fix(proxy): preserve fetch semantics through the proxy adapter

Addresses review round 4 on #2099 — three posted comments and three suppressed
ones, all valid.

- A rebuilt Response reported `url: ""`, `redirected: false`, and
  `type: "default"` even after undici followed a redirect: a fetch-contract
  regression visible only to proxy users, and one `core/auth/endpointOverrides.ts`
  explicitly guards against by declining to rebuild at all. This adapter has no
  such choice — the response arrives from the wrong realm — so it now reinstates
  the three with `defineProperty`.

- Normalizing a `Request` input dropped every option the Request itself carried
  beyond method/headers/body/signal. Most visibly `redirect`: a
  `new Request(url, { redirect: "manual" })` started following redirects instead
  of surfacing them. All of them are now carried through, with an explicit
  `init` still winning per the fetch contract.

  Both are covered by tests driving a real redirect through a real proxy, and
  both were confirmed to fail without the fix.

- `@modelcontextprotocol/ext-apps` was added only to the CLI's `external` list.
  AGENTS.md requires every root-declared runtime dependency in all three, since
  which client reaches it is a function of what `core/` imports rather than of
  the client's own code. Added to the web runner and TUI lists too.

- The `BUNDLED_CLIENTS` test compared the hard-coded list against a hard-coded
  copy of itself, so a new tsup-bundled client would have left it green while
  going unchecked. It now discovers clients from disk (a client is tsup-bundled
  iff it ships a tsup config) and compares. Verified it fails when a stub client
  is added.

- AGENTS.md's project tree still described `core/mcp/node/` as only the stdio
  transport factory; it now carries `proxyFetch.ts` and its role.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JTHVxSu8AUgHRLvo1ntZ8H
Signed-off-by: cliffhall <cliff@futurescale.com>

AGENTS.md

@@ -169,7 +169,11 @@ v2/main/
 │   │   │                               #   server.json parser, strategy registry + well-known
 │   │   │                               #   paths, strategy-agnostic merge. Pure/isomorphic;
 │   │   │                               #   used by the web file-upload path + /api/import-source.
-│   │   ├── node/                       # Node stdio transport factory
+│   │   ├── node/                       # Node stdio transport factory, plus proxyFetch.ts
+│   │   │                               #   (the shared HTTPS_PROXY/HTTP_PROXY/NO_PROXY
+│   │   │                               #   fetch every Node client installs as its
+│   │   │                               #   `environment.fetch`, and the web backend as the
+│   │   │                               #   createTransportNode default — #2067)
 │   │   ├── remote/                     # Browser HTTP/SSE transport + remote logger/fetch
 │   │   │   └── node/                   # Hono-based remote server backend (used by remote/ above)
 │   │   └── state/                      # InspectorClient state stores consumed by core/react/

clients/cli/tsup.config.ts

@@ -39,10 +39,10 @@ export default defineConfig({
     "@modelcontextprotocol/client",
     "@modelcontextprotocol/core",
     // Root-declared and reached through `core/mcp/apps.ts`, so it must be
-    // external here like every other root runtime dependency — it was being
+    // external here like every other root runtime dependency (AGENTS.md). This
-    // inlined (dragging the transitive v1 SDK and zod-to-json-schema with it),
+    // client was actually inlining it — dragging the transitive v1 SDK and
-    // which the #2067 guard surfaced. ESM, so it was not failing the way
+    // zod-to-json-schema in with it — which the #2067 guard surfaced. ESM, so it
-    // `undici` did; the rule is what it violated.
+    // was not failing the way `undici` did; the rule is what it violated.
     "@modelcontextprotocol/ext-apps",
     "commander",
     "pino",

clients/tui/tsup.config.ts

@@ -127,6 +127,12 @@ export default defineConfig({
     "pino",
     "@modelcontextprotocol/client",
     "@modelcontextprotocol/core",
+    // Root-declared and reached through `core/mcp/apps.ts`, so it must be
+    // external here like every other root runtime dependency — which client
+    // actually reaches it is a function of what `core/` imports, not of this
+    // client's own code, so all three lists carry it (AGENTS.md). The CLI was
+    // inlining it; the #2067 guard surfaced that.
+    "@modelcontextprotocol/ext-apps",
     "@napi-rs/keyring",
     // Root-declared (see the repo's dependency-placement rule) and CJS, which
     // is the combination that bites: tsup externalizes what the *client's*

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

@@ -467,6 +467,11 @@ describe("Transport", () => {
             res.end();
             return;
           }
+          if (req.url === "/redirect") {
+            res.writeHead(302, { location: `${originUrl}/landed` });
+            res.end();
+            return;
+          }
           res.writeHead(200, { "content-type": "text/plain", "x-origin": "1" });
           res.end("hello from origin");
         });
@@ -552,6 +557,37 @@ describe("Transport", () => {
         expect(res.body).toBeNull();
       });
 
+      it("reports url/redirected/type after following a redirect", async () => {
+        // A rebuilt Response cannot carry these through its constructor, so they
+        // are reinstated explicitly. Without that, a proxied response reports
+        // `url: ""` and `redirected: false` even when undici did follow a
+        // redirect — a fetch-contract regression only proxy users would see.
+        clearProxyEnv();
+        process.env.HTTP_PROXY = proxyUrl;
+        const proxyFetch = createProxyFetch();
+
+        const res = await proxyFetch!(`${originUrl}/redirect`);
+        expect(res.status).toBe(200);
+        expect(res.url).toBe(`${originUrl}/landed`);
+        expect(res.redirected).toBe(true);
+        expect(res.type).toBe("basic");
+      });
+
+      it("honors a Request's own redirect mode", async () => {
+        // The Request carries options the adapter must not drop on the way into
+        // undici. `redirect: "manual"` is the visible one: lose it and a caller
+        // asking to see the 302 silently gets the followed 200 instead.
+        clearProxyEnv();
+        process.env.HTTP_PROXY = proxyUrl;
+        const proxyFetch = createProxyFetch();
+
+        const res = await proxyFetch!(
+          new Request(`${originUrl}/redirect`, { redirect: "manual" }),
+        );
+        expect(res.status).toBe(302);
+        expect(res.redirected).toBe(false);
+      });
+
       it("accepts a URL instance and forwards the init", async () => {
         clearProxyEnv();
         process.env.HTTP_PROXY = proxyUrl;

clients/web/tsup.runner.config.ts

@@ -44,6 +44,12 @@ export default defineConfig({
     "proper-lockfile",
     "@modelcontextprotocol/client",
     "@modelcontextprotocol/core",
+    // Root-declared and reached through `core/mcp/apps.ts`, so it must be
+    // external here like every other root runtime dependency — which client
+    // actually reaches it is a function of what `core/` imports, not of this
+    // client's own code, so all three lists carry it (AGENTS.md). The CLI was
+    // inlining it; the #2067 guard surfaced that.
+    "@modelcontextprotocol/ext-apps",
   ],
   esbuildOptions(options) {
     options.alias = {

core/mcp/node/proxyFetch.ts

@@ -67,14 +67,22 @@ const NULL_BODY_STATUSES: ReadonlySet<number> = new Set([
  *
  * The body passes through as a `ReadableStream` rather than being buffered, so
  * SSE responses (the streamable-HTTP transport's whole event channel) still
- * stream. `url` and `redirected` are not carried over — the `Response`
+ * stream.
- * constructor cannot set them — which is safe here only because nothing in this
+ *
- * repo or the SDK reads either.
+ * `url`, `redirected`, and `type` are restored afterwards with
+ * `defineProperty`, because the `Response` constructor cannot set them and a
+ * rebuilt response would otherwise report `url: ""` and `redirected: false`
+ * even after undici followed a redirect — a fetch-contract regression visible
+ * only to proxy users. `core/auth/endpointOverrides.ts` guards the same three
+ * properties by declining to rebuild at all; this adapter has no such choice,
+ * since the response arrives from the wrong realm, so it reinstates them
+ * instead. They shadow the prototype getters as own properties, which is what
+ * `Response` exposes them as to a consumer.
  */
 function toGlobalResponse(res: UndiciResponse): Response {
   const headers = new Headers();
   for (const [key, value] of res.headers) headers.append(key, value);
-  return new Response(
+  const out = new Response(
     NULL_BODY_STATUSES.has(res.status) ? null : asBodyInit(res.body),
     {
       status: res.status,
@@ -82,6 +90,18 @@ function toGlobalResponse(res: UndiciResponse): Response {
       headers,
     },
   );
+  for (const [key, value] of [
+    ["url", res.url],
+    ["redirected", res.redirected],
+    ["type", res.type],
+  ] as const) {
+    Object.defineProperty(out, key, {
+      value,
+      configurable: true,
+      enumerable: false,
+    });
+  }
+  return out;
 }
 
 /**
@@ -153,6 +173,19 @@ function toUndiciRequest(
         // the DOM `RequestInit`, so it rides along as an extra property.
         ...(input.body ? { body: input.body, duplex: "half" } : {}),
         signal: input.signal,
+        // Everything else the Request carries. Dropping these would silently
+        // change fetch semantics under a proxy — most visibly `redirect`, where
+        // a `new Request(url, { redirect: "manual" })` would start following
+        // redirects instead of surfacing them.
+        redirect: input.redirect,
+        cache: input.cache,
+        credentials: input.credentials,
+        integrity: input.integrity,
+        keepalive: input.keepalive,
+        mode: input.mode,
+        referrer: input.referrer,
+        referrerPolicy: input.referrerPolicy,
+        // An explicit `init` still wins, per the fetch contract.
         ...init,
       },
     ];

scripts/verify-bundle-externals.test.mjs

@@ -9,6 +9,14 @@
 
 import { test, describe } from "node:test";
 import assert from "node:assert/strict";
+import { existsSync, readdirSync } from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+const repoRoot = path.resolve(
+  path.dirname(fileURLToPath(import.meta.url)),
+  "..",
+);
 
 import {
   BUNDLED_CLIENTS,
@@ -263,13 +271,38 @@ describe("evaluateClient", () => {
 });
 
 describe("BUNDLED_CLIENTS", () => {
-  test("covers every tsup-bundled client", () => {
+  test("is enrolled from what is actually on disk, not from itself", () => {
-    // `clients/launcher` is plain tsc and emits no bundle, so it is deliberately
+    // Asserting the hard-coded list equals a hard-coded copy of itself proves
-    // absent. If a fourth bundled client appears, it must be enrolled here.
+    // nothing — adding `clients/foo` with a tsup build would leave it green
-    assert.deepEqual(BUNDLED_CLIENTS.map((c) => c.name).sort(), [
+    // while the new bundle went unchecked. So the expected set is DISCOVERED:
-      "cli",
+    // a client is tsup-bundled iff it ships a tsup config. `clients/launcher`
-      "tui",
+    // is plain tsc, emits no bundle, and is correctly absent.
-      "web",
+    const clientsDir = path.join(repoRoot, "clients");
-    ]);
+    const discovered = readdirSync(clientsDir)
+      .filter((name) =>
+        readdirSync(path.join(clientsDir, name)).some((f) =>
+          /^tsup(\..+)?\.config\.ts$/.test(f),
+        ),
+      )
+      .sort();
+
+    assert.ok(discovered.length > 0, "no tsup-bundled clients discovered");
+    assert.deepEqual(
+      BUNDLED_CLIENTS.map((c) => c.name).sort(),
+      discovered,
+      "BUNDLED_CLIENTS is out of sync with the clients that ship a tsup config",
+    );
+  });
+
+  test("points each client at a config and build dir that exist", () => {
+    for (const client of BUNDLED_CLIENTS) {
+      assert.ok(
+        existsSync(path.join(repoRoot, client.config)),
+        `${client.name}: missing ${client.config}`,
+      );
+      // The build dir is produced by `npm run build`; only assert the path is
+      // under the client, since the guard itself reports a missing build.
+      assert.match(client.build, new RegExp(`^clients/${client.name}/`));
+    }
   });
 });