diff --git a/src/client/sse.test.ts b/src/client/sse.test.ts
new file mode 100644
index 00000000..02dfac43
--- /dev/null
+++ b/src/client/sse.test.ts
@@ -0,0 +1,107 @@
+import { SSEClientTransport } from "./sse.js";
+import { createServer, type Server } from "http";
+import { JSONRPCMessage } from "../types.js";
+import { AddressInfo } from "net";
+
+describe("SSEClientTransport", () => {
+  let server: Server;
+  let transport: SSEClientTransport;
+  let baseUrl: URL;
+
+  beforeEach((done) => {
+    // Create a test server that will receive the EventSource connection
+    server = createServer((req, res) => {
+      // Store the received headers for verification
+      (server as any).lastRequest = req;
+
+      // Send SSE headers
+      res.writeHead(200, {
+        "Content-Type": "text/event-stream",
+        "Cache-Control": "no-cache",
+        "Connection": "keep-alive"
+      });
+
+      // Send the endpoint event
+      res.write("event: endpoint\n");
+      res.write(`data: ${baseUrl.href}\n\n`);
+    });
+
+    // Start server on random port
+    server.listen(0, "127.0.0.1", () => {
+      const addr = server.address() as AddressInfo;
+      baseUrl = new URL(`http://127.0.0.1:${addr.port}`);
+      done();
+    });
+  });
+
+  afterEach((done) => {
+    transport?.close().then(() => {
+      server.close(done);
+    });
+  });
+
+  it("uses custom fetch implementation from EventSourceInit to add auth headers", async () => {
+    const authToken = "Bearer test-token";
+
+    // Create a fetch wrapper that adds auth header
+    const fetchWithAuth = (url: string | URL, init?: RequestInit) => {
+      const headers = new Headers(init?.headers);
+      headers.set("Authorization", authToken);
+      return fetch(url.toString(), { ...init, headers });
+    };
+
+    transport = new SSEClientTransport(baseUrl, {
+      eventSourceInit: {
+        fetch: fetchWithAuth
+      }
+    });
+
+    await transport.start();
+
+    // Verify the auth header was received by the server
+    const headers = (server as any).lastRequest.headers;
+    expect(headers.authorization).toBe(authToken);
+  });
+
+  it("passes custom headers to fetch requests", async () => {
+    const customHeaders = {
+      Authorization: "Bearer test-token",
+      "X-Custom-Header": "custom-value"
+    };
+
+    transport = new SSEClientTransport(baseUrl, {
+      requestInit: {
+        headers: customHeaders
+      }
+    });
+
+    await transport.start();
+
+    // Mock fetch for the message sending test
+    global.fetch = jest.fn().mockResolvedValue({
+      ok: true
+    });
+
+    const message: JSONRPCMessage = {
+      jsonrpc: "2.0",
+      id: "1",
+      method: "test",
+      params: {}
+    };
+
+    await transport.send(message);
+
+    // Verify fetch was called with correct headers
+    expect(global.fetch).toHaveBeenCalledWith(
+      expect.any(URL),
+      expect.objectContaining({
+        headers: expect.any(Headers)
+      })
+    );
+
+    const calledHeaders = (global.fetch as jest.Mock).mock.calls[0][1].headers;
+    expect(calledHeaders.get("Authorization")).toBe(customHeaders.Authorization);
+    expect(calledHeaders.get("X-Custom-Header")).toBe(customHeaders["X-Custom-Header"]);
+    expect(calledHeaders.get("content-type")).toBe("application/json");
+  });
+});
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