fix: client initial GET sse include requestInit (#895)

Author: De Wildt van Reenen

src/client/streamableHttp.test.ts

@@ -502,6 +502,30 @@ describe("StreamableHTTPClientTransport", () => {
     expect(global.fetch).toHaveBeenCalledTimes(2);
   });
 
+  it("should always include requestInit options for initial connection in SSE", async () => {
+    const requestInit: RequestInit = {
+      credentials: 'include'
+    };
+
+    transport = new StreamableHTTPClientTransport(new URL("http://localhost:1234/mcp"), {
+      requestInit: requestInit
+    });
+
+    let actualReqInit: RequestInit = {};
+
+    ((global.fetch as jest.Mock)).mockImplementation(
+      async (_url, reqInit) => {
+        actualReqInit = reqInit;
+        return new Response(null, { status: 200, headers: { "content-type": "text/event-stream" } });
+      }
+    );
+
+    await transport.start();
+
+    await transport["_startOrAuthSse"]({});
+    expect(actualReqInit.credentials).toBe("include");
+  });
+
   it("should always send specified custom headers (Headers class)", async () => {
     const requestInit = {
       headers: new Headers({

src/client/streamableHttp.ts

@@ -208,6 +208,7 @@ export class StreamableHTTPClientTransport implements Transport {
       }
 
       const response = await (this._fetch ?? fetch)(this._url, {
+        ...this._requestInit,
         method: "GET",
         headers,
         signal: this._abortController?.signal,
@@ -301,7 +302,7 @@ export class StreamableHTTPClientTransport implements Transport {
   }
 
   private _handleSseStream(
-    stream: ReadableStream<Uint8Array> | null, 
+    stream: ReadableStream<Uint8Array> | null,
     options: StartSSEOptions,
     isReconnectable: boolean,
   ): void {
@@ -352,8 +353,8 @@ export class StreamableHTTPClientTransport implements Transport {
 
         // Attempt to reconnect if the stream disconnects unexpectedly and we aren't closing
         if (
-          isReconnectable && 
-          this._abortController && 
+          isReconnectable &&
+          this._abortController &&
           !this._abortController.signal.aborted
         ) {
           // Use the exponential backoff reconnection strategy