From 8797754d0b9c07ae7f6f53a48bc223f0e0e489ca Mon Sep 17 00:00:00 2001
From: "astro-factory[bot]"
 <316791938+astro-factory[bot]@users.noreply.github.com>
Date: Sun, 23 Aug 2026 11:12:11 +0100
Subject: [PATCH] Fix i18n fallback rewrite returning 500 instead of 404 for
 missing prerendered rest-param routes (#17799)

Co-authored-by: factory[bot] <factory[bot]@users.noreply.github.com>
---
 .changeset/strong-lines-sit.md                |  5 ++
 packages/astro/src/core/i18n/handler.ts       | 10 ++-
 .../astro/test/units/i18n/i18n-app.test.ts    | 62 ++++++++++++++++++-
 3 files changed, 75 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/strong-lines-sit.md

diff --git a/.changeset/strong-lines-sit.md b/.changeset/strong-lines-sit.md
new file mode 100644
index 000000000000..fbb08da55ce8
--- /dev/null
+++ b/.changeset/strong-lines-sit.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fixes i18n `fallbackType: "rewrite"` returning 500 instead of 404 when the fallback locale also has no matching static path for a prerendered dynamic route
diff --git a/packages/astro/src/core/i18n/handler.ts b/packages/astro/src/core/i18n/handler.ts
index 31497198a663..3e5563f8422c 100644
--- a/packages/astro/src/core/i18n/handler.ts
+++ b/packages/astro/src/core/i18n/handler.ts
@@ -180,7 +180,15 @@ export async function finalizeI18n(
 					headers: { Location: fallbackDecision.pathname + url.search },
 				});
 			case 'rewrite':
-				return await state.rewrite(fallbackDecision.pathname + url.search);
+				try {
+					return await state.rewrite(fallbackDecision.pathname + url.search);
+				} catch {
+					// The rewrite target has no renderable page either (e.g. a
+					// prerendered dynamic route where the slug is excluded from
+					// getStaticPaths in the fallback locale too). Fall through
+					// so the original 404 response is returned. See #17778.
+					break;
+				}
 			case 'none':
 				break;
 		}
diff --git a/packages/astro/test/units/i18n/i18n-app.test.ts b/packages/astro/test/units/i18n/i18n-app.test.ts
index c172155e6d5f..fdd408408271 100644
--- a/packages/astro/test/units/i18n/i18n-app.test.ts
+++ b/packages/astro/test/units/i18n/i18n-app.test.ts
@@ -5,7 +5,10 @@ import type { RoutingStrategies } from '../../../dist/core/app/common.js';
 import { createI18nMiddleware } from '../../../dist/i18n/middleware.js';
 import { createComponent, render } from '../../../dist/runtime/server/index.js';
 import type { Locales } from '../../../dist/types/public/config.js';
-import { createPage, createTestApp } from '../mocks.ts';
+import { createPage, createTestApp, createRouteData } from '../mocks.ts';
+import { createManifest, createRouteInfo } from '../app/test-helpers.ts';
+import { App } from '../../../dist/core/app/app.js';
+import type { SSRManifest, RouteInfo } from '../../../dist/core/app/types.js';
 import { dynamicPart, spreadPart, staticPart } from '../routing/test-helpers.ts';
 
 interface I18nConfigOverrides {
@@ -666,3 +669,60 @@ describe('i18n via AstroHandler (no middleware) - prefix-always (#16800)', () =>
 		assert.equal(res.status, 404);
 	});
 });
+
+// #17778: i18n fallback rewrite returns 404 (not 500) when the fallback target
+// is a prerendered route whose component is not available at runtime.
+describe('i18n fallback rewrite with prerendered routes not in pageMap', () => {
+	it('returns 404 when fallback rewrite targets a prerendered route with no runtime component', async () => {
+		const i18n = makeI18nConfig({
+			defaultLocale: 'en',
+			locales: ['en', 'es'],
+			strategy: 'pathname-prefix-other-locales' as RoutingStrategies,
+			fallbackType: 'rewrite',
+			fallback: { es: 'en' },
+		});
+
+		// Build a manifest where the prerendered blog routes appear in
+		// manifest.routes (so findRouteToRewrite can match them) but NOT
+		// in pageMap (simulating the production runtime where prerendered
+		// components are not bundled into the server entry).
+		const indexPage = createPage(localePage, {
+			route: '/',
+			component: 'src/pages/index.astro',
+		});
+		const enBlogRoute = createRouteData({
+			route: '/blog/[...slug]',
+			component: 'src/pages/blog/[...slug].astro',
+			prerender: true,
+			pathname: undefined,
+			segments: [[staticPart('blog')], [spreadPart('slug')]],
+		});
+		const esBlogRoute = createRouteData({
+			route: '/es/blog/[...slug]',
+			component: 'src/pages/es/blog/[...slug].astro',
+			prerender: true,
+			pathname: undefined,
+			segments: [[staticPart('es')], [staticPart('blog')], [spreadPart('slug')]],
+		});
+
+		const routes: RouteInfo[] = [
+			createRouteInfo(indexPage.routeData) as RouteInfo,
+			createRouteInfo(enBlogRoute) as RouteInfo,
+			createRouteInfo(esBlogRoute) as RouteInfo,
+		];
+		// Only the SSR index page goes into pageMap; prerendered routes
+		// are excluded, matching real production behavior.
+		const pageMap = new Map<string, () => Promise<Record<string, unknown>>>();
+		pageMap.set(indexPage.routeData.component, indexPage.module);
+
+		const manifest = createManifest({
+			routes,
+			pageMap: pageMap as unknown as SSRManifest['pageMap'],
+			i18n,
+		});
+
+		const app = new App(manifest as unknown as SSRManifest);
+		const res = await app.render(new Request('http://example.com/es/blog/missing-post'));
+		assert.equal(res.status, 404, 'should return 404, not 500');
+	});
+});