diff --git a/src/client/sse.test.ts b/src/client/sse.test.ts
index 714e1fdd..d01aaff9 100644
--- a/src/client/sse.test.ts
+++ b/src/client/sse.test.ts
@@ -41,24 +41,32 @@ describe("SSEClientTransport", () => {
     resourceServer = createServer((req, res) => {
       lastServerRequest = req;
 
-      // Send SSE headers
-      res.writeHead(200, {
-        "Content-Type": "text/event-stream",
-        "Cache-Control": "no-cache, no-transform",
-        Connection: "keep-alive",
-      });
-
-      // Send the endpoint event
-      res.write("event: endpoint\n");
-      res.write(`data: ${resourceBaseUrl.href}\n\n`);
-
-      // Store reference to send function for tests
-      sendServerMessage = (message: string) => {
-        res.write(`data: ${message}\n\n`);
-      };
-
-      // Handle request body for POST endpoints
-      if (req.method === "POST") {
+      // Extract the path from the URL for proper endpoint handling
+      const urlPath = req.url || '/';
+      const isSSERequest = req.method === 'GET';
+      
+      if (isSSERequest) {
+        // Send SSE headers
+        res.writeHead(200, {
+          "Content-Type": "text/event-stream",
+          "Cache-Control": "no-cache, no-transform",
+          Connection: "keep-alive",
+        });
+  
+        // Determine the message endpoint path by replacing /sse with /messages in the path
+        // This preserves any subpath structure
+        const messagesPath = urlPath.replace(/\/sse$/, '/messages');
+  
+        // Send the endpoint event with the correct subpath
+        res.write("event: endpoint\n");
+        res.write(`data: ${messagesPath}\n\n`);
+  
+        // Store reference to send function for tests
+        sendServerMessage = (message: string) => {
+          res.write(`data: ${message}\n\n`);
+        };
+      } else if (req.method === "POST") {
+        // Handle request body for POST endpoints
         let body = "";
         req.on("data", (chunk) => {
           body += chunk;
@@ -89,6 +97,232 @@ describe("SSEClientTransport", () => {
   });
 
   describe("connection handling", () => {
+    it("maintains custom path when constructing endpoint URL", async () => {
+      // Create a URL with a custom path
+      const customPathUrl = new URL("/custom/path/sse", resourceBaseUrl);
+      transport = new SSEClientTransport(customPathUrl);
+      
+      // Start the transport
+      await transport.start();
+
+      // Send a test message to verify the endpoint URL
+      const message: JSONRPCMessage = {
+        jsonrpc: "2.0",
+        id: "test-1",
+        method: "test",
+        params: {}
+      };
+
+      await transport.send(message);
+
+      // Verify the POST request maintains the custom path
+      expect(lastServerRequest.url).toBe("/custom/path/messages");
+    });
+
+    it("properly preserves complex subpaths in endpoint URL", async () => {
+      // Create a server with a complex subpath structure
+      await resourceServer.close();
+
+      const complexSubpath = "/api/v2/services/mcp";
+      resourceServer = createServer((req, res) => {
+        lastServerRequest = req;
+
+        // For the initial SSE connection
+        if (req.method === "GET" && req.url === `${complexSubpath}/sse`) {
+          res.writeHead(200, {
+            "Content-Type": "text/event-stream",
+            "Cache-Control": "no-cache, no-transform",
+            Connection: "keep-alive",
+          });
+
+          // Send the endpoint event with the messages path that includes the subpath
+          res.write("event: endpoint\n");
+          res.write(`data: ${complexSubpath}/messages\n\n`);
+          return;
+        }
+
+        // For POST requests to send messages, should include the full subpath
+        if (req.method === "POST" && req.url && req.url.startsWith(`${complexSubpath}/messages`)) {
+          res.writeHead(202).end("Accepted");
+          return;
+        }
+
+        res.writeHead(404).end("Not Found");
+      });
+
+      await new Promise<void>((resolve) => {
+        resourceServer.listen(0, "127.0.0.1", () => {
+          const addr = resourceServer.address() as AddressInfo;
+          resourceBaseUrl = new URL(`http://127.0.0.1:${addr.port}`);
+          resolve();
+        });
+      });
+
+      // Connect to the server using the complex subpath
+      const sseUrl = new URL(`${complexSubpath}/sse`, resourceBaseUrl);
+      transport = new SSEClientTransport(sseUrl);
+      await transport.start();
+
+      // Send a message to verify the correct endpoint path is used
+      const message: JSONRPCMessage = {
+        jsonrpc: "2.0",
+        id: "test-1",
+        method: "test",
+        params: {}
+      };
+
+      await transport.send(message);
+
+      // Verify the POST request maintains the complex subpath
+      expect(lastServerRequest.url).toContain(complexSubpath);
+      expect(lastServerRequest.url).toContain("/messages");
+    });
+
+    it("correctly preserves subpath when server URL has a trailing slash", async () => {
+      // Create a server with a subpath ending in a trailing slash
+      await resourceServer.close();
+
+      const subpathWithSlash = "/api/v3/";
+      resourceServer = createServer((req, res) => {
+        lastServerRequest = req;
+
+        // For the initial SSE connection
+        if (req.method === "GET" && req.url === `${subpathWithSlash}sse`) {
+          res.writeHead(200, {
+            "Content-Type": "text/event-stream",
+            "Cache-Control": "no-cache, no-transform",
+            Connection: "keep-alive",
+          });
+
+          // Send the endpoint event with the messages path
+          res.write("event: endpoint\n");
+          res.write(`data: ${subpathWithSlash}messages\n\n`);
+          return;
+        }
+
+        // For POST requests to send messages
+        if (req.method === "POST" && req.url && req.url.startsWith(`${subpathWithSlash}messages`)) {
+          res.writeHead(202).end("Accepted");
+          return;
+        }
+
+        res.writeHead(404).end("Not Found");
+      });
+
+      await new Promise<void>((resolve) => {
+        resourceServer.listen(0, "127.0.0.1", () => {
+          const addr = resourceServer.address() as AddressInfo;
+          resourceBaseUrl = new URL(`http://127.0.0.1:${addr.port}`);
+          resolve();
+        });
+      });
+
+      // Connect to the server using the subpath with trailing slash
+      const sseUrl = new URL(`${subpathWithSlash}sse`, resourceBaseUrl);
+      transport = new SSEClientTransport(sseUrl);
+      await transport.start();
+
+      // Send a message to verify the correct endpoint path is used
+      const message: JSONRPCMessage = {
+        jsonrpc: "2.0",
+        id: "test-1",
+        method: "test",
+        params: {}
+      };
+
+      await transport.send(message);
+
+      // Verify the POST request maintains the subpath with trailing slash
+      expect(lastServerRequest.url).toContain(subpathWithSlash);
+      expect(lastServerRequest.url).toContain("messages");
+    });
+
+    it("handles multiple levels of custom paths", async () => {
+      // Test with a deeper nested path
+      const nestedPathUrl = new URL("/api/v1/custom/deep/path/sse", resourceBaseUrl);
+      transport = new SSEClientTransport(nestedPathUrl);
+      
+      await transport.start();
+
+      const message: JSONRPCMessage = {
+        jsonrpc: "2.0",
+        id: "test-1",
+        method: "test",
+        params: {}
+      };
+
+      await transport.send(message);
+
+      // Verify the POST request maintains the full custom path
+      expect(lastServerRequest.url).toBe("/api/v1/custom/deep/path/messages");
+    });
+
+    it("maintains custom path for SSE connection", async () => {
+      const customPathUrl = new URL("/custom/path/sse", resourceBaseUrl);
+      transport = new SSEClientTransport(customPathUrl);
+      await transport.start();
+      expect(lastServerRequest.url).toBe("/custom/path/sse");
+    });
+
+    it("handles URLs with query parameters", async () => {
+      // For this test, we need a special server setup that correctly handles
+      // the query parameters in both directions
+      await resourceServer.close();
+      
+      resourceServer = createServer((req, res) => {
+        lastServerRequest = req;
+        
+        if (req.method === "GET" && req.url?.startsWith("/custom/path/sse")) {
+          res.writeHead(200, {
+            "Content-Type": "text/event-stream",
+            "Cache-Control": "no-cache, no-transform",
+            Connection: "keep-alive",
+          });
+          
+          // Send a modified endpoint that replaces /sse with /messages but preserves query params
+          const endpoint = "/custom/path/messages?sessionId=test-session";
+          res.write("event: endpoint\n");
+          res.write(`data: ${endpoint}\n\n`);
+        } else if (req.method === "POST" && req.url?.startsWith("/custom/path/messages")) {
+          // The POST endpoint should include the sessionId parameter
+          res.writeHead(200).end();
+        } else {
+          res.writeHead(404).end();
+        }
+      });
+      
+      await new Promise<void>((resolve) => {
+        resourceServer.listen(0, "127.0.0.1", () => {
+          const addr = resourceServer.address() as AddressInfo;
+          resourceBaseUrl = new URL(`http://127.0.0.1:${addr.port}`);
+          resolve();
+        });
+      });
+      
+      // Connect with query params in the URL
+      const urlWithQuery = new URL("/custom/path/sse?param=value", resourceBaseUrl);
+      transport = new SSEClientTransport(urlWithQuery);
+      await transport.start();
+      
+      // Verify the SSE connection includes the query parameters
+      expect(lastServerRequest.url).toBe("/custom/path/sse?param=value");
+      
+      // Send a message
+      const message: JSONRPCMessage = {
+        jsonrpc: "2.0",
+        id: "test-1",
+        method: "test",
+        params: {}
+      };
+      
+      await transport.send(message);
+      
+      // Verify the POST request went to the correct endpoint
+      expect(lastServerRequest.method).toBe("POST");
+      expect(lastServerRequest.url).toContain("/custom/path/messages");
+      expect(lastServerRequest.url).toContain("sessionId=");
+    });
+
     it("establishes SSE connection and receives endpoint", async () => {
       transport = new SSEClientTransport(resourceBaseUrl);
       await transport.start();
diff --git a/src/client/sse.ts b/src/client/sse.ts
index 878a4919..3090f868 100644
--- a/src/client/sse.ts
+++ b/src/client/sse.ts
@@ -146,6 +146,7 @@ export class SSEClientTransport implements Transport {
 
         try {
           this._endpoint = new URL(messageEvent.data, this._url);
+          
           if (this._endpoint.origin !== this._url.origin) {
             throw new Error(
               `Endpoint origin does not match connection origin: ${this._endpoint.origin}`,
diff --git a/src/server/sse.test.ts b/src/server/sse.test.ts
index 2fd2c042..ae3ba44a 100644
--- a/src/server/sse.test.ts
+++ b/src/server/sse.test.ts
@@ -105,5 +105,65 @@ describe('SSEServerTransport', () => {
         `event: endpoint\ndata: /?sessionId=${expectedSessionId}\n\n`
       );
     });
+    
+    it('should correctly preserve the subpath in endpoint URL', async () => {
+      const mockRes = createMockResponse();
+      const endpoint = '/subpath/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: /subpath/messages?sessionId=${expectedSessionId}\n\n`
+      );
+    });
+
+    it('should correctly preserve nested subpaths in endpoint URL', async () => {
+      const mockRes = createMockResponse();
+      const endpoint = '/api/v1/subpath/messages';
+      const transport = new SSEServerTransport(endpoint, mockRes);
+      const expectedSessionId = transport.sessionId;
+
+      await transport.start();
+
+      expect(mockRes.writeHead).toHaveBeenCalledWith(200, expect.any(Object));
+      expect(mockRes.write).toHaveBeenCalledTimes(1);
+      expect(mockRes.write).toHaveBeenCalledWith(
+        `event: endpoint\ndata: /api/v1/subpath/messages?sessionId=${expectedSessionId}\n\n`
+      );
+    });
+
+    it('should correctly preserve the subpath with existing query parameters', async () => {
+      const mockRes = createMockResponse();
+      const endpoint = '/api/v1/subpath/messages?foo=bar';
+      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: /api/v1/subpath/messages?foo=bar&sessionId=${expectedSessionId}\n\n`
+      );
+    });
+
+    it('should correctly handle absolute URLs as endpoints', async () => {
+      const mockRes = createMockResponse();
+      const endpoint = 'https://example.com/subpath/messages';
+      const transport = new SSEServerTransport(endpoint, mockRes);
+      const expectedSessionId = transport.sessionId;
+
+      await transport.start();
+
+      expect(mockRes.writeHead).toHaveBeenCalledWith(200, expect.any(Object));
+      expect(mockRes.write).toHaveBeenCalledTimes(1);
+      expect(mockRes.write).toHaveBeenCalledWith(
+        `event: endpoint\ndata: /subpath/messages?sessionId=${expectedSessionId}\n\n`
+      );
+    });
   });
 });
diff --git a/src/server/sse.ts b/src/server/sse.ts
index 03f6fefc..388e513f 100644
--- a/src/server/sse.ts
+++ b/src/server/sse.ts
@@ -50,11 +50,21 @@ export class SSEServerTransport implements Transport {
       Connection: "keep-alive",
     });
 
-    // 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);
+    // Send the endpoint event with a properly formatted URL that preserves any path prefix
+    let endpointUrl: URL;
+    
+    if (this._endpoint === '' || this._endpoint.startsWith('/')) {
+      const dummyBase = 'http://localhost';
+      endpointUrl = new URL(this._endpoint || '/', dummyBase);
+    } else {
+      try {
+        endpointUrl = new URL(this._endpoint);
+      } catch (error) {
+        const dummyBase = 'http://localhost';
+        endpointUrl = new URL('/', dummyBase);
+      }
+    }
+    
     endpointUrl.searchParams.set('sessionId', this._sessionId);
 
     // Reconstruct the relative URL string (pathname + search + hash)