diff --git a/src/client/sse.test.ts b/src/client/sse.test.ts
index 2d116344..3e3abe68 100644
--- a/src/client/sse.test.ts
+++ b/src/client/sse.test.ts
@@ -382,6 +382,29 @@ 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(resourceBaseUrl, {
+        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 faffecc4..568a5159 100644
--- a/src/client/sse.ts
+++ b/src/client/sse.ts
@@ -106,10 +106,8 @@ export class SSEClientTransport implements Transport {
     return await this._startOrAuth();
   }
 
-  private async _commonHeaders(): Promise<HeadersInit> {
-    const headers = {
-      ...this._requestInit?.headers,
-    } as HeadersInit & Record<string, string>;
+  private async _commonHeaders(): Promise<Headers> {
+    const headers: HeadersInit = {};
     if (this._authProvider) {
       const tokens = await this._authProvider.tokens();
       if (tokens) {
@@ -120,24 +118,24 @@ export class SSEClientTransport implements Transport {
       headers["mcp-protocol-version"] = this._protocolVersion;
     }
 
-    return headers;
+    return new Headers(
+      { ...headers, ...this._requestInit?.headers }
+    );
   }
 
   private _startOrAuth(): Promise<void> {
-const fetchImpl = (this?._eventSourceInit?.fetch ?? this._fetch ?? fetch) as typeof fetch
+    const fetchImpl = (this?._eventSourceInit?.fetch ?? this._fetch ?? fetch) as typeof fetch
     return new Promise((resolve, reject) => {
       this._eventSource = new EventSource(
         this._url.href,
         {
           ...this._eventSourceInit,
           fetch: async (url, init) => {
-            const headers = await this._commonHeaders()
+            const headers = await this._commonHeaders();
+            headers.set("Accept", "text/event-stream");
             const response = await fetchImpl(url, {
               ...init,
-              headers: new Headers({
-                ...headers,
-                Accept: "text/event-stream"
-              })
+              headers,
             })
 
             if (response.status === 401 && response.headers.has('www-authenticate')) {
@@ -238,8 +236,7 @@ const fetchImpl = (this?._eventSourceInit?.fetch ?? this._fetch ?? fetch) as typ
     }
 
     try {
-      const commonHeaders = await this._commonHeaders();
-      const headers = new Headers(commonHeaders);
+      const headers = await this._commonHeaders();
       headers.set("content-type", "application/json");
       const init = {
         ...this._requestInit,