transport.go: disable stdio batching for newer protocols

mcp/streamable.go

@@ -624,6 +624,7 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
 	if protocolVersion == "" {
 		protocolVersion = protocolVersion20250326
 	}
+	protocolVersion = negotiatedVersion(protocolVersion)
 
 	if isBatch && protocolVersion >= protocolVersion20250618 {
 		http.Error(w, fmt.Sprintf("JSON-RPC batching is not supported in %s and later (request version: %s)", protocolVersion20250618, protocolVersion), http.StatusBadRequest)

mcp/transport.go

@@ -283,6 +283,8 @@ func (r rwc) Close() error {
 //
 // See [msgBatch] for more discussion of message batching.
 type ioConn struct {
+	protocolVersion string
+
 	writeMu sync.Mutex         // guards Write, which must be concurrency safe.
 	rwc     io.ReadWriteCloser // the underlying stream
 
@@ -360,6 +362,17 @@ func newIOConn(rwc io.ReadWriteCloser) *ioConn {
 
 func (c *ioConn) SessionID() string { return "" }
 
+func (c *ioConn) sessionUpdated(state ServerSessionState) {
+	protocolVersion := ""
+	if state.InitializeParams != nil {
+		protocolVersion = state.InitializeParams.ProtocolVersion
+	}
+	if protocolVersion == "" {
+		protocolVersion = protocolVersion20250326
+	}
+	c.protocolVersion = negotiatedVersion(protocolVersion)
+}
+
 // addBatch records a msgBatch for an incoming batch payload.
 // It returns an error if batch is malformed, containing previously seen IDs.
 //
@@ -458,6 +471,10 @@ func (t *ioConn) Read(ctx context.Context) (jsonrpc.Message, error) {
 	if err != nil {
 		return nil, err
 	}
+	if batch && t.protocolVersion >= protocolVersion20250618 {
+		return nil, fmt.Errorf("JSON-RPC batching is not supported in %s and later (request version: %s)", protocolVersion20250618, t.protocolVersion)
+	}
+
 	t.queue = msgs[1:]
 
 	if batch {

mcp/transport_test.go

@@ -55,17 +55,16 @@ func TestBatchFraming(t *testing.T) {
 
 func TestIOConnRead(t *testing.T) {
 	tests := []struct {
-		name  string
-		input string
-		want  string
+		name            string
+		input           string
+		want            string
+		protocolVersion string
 	}{
-
 		{
 			name:  "valid json input",
 			input: `{"jsonrpc":"2.0","id":1,"method":"test","params":{}}`,
 			want:  "",
 		},
-
 		{
 			name: "newline at the end of first valid json input",
 			input: `{"jsonrpc":"2.0","id":1,"method":"test","params":{}}
@@ -77,13 +76,41 @@ func TestIOConnRead(t *testing.T) {
 			input: `{"jsonrpc":"2.0","id":1,"method":"test","params":{}},`,
 			want:  "invalid trailing data at the end of stream",
 		},
+		{
+			name:            "batching unknown protocol",
+			input:           `[{"jsonrpc":"2.0","id":1,"method":"test1"},{"jsonrpc":"2.0","id":2,"method":"test2"}]`,
+			want:            "",
+			protocolVersion: "",
+		},
+		{
+			name:            "batching old protocol",
+			input:           `[{"jsonrpc":"2.0","id":1,"method":"test1"},{"jsonrpc":"2.0","id":2,"method":"test2"}]`,
+			want:            "",
+			protocolVersion: protocolVersion20241105,
+		},
+		{
+			name:            "batching new protocol",
+			input:           `[{"jsonrpc":"2.0","id":1,"method":"test1"},{"jsonrpc":"2.0","id":2,"method":"test2"}]`,
+			want:            "JSON-RPC batching is not supported in 2025-06-18 and later (request version: 2025-06-18)",
+			protocolVersion: protocolVersion20250618,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			tr := newIOConn(rwc{
 				rc: io.NopCloser(strings.NewReader(tt.input)),
 			})
+			if tt.protocolVersion != "" {
+				tr.sessionUpdated(ServerSessionState{
+					InitializeParams: &InitializeParams{
+						ProtocolVersion: tt.protocolVersion,
+					},
+				})
+			}
 			_, err := tr.Read(context.Background())
+			if err == nil && tt.want != "" {
+				t.Errorf("ioConn.Read() got nil error but wanted %v", tt.want)
+			}
 			if err != nil && err.Error() != tt.want {
 				t.Errorf("ioConn.Read() = %v, want %v", err.Error(), tt.want)
 			}