fix(mcp): treat GET 404 as 405 in StreamableHTTPClientTransport (#24847)

Co-authored-by: Coco Sheng <cocosheng@google.com>
Co-authored-by: Spencer <spencertang@google.com>
Co-authored-by: Tommaso Sciortino <sciortino@gmail.com>

packages/core/src/tools/mcp-client.test.ts

@@ -1816,6 +1816,48 @@ describe('mcp-client', () => {
           },
         });
       });
+
+      it('wraps fetch to convert GET 404 to 405 for POST-only servers (e.g. n8n)', async () => {
+        const mockFetch = vi
+          .fn()
+          .mockResolvedValue(
+            new Response(null, { status: 404, statusText: 'Not Found' }),
+          );
+        vi.stubGlobal('fetch', mockFetch);
+
+        try {
+          const transport = await createTransport(
+            'test-server',
+            { httpUrl: 'http://test-server' },
+            false,
+            MOCK_CONTEXT,
+          );
+
+          const wrappedFetch = (
+            transport as unknown as {
+              _fetch: (
+                url: URL | string,
+                init?: RequestInit,
+              ) => Promise<Response>;
+            }
+          )._fetch;
+
+          // GET 404 → 405: server doesn't support optional SSE GET stream
+          const getRes = await wrappedFetch('http://test-server', {
+            method: 'GET',
+          });
+          expect(getRes.status).toBe(405);
+          expect(getRes.statusText).toBe('Method Not Allowed');
+
+          // POST 404 → unchanged: real "not found" errors must still propagate
+          const postRes = await wrappedFetch('http://test-server', {
+            method: 'POST',
+          });
+          expect(postRes.status).toBe(404);
+        } finally {
+          vi.unstubAllGlobals();
+        }
+      });
     });
 
     describe('should connect via url', () => {

packages/core/src/tools/mcp-client.ts

@@ -2123,6 +2123,22 @@ function createUrlTransport(
     | StreamableHTTPClientTransportOptions
     | SSEClientTransportOptions,
 ): StreamableHTTPClientTransport | SSEClientTransport {
+  // Wrap fetch to treat GET 404 as 405 so servers that do not support the
+  // optional SSE GET stream (e.g. n8n native MCP) are handled gracefully.
+  // The SDK already silently ignores 405; 404 is semantically equivalent here.
+  const baseFetch =
+    (transportOptions as StreamableHTTPClientTransportOptions).fetch ??
+    globalThis.fetch;
+  const httpOptions: StreamableHTTPClientTransportOptions = {
+    ...transportOptions,
+    fetch: async (url, init) => {
+      const res = await baseFetch(url, init);
+      return init?.method === 'GET' && res.status === 404
+        ? new Response(null, { status: 405, statusText: 'Method Not Allowed' })
+        : res;
+    },
+  };
+
   // Priority 1: httpUrl (deprecated)
   if (mcpServerConfig.httpUrl) {
     if (mcpServerConfig.url) {
@@ -2133,7 +2149,7 @@ function createUrlTransport(
     }
     return new StreamableHTTPClientTransport(
       new URL(mcpServerConfig.httpUrl),
-      transportOptions,
+      httpOptions,
     );
   }
 
@@ -2142,7 +2158,7 @@ function createUrlTransport(
     if (mcpServerConfig.type === 'http') {
       return new StreamableHTTPClientTransport(
         new URL(mcpServerConfig.url),
-        transportOptions,
+        httpOptions,
       );
     } else if (mcpServerConfig.type === 'sse') {
       return new SSEClientTransport(
@@ -2156,7 +2172,7 @@ function createUrlTransport(
   if (mcpServerConfig.url) {
     return new StreamableHTTPClientTransport(
       new URL(mcpServerConfig.url),
-      transportOptions,
+      httpOptions,
     );
   }