[fix] more informative serialization error messages

.changeset/serious-llamas-smoke.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[fix] more informative serialization error messages

packages/kit/src/runtime/server/page/actions.js

@@ -248,5 +248,9 @@ function check_serializability(value, id, path) {
 		}
 	}
 
-	throw new Error(`${path} returned from action in ${id} cannot be serialized as JSON`);
+	throw new Error(
+		`${path} returned from action in ${id} cannot be serialized as JSON without loosing its original type` +
+			// probably the most common case, so let's give a hint
+			(value instanceof Date ? ' (Date objects are serialized to strings)' : '')
+	);
 }

packages/kit/src/runtime/server/page/render.js

@@ -177,7 +177,10 @@ export async function render_response({
 		// function, but it would mean passing more stuff around than we currently do
 		const error = /** @type {any} */ (e);
 		const match = /\[(\d+)\]\.data\.(.+)/.exec(error.path);
-		if (match) throw new Error(`${error.message} (data.${match[2]})`);
+		if (match)
+			throw new Error(
+				`Returned data from a load function related to routeId ${event.routeId} is not serializable: ${error.message} (data.${match[2]})`
+			);
 		throw error;
 	}