fix: record an intercepted 401's status, headers and body (#2297)

The network tracker wrapped the auth-challenge interceptor, so a 401/403
never reached it as a response: the interceptor cancelled the body and
threw, and the tracker logged only the thrown error. NetworkEntry renders
its response section only when a status is present, so the body — and the
WWW-Authenticate resource_metadata and scope — were never shown.

Move the tracker below the interceptor so it records the real response and
clones its body before the interceptor cancels it, and stop awaiting that
cancel: with the tracker's clone as the other tee branch, the cancel settles
only once the clone reads to the end, which would hang on an unending body.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SG6q6YEDJyJB5H2Hr8nBDS
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/test/integration/mcp/node/transport.test.ts

@@ -242,6 +242,85 @@ describe("Transport", () => {
       }
     });
 
+    it("records an intercepted 401's status, WWW-Authenticate and body (#2297)", async () => {
+      const challengeBody = JSON.stringify({
+        error: "invalid_token",
+        error_description: "token expired",
+      });
+      const wwwAuthenticate =
+        'Bearer resource_metadata="https://mcp.example/.well-known/oauth-protected-resource", scope="files:read"';
+      const fetchFn: typeof fetch = async () =>
+        new Response(challengeBody, {
+          status: 401,
+          statusText: "Unauthorized",
+          headers: {
+            "content-type": "application/json",
+            "www-authenticate": wwwAuthenticate,
+          },
+        });
+
+      const fetchRequests: FetchRequestEntryBase[] = [];
+      const bodies = new Map<string, string>();
+      const result = createTransportNode(
+        { type: "streamable-http", url: "https://mcp.example/mcp" },
+        {
+          fetchFn,
+          interceptAuthChallenges: true,
+          onFetchRequest: (entry) => fetchRequests.push(entry),
+          onFetchResponseBody: (id, body) => bodies.set(id, body),
+        },
+      );
+
+      const client = new Client(
+        { name: "test-client", version: "1.0.0" },
+        { capabilities: {} },
+      );
+      await expect(client.connect(result.transport)).rejects.toThrow(
+        "MCP auth challenge (401)",
+      );
+
+      expect(fetchRequests).toHaveLength(1);
+      const [entry] = fetchRequests;
+      expect(entry.error).toBeUndefined();
+      expect(entry.responseStatus).toBe(401);
+      expect(entry.responseHeaders?.["www-authenticate"]).toBe(wwwAuthenticate);
+      await vi.waitFor(() => expect(bodies.get(entry.id)).toBe(challengeBody));
+    });
+
+    it("still throws an intercepted 401 whose body never ends (#2297)", async () => {
+      const fetchFn: typeof fetch = async () =>
+        new Response(
+          new ReadableStream<Uint8Array>({
+            start(controller) {
+              controller.enqueue(new TextEncoder().encode("partial"));
+            },
+          }),
+          {
+            status: 401,
+            headers: { "www-authenticate": 'Bearer error="invalid_token"' },
+          },
+        );
+
+      const fetchRequests: FetchRequestEntryBase[] = [];
+      const result = createTransportNode(
+        { type: "streamable-http", url: "https://mcp.example/mcp" },
+        {
+          fetchFn,
+          interceptAuthChallenges: true,
+          onFetchRequest: (entry) => fetchRequests.push(entry),
+        },
+      );
+
+      const client = new Client(
+        { name: "test-client", version: "1.0.0" },
+        { capabilities: {} },
+      );
+      await expect(client.connect(result.transport)).rejects.toThrow(
+        "MCP auth challenge (401)",
+      );
+      expect(fetchRequests[0]?.responseStatus).toBe(401);
+    });
+
     it("applies settings.headers to the outgoing streamable-http request", async () => {
       const server = createTestServerHttp({
         serverInfo: createTestServerInfo(),

core/mcp/node/authChallengeFetch.ts

@@ -24,8 +24,11 @@ export function createAuthChallengeInterceptFetch(
     }
 
     // Release the connection before throwing so the SDK transport is not left
-    // with a half-read 401/403 body on streamable HTTP.
+    // with a half-read 401/403 body on streamable HTTP. Not awaited: when the
-    await response.body?.cancel().catch(() => {});
+    // network tracker sits below this wrapper (#2297) the body is one branch of
+    // a tee, and cancelling a branch settles only once the tracker's clone has
+    // read to the end — so a 401 whose body never ends would hang the throw.
+    void response.body?.cancel().catch(() => {});
 
     throw new AuthChallengeError(
       challenge,

core/mcp/node/transport.ts

@@ -67,12 +67,27 @@ export function createTransportNode(
     onAuthChallengeObserved
       ? createAuthChallengeObserverFetch(inner, onAuthChallengeObserved)
       : inner;
+  const withTracking = (inner: typeof fetch): typeof fetch =>
+    onFetchRequest
+      ? createFetchTracker(inner, {
+          trackRequest: onFetchRequest,
+          updateResponseBody: onFetchResponseBody,
+        })
+      : inner;
   // The observer sits *under* the interceptor so it still reports the
   // challenge on the path where interception throws — and, more to the point,
   // on the legacy first-auth path where interception is off entirely.
+  //
+  // The network tracker sits under the interceptor too (#2297). Above it, the
+  // tracker never saw the 401/403 at all: the interceptor cancels the body and
+  // throws, so the entry was recorded from the thrown error alone — no status,
+  // no `WWW-Authenticate`, and no response body, which is exactly the detail
+  // someone debugging an OAuth challenge needs. Below it, the tracker records
+  // the real response and clones its body before the interceptor cancels it.
+  const trackedBase = withTracking(withChallengeObserver(baseFetch));
   const fetchWithOptionalAuthIntercept = interceptAuthChallenges
-    ? createAuthChallengeInterceptFetch(withChallengeObserver(baseFetch))
+    ? createAuthChallengeInterceptFetch(trackedBase)
-    : withChallengeObserver(baseFetch);
+    : trackedBase;
 
   if (serverType === "stdio") {
     const stdioConfig = config as StdioServerConfig;
@@ -111,15 +126,9 @@ export function createTransportNode(
     const configuredSseFetch = sseConfig.eventSourceInit?.fetch as
       | typeof fetch
       | undefined;
-    const sseFetch = configuredSseFetch
+    const trackedFetch = configuredSseFetch
-      ? withChallengeObserver(configuredSseFetch)
+      ? withTracking(withChallengeObserver(configuredSseFetch))
       : fetchWithOptionalAuthIntercept;
-    const trackedFetch = onFetchRequest
-      ? createFetchTracker(sseFetch, {
-          trackRequest: onFetchRequest,
-          updateResponseBody: onFetchResponseBody,
-        })
-      : sseFetch;
 
     const headers = headersFromSettings(settings);
 
@@ -134,18 +143,11 @@ export function createTransportNode(
       ...(headers && { headers }),
     };
 
-    const postFetch = onFetchRequest
-      ? createFetchTracker(fetchWithOptionalAuthIntercept, {
-          trackRequest: onFetchRequest,
-          updateResponseBody: onFetchResponseBody,
-        })
-      : fetchWithOptionalAuthIntercept;
-
     const transport = new SSEClientTransport(url, {
       authProvider,
       eventSourceInit,
       requestInit,
-      fetch: postFetch,
+      fetch: fetchWithOptionalAuthIntercept,
     });
 
     return { transport };
@@ -161,17 +163,10 @@ export function createTransportNode(
       ...(headers && { headers }),
     };
 
-    const transportFetch = onFetchRequest
-      ? createFetchTracker(fetchWithOptionalAuthIntercept, {
-          trackRequest: onFetchRequest,
-          updateResponseBody: onFetchResponseBody,
-        })
-      : fetchWithOptionalAuthIntercept;
-
     const transport = new StreamableHTTPClientTransport(url, {
       authProvider,
       requestInit,
-      fetch: transportFetch,
+      fetch: fetchWithOptionalAuthIntercept,
       // SEP-2350: how the transport reacts to a `403 insufficient_scope`
       // challenge. Defaults to the SDK's `reauthorize` when unset.
       ...(settings?.oauthOnInsufficientScope && {