fix: server islands broken in dev mode with adapters that don't set buildOutput

.changeset/fix-server-island-dev-build-output.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fixes server islands returning a 500 error in dev mode for adapters that do not set `adapterFeatures.buildOutput` (e.g. `@astrojs/netlify`)

packages/astro/src/core/dev/container.ts

@@ -78,6 +78,13 @@ export async function createContainer({
 		.map((r) => r.clientEntrypoint)
 		.filter(Boolean) as string[];
 
+	// Set the initial buildOutput default before runHookConfigDone, so that
+	// setAdapter() inside astro:config:done can upgrade it to 'server'.
+	// This matches the ordering in the build path (packages/astro/src/core/build/index.ts).
+	if (!settings.adapter?.adapterFeatures?.buildOutput) {
+		settings.buildOutput = getPrerenderDefault(settings.config) ? 'static' : 'server';
+	}
+
 	// Create the route manifest already outside of Vite so that `runHookConfigDone` can use it to inform integrations of the build output
 	await runHookConfigDone({ settings, logger, command: 'dev' });
 
@@ -94,10 +101,6 @@ export async function createContainer({
 			dev: true,
 		},
 	);
-	// If the adapter explicitly set a buildOutput, don't override it
-	if (!settings.adapter?.adapterFeatures?.buildOutput) {
-		settings.buildOutput = getPrerenderDefault(settings.config) ? 'static' : 'server';
-	}
 	const viteConfig = await createVite(
 		{
 			server: { host, headers, open, allowedHosts },

packages/astro/test/server-islands.test.js

@@ -394,6 +394,32 @@ describe('Server islands', () => {
 			});
 		});
 
+		it('can fetch the server island endpoint in dev with adapter that does not set buildOutput', async () => {
+			// Use an adapter that does NOT set adapterFeatures.buildOutput,
+			// like @astrojs/netlify. This triggers the bug in container.ts where
+			// buildOutput is reset to 'static' after runHookConfigDone sets it to 'server'.
+			const devFixture = await loadFixture({
+				root: './fixtures/server-islands/hybrid',
+				adapter: testAdapter({
+					extendAdapter: {
+						adapterFeatures: {},
+					},
+				}),
+			});
+			const devServer = await devFixture.startDevServer();
+			try {
+				const res = await devFixture.fetch('/');
+				assert.equal(res.status, 200);
+				const html = await res.text();
+				const fetchMatch = /fetch\('(\/_server-islands\/Island[^']*)/.exec(html);
+				assert.ok(fetchMatch, 'should have a server island fetch URL');
+				const islandRes = await devFixture.fetch(fetchMatch[1]);
+				assert.equal(islandRes.status, 200, 'server island endpoint should return 200, not GetStaticPathsRequired error');
+			} finally {
+				await devServer.stop();
+			}
+		});
+
 		describe('with no adapter', () => {
 			let devServer;