diff --git a/src/server/sse.test.ts b/src/server/sse.test.ts
new file mode 100644
index 00000000..2fd2c042
--- /dev/null
+++ b/src/server/sse.test.ts
@@ -0,0 +1,109 @@
+import http from 'http'; 
+import { jest } from '@jest/globals';
+import { SSEServerTransport } from './sse.js'; 
+
+const createMockResponse = () => {
+  const res = {
+    writeHead: jest.fn<http.ServerResponse['writeHead']>(),
+    write: jest.fn<http.ServerResponse['write']>().mockReturnValue(true),
+    on: jest.fn<http.ServerResponse['on']>(),
+  };
+  res.writeHead.mockReturnThis();
+  res.on.mockReturnThis();
+  
+  return res as unknown as http.ServerResponse;
+};
+
+describe('SSEServerTransport', () => {
+  describe('start method', () => { 
+    it('should correctly append sessionId to a simple relative endpoint', async () => { 
+      const mockRes = createMockResponse();
+      const endpoint = '/messages';
+      const transport = new SSEServerTransport(endpoint, mockRes);
+      const expectedSessionId = transport.sessionId;
+
+      await transport.start();
+
+      expect(mockRes.writeHead).toHaveBeenCalledWith(200, expect.any(Object));
+      expect(mockRes.write).toHaveBeenCalledTimes(1);
+      expect(mockRes.write).toHaveBeenCalledWith(
+        `event: endpoint\ndata: /messages?sessionId=${expectedSessionId}\n\n`
+      );
+    });
+
+    it('should correctly append sessionId to an endpoint with existing query parameters', async () => { 
+      const mockRes = createMockResponse();
+      const endpoint = '/messages?foo=bar&baz=qux';
+      const transport = new SSEServerTransport(endpoint, mockRes);
+      const expectedSessionId = transport.sessionId;
+
+      await transport.start();
+
+      expect(mockRes.writeHead).toHaveBeenCalledWith(200, expect.any(Object));
+      expect(mockRes.write).toHaveBeenCalledTimes(1);
+      expect(mockRes.write).toHaveBeenCalledWith(
+        `event: endpoint\ndata: /messages?foo=bar&baz=qux&sessionId=${expectedSessionId}\n\n`
+      );
+    });
+
+    it('should correctly append sessionId to an endpoint with a hash fragment', async () => { 
+      const mockRes = createMockResponse();
+      const endpoint = '/messages#section1';
+      const transport = new SSEServerTransport(endpoint, mockRes);
+      const expectedSessionId = transport.sessionId;
+
+      await transport.start();
+
+      expect(mockRes.writeHead).toHaveBeenCalledWith(200, expect.any(Object));
+      expect(mockRes.write).toHaveBeenCalledTimes(1);
+      expect(mockRes.write).toHaveBeenCalledWith(
+        `event: endpoint\ndata: /messages?sessionId=${expectedSessionId}#section1\n\n`
+      );
+    });
+
+    it('should correctly append sessionId to an endpoint with query parameters and a hash fragment', async () => { 
+      const mockRes = createMockResponse();
+      const endpoint = '/messages?key=value#section2';
+      const transport = new SSEServerTransport(endpoint, mockRes);
+      const expectedSessionId = transport.sessionId;
+
+      await transport.start();
+
+      expect(mockRes.writeHead).toHaveBeenCalledWith(200, expect.any(Object));
+      expect(mockRes.write).toHaveBeenCalledTimes(1);
+      expect(mockRes.write).toHaveBeenCalledWith(
+        `event: endpoint\ndata: /messages?key=value&sessionId=${expectedSessionId}#section2\n\n`
+      );
+    });
+
+    it('should correctly handle the root path endpoint "/"', async () => { 
+      const mockRes = createMockResponse();
+      const endpoint = '/';
+      const transport = new SSEServerTransport(endpoint, mockRes);
+      const expectedSessionId = transport.sessionId;
+
+      await transport.start();
+
+      expect(mockRes.writeHead).toHaveBeenCalledWith(200, expect.any(Object));
+      expect(mockRes.write).toHaveBeenCalledTimes(1);
+      expect(mockRes.write).toHaveBeenCalledWith(
+        `event: endpoint\ndata: /?sessionId=${expectedSessionId}\n\n`
+      );
+    });
+
+    it('should correctly handle an empty string endpoint ""', async () => { 
+      const mockRes = createMockResponse();
+      const endpoint = ''; 
+      const transport = new SSEServerTransport(endpoint, mockRes);
+      const expectedSessionId = transport.sessionId;
+
+      await transport.start();
+
+      expect(mockRes.writeHead).toHaveBeenCalledWith(200, expect.any(Object));
+      expect(mockRes.write).toHaveBeenCalledTimes(1);
+      expect(mockRes.write).toHaveBeenCalledWith(
+        `event: endpoint\ndata: /?sessionId=${expectedSessionId}\n\n`
+      );
+    });
+  });
+});
diff --git a/src/server/sse.ts b/src/server/sse.ts
index 84c1cbb9..701b9e91 100644
--- a/src/server/sse.ts
+++ b/src/server/sse.ts
@@ -4,6 +4,7 @@ import { Transport } from "../shared/transport.js";
 import { JSONRPCMessage, JSONRPCMessageSchema } from "../types.js";
 import getRawBody from "raw-body";
 import contentType from "content-type";
+import { URL } from 'url';
 
 const MAXIMUM_MESSAGE_SIZE = "4mb";
 
@@ -49,8 +50,17 @@ export class SSEServerTransport implements Transport {
     });
 
     // Send the endpoint event
+    // Use a dummy base URL because this._endpoint is relative.
+    // This allows using URL/URLSearchParams for robust parameter handling.
+    const dummyBase = 'http://localhost'; // Any valid base works
+    const endpointUrl = new URL(this._endpoint, dummyBase);
+    endpointUrl.searchParams.set('sessionId', this._sessionId);
+
+    // Reconstruct the relative URL string (pathname + search + hash)
+    const relativeUrlWithSession = endpointUrl.pathname + endpointUrl.search + endpointUrl.hash;
+
     this.res.write(
-      `event: endpoint\ndata: ${encodeURI(this._endpoint)}?sessionId=${this._sessionId}\n\n`,
+      `event: endpoint\ndata: ${relativeUrlWithSession}\n\n`,
     );
 
     this._sseResponse = this.res;