fix: carry the proxy deadline's identity, correct the rounding note (#2319 round 9)

A deadline enforced by the backend could not arrive as an
OAuthRequestTimeoutError: /api/fetch serialized it as a 500 and
createRemoteFetch turned that into a plain Error, so every instanceof
check downstream was false — including the one that lets a stalled
probe escape withRfc8414OidcCompat. That matters most where no
client-side wrapper exists to have fired first: the discovery the SDK
runs from inside the transport. The route now answers 504 with a typed
marker (OAUTH_TIMEOUT_WIRE_CODE plus url and timeoutMs) and
createRemoteFetch reconstructs the error from it, guarded by a shape
check so an upstream's own JSON cannot forge one.

The route's deadline becomes a `fetchTimeoutMs` option defaulting to
the shared constant, so the backstop can be proven to fire in
milliseconds instead of thirty seconds of wall clock — and because a
very slow authorization server is a legitimate reason to want a longer
one.

The rounding rationale was wrong: setTimeout accepts a fractional delay
and truncates it (verified on Node 26); ERR_OUT_OF_RANGE is
AbortSignal.timeout's behaviour, which this wrapper never calls. The
rounding stays, for the reason that is actually true — the budget is
interpolated into the message and exposed as timeoutMs, and a
performance.now() budget would otherwise report 1000.4000000953674ms.

Also drops a sentence in the discarded-stream test that contradicted
the qualification above it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YFnM6RqB4nXqNysgUYmjmq
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/test/core/auth/requestTimeout.test.ts

@@ -318,10 +318,12 @@ describe("withOAuthRequestTimeout", () => {
     await expect(wrapped(URL_UNDER_TEST)).rejects.toBe(boom);
   });
 
-  it("rounds a fractional budget, because setTimeout takes an integer", async () => {
+  it("rounds a fractional budget, which is reported to the caller", async () => {
     vi.useFakeTimers();
-    // A budget derived from `performance.now()` arrives fractional; a fractional
-    // delay throws ERR_OUT_OF_RANGE before the request is ever made.
+    // `setTimeout` would accept the fraction and truncate it. The rounding is
+    // for the budget's *reported* form: a `performance.now()` subtraction would
+    // otherwise name "1000.4000000953674ms" in the message and expose a
+    // non-integer `timeoutMs`.
     const wrapped = withOAuthRequestTimeout(neverSettles, 1000.4);
 
     const assertion = wrapped(URL_UNDER_TEST).catch((err: unknown) => err);

clients/web/src/test/core/mcp/remote/createRemoteFetch.test.ts

@@ -1,5 +1,9 @@
 import { describe, it, expect, vi } from "vitest";
 import { createRemoteFetch } from "@inspector/core/mcp/remote/createRemoteFetch.js";
+import {
+  OAUTH_TIMEOUT_WIRE_CODE,
+  OAuthRequestTimeoutError,
+} from "@inspector/core/auth/requestTimeout.js";
 
 function ok(body: object): Response {
   return new Response(JSON.stringify(body), {
@@ -241,4 +245,83 @@ describe("createRemoteFetch", () => {
       expect(signalOf(fetchFn)).toBeUndefined();
     });
   });
+
+  describe("proxy-side deadline (#2319)", () => {
+    function remoteReturning(status: number, body: string) {
+      const fetchFn = vi.fn<typeof fetch>().mockResolvedValue(
+        new Response(body, {
+          status,
+          headers: { "content-type": "application/json" },
+        }),
+      );
+      return createRemoteFetch({
+        baseUrl: "http://remote.example",
+        fetchFn: fetchFn as unknown as typeof fetch,
+      });
+    }
+
+    it("rebuilds an OAuthRequestTimeoutError from the route's marker", async () => {
+      // The one path where a timeout reaches the browser without a client-side
+      // wrapper having fired first: the discovery the SDK runs from inside the
+      // transport has no wrapper at all, so the backend's deadline is the only
+      // one, and it arrives as an ordinary error response.
+      const remoteFetch = remoteReturning(
+        504,
+        JSON.stringify({
+          error: "proxied request to https://as.example.com/x timed out",
+          code: OAUTH_TIMEOUT_WIRE_CODE,
+          url: "https://as.example.com/x",
+          timeoutMs: 30000,
+        }),
+      );
+
+      const err = await remoteFetch("https://as.example.com/x").catch(
+        (e: unknown) => e,
+      );
+
+      expect(err).toBeInstanceOf(OAuthRequestTimeoutError);
+      const timeout = err as OAuthRequestTimeoutError;
+      expect(timeout.url).toBe("https://as.example.com/x");
+      expect(timeout.timeoutMs).toBe(30000);
+    });
+
+    it("leaves an ordinary error response as a plain Error", async () => {
+      const remoteFetch = remoteReturning(
+        500,
+        JSON.stringify({ error: "connect ECONNREFUSED" }),
+      );
+
+      const err = await remoteFetch("https://as.example.com/x").catch(
+        (e: unknown) => e,
+      );
+
+      expect(err).not.toBeInstanceOf(OAuthRequestTimeoutError);
+      expect((err as Error).message).toMatch(/Remote fetch failed \(500\)/);
+    });
+
+    it("ignores a non-JSON error body rather than throwing on the parse", async () => {
+      const remoteFetch = remoteReturning(502, "<html>bad gateway</html>");
+
+      const err = await remoteFetch("https://as.example.com/x").catch(
+        (e: unknown) => e,
+      );
+
+      expect((err as Error).message).toMatch(/Remote fetch failed \(502\)/);
+    });
+
+    it("ignores a marker whose fields are the wrong shape", async () => {
+      // An upstream could serve JSON of its own through a failing proxy; the
+      // guard checks the field types, not just the code.
+      const remoteFetch = remoteReturning(
+        504,
+        JSON.stringify({ code: OAUTH_TIMEOUT_WIRE_CODE, url: 5 }),
+      );
+
+      const err = await remoteFetch("https://as.example.com/x").catch(
+        (e: unknown) => e,
+      );
+
+      expect(err).not.toBeInstanceOf(OAuthRequestTimeoutError);
+    });
+  });
 });

clients/web/src/test/integration/mcp/remote/server-extra-coverage.test.ts

@@ -51,6 +51,7 @@ interface StartOpts {
   secretStore?: SecretStore;
   logger?: pinoType.Logger;
   seedConfig?: string;
+  fetchTimeoutMs?: number;
 }
 
 async function start(opts: StartOpts = {}): Promise<Harness> {
@@ -65,6 +66,9 @@ async function start(opts: StartOpts = {}): Promise<Harness> {
     initialConfig: { defaultEnvironment: {} },
     secretStore: opts.secretStore ?? new InMemorySecretStore(),
     logger: opts.logger,
+    ...(opts.fetchTimeoutMs !== undefined && {
+      fetchTimeoutMs: opts.fetchTimeoutMs,
+    }),
   });
   const { baseUrl, server } = await new Promise<{
     baseUrl: string;
@@ -465,11 +469,42 @@ describe("server.ts supplemental coverage", () => {
       const payload = (await res.json()) as { status: number; body?: string };
       expect(payload.status).toBe(200);
       expect(payload.body).toBeUndefined();
-      // The upstream sees its connection go away once the handler returns.
-      // Without the cancel this never resolves and the test times out.
+      // The observable contract: the route does not leave the upstream stream
+      // running after it has answered. (Per the qualification above, this does
+      // not isolate the explicit `cancel()` as the cause.)
       await openStreamClosed;
     });
 
+    it("answers 504 with a typed marker when its own deadline fires (#2319)", async () => {
+      // The backstop, on its own short budget so this costs milliseconds rather
+      // than the production thirty seconds. It matters because this is the only
+      // deadline on the transport-internal discovery path, which has no
+      // client-side wrapper — so its error has to survive the hop as something
+      // an `instanceof OAuthRequestTimeoutError` check can still recognize.
+      await stop(h);
+      h = await start({ fetchTimeoutMs: 120 });
+
+      const res = await fetch(`${h.baseUrl}/api/fetch`, {
+        method: "POST",
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({ url: `${targetUrl}/stall` }),
+      });
+
+      expect(res.status).toBe(504);
+      const payload = (await res.json()) as {
+        error: string;
+        code: string;
+        url: string;
+        timeoutMs: number;
+      };
+      expect(payload.code).toBe("oauth_request_timeout");
+      expect(payload.url).toBe(`${targetUrl}/stall`);
+      expect(payload.timeoutMs).toBe(120);
+      expect(payload.error).toContain("timed out after 120ms");
+      // And the upstream is released rather than left running.
+      await stallClosed;
+    });
+
     it("routes the outbound request through HTTP_PROXY (#2067)", async () => {
       // /api/fetch is the browser's ONLY way out to the network — the web
       // client's `environment.fetch` is `createRemoteFetch()`, which forwards