fix: SSEClientTransport drops path prefix when endpoint URL is not on the HTTP root

mcp/sse.go

@@ -363,6 +363,13 @@ func (c *SSEClientTransport) Connect(ctx context.Context) (Connection, error) {
 			return nil, fmt.Errorf("first event is %q, want %q", evt.Name, "endpoint")
 		}
 		raw := string(evt.Data)
+		// If the server sends an absolute path (starting with "/"), convert it
+		// to a relative path to preserve the base URL's path prefix. This is
+		// necessary when the server is behind a reverse proxy with path-based
+		// routing, where the server may not know about the proxy's path prefix.
+		if len(raw) > 0 && raw[0] == '/' {
+			raw = raw[1:]
+		}
 		return parsedURL.Parse(raw)
 	}()
 	if err != nil {

mcp/sse_test.go

@@ -131,3 +131,60 @@ type roundTripperFunc func(*http.Request) (*http.Response, error)
 func (f roundTripperFunc) RoundTrip(req *http.Request) (*http.Response, error) {
 	return f(req)
 }
+
+// TestSSEClientPreservesPathPrefix verifies that the SSE client preserves the
+// path prefix from the endpoint URL when resolving the message endpoint.
+// This is important when the server is behind a reverse proxy with path-based
+// routing. See https://github.com/modelcontextprotocol/go-sdk/issues/687.
+func TestSSEClientPreservesPathPrefix(t *testing.T) {
+	ctx := context.Background()
+	server := NewServer(testImpl, nil)
+	AddTool(server, &Tool{Name: "greet"}, sayHi)
+
+	sseHandler := NewSSEHandler(func(*http.Request) *Server { return server }, nil)
+
+	// Create a server with a path prefix to simulate a reverse proxy setup.
+	mux := http.NewServeMux()
+	const pathPrefix = "/api/mcp/backend"
+	mux.Handle(pathPrefix+"/", http.StripPrefix(pathPrefix, sseHandler))
+	httpServer := httptest.NewServer(mux)
+	defer httpServer.Close()
+
+	// Track the paths that the client POSTs to.
+	var postedPaths []string
+	customClient := &http.Client{
+		Transport: roundTripperFunc(func(req *http.Request) (*http.Response, error) {
+			if req.Method == http.MethodPost {
+				postedPaths = append(postedPaths, req.URL.Path)
+			}
+			return http.DefaultTransport.RoundTrip(req)
+		}),
+	}
+
+	clientTransport := &SSEClientTransport{
+		Endpoint:   httpServer.URL + pathPrefix + "/",
+		HTTPClient: customClient,
+	}
+
+	c := NewClient(testImpl, nil)
+	cs, err := c.Connect(ctx, clientTransport, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cs.Close()
+
+	// Ping to trigger a POST request.
+	if err := cs.Ping(ctx, nil); err != nil {
+		t.Fatal(err)
+	}
+
+	// Verify that the POST request was sent to a path that preserves the prefix.
+	if len(postedPaths) == 0 {
+		t.Fatal("expected at least one POST request")
+	}
+	for _, path := range postedPaths {
+		if len(path) < len(pathPrefix) || path[:len(pathPrefix)] != pathPrefix {
+			t.Errorf("POST path %q does not preserve prefix %q", path, pathPrefix)
+		}
+	}
+}