Permit 204/205 status codes on document requests as well

integration/single-fetch-test.ts

@@ -1700,7 +1700,7 @@ test.describe("single-fetch", () => {
     ]);
   });
 
-  test("does not try to encode a turbo-stream body into 204 responses", async ({
+  test.only("does not try to encode a turbo-stream body into 204 responses", async ({
     page,
   }) => {
     let fixture = await createFixture({
@@ -1743,6 +1743,14 @@ test.describe("single-fetch", () => {
       }
     });
 
+    // Document requests
+    let documentRes = await fixture.requestDocument("/?index", {
+      method: "post",
+    });
+    expect(documentRes.status).toBe(204);
+    expect(await documentRes.text()).toBe("");
+
+    // Data requests
     await app.goto("/");
     (await page.$("[data-submit]"))?.click();
     await page.waitForSelector("[data-active]");

packages/react-router/lib/server-runtime/server.ts

@@ -356,9 +356,17 @@ async function handleDocumentRequest(
 
   let headers = getDocumentHeaders(build, context);
 
-  // 304 responses should not have a body or a content-type
-  if (context.statusCode === 304) {
-    return new Response(null, { status: 304, headers });
+  // Do not include a response body if the status code is one of these,
+  // otherwise `undici` will throw an error when constructing the Response:
+  //   /~https://github.com/nodejs/undici/blob/bd98a6303e45d5e0d44192a93731b1defdb415f3/lib/web/fetch/response.js#L522-L528
+  //
+  // Specs:
+  //   https://datatracker.ietf.org/doc/html/rfc9110#name-204-no-content
+  //   https://datatracker.ietf.org/doc/html/rfc9110#name-205-reset-content
+  //   https://datatracker.ietf.org/doc/html/rfc9110#name-304-not-modified
+  let NO_BODY_STATUS_CODES = new Set([204, 205, 304]);
+  if (NO_BODY_STATUS_CODES.has(context.statusCode)) {
+    return new Response(null, { status: context.statusCode, headers });
   }
 
   // Sanitize errors outside of development environments