diff --git a/packages/docusaurus/src/commands/serve/__tests__/requestHandler.test.ts b/packages/docusaurus/src/commands/serve/__tests__/requestHandler.test.ts
index 57edd8940c1b..73bc3ffc5bfc 100644
--- a/packages/docusaurus/src/commands/serve/__tests__/requestHandler.test.ts
+++ b/packages/docusaurus/src/commands/serve/__tests__/requestHandler.test.ts
@@ -727,15 +727,27 @@ describe('createServeRequestHandler', () => {
       ).resolves.toBe('200 file.html');
     });
 
-    // TODO fix: keep the baseUrl, "301 /base/missing → 404 404.html"
-    // serve-handler decodes the path before its clean URL redirect
-    it('drops the baseUrl in clean URL redirects of encoded paths', async () => {
-      await expect(
-        serveRequest(
-          {baseUrl: '/base/', trailingSlash: undefined},
-          '/base/missing%2Ehtml',
-        ),
-      ).resolves.toBe('301 /missing → 302 /base/ → 200 index.html');
+    // serve-handler decodes the path before its clean URL redirect, which
+    // would drop the baseUrl
+    describe('answers 404 to encoded clean URL paths', () => {
+      it('with an encoded .html extension', async () => {
+        await expect(
+          serveRequest(
+            {baseUrl: '/base/', trailingSlash: undefined},
+            '/base/missing%2Ehtml',
+          ),
+        ).resolves.toBe('404 404.html');
+      });
+
+      // cspell:ignore Findex
+      it('with an encoded /index segment', async () => {
+        await expect(
+          serveRequest(
+            {baseUrl: '/base/', trailingSlash: undefined},
+            '/base/folder%2Findex',
+          ),
+        ).resolves.toBe('404 404.html');
+      });
     });
 
     it('keeps the query string in redirects without a baseUrl', async () => {
diff --git a/packages/docusaurus/src/commands/serve/requestHandler.ts b/packages/docusaurus/src/commands/serve/requestHandler.ts
index b2f6729a4ea2..92c7426a900f 100644
--- a/packages/docusaurus/src/commands/serve/requestHandler.ts
+++ b/packages/docusaurus/src/commands/serve/requestHandler.ts
@@ -76,8 +76,11 @@ export function createServeRequestHandler({
 
     await serveHandler(req, res, {
       // cleanUrls serves /file with file.html, but also redirects /file.html
-      // to /file, even when the file exists
-      cleanUrls: !(await isHtmlFileRequest(outDir, req.url)),
+      // to /file, even when the file exists, and drops the baseUrl
+      // We already did our clean URL redirects: only existing .html files and
+      // paths with an encoded ".html" or "/index" get there
+      // See https://github.com/meta/docusaurus/pull/12916
+      cleanUrls: !hasCleanUrlSuffix(req.url),
       public: outDir,
       // We do the trailing slash redirects: serve-handler would decode
       // encoded slashes, merge them and drop the baseUrl
@@ -194,6 +197,16 @@ function hasNullByte(url: string): boolean {
   return pathname.includes('%00');
 }
 
+// The decoded paths that serve-handler's cleanUrls redirects
+function hasCleanUrlSuffix(url: string): boolean {
+  const pathname = url.split(/[?#]/)[0]!;
+  try {
+    return /(?:\.html|\/index)$/.test(decodeURIComponent(pathname));
+  } catch {
+    return false; // Malformed percent-encoding, serve-handler answers 400
+  }
+}
+
 // Pages with a dot in their name look like assets, like "docs/2.x"
 async function isPage(outDir: string, urlPath: string): Promise<boolean> {
   const filePath = getFilePath(outDir, urlPath);