fix: extra headers when they are a Headers object

src/client/streamableHttp.test.ts

@@ -476,6 +476,37 @@ describe("StreamableHTTPClientTransport", () => {
     expect(global.fetch).toHaveBeenCalledTimes(2);
   });
 
+  it("should always send specified custom headers (Headers class)", async () => {
+    const requestInit = {
+      headers: new Headers({
+        "X-Custom-Header": "CustomValue"
+      })
+    };
+    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.headers as Headers).get("x-custom-header")).toBe("CustomValue");
+
+    (requestInit.headers as Headers).set("X-Custom-Header","SecondCustomValue");
+
+    await transport.send({ jsonrpc: "2.0", method: "test", params: {} } as JSONRPCMessage);
+    expect((actualReqInit.headers as Headers).get("x-custom-header")).toBe("SecondCustomValue");
+
+    expect(global.fetch).toHaveBeenCalledTimes(2);
+  });
 
   it("should have exponential backoff with configurable maxRetries", () => {
     // This test verifies the maxRetries and backoff calculation directly

src/client/streamableHttp.ts

@@ -174,9 +174,12 @@ export class StreamableHTTPClientTransport implements Transport {
       headers["mcp-session-id"] = this._sessionId;
     }
 
-    return new Headers(
-      { ...headers, ...this._requestInit?.headers }
-    );
+    const extraHeaders = this._normalizeHeaders(this._requestInit?.headers);
+
+    return new Headers({
+      ...headers,
+      ...extraHeaders,
+    });
   }
 
 
@@ -242,6 +245,20 @@ export class StreamableHTTPClientTransport implements Transport {
 
   }
 
+    private _normalizeHeaders(headers: HeadersInit | undefined): Record<string, string> {
+    if (!headers) return {};
+
+    if (headers instanceof Headers) {
+      return Object.fromEntries(headers.entries());
+    }
+
+    if (Array.isArray(headers)) {
+      return Object.fromEntries(headers);
+    }
+
+    return { ...headers as Record<string, string> };
+  }
+
   /**
    * Schedule a reconnection attempt with exponential backoff
    *