .../errors/page-endpoint/post-explicit.svelte |   1 +
 .../errors/page-endpoint/post-implicit.svelte |   4 +-
 packages/kit/test/apps/basics/test/test.js    | 101 ++++++++++++++++--
 9 files changed, 126 insertions(+), 13 deletions(-)
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.svelte
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.svelte

diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
new file mode 100644
index 000000000000..9dde61c6fe9c
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
@@ -0,0 +1,13 @@
+<script>
+	import { page } from '$app/stores';
+</script>
+
+<pre>{JSON.stringify(
+		{
+			status: $page.status,
+			message: $page.error.message,
+			stack: $page.error.stack
+		},
+		null,
+		'  '
+	)}</pre>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js
new file mode 100644
index 000000000000..409a33fb5714
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js
@@ -0,0 +1,6 @@
+export const get = () => {
+	return {
+		status: 400,
+		body: new Error('oops')
+	};
+};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.svelte
new file mode 100644
index 000000000000..52cd0974062b
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.svelte
@@ -0,0 +1 @@
+<h1>if you see this something went wrong</h1>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.svelte
index d5e02b47db24..52cd0974062b 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.svelte
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.svelte
@@ -1 +1 @@
-<p>Should not show up.</p>
+<h1>if you see this something went wrong</h1>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/index.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/index.svelte
index 4c220bc7c85c..b2158ab412f8 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/index.svelte
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/index.svelte
@@ -1,5 +1,10 @@
 <a id="get-implicit" href="/errors/page-endpoint/get-implicit">GET (implicit)</a>
+<a id="get-explicit" href="/errors/page-endpoint/get-explicit">GET (explicit)</a>
 
 <form action="/errors/page-endpoint/post-implicit" method="post">
 	<button type="submit" id="post-implicit">POST (implicit)</button>
 </form>
+
+<form action="/errors/page-endpoint/post-explicit" method="post">
+	<button type="submit" id="post-explicit">POST (explicit)</button>
+</form>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js
new file mode 100644
index 000000000000..409a33fb5714
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js
@@ -0,0 +1,6 @@
+export const get = () => {
+	return {
+		status: 400,
+		body: new Error('oops')
+	};
+};
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.svelte
new file mode 100644
index 000000000000..52cd0974062b
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.svelte
@@ -0,0 +1 @@
+<h1>if you see this something went wrong</h1>
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.svelte
index ad00bb6bf699..52cd0974062b 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.svelte
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.svelte
@@ -1,3 +1 @@
-<form method="post">
-	<button type="submit" id="submit">Submit</button>
-</form>
+<h1>if you see this something went wrong</h1>
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 4aed569b3d32..6b02406d19c1 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1133,14 +1133,21 @@ test.describe('Errors', () => {
 		);
 	});
 
-	test('page endpoint GET thrown error message is preserved', async ({ page, read_errors }) => {
+	test.only('page endpoint GET thrown error message is preserved', async ({
+		page,
+		clicknav,
+		read_errors
+	}) => {
 		await page.goto('/errors/page-endpoint');
-		await page.click('#get-implicit');
-		await page.waitForSelector('#message');
-		expect(await page.textContent('#message')).toContain('oops');
+		await clicknav('#get-implicit');
+		const json = await page.textContent('pre');
+		const { status, message, stack } = JSON.parse(json);
+
+		expect(status).toBe(500);
+		expect(message).toBe('oops');
 
 		if (process.env.DEV) {
-			const lines = (await page.textContent('pre')).split('\n');
+			const lines = stack.split('\n');
 			expect(lines[1]).toContain('get-implicit.js:2:8');
 		}
 
@@ -1148,15 +1155,44 @@ test.describe('Errors', () => {
 		expect(error).toContain('oops');
 	});
 
-	test('page endpoint POST thrown error message is preserved', async ({ page, read_errors }) => {
+	test.only('page endpoint GET returned error message is preserved', async ({
+		page,
+		clicknav,
+		read_errors
+	}) => {
+		await page.goto('/errors/page-endpoint');
+		await clicknav('#get-explicit');
+		const json = await page.textContent('pre');
+		const { status, message, stack } = JSON.parse(json);
+
+		expect(status).toBe(400);
+		expect(message).toBe('oops');
+
+		if (process.env.DEV) {
+			const lines = stack.split('\n');
+			expect(lines[1]).toContain('get-explicit.js:4:9');
+		}
+
+		const error = read_errors('/errors/page-endpoint/get-explicit');
+		expect(error).toContain('oops');
+	});
+
+	test.only('page endpoint POST thrown error message is preserved', async ({
+		page,
+		read_errors
+	}) => {
 		// The case where we're submitting a POST request via a form.
 		// It should show the __error template with our message.
 		await page.goto('/errors/page-endpoint');
 		await Promise.all([page.waitForNavigation(), page.click('#post-implicit')]);
-		expect(await page.textContent('#message')).toContain('oops');
+		const json = await page.textContent('pre');
+		const { status, message, stack } = JSON.parse(json);
+
+		expect(status).toBe(500);
+		expect(message).toBe('oops');
 
 		if (process.env.DEV) {
-			const lines = (await page.textContent('pre')).split('\n');
+			const lines = stack.split('\n');
 			expect(lines[1]).toContain('post-implicit.js:2:8');
 		}
 
@@ -1164,7 +1200,32 @@ test.describe('Errors', () => {
 		expect(error).toContain('oops');
 	});
 
-	test('page endpoint thrown error respects `accept: application/json`', async ({ request }) => {
+	test.only('page endpoint POST returned error message is preserved', async ({
+		page,
+		read_errors
+	}) => {
+		// The case where we're submitting a POST request via a form.
+		// It should show the __error template with our message.
+		await page.goto('/errors/page-endpoint');
+		await Promise.all([page.waitForNavigation(), page.click('#post-explicit')]);
+		const json = await page.textContent('pre');
+		const { status, message, stack } = JSON.parse(json);
+
+		expect(status).toBe(400);
+		expect(message).toBe('oops');
+
+		if (process.env.DEV) {
+			const lines = stack.split('\n');
+			expect(lines[1]).toContain('post-explicit.js:4:9');
+		}
+
+		const error = read_errors('/errors/page-endpoint/post-explicit');
+		expect(error).toContain('oops');
+	});
+
+	test.only('page endpoint thrown error respects `accept: application/json`', async ({
+		request
+	}) => {
 		const response = await request.get('/errors/page-endpoint/get-implicit', {
 			headers: {
 				accept: 'application/json'
@@ -1173,6 +1234,28 @@ test.describe('Errors', () => {
 
 		const { message, stack } = await response.json();
 
+		expect(response.status()).toBe(500);
+		expect(message).toBe('oops');
+
+		if (process.env.DEV) {
+			expect(stack.split('\n').length).toBeGreaterThan(1);
+		} else {
+			expect(stack.split('\n').length).toBe(1);
+		}
+	});
+
+	test.only('page endpoint returned error respects `accept: application/json`', async ({
+		request
+	}) => {
+		const response = await request.get('/errors/page-endpoint/get-explicit', {
+			headers: {
+				accept: 'application/json'
+			}
+		});
+
+		const { message, stack } = await response.json();
+
+		expect(response.status()).toBe(400);
 		expect(message).toBe('oops');
 
 		if (process.env.DEV) {

From 30fd42f7a89fff9a5037bb848f8aef754f3b9cfc Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 11:47:29 -0400
Subject: [PATCH 13/29] update tests

---
 packages/kit/src/runtime/server/endpoint.js   | 19 +++++++++++++++++--
 packages/kit/src/runtime/server/index.js      |  4 ++--
 packages/kit/src/runtime/server/page/index.js |  2 +-
 .../kit/src/runtime/server/page/load_node.js  |  4 +++-
 .../errors/page-endpoint/get-explicit.js      | 10 ++++------
 .../errors/page-endpoint/post-explicit.js     | 10 ++++------
 packages/kit/test/apps/basics/test/test.js    |  8 ++++----
 7 files changed, 35 insertions(+), 22 deletions(-)

diff --git a/packages/kit/src/runtime/server/endpoint.js b/packages/kit/src/runtime/server/endpoint.js
index 8ab8d2dd312a..d437b64e9fd4 100644
--- a/packages/kit/src/runtime/server/endpoint.js
+++ b/packages/kit/src/runtime/server/endpoint.js
@@ -39,9 +39,10 @@ export function is_text(content_type) {
 /**
  * @param {import('types').RequestEvent} event
  * @param {{ [method: string]: import('types').RequestHandler }} mod
+ * @param {import('types').SSROptions} options
  * @returns {Promise<Response>}
  */
-export async function render_endpoint(event, mod) {
+export async function render_endpoint(event, mod, options) {
 	const method = normalize_request_method(event);
 
 	/** @type {import('types').RequestHandler} */
@@ -111,7 +112,7 @@ export async function render_endpoint(event, mod) {
 
 	if (is_pojo(body) && (!type || type.startsWith('application/json'))) {
 		headers.set('content-type', 'application/json; charset=utf-8');
-		normalized_body = JSON.stringify(body);
+		normalized_body = body instanceof Error ? serialize_error(body, options) : JSON.stringify(body);
 	} else {
 		normalized_body = /** @type {import('types').StrictBody} */ (body);
 	}
@@ -134,3 +135,17 @@ export async function render_endpoint(event, mod) {
 		}
 	);
 }
+
+/**
+ * @param {Error} error
+ * @param {import('types').SSROptions} options
+ */
+function serialize_error(error, options) {
+	const object = {
+		name: error.name,
+		message: error.message,
+		stack: options.get_stack(error)
+	};
+
+	return JSON.stringify(object);
+}
diff --git a/packages/kit/src/runtime/server/index.js b/packages/kit/src/runtime/server/index.js
index f560783a2eed..8aa18fa3d0bf 100644
--- a/packages/kit/src/runtime/server/index.js
+++ b/packages/kit/src/runtime/server/index.js
@@ -210,7 +210,7 @@ export async function respond(request, options, state) {
 					let response;
 
 					if (is_data_request && route.type === 'page' && route.shadow) {
-						response = await render_endpoint(event, await route.shadow());
+						response = await render_endpoint(event, await route.shadow(), options);
 
 						// loading data for a client-side transition is a special case
 						if (request.headers.has('x-sveltekit-load')) {
@@ -232,7 +232,7 @@ export async function respond(request, options, state) {
 					} else {
 						response =
 							route.type === 'endpoint'
-								? await render_endpoint(event, await route.load())
+								? await render_endpoint(event, await route.load(), options)
 								: await render_page(event, route, options, state, resolve_opts);
 					}
 
diff --git a/packages/kit/src/runtime/server/page/index.js b/packages/kit/src/runtime/server/page/index.js
index df62270b1724..c56e97b05270 100644
--- a/packages/kit/src/runtime/server/page/index.js
+++ b/packages/kit/src/runtime/server/page/index.js
@@ -25,7 +25,7 @@ export async function render_page(event, route, options, state, resolve_opts) {
 		]);
 
 		if (type === 'application/json') {
-			return render_endpoint(event, await route.shadow());
+			return render_endpoint(event, await route.shadow(), options);
 		}
 	}
 
diff --git a/packages/kit/src/runtime/server/page/load_node.js b/packages/kit/src/runtime/server/page/load_node.js
index 5ce5abd4d614..4bb1bd5358ef 100644
--- a/packages/kit/src/runtime/server/page/load_node.js
+++ b/packages/kit/src/runtime/server/page/load_node.js
@@ -541,7 +541,9 @@ function validate_shadow_output(result) {
 	}
 
 	if (!is_pojo(body)) {
-		throw new Error('Body returned from endpoint request handler must be a plain object');
+		throw new Error(
+			'Body returned from endpoint request handler must be a plain object or an Error'
+		);
 	}
 
 	return { status, headers, body };
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js
index 409a33fb5714..ecd85beed882 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js
@@ -1,6 +1,4 @@
-export const get = () => {
-	return {
-		status: 400,
-		body: new Error('oops')
-	};
-};
+export const get = () => ({
+	status: 400,
+	body: new Error('oops')
+});
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js
index 409a33fb5714..13a1d4c54bde 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js
@@ -1,6 +1,4 @@
-export const get = () => {
-	return {
-		status: 400,
-		body: new Error('oops')
-	};
-};
+export const post = () => ({
+	status: 400,
+	body: new Error('oops')
+});
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 6b02406d19c1..986bfefab0ee 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1170,11 +1170,11 @@ test.describe('Errors', () => {
 
 		if (process.env.DEV) {
 			const lines = stack.split('\n');
-			expect(lines[1]).toContain('get-explicit.js:4:9');
+			expect(lines[1]).toContain('get-explicit.js:3:8');
 		}
 
 		const error = read_errors('/errors/page-endpoint/get-explicit');
-		expect(error).toContain('oops');
+		expect(error).toBe(undefined);
 	});
 
 	test.only('page endpoint POST thrown error message is preserved', async ({
@@ -1216,11 +1216,11 @@ test.describe('Errors', () => {
 
 		if (process.env.DEV) {
 			const lines = stack.split('\n');
-			expect(lines[1]).toContain('post-explicit.js:4:9');
+			expect(lines[1]).toContain('post-explicit.js:3:8');
 		}
 
 		const error = read_errors('/errors/page-endpoint/post-explicit');
-		expect(error).toContain('oops');
+		expect(error).toBe(undefined);
 	});
 
 	test.only('page endpoint thrown error respects `accept: application/json`', async ({

From ec1cddece8770bd8829a5fb5114f790404f2bf5c Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 11:56:23 -0400
Subject: [PATCH 14/29] render error page if body instanceof Error

---
 packages/kit/src/runtime/server/page/load_node.js | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/packages/kit/src/runtime/server/page/load_node.js b/packages/kit/src/runtime/server/page/load_node.js
index 4bb1bd5358ef..590e981947c4 100644
--- a/packages/kit/src/runtime/server/page/load_node.js
+++ b/packages/kit/src/runtime/server/page/load_node.js
@@ -459,6 +459,18 @@ async function load_shadow_data(route, event, options, prerender) {
 				return data;
 			}
 
+			// explicit errors cause an error page...
+			if (body instanceof Error) {
+				if (status < 400) {
+					data.status = 500;
+					data.error = new Error('A non-error status code was returned with an error body');
+				} else {
+					data.error = body;
+				}
+
+				return data;
+			}
+
 			// ...but 4xx and 5xx status codes _don't_ result in the error page
 			// rendering for non-GET requests — instead, we allow the page
 			// to render with any validation errors etc that were returned

From bb1c710b70c2b990c906a8cad061b9524d943d9f Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 12:02:36 -0400
Subject: [PATCH 15/29] preserve errors returned from page endpoint GET
 handlers

---
 packages/kit/src/runtime/server/page/load_node.js | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/packages/kit/src/runtime/server/page/load_node.js b/packages/kit/src/runtime/server/page/load_node.js
index 590e981947c4..8788aa34805d 100644
--- a/packages/kit/src/runtime/server/page/load_node.js
+++ b/packages/kit/src/runtime/server/page/load_node.js
@@ -493,6 +493,17 @@ async function load_shadow_data(route, event, options, prerender) {
 			add_cookies(/** @type {string[]} */ (data.cookies), headers);
 			data.status = status;
 
+			if (body instanceof Error) {
+				if (status < 400) {
+					data.status = 500;
+					data.error = new Error('A non-error status code was returned with an error body');
+				} else {
+					data.error = body;
+				}
+
+				return data;
+			}
+
 			if (status >= 400) {
 				data.error = new Error('Failed to load data');
 				return data;

From 3c8360e9970daf84581f40590193051de8937ac6 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 12:38:17 -0400
Subject: [PATCH 16/29] serialize errors consistently

---
 packages/kit/src/runtime/server/endpoint.js    | 16 +---------------
 packages/kit/src/runtime/server/index.js       | 16 +++++-----------
 packages/kit/src/runtime/server/utils.js       | 14 ++++++++++++++
 .../routes/errors/page-endpoint/__error.svelte |  1 +
 packages/kit/test/apps/basics/test/test.js     | 18 ++++++++++++------
 5 files changed, 33 insertions(+), 32 deletions(-)

diff --git a/packages/kit/src/runtime/server/endpoint.js b/packages/kit/src/runtime/server/endpoint.js
index d437b64e9fd4..1ff4737f9f51 100644
--- a/packages/kit/src/runtime/server/endpoint.js
+++ b/packages/kit/src/runtime/server/endpoint.js
@@ -1,6 +1,6 @@
 import { to_headers } from '../../utils/http.js';
 import { hash } from '../hash.js';
-import { is_pojo, normalize_request_method } from './utils.js';
+import { is_pojo, normalize_request_method, serialize_error } from './utils.js';
 
 /** @param {string} body */
 function error(body) {
@@ -135,17 +135,3 @@ export async function render_endpoint(event, mod, options) {
 		}
 	);
 }
-
-/**
- * @param {Error} error
- * @param {import('types').SSROptions} options
- */
-function serialize_error(error, options) {
-	const object = {
-		name: error.name,
-		message: error.message,
-		stack: options.get_stack(error)
-	};
-
-	return JSON.stringify(object);
-}
diff --git a/packages/kit/src/runtime/server/index.js b/packages/kit/src/runtime/server/index.js
index 8aa18fa3d0bf..28ac87390d86 100644
--- a/packages/kit/src/runtime/server/index.js
+++ b/packages/kit/src/runtime/server/index.js
@@ -3,7 +3,7 @@ import { render_page } from './page/index.js';
 import { render_response } from './page/render.js';
 import { respond_with_error } from './page/respond_with_error.js';
 import { coalesce_to_error } from '../../utils/error.js';
-import { decode_params } from './utils.js';
+import { decode_params, serialize_error } from './utils.js';
 import { normalize_path } from '../../utils/url.js';
 import { exec } from '../../utils/routing.js';
 import { negotiate } from '../../utils/http.js';
@@ -322,16 +322,10 @@ export async function respond(request, options, state) {
 		]);
 
 		if (is_data_request || type === 'application/json') {
-			return new Response(
-				JSON.stringify({
-					message: error.message,
-					stack: options.get_stack(error)
-				}),
-				{
-					status: 500,
-					headers: { 'content-type': 'application/json; charset=utf-8' }
-				}
-			);
+			return new Response(serialize_error(error, options), {
+				status: 500,
+				headers: { 'content-type': 'application/json; charset=utf-8' }
+			});
 		}
 
 		try {
diff --git a/packages/kit/src/runtime/server/utils.js b/packages/kit/src/runtime/server/utils.js
index 69831be459c1..290f225c191d 100644
--- a/packages/kit/src/runtime/server/utils.js
+++ b/packages/kit/src/runtime/server/utils.js
@@ -55,3 +55,17 @@ export function normalize_request_method(event) {
 	const method = event.request.method.toLowerCase();
 	return method === 'delete' ? 'del' : method; // 'delete' is a reserved word
 }
+
+/**
+ * @param {Error} error
+ * @param {import('types').SSROptions} options
+ */
+export function serialize_error(error, options) {
+	const object = {
+		name: error.name,
+		message: error.message,
+		stack: options.get_stack(error)
+	};
+
+	return JSON.stringify(object);
+}
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
index 9dde61c6fe9c..5b4cfb3c2ed6 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
@@ -5,6 +5,7 @@
 <pre>{JSON.stringify(
 		{
 			status: $page.status,
+			name: $page.error.name,
 			message: $page.error.message,
 			stack: $page.error.stack
 		},
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 986bfefab0ee..18f02d94e095 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1141,9 +1141,10 @@ test.describe('Errors', () => {
 		await page.goto('/errors/page-endpoint');
 		await clicknav('#get-implicit');
 		const json = await page.textContent('pre');
-		const { status, message, stack } = JSON.parse(json);
+		const { status, name, message, stack } = JSON.parse(json);
 
 		expect(status).toBe(500);
+		expect(name).toBe('Error');
 		expect(message).toBe('oops');
 
 		if (process.env.DEV) {
@@ -1163,9 +1164,10 @@ test.describe('Errors', () => {
 		await page.goto('/errors/page-endpoint');
 		await clicknav('#get-explicit');
 		const json = await page.textContent('pre');
-		const { status, message, stack } = JSON.parse(json);
+		const { status, name, message, stack } = JSON.parse(json);
 
 		expect(status).toBe(400);
+		expect(name).toBe('Error');
 		expect(message).toBe('oops');
 
 		if (process.env.DEV) {
@@ -1186,9 +1188,10 @@ test.describe('Errors', () => {
 		await page.goto('/errors/page-endpoint');
 		await Promise.all([page.waitForNavigation(), page.click('#post-implicit')]);
 		const json = await page.textContent('pre');
-		const { status, message, stack } = JSON.parse(json);
+		const { status, name, message, stack } = JSON.parse(json);
 
 		expect(status).toBe(500);
+		expect(name).toBe('Error');
 		expect(message).toBe('oops');
 
 		if (process.env.DEV) {
@@ -1209,9 +1212,10 @@ test.describe('Errors', () => {
 		await page.goto('/errors/page-endpoint');
 		await Promise.all([page.waitForNavigation(), page.click('#post-explicit')]);
 		const json = await page.textContent('pre');
-		const { status, message, stack } = JSON.parse(json);
+		const { status, name, message, stack } = JSON.parse(json);
 
 		expect(status).toBe(400);
+		expect(name).toBe('Error');
 		expect(message).toBe('oops');
 
 		if (process.env.DEV) {
@@ -1232,9 +1236,10 @@ test.describe('Errors', () => {
 			}
 		});
 
-		const { message, stack } = await response.json();
+		const { message, name, stack } = await response.json();
 
 		expect(response.status()).toBe(500);
+		expect(name).toBe('Error');
 		expect(message).toBe('oops');
 
 		if (process.env.DEV) {
@@ -1253,9 +1258,10 @@ test.describe('Errors', () => {
 			}
 		});
 
-		const { message, stack } = await response.json();
+		const { message, name, stack } = await response.json();
 
 		expect(response.status()).toBe(400);
+		expect(name).toBe('Error');
 		expect(message).toBe('oops');
 
 		if (process.env.DEV) {

From 3a2127f6200c5fabb9d0fc6c7f9e38bf91bca4f8 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 13:09:18 -0400
Subject: [PATCH 17/29] better error serialization

---
 packages/kit/src/runtime/server/endpoint.js   |  3 +-
 packages/kit/src/runtime/server/index.js      |  2 +-
 .../kit/src/runtime/server/page/render.js     |  1 +
 packages/kit/src/runtime/server/utils.js      | 37 +++++++++++++----
 packages/kit/src/runtime/server/utils.spec.js | 41 ++++++++++++++++++-
 5 files changed, 73 insertions(+), 11 deletions(-)

diff --git a/packages/kit/src/runtime/server/endpoint.js b/packages/kit/src/runtime/server/endpoint.js
index 1ff4737f9f51..d9bfb3e9c74f 100644
--- a/packages/kit/src/runtime/server/endpoint.js
+++ b/packages/kit/src/runtime/server/endpoint.js
@@ -112,7 +112,8 @@ export async function render_endpoint(event, mod, options) {
 
 	if (is_pojo(body) && (!type || type.startsWith('application/json'))) {
 		headers.set('content-type', 'application/json; charset=utf-8');
-		normalized_body = body instanceof Error ? serialize_error(body, options) : JSON.stringify(body);
+		normalized_body =
+			body instanceof Error ? serialize_error(body, options.get_stack) : JSON.stringify(body);
 	} else {
 		normalized_body = /** @type {import('types').StrictBody} */ (body);
 	}
diff --git a/packages/kit/src/runtime/server/index.js b/packages/kit/src/runtime/server/index.js
index 28ac87390d86..9140a00018fb 100644
--- a/packages/kit/src/runtime/server/index.js
+++ b/packages/kit/src/runtime/server/index.js
@@ -322,7 +322,7 @@ export async function respond(request, options, state) {
 		]);
 
 		if (is_data_request || type === 'application/json') {
-			return new Response(serialize_error(error, options), {
+			return new Response(serialize_error(error, options.get_stack), {
 				status: 500,
 				headers: { 'content-type': 'application/json; charset=utf-8' }
 			});
diff --git a/packages/kit/src/runtime/server/page/render.js b/packages/kit/src/runtime/server/page/render.js
index dc231a546749..f19b279f791b 100644
--- a/packages/kit/src/runtime/server/page/render.js
+++ b/packages/kit/src/runtime/server/page/render.js
@@ -330,6 +330,7 @@ function try_serialize(data, fail) {
 
 /** @param {(Error & {frame?: string} & {loc?: object}) | undefined | null} error */
 function serialize_error(error) {
+	// TODO use other serialize_error
 	if (!error) return null;
 	let serialized = try_serialize(error);
 	if (!serialized) {
diff --git a/packages/kit/src/runtime/server/utils.js b/packages/kit/src/runtime/server/utils.js
index 290f225c191d..020bdca56a0f 100644
--- a/packages/kit/src/runtime/server/utils.js
+++ b/packages/kit/src/runtime/server/utils.js
@@ -58,14 +58,35 @@ export function normalize_request_method(event) {
 
 /**
  * @param {Error} error
- * @param {import('types').SSROptions} options
+ * @param {(error: Error) => string | undefined} get_stack
  */
-export function serialize_error(error, options) {
-	const object = {
-		name: error.name,
-		message: error.message,
-		stack: options.get_stack(error)
-	};
+export function serialize_error(error, get_stack) {
+	return JSON.stringify(clone_error(error, get_stack));
+}
+
+/**
+ * @param {Error} error
+ * @param {(error: Error) => string | undefined} get_stack
+ */
+function clone_error(error, get_stack) {
+	const {
+		name,
+		message,
+		stack,
+		// @ts-expect-error i guess typescript doesn't know about error.cause yet
+		cause,
+		...custom
+	} = error;
+
+	/** @type {Record<string, any>} */
+	const object = { name, message, stack: get_stack(error) };
+
+	if (cause) object.cause = clone_error(cause, get_stack);
+
+	for (const key in custom) {
+		// @ts-expect-error
+		object[key] = custom[key];
+	}
 
-	return JSON.stringify(object);
+	return object;
 }
diff --git a/packages/kit/src/runtime/server/utils.spec.js b/packages/kit/src/runtime/server/utils.spec.js
index 34ce41d8ba04..40cbb484767e 100644
--- a/packages/kit/src/runtime/server/utils.spec.js
+++ b/packages/kit/src/runtime/server/utils.spec.js
@@ -1,6 +1,6 @@
 import { test } from 'uvu';
 import * as assert from 'uvu/assert';
-import { lowercase_keys } from './utils.js';
+import { lowercase_keys, serialize_error } from './utils.js';
 
 test('lowercase_keys', () => {
 	assert.equal(lowercase_keys({ KEY: 'value' }), { key: 'value' });
@@ -9,4 +9,43 @@ test('lowercase_keys', () => {
 	assert.equal(lowercase_keys({ 1: 'Hello World' }), { 1: 'Hello World' });
 });
 
+test('serialize_error', () => {
+	class FancyError extends Error {
+		name = 'FancyError';
+		fancy = true;
+
+		/**
+		 * @param {string} message
+		 * @param {{
+		 *   cause?: Error
+		 * }} [options]
+		 */
+		constructor(message, options) {
+			// @ts-ignore go home typescript ur drunk
+			super(message, options);
+		}
+	}
+
+	const error = new FancyError('something went wrong', {
+		cause: new Error('sorry')
+	});
+
+	const serialized = serialize_error(error, (error) => error.stack);
+
+	assert.equal(
+		serialized,
+		JSON.stringify({
+			name: 'FancyError',
+			message: 'something went wrong',
+			stack: error.stack,
+			cause: {
+				name: 'Error',
+				message: 'sorry',
+				stack: error.cause.stack
+			},
+			fancy: true
+		})
+	);
+});
+
 test.run();

From c8b9febd9e78906a306e13fa90175e5778a50c36 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 13:17:52 -0400
Subject: [PATCH 18/29] beef up tests

---
 .../errors/page-endpoint/__error.svelte       |  3 +-
 .../routes/errors/page-endpoint/_shared.js    |  8 +++++
 .../errors/page-endpoint/get-explicit.js      |  4 ++-
 .../errors/page-endpoint/get-implicit.js      |  4 ++-
 .../errors/page-endpoint/post-explicit.js     |  4 ++-
 .../errors/page-endpoint/post-implicit.js     |  4 ++-
 packages/kit/test/apps/basics/test/test.js    | 35 +++++++++++--------
 7 files changed, 42 insertions(+), 20 deletions(-)
 create mode 100644 packages/kit/test/apps/basics/src/routes/errors/page-endpoint/_shared.js

diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
index 5b4cfb3c2ed6..d07904a83968 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
@@ -7,7 +7,8 @@
 			status: $page.status,
 			name: $page.error.name,
 			message: $page.error.message,
-			stack: $page.error.stack
+			stack: $page.error.stack,
+			fancy: $page.error.fancy
 		},
 		null,
 		'  '
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/_shared.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/_shared.js
new file mode 100644
index 000000000000..3ae26117e794
--- /dev/null
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/_shared.js
@@ -0,0 +1,8 @@
+export class FancyError extends Error {
+	name = 'FancyError';
+	fancy = true;
+
+	constructor(message, options) {
+		super(message, options);
+	}
+}
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js
index ecd85beed882..21e6c7139780 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-explicit.js
@@ -1,4 +1,6 @@
+import { FancyError } from './_shared.js';
+
 export const get = () => ({
 	status: 400,
-	body: new Error('oops')
+	body: new FancyError('oops')
 });
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.js
index a3f81411957c..abf242945a3a 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.js
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/get-implicit.js
@@ -1,3 +1,5 @@
+import { FancyError } from './_shared.js';
+
 export const get = () => {
-	throw new Error('oops');
+	throw new FancyError('oops');
 };
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js
index 13a1d4c54bde..302a2992c464 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-explicit.js
@@ -1,4 +1,6 @@
+import { FancyError } from './_shared.js';
+
 export const post = () => ({
 	status: 400,
-	body: new Error('oops')
+	body: new FancyError('oops')
 });
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.js b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.js
index 4cdd25946513..da5561fe7696 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.js
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/post-implicit.js
@@ -1,3 +1,5 @@
+import { FancyError } from './_shared.js';
+
 export const post = () => {
-	throw new Error('oops');
+	throw new FancyError('oops');
 };
diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index 18f02d94e095..c84cb61f4359 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1141,15 +1141,16 @@ test.describe('Errors', () => {
 		await page.goto('/errors/page-endpoint');
 		await clicknav('#get-implicit');
 		const json = await page.textContent('pre');
-		const { status, name, message, stack } = JSON.parse(json);
+		const { status, name, message, stack, fancy } = JSON.parse(json);
 
 		expect(status).toBe(500);
-		expect(name).toBe('Error');
+		expect(name).toBe('FancyError');
 		expect(message).toBe('oops');
+		expect(fancy).toBe(true);
 
 		if (process.env.DEV) {
 			const lines = stack.split('\n');
-			expect(lines[1]).toContain('get-implicit.js:2:8');
+			expect(lines[1]).toContain('get-implicit.js:4:8');
 		}
 
 		const error = read_errors('/errors/page-endpoint/get-implicit');
@@ -1164,15 +1165,16 @@ test.describe('Errors', () => {
 		await page.goto('/errors/page-endpoint');
 		await clicknav('#get-explicit');
 		const json = await page.textContent('pre');
-		const { status, name, message, stack } = JSON.parse(json);
+		const { status, name, message, stack, fancy } = JSON.parse(json);
 
 		expect(status).toBe(400);
-		expect(name).toBe('Error');
+		expect(name).toBe('FancyError');
 		expect(message).toBe('oops');
+		expect(fancy).toBe(true);
 
 		if (process.env.DEV) {
 			const lines = stack.split('\n');
-			expect(lines[1]).toContain('get-explicit.js:3:8');
+			expect(lines[1]).toContain('get-explicit.js:5:8');
 		}
 
 		const error = read_errors('/errors/page-endpoint/get-explicit');
@@ -1188,15 +1190,16 @@ test.describe('Errors', () => {
 		await page.goto('/errors/page-endpoint');
 		await Promise.all([page.waitForNavigation(), page.click('#post-implicit')]);
 		const json = await page.textContent('pre');
-		const { status, name, message, stack } = JSON.parse(json);
+		const { status, name, message, stack, fancy } = JSON.parse(json);
 
 		expect(status).toBe(500);
-		expect(name).toBe('Error');
+		expect(name).toBe('FancyError');
 		expect(message).toBe('oops');
+		expect(fancy).toBe(true);
 
 		if (process.env.DEV) {
 			const lines = stack.split('\n');
-			expect(lines[1]).toContain('post-implicit.js:2:8');
+			expect(lines[1]).toContain('post-implicit.js:4:8');
 		}
 
 		const error = read_errors('/errors/page-endpoint/post-implicit');
@@ -1212,15 +1215,16 @@ test.describe('Errors', () => {
 		await page.goto('/errors/page-endpoint');
 		await Promise.all([page.waitForNavigation(), page.click('#post-explicit')]);
 		const json = await page.textContent('pre');
-		const { status, name, message, stack } = JSON.parse(json);
+		const { status, name, message, stack, fancy } = JSON.parse(json);
 
 		expect(status).toBe(400);
-		expect(name).toBe('Error');
+		expect(name).toBe('FancyError');
 		expect(message).toBe('oops');
+		expect(fancy).toBe(true);
 
 		if (process.env.DEV) {
 			const lines = stack.split('\n');
-			expect(lines[1]).toContain('post-explicit.js:3:8');
+			expect(lines[1]).toContain('post-explicit.js:5:8');
 		}
 
 		const error = read_errors('/errors/page-endpoint/post-explicit');
@@ -1236,11 +1240,12 @@ test.describe('Errors', () => {
 			}
 		});
 
-		const { message, name, stack } = await response.json();
+		const { message, name, stack, fancy } = await response.json();
 
 		expect(response.status()).toBe(500);
-		expect(name).toBe('Error');
+		expect(name).toBe('FancyError');
 		expect(message).toBe('oops');
+		expect(fancy).toBe(true);
 
 		if (process.env.DEV) {
 			expect(stack.split('\n').length).toBeGreaterThan(1);
@@ -1261,7 +1266,7 @@ test.describe('Errors', () => {
 		const { message, name, stack } = await response.json();
 
 		expect(response.status()).toBe(400);
-		expect(name).toBe('Error');
+		expect(name).toBe('FancyError');
 		expect(message).toBe('oops');
 
 		if (process.env.DEV) {

From 2694afa4e8ddb9dfa961e8e43b87e46ff521d2c3 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 13:18:09 -0400
Subject: [PATCH 19/29] remove test.only

---
 packages/kit/test/apps/basics/test/test.js | 22 ++++++----------------
 1 file changed, 6 insertions(+), 16 deletions(-)

diff --git a/packages/kit/test/apps/basics/test/test.js b/packages/kit/test/apps/basics/test/test.js
index c84cb61f4359..6d8fbbbaba30 100644
--- a/packages/kit/test/apps/basics/test/test.js
+++ b/packages/kit/test/apps/basics/test/test.js
@@ -1133,7 +1133,7 @@ test.describe('Errors', () => {
 		);
 	});
 
-	test.only('page endpoint GET thrown error message is preserved', async ({
+	test('page endpoint GET thrown error message is preserved', async ({
 		page,
 		clicknav,
 		read_errors
@@ -1157,7 +1157,7 @@ test.describe('Errors', () => {
 		expect(error).toContain('oops');
 	});
 
-	test.only('page endpoint GET returned error message is preserved', async ({
+	test('page endpoint GET returned error message is preserved', async ({
 		page,
 		clicknav,
 		read_errors
@@ -1181,10 +1181,7 @@ test.describe('Errors', () => {
 		expect(error).toBe(undefined);
 	});
 
-	test.only('page endpoint POST thrown error message is preserved', async ({
-		page,
-		read_errors
-	}) => {
+	test('page endpoint POST thrown error message is preserved', async ({ page, read_errors }) => {
 		// The case where we're submitting a POST request via a form.
 		// It should show the __error template with our message.
 		await page.goto('/errors/page-endpoint');
@@ -1206,10 +1203,7 @@ test.describe('Errors', () => {
 		expect(error).toContain('oops');
 	});
 
-	test.only('page endpoint POST returned error message is preserved', async ({
-		page,
-		read_errors
-	}) => {
+	test('page endpoint POST returned error message is preserved', async ({ page, read_errors }) => {
 		// The case where we're submitting a POST request via a form.
 		// It should show the __error template with our message.
 		await page.goto('/errors/page-endpoint');
@@ -1231,9 +1225,7 @@ test.describe('Errors', () => {
 		expect(error).toBe(undefined);
 	});
 
-	test.only('page endpoint thrown error respects `accept: application/json`', async ({
-		request
-	}) => {
+	test('page endpoint thrown error respects `accept: application/json`', async ({ request }) => {
 		const response = await request.get('/errors/page-endpoint/get-implicit', {
 			headers: {
 				accept: 'application/json'
@@ -1254,9 +1246,7 @@ test.describe('Errors', () => {
 		}
 	});
 
-	test.only('page endpoint returned error respects `accept: application/json`', async ({
-		request
-	}) => {
+	test('page endpoint returned error respects `accept: application/json`', async ({ request }) => {
 		const response = await request.get('/errors/page-endpoint/get-explicit', {
 			headers: {
 				accept: 'application/json'

From 75e69e193fe5065d807e90172eefee837284c948 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 13:23:39 -0400
Subject: [PATCH 20/29] reuse serialize_error

---
 .../kit/src/runtime/server/page/render.js     | 20 ++-----------------
 1 file changed, 2 insertions(+), 18 deletions(-)

diff --git a/packages/kit/src/runtime/server/page/render.js b/packages/kit/src/runtime/server/page/render.js
index f19b279f791b..7166e5838165 100644
--- a/packages/kit/src/runtime/server/page/render.js
+++ b/packages/kit/src/runtime/server/page/render.js
@@ -6,6 +6,7 @@ import { render_json_payload_script } from '../../../utils/escape.js';
 import { s } from '../../../utils/misc.js';
 import { Csp, csp_ready } from './csp.js';
 import { PrerenderingURL } from '../../../utils/url.js';
+import { serialize_error } from '../utils.js';
 
 // TODO rename this function/module
 
@@ -179,7 +180,7 @@ export async function render_response({
 			trailing_slash: ${s(options.trailing_slash)},
 			hydrate: ${resolve_opts.ssr && page_config.hydrate ? `{
 				status: ${status},
-				error: ${serialize_error(error)},
+				error: ${error && serialize_error(error, options.get_stack)},
 				nodes: [${branch.map(({ node }) => node.index).join(', ')}],
 				params: ${devalue(event.params)},
 				routeId: ${s(event.routeId)}
@@ -325,20 +326,3 @@ function try_serialize(data, fail) {
 		return null;
 	}
 }
-
-// Ensure we return something truthy so the client will not re-render the page over the error
-
-/** @param {(Error & {frame?: string} & {loc?: object}) | undefined | null} error */
-function serialize_error(error) {
-	// TODO use other serialize_error
-	if (!error) return null;
-	let serialized = try_serialize(error);
-	if (!serialized) {
-		const { name, message, stack } = error;
-		serialized = try_serialize({ ...error, name, message, stack });
-	}
-	if (!serialized) {
-		serialized = '{}';
-	}
-	return serialized;
-}

From 3940383d97c7336eea0d1436db3606a5e6750cf4 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 13:30:54 -0400
Subject: [PATCH 21/29] shut up eslint, you big dummy

---
 packages/kit/src/runtime/server/utils.js | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/packages/kit/src/runtime/server/utils.js b/packages/kit/src/runtime/server/utils.js
index 020bdca56a0f..717dc85c05ea 100644
--- a/packages/kit/src/runtime/server/utils.js
+++ b/packages/kit/src/runtime/server/utils.js
@@ -72,6 +72,8 @@ function clone_error(error, get_stack) {
 	const {
 		name,
 		message,
+		// this should constitute 'using' a var, since it affects `custom`
+		// eslint-disable-next-line
 		stack,
 		// @ts-expect-error i guess typescript doesn't know about error.cause yet
 		cause,

From 091c09fb867a0fa8587024df71f32a2f1e389426 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 13:38:31 -0400
Subject: [PATCH 22/29] bah typescript

---
 packages/kit/src/runtime/server/utils.spec.js                    | 1 +
 .../apps/basics/src/routes/errors/page-endpoint/__error.svelte   | 1 +
 2 files changed, 2 insertions(+)

diff --git a/packages/kit/src/runtime/server/utils.spec.js b/packages/kit/src/runtime/server/utils.spec.js
index 40cbb484767e..458338ecc7db 100644
--- a/packages/kit/src/runtime/server/utils.spec.js
+++ b/packages/kit/src/runtime/server/utils.spec.js
@@ -41,6 +41,7 @@ test('serialize_error', () => {
 			cause: {
 				name: 'Error',
 				message: 'sorry',
+				// @ts-expect-error
 				stack: error.cause.stack
 			},
 			fancy: true
diff --git a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
index d07904a83968..5fdd3f068c7c 100644
--- a/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
+++ b/packages/kit/test/apps/basics/src/routes/errors/page-endpoint/__error.svelte
@@ -8,6 +8,7 @@
 			name: $page.error.name,
 			message: $page.error.message,
 			stack: $page.error.stack,
+			// @ts-expect-error
 			fancy: $page.error.fancy
 		},
 		null,

From 91aa2dd61733476a0e32ff5a30d1a798435f2042 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 15:02:35 -0400
Subject: [PATCH 23/29] stack is already fixed

---
 packages/kit/src/runtime/server/page/render.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/kit/src/runtime/server/page/render.js b/packages/kit/src/runtime/server/page/render.js
index 7166e5838165..e4bdae307f5e 100644
--- a/packages/kit/src/runtime/server/page/render.js
+++ b/packages/kit/src/runtime/server/page/render.js
@@ -180,7 +180,7 @@ export async function render_response({
 			trailing_slash: ${s(options.trailing_slash)},
 			hydrate: ${resolve_opts.ssr && page_config.hydrate ? `{
 				status: ${status},
-				error: ${error && serialize_error(error, options.get_stack)},
+				error: ${error && serialize_error(error, e => e.stack)},
 				nodes: [${branch.map(({ node }) => node.index).join(', ')}],
 				params: ${devalue(event.params)},
 				routeId: ${s(event.routeId)}

From 01fdc7eee8b66200bc75c1ca7b3690f96a6ffd18 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 15:15:40 -0400
Subject: [PATCH 24/29] explicitly add Error to ResponseBody

---
 packages/kit/types/index.d.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/kit/types/index.d.ts b/packages/kit/types/index.d.ts
index 36d64f6fa2db..e58d45caf09e 100644
--- a/packages/kit/types/index.d.ts
+++ b/packages/kit/types/index.d.ts
@@ -267,7 +267,7 @@ export interface ResolveOptions {
 	transformPage?: ({ html }: { html: string }) => MaybePromise<string>;
 }
 
-export type ResponseBody = JSONValue | Uint8Array | ReadableStream;
+export type ResponseBody = JSONValue | Uint8Array | ReadableStream | Error;
 
 export class Server {
 	constructor(manifest: SSRManifest);

From fcbb8998af66b1787b86611b656fdd9122817611 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 15:54:46 -0400
Subject: [PATCH 25/29] overhaul endpoint docs to mention Error

---
 documentation/docs/02-routing.md | 59 ++++++++++++++++++++++----------
 1 file changed, 40 insertions(+), 19 deletions(-)

diff --git a/documentation/docs/02-routing.md b/documentation/docs/02-routing.md
index d4bf789c4628..01012e162bcc 100644
--- a/documentation/docs/02-routing.md
+++ b/documentation/docs/02-routing.md
@@ -51,7 +51,41 @@ A route can have multiple dynamic parameters, for example `src/routes/[category]
 
 ### Endpoints
 
-Endpoints are modules written in `.js` (or `.ts`) files that export [request handler](/docs/types#sveltejs-kit-requesthandler) functions corresponding to HTTP methods. Their job is to make it possible to read and write data that is only available on the server (for example in a database, or on the filesystem).
+Endpoints are modules written in `.js` (or `.ts`) files that export [request handler](/docs/types#sveltejs-kit-requesthandler) functions corresponding to HTTP methods. Request handlers make it possible to read and write data that is only available on the server (for example in a database, or on the filesystem).
+
+Their job is to return a `{ status, headers, body }` object representing the response.
+
+```js
+/// file: src/routes/random.js
+/** @type {import('@sveltejs/kit').RequestHandler} */
+export async function get() {
+	return {
+		status: 200,
+		headers: {
+			'access-control-allow-origin': '*'
+		},
+		body: {
+			number: Math.random()
+		}
+	};
+}
+```
+
+- `status` is an [HTTP status code](https://httpstatusdogs.com):
+  - `2xx` — successful response (default is `200`)
+  - `3xx` — redirection (should be accompanied by a `location` header)
+  - `4xx` — client error
+  - `5xx` — server error
+- `headers` can either be a plain object, as above, or an instance of the [`Headers`](https://developer.mozilla.org/en-US/docs/Web/API/Headers) class
+- `body` can be a plain object or, if something goes wrong, an [`Error`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error). It will be serialized as JSON
+
+A `GET` or `HEAD` response must include a `body`, but beyond that restriction all three properties are optional.
+
+#### Page endpoints
+
+If an endpoint has the same filename as a page (except for the extension), the page gets its props from the endpoint — via `fetch` during client-side navigation, or via direct function call during SSR. If a page uses syntax for [named layouts](/docs/layouts#named-layouts) or [matchers](/docs/routing#advanced-routing-matching) in its filename then the corresponding page endpoint's filename must also include them.
+
+For example, you might have a `src/routes/items/[id].svelte` page paired with a `src/routes/items/[id].js` endpoint:
 
 ```js
 /// file: src/routes/items/[id].js
@@ -77,6 +111,8 @@ export async function get({ params }) {
 
 	if (item) {
 		return {
+			status: 200,
+			headers: {},
 			body: { item }
 		};
 	}
@@ -89,20 +125,7 @@ export async function get({ params }) {
 
 > Don't worry about the `$lib` import, we'll get to that [later](/docs/modules#$lib).
 
-The type of the `get` function above comes from `./[id].d.ts`, which is a file generated by SvelteKit (inside your [`outDir`](/docs/configuration#outdir), using the [`rootDirs`](https://www.typescriptlang.org/tsconfig#rootDirs) option) that provides type safety when accessing `params`. See the section on [generated types](/docs/types#generated-types) for more detail.
-
-The job of a [request handler](/docs/types#sveltejs-kit-requesthandler) is to return a `{ status, headers, body }` object representing the response, where `status` is an [HTTP status code](https://httpstatusdogs.com):
-
-- `2xx` — successful response (default is `200`)
-- `3xx` — redirection (should be accompanied by a `location` header)
-- `4xx` — client error
-- `5xx` — server error
-
-#### Page endpoints
-
-If an endpoint has the same filename as a page (except for the extension), the page gets its props from the endpoint — via `fetch` during client-side navigation, or via direct function call during SSR. If a page uses syntax for [named layouts](/docs/layouts#named-layouts) or [matchers](/docs/routing#advanced-routing-matching) in its filename then the corresponding page endpoint's filename must also include them.
-
-A page like `src/routes/items/[id].svelte` could get its props from the `body` in the endpoint above:
+The type of the `get` function above comes from `./__types/[id].d.ts`, which is a file generated by SvelteKit (inside your [`outDir`](/docs/configuration#outdir), using the [`rootDirs`](https://www.typescriptlang.org/tsconfig#rootDirs) option) that provides type safety when accessing `params`. See the section on [generated types](/docs/types#generated-types) for more detail.
 
 ```svelte
 /// file: src/routes/items/[id].svelte
@@ -114,11 +137,11 @@ A page like `src/routes/items/[id].svelte` could get its props from the `body` i
 <h1>{item.title}</h1>
 ```
 
-Because the page and route have the same URL, you will need to include an `accept: application/json` header to get JSON from the endpoint rather than HTML from the page. You can also get the raw data by appending `/__data.json` to the URL, e.g. `/items/[id]/__data.json`.
+To get the raw data instead of the page, you can include an `accept: application/json` header in the request, or — for convenience — append `/__data.json` to the URL, e.g. `/items/[id]/__data.json`.
 
 #### Standalone endpoints
 
-Most commonly, endpoints exist to provide data to the page with which they're paired. They can, however, exist separately from pages. Standalone endpoints have slightly more flexibility over the returned `body` type — in addition to objects, they can return a `Uint8Array`.
+Most commonly, endpoints exist to provide data to the page with which they're paired. They can, however, exist separately from pages. Standalone endpoints have slightly more flexibility over the returned `body` type — in addition to objects and [`Error`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error) instances, they can return a [`Uint8Array`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array) or a [`ReadableStream`](https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream).
 
 Standalone endpoints can be given a file extension if desired, or accessed directly if not:
 
@@ -129,8 +152,6 @@ Standalone endpoints can be given a file extension if desired, or accessed direc
 | src/routes/data/index.js      | /data      |
 | src/routes/data.js            | /data      |
 
-> Support for streaming request and response bodies is [coming soon](https://github.com/sveltejs/kit/issues/3419).
-
 #### POST, PUT, PATCH, DELETE
 
 Endpoints can handle any HTTP method — not just `GET` — by exporting the corresponding function:

From ca762d89a60b1f75aaf607b57ce895019c74061c Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 16:01:21 -0400
Subject: [PATCH 26/29] more doc tweaks

---
 documentation/docs/02-routing.md              | 30 +++++++++----------
 .../src/lib/docs/client/docs.css              | 10 ++++++-
 2 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/documentation/docs/02-routing.md b/documentation/docs/02-routing.md
index 01012e162bcc..c3f31062d0a2 100644
--- a/documentation/docs/02-routing.md
+++ b/documentation/docs/02-routing.md
@@ -83,9 +83,21 @@ A `GET` or `HEAD` response must include a `body`, but beyond that restriction al
 
 #### Page endpoints
 
-If an endpoint has the same filename as a page (except for the extension), the page gets its props from the endpoint — via `fetch` during client-side navigation, or via direct function call during SSR. If a page uses syntax for [named layouts](/docs/layouts#named-layouts) or [matchers](/docs/routing#advanced-routing-matching) in its filename then the corresponding page endpoint's filename must also include them.
+If an endpoint has the same filename as a page (except for the extension), the page gets its props from the endpoint — via `fetch` during client-side navigation, or via direct function call during SSR. (If a page uses syntax for [named layouts](/docs/layouts#named-layouts) or [matchers](/docs/routing#advanced-routing-matching) in its filename then the corresponding page endpoint's filename must also include them.)
 
-For example, you might have a `src/routes/items/[id].svelte` page paired with a `src/routes/items/[id].js` endpoint:
+For example, you might have a `src/routes/items/[id].svelte` page...
+
+```svelte
+/// file: src/routes/items/[id].svelte
+<script>
+	// populated with data from the endpoint
+	export let item;
+</script>
+
+<h1>{item.title}</h1>
+```
+
+...paired with a `src/routes/items/[id].js` endpoint (don't worry about the `$lib` import, we'll get to that [later](/docs/modules#$lib)):
 
 ```js
 /// file: src/routes/items/[id].js
@@ -123,19 +135,7 @@ export async function get({ params }) {
 }
 ```
 
-> Don't worry about the `$lib` import, we'll get to that [later](/docs/modules#$lib).
-
-The type of the `get` function above comes from `./__types/[id].d.ts`, which is a file generated by SvelteKit (inside your [`outDir`](/docs/configuration#outdir), using the [`rootDirs`](https://www.typescriptlang.org/tsconfig#rootDirs) option) that provides type safety when accessing `params`. See the section on [generated types](/docs/types#generated-types) for more detail.
-
-```svelte
-/// file: src/routes/items/[id].svelte
-<script>
-	// populated with data from the endpoint
-	export let item;
-</script>
-
-<h1>{item.title}</h1>
-```
+> The type of the `get` function above comes from `./__types/[id].d.ts`, which is a file generated by SvelteKit (inside your [`outDir`](/docs/configuration#outdir), using the [`rootDirs`](https://www.typescriptlang.org/tsconfig#rootDirs) option) that provides type safety when accessing `params`. See the section on [generated types](/docs/types#generated-types) for more detail.
 
 To get the raw data instead of the page, you can include an `accept: application/json` header in the request, or — for convenience — append `/__data.json` to the URL, e.g. `/items/[id]/__data.json`.
 
diff --git a/sites/kit.svelte.dev/src/lib/docs/client/docs.css b/sites/kit.svelte.dev/src/lib/docs/client/docs.css
index 4c14f89019bc..f6e9da40a77c 100644
--- a/sites/kit.svelte.dev/src/lib/docs/client/docs.css
+++ b/sites/kit.svelte.dev/src/lib/docs/client/docs.css
@@ -205,13 +205,21 @@
 	background: rgba(255, 62, 0, 0.1) !important;
 }
 
+.content ul ul {
+	margin-bottom: 0;
+}
+
+.content ul > li {
+	margin: 0.5em 0;
+}
+
 /** hacky overrides to allow filenames inside code blocks —
     TODO change the CSS in site-kit so we can get rid of this */
 .code-block {
 	background-color: var(--code-bg);
 	color: var(--code-base);
 	border-radius: 0.5rem;
-	margin: 0 0 1rem 0;
+	margin: 0 0 2rem 0;
 	font-size: 14px;
 	max-width: var(--linemax);
 	box-shadow: inset 1px 1px 6px hsla(205.7, 63.6%, 30.8%, 0.06);

From b19c3f54f9ea1aac3ac9ac2367a4a799014dc81a Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 17:31:45 -0400
Subject: [PATCH 27/29] Update packages/kit/src/runtime/server/utils.spec.js

Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>
---
 packages/kit/src/runtime/server/utils.spec.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/kit/src/runtime/server/utils.spec.js b/packages/kit/src/runtime/server/utils.spec.js
index 458338ecc7db..2748a8b24d6c 100644
--- a/packages/kit/src/runtime/server/utils.spec.js
+++ b/packages/kit/src/runtime/server/utils.spec.js
@@ -21,7 +21,7 @@ test('serialize_error', () => {
 		 * }} [options]
 		 */
 		constructor(message, options) {
-			// @ts-ignore go home typescript ur drunk
+			// @ts-expect-error go home typescript ur drunk
 			super(message, options);
 		}
 	}

From 402ae5c5deb9d81349f760d798a7dc871dc23b3e Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 17:40:50 -0400
Subject: [PATCH 28/29] add comments

---
 packages/kit/src/runtime/server/utils.js | 4 ++++
 packages/kit/src/utils/http.js           | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/packages/kit/src/runtime/server/utils.js b/packages/kit/src/runtime/server/utils.js
index 717dc85c05ea..de9da9cf660c 100644
--- a/packages/kit/src/runtime/server/utils.js
+++ b/packages/kit/src/runtime/server/utils.js
@@ -57,6 +57,10 @@ export function normalize_request_method(event) {
 }
 
 /**
+ * Serialize an error into a JSON string, by copying its `name`, `message`
+ * and (in dev) `stack`, plus any custom properties, plus recursively
+ * serialized `cause` properties. This is necessary because
+ * `JSON.stringify(error) === '{}'`
  * @param {Error} error
  * @param {(error: Error) => string | undefined} get_stack
  */
diff --git a/packages/kit/src/utils/http.js b/packages/kit/src/utils/http.js
index 8ddd7b07a1eb..29fb7ce6cad7 100644
--- a/packages/kit/src/utils/http.js
+++ b/packages/kit/src/utils/http.js
@@ -21,6 +21,8 @@ export function to_headers(object) {
 }
 
 /**
+ * Given an Accept header and a list of possible content types, pick
+ * the most suitable one to respond with
  * @param {string} accept
  * @param {string[]} types
  */

From 83c6252817ae04753c37f1856e678b8e049d8869 Mon Sep 17 00:00:00 2001
From: Rich Harris <hello@rich-harris.dev>
Date: Thu, 7 Jul 2022 17:52:09 -0400
Subject: [PATCH 29/29] DRY some stuff out

---
 .../kit/src/runtime/server/page/load_node.js  | 51 +++++++------------
 1 file changed, 19 insertions(+), 32 deletions(-)

diff --git a/packages/kit/src/runtime/server/page/load_node.js b/packages/kit/src/runtime/server/page/load_node.js
index 8788aa34805d..43299934fb54 100644
--- a/packages/kit/src/runtime/server/page/load_node.js
+++ b/packages/kit/src/runtime/server/page/load_node.js
@@ -436,28 +436,9 @@ async function load_shadow_data(route, event, options, prerender) {
 		};
 
 		if (!is_get) {
-			const result = await handler(event);
-
-			// TODO remove for 1.0
-			// @ts-expect-error
-			if (result.fallthrough) {
-				throw new Error(
-					'fallthrough is no longer supported. Use matchers instead: https://kit.svelte.dev/docs/routing#advanced-routing-matching'
-				);
-			}
-
-			const { status, headers, body } = validate_shadow_output(result);
-			data.status = status;
-
+			const { status, headers, body } = validate_shadow_output(await handler(event));
 			add_cookies(/** @type {string[]} */ (data.cookies), headers);
-
-			// Redirects are respected...
-			if (status >= 300 && status < 400) {
-				data.redirect = /** @type {string} */ (
-					headers instanceof Headers ? headers.get('location') : headers.location
-				);
-				return data;
-			}
+			data.status = status;
 
 			// explicit errors cause an error page...
 			if (body instanceof Error) {
@@ -471,6 +452,14 @@ async function load_shadow_data(route, event, options, prerender) {
 				return data;
 			}
 
+			// ...redirects are respected...
+			if (status >= 300 && status < 400) {
+				data.redirect = /** @type {string} */ (
+					headers instanceof Headers ? headers.get('location') : headers.location
+				);
+				return data;
+			}
+
 			// ...but 4xx and 5xx status codes _don't_ result in the error page
 			// rendering for non-GET requests — instead, we allow the page
 			// to render with any validation errors etc that were returned
@@ -479,17 +468,7 @@ async function load_shadow_data(route, event, options, prerender) {
 
 		const get = (method === 'head' && mod.head) || mod.get;
 		if (get) {
-			const result = await get(event);
-
-			// TODO remove for 1.0
-			// @ts-expect-error
-			if (result.fallthrough) {
-				throw new Error(
-					'fallthrough is no longer supported. Use matchers instead: https://kit.svelte.dev/docs/routing#advanced-routing-matching'
-				);
-			}
-
-			const { status, headers, body } = validate_shadow_output(result);
+			const { status, headers, body } = validate_shadow_output(await get(event));
 			add_cookies(/** @type {string[]} */ (data.cookies), headers);
 			data.status = status;
 
@@ -550,6 +529,14 @@ function add_cookies(target, headers) {
  * @param {import('types').ShadowEndpointOutput} result
  */
 function validate_shadow_output(result) {
+	// TODO remove for 1.0
+	// @ts-expect-error
+	if (result.fallthrough) {
+		throw new Error(
+			'fallthrough is no longer supported. Use matchers instead: https://kit.svelte.dev/docs/routing#advanced-routing-matching'
+		);
+	}
+
 	const { status = 200, body = {} } = result;
 	let headers = result.headers || {};