From 49b575cc714966b6074b0271c5cdc01f06cc1d51 Mon Sep 17 00:00:00 2001
From: zh-ffmc <zhakak@fullerton.com.sg>
Date: Wed, 28 Oct 2026 17:57:06 +0800
Subject: [PATCH] fix(mcp): drop X-Glean-Internal-Service header from gateway
 requests

buildTransport() sends X-Glean-Internal-Service: true on every request to
the public MCP gateway (/mcp/gateway/proxy). Glean's public edge refuses
any external request carrying that internal-service marker with a bare
HTTP 403 before auth, so plugin setup fails before the OAuth sign-in
prompt is ever reached - on every instance.

Reproduced live (plugin 3.4.2 and 3.5.1, affected since the header's
introduction):

  POST /mcp/gateway/proxy  (MCP initialize, NO header)
    -> 401 + Www-Authenticate: Bearer resource_metadata=...  (correct)
  same POST with X-Glean-Internal-Service: true
    -> 403 Forbidden (GCP LB HTML page; no X-Glean-Trace-Id, request
       never reaches the Glean app)
  same POST with browser User-Agent + the header
    -> 403  (User-Agent is irrelevant; the header is the trigger)

Removing only the header restores the full flow against a live instance:
401 -> OAuth discovery (PRM/ASM 200) -> dynamic registration 201 ->
token 200 -> gateway initialize 200 -> tools enabled.

The X-Glean-Gateway-Request-Metadata attribution header is untouched.
Adds a regression test that captures the transport's request headers and
asserts the internal-service marker is absent while gateway metadata
remains (fails before this change, passes after). Full bundle suite:
435/435 green; typecheck clean.
---
 shared/glean/mcp/src/remote-client.ts        |  6 ++++-
 shared/glean/mcp/tests/remote-client.test.ts | 28 ++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/shared/glean/mcp/src/remote-client.ts b/shared/glean/mcp/src/remote-client.ts
index cb19530..99339b5 100644
--- a/shared/glean/mcp/src/remote-client.ts
+++ b/shared/glean/mcp/src/remote-client.ts
@@ -154,7 +154,11 @@ function buildTransport(
 ): StreamableHTTPClientTransport {
   const parsedUrl = new URL(serverUrl);
   const headers: Record<string, string> = {
-    "X-Glean-Internal-Service": "true",
+    // Note: do NOT add an "X-Glean-Internal-Service" header here. Glean's
+    // public edge (GCP LB) refuses any external request carrying that
+    // internal-service marker with HTTP 403 before auth, which breaks the
+    // plugin's OAuth flow entirely (gleanwork/claude-plugins installs, 3.4.x
+    // and 3.5.x, were blocked on every instance).
     "X-Glean-Gateway-Request-Metadata": buildGatewayMetadataHeader(chatSessionId),
   };
 
diff --git a/shared/glean/mcp/tests/remote-client.test.ts b/shared/glean/mcp/tests/remote-client.test.ts
index c500bc2..4d13516 100644
--- a/shared/glean/mcp/tests/remote-client.test.ts
+++ b/shared/glean/mcp/tests/remote-client.test.ts
@@ -70,6 +70,34 @@ describe("buildGatewayMetadataHeader", () => {
   });
 });
 
+describe("transport request headers", () => {
+  // Regression: plugin builds up to 3.5.1 sent X-Glean-Internal-Service: true
+  // on every gateway request. Glean's public edge 403s any external request
+  // carrying that internal marker, before auth, breaking setup on every
+  // instance (401 OAuth challenge without it, 403 with it).
+  it("omits the internal-service marker and keeps gateway metadata", async () => {
+    let captured: Headers | undefined;
+    const handler = createMcpHandler(() => {
+      return new McpServer({ name: "test", version: "1.0.0" });
+    });
+    const client = await createRemoteClient("http://test.local/mcp", {
+      fetch: (url, init) => {
+        const request = new Request(url, init);
+        captured ??= request.headers;
+        return handler.fetch(request);
+      },
+    });
+    try {
+      expect(captured).toBeDefined();
+      expect(captured!.has("x-glean-internal-service")).toBe(false);
+      expect(captured!.get("x-glean-gateway-request-metadata")).not.toBeNull();
+    } finally {
+      await client.close();
+      await handler.close();
+    }
+  });
+});
+
 describe("remoteToolTimeoutMs", () => {
   const KEY = "GLEAN_REMOTE_TOOL_TIMEOUT_MS";
   const original = process.env[KEY];