diff --git a/.changeset/fix-miniflare-post-401.md b/.changeset/fix-miniflare-post-401.md
new file mode 100644
index 0000000000..d15ab91c9f
--- /dev/null
+++ b/.changeset/fix-miniflare-post-401.md
@@ -0,0 +1,7 @@
+---
+"miniflare": patch
+---
+
+fix(miniflare): prevent fetch failed on POST 401 responses
+
+Set `credentials: "omit"` in miniflare's fetch wrapper to bypass undici's HTTP 401 authentication retry logic. Workerd uses streamed request bodies where `body.source` is null, causing undici to throw "expected non-null body source" on POST 401 responses.
diff --git a/packages/miniflare/src/http/fetch.ts b/packages/miniflare/src/http/fetch.ts
index 8343f26a20..08df48222c 100644
--- a/packages/miniflare/src/http/fetch.ts
+++ b/packages/miniflare/src/http/fetch.ts
@@ -83,6 +83,15 @@ export async function fetch(
 
 	const response = await undici.fetch(request, {
 		dispatcher: requestInit?.dispatcher,
+		// Prevent undici's HTTP 401 authentication retry logic from triggering.
+		// When a POST request with a streamed body (body.source === null) receives
+		// a 401, undici tries to re-extract the request body for re-sending with
+		// credentials, but fails with "expected non-null body source" because
+		// workerd uses streamed bodies. Setting credentials to "omit" bypasses
+		// the 401 handling entirely (Fetch spec §4.4 step 14). This is safe
+		// because miniflare dispatches to workerd directly — there is no HTTP
+		// authentication to retry.
+		credentials: "omit",
 	});
 	return new Response(response.body, response);
 }
diff --git a/packages/miniflare/src/index.ts b/packages/miniflare/src/index.ts
index 76cf0c9c2b..5da4c0030d 100644
--- a/packages/miniflare/src/index.ts
+++ b/packages/miniflare/src/index.ts
@@ -2707,6 +2707,14 @@ export class Miniflare {
 
 		const forwardInit = forward as RequestInit;
 		forwardInit.dispatcher = dispatcher;
+		// Omit credentials to prevent undici's HTTP 401 authentication retry
+		// logic from triggering. When a POST request with a streamed body
+		// receives a 401, undici tries to re-extract the body source for
+		// credential re-authentication. Since workerd uses streamed bodies
+		// (body.source === null), this causes a "fetch failed" network error
+		// with "expected non-null body source". Setting credentials to "omit"
+		// bypasses the 401 handling entirely (see Fetch spec step 14).
+		forwardInit.credentials = "omit";
 		const response = await fetch(url, forwardInit);
 
 		// If the Worker threw an uncaught exception, propagate it to the caller