fix(server): return 200 for HEAD requests instead of 404 (#937)

StreamableHTTPServer.ServeHTTP and Handle only switch on POST/GET/DELETE;
all other methods fall through to http.NotFound → 404. Many MCP clients
and health-checkers probe with HEAD before issuing POST.

Add HEAD case to both ServeHTTP and Handle method switches, returning
200 with an empty body.

Co-authored-by: huxiaoyang <huxiaoyang@lingyu.local>

Author: suxiaohun

server/streamable_http.go

@@ -429,6 +429,8 @@ func (s *StreamableHTTPServer) ServeHTTP(w http.ResponseWriter, r *http.Request)
 		s.handleGet(hw, hr)
 	case http.MethodDelete:
 		s.handleDelete(hw, hr)
+	case http.MethodHead:
+		w.WriteHeader(http.StatusOK)
 	default:
 		http.NotFound(w, r)
 	}

server/streamable_http_handle.go

@@ -207,6 +207,8 @@ func (s *StreamableHTTPServer) Handle(w HTTPResponseWriter, r *HTTPRequest) {
 		s.handleGet(w, r)
 	case http.MethodDelete:
 		s.handleDelete(w, r)
+	case http.MethodHead:
+		w.WriteHeader(http.StatusOK)
 	default:
 		writeHTTPError(w, "404 page not found", http.StatusNotFound)
 	}

server/streamable_http_handle_test.go

@@ -203,6 +203,25 @@ func TestStreamableHTTPServer_Handle_UnknownMethod(t *testing.T) {
 	assert.Equal(t, http.StatusNotFound, w.status)
 }
 
+func TestStreamableHTTPServer_Handle_HeadReturns200(t *testing.T) {
+	mcpServer := NewMCPServer("test-mcp-server", "1.0")
+	srv := NewStreamableHTTPServer(mcpServer)
+
+	w := newBufferingHTTPResponseWriter()
+	r := &HTTPRequest{
+		Method:  http.MethodHead,
+		URL:     &url.URL{Path: "/mcp"},
+		Header:  http.Header{},
+		Context: t.Context(),
+	}
+
+	srv.Handle(w, r)
+
+	assert.Equal(t, http.StatusOK, w.status)
+	assert.True(t, w.wrote)
+	assert.Empty(t, w.body)
+}
+
 func TestStreamableHTTPServer_Handle_FlushableWriterMatchesServeHTTP(t *testing.T) {
 	// A streamable HTTPResponseWriter going through Handle should produce
 	// the same JSON body as net/http's default writer going through ServeHTTP

server/streamable_http_test.go

@@ -2525,6 +2525,20 @@ func TestStreamableHTTP_Delete(t *testing.T) {
 	assert.Equal(t, sessionID, hookSession.SessionID())
 }
 
+func TestStreamableHTTP_HeadReturns200(t *testing.T) {
+	mcpServer := NewMCPServer("test-mcp-server", "1.0")
+	sseServer := NewStreamableHTTPServer(mcpServer)
+	testServer := httptest.NewServer(sseServer)
+	defer testServer.Close()
+
+	req, _ := http.NewRequest(http.MethodHead, testServer.URL, nil)
+	resp, err := testServer.Client().Do(req)
+	require.NoError(t, err)
+	defer resp.Body.Close()
+
+	assert.Equal(t, http.StatusOK, resp.StatusCode)
+}
+
 func TestStreamableHTTP_DrainNotifications(t *testing.T) {
 	t.Run("drain pending notifications after response is computed", func(t *testing.T) {
 		mcpServer := NewMCPServer("test-mcp-server", "1.0")