fix(mcp): avoid duplicating headers in SSE fallback (#7447)

packages/service/core/app/mcp.ts

@@ -269,20 +269,6 @@ export class MCPClient {
             fetch: safeFetch,
             requestInit: {
               headers: this.headers
-            },
-            eventSourceInit: {
-              fetch: (url, init) => {
-                const mergedHeaders = {
-                  ...this.headers
-                };
-
-                Object.assign(mergedHeaders, headersInitToRecord(init?.headers));
-
-                return safeFetch(url, {
-                  ...init,
-                  headers: mergedHeaders
-                });
-              }
             }
           })
         );

packages/service/test/core/app/mcp.test.ts

@@ -495,6 +495,24 @@ describe('MCPClient', () => {
       expect(result).toBe(client);
     });
 
+    it('should pass custom headers once to the SSE fallback transport', async () => {
+      const mcpClient = new MCPClient(config);
+      const client = getPrivateClient(mcpClient);
+      client.connect = vi
+        .fn()
+        .mockRejectedValueOnce(new StreamableHTTPError(405, 'Method Not Allowed'))
+        .mockResolvedValueOnce(undefined);
+
+      await (mcpClient as any).getConnection();
+
+      const sseTransport = client.connect.mock.calls[1][0] as {
+        _requestInit?: RequestInit;
+        _eventSourceInit?: EventSourceInit;
+      };
+      expect(sseTransport._requestInit?.headers).toEqual(config.headers);
+      expect(sseTransport._eventSourceInit).toBeUndefined();
+    });
+
     it('should not fallback to SSE on a non-HTTP (e.g. network) error', async () => {
       const mcpClient = new MCPClient(config);
       const client = getPrivateClient(mcpClient);