diff --git a/src/client/sse.test.ts b/src/client/sse.test.ts
index 77b28508..5976633a 100644
--- a/src/client/sse.test.ts
+++ b/src/client/sse.test.ts
@@ -329,6 +329,29 @@ describe("SSEClientTransport", () => {
       expect(mockAuthProvider.tokens).toHaveBeenCalled();
     });
 
+    it("attaches custom header from provider on initial SSE connection", async () => {
+      mockAuthProvider.tokens.mockResolvedValue({
+        access_token: "test-token",
+        token_type: "Bearer"
+      });
+      const customHeaders = {
+        "X-Custom-Header": "custom-value",
+      };
+
+      transport = new SSEClientTransport(baseUrl, {
+        authProvider: mockAuthProvider,
+        requestInit: {
+          headers: customHeaders,
+        },
+      });
+
+      await transport.start();
+
+      expect(lastServerRequest.headers.authorization).toBe("Bearer test-token");
+      expect(lastServerRequest.headers["x-custom-header"]).toBe("custom-value");
+      expect(mockAuthProvider.tokens).toHaveBeenCalled();
+    });
+
     it("attaches auth header from provider on POST requests", async () => {
       mockAuthProvider.tokens.mockResolvedValue({
         access_token: "test-token",
diff --git a/src/client/sse.ts b/src/client/sse.ts
index 5e9f0cf0..7e2c2d81 100644
--- a/src/client/sse.ts
+++ b/src/client/sse.ts
@@ -113,13 +113,17 @@ export class SSEClientTransport implements Transport {
       this._eventSource = new EventSource(
         this._url.href,
         this._eventSourceInit ?? {
-          fetch: (url, init) => this._commonHeaders().then((headers) => fetch(url, {
-            ...init,
-            headers: {
-              ...headers,
-              Accept: "text/event-stream"
-            }
-          })),
+          fetch: async (url, init) => {
+            const commonHeaders = await this._commonHeaders();
+            const allHeaders = { ...commonHeaders, ...this._requestInit?.headers};
+            return fetch(url, {
+              ...init,
+              headers: {
+                ...allHeaders,
+                Accept: "text/event-stream"
+              }
+            })
+          }
         },
       );
       this._abortController = new AbortController();