mcp: add support for logging level (SEP-2575)

mcp/protocol.go

@@ -2102,6 +2102,8 @@ const (
 	MetaKeyClientInfo = "io.modelcontextprotocol/clientInfo"
 	// MetaKeyClientCapabilities carries the client's [ClientCapabilities].
 	MetaKeyClientCapabilities = "io.modelcontextprotocol/clientCapabilities"
+	// MetaKeyLogLevel identifies the desired log level for the request.
+	MetaKeyLogLevel = "io.modelcontextprotocol/logLevel"
 )
 
 // UnsupportedProtocolVersionData is the SEP-2575 payload carried in the

mcp/server.go

@@ -1367,13 +1367,28 @@ func (ss *ServerSession) Elicit(ctx context.Context, params *ElicitParams) (*Eli
 	return res, nil
 }
 
+// logLevelContextKey carries the per-request log level from
+// [ServerSession.handle] to [ServerSession.Log] for new-protocol
+// (>= 2026-06-30) requests. The level is scoped to a single in-flight request
+// — including handler goroutines that call [ServerSession.Log] concurrently —
+// rather than to the session, which avoids races between concurrent requests
+// and aligns with SEP-2575's per-request opt-in model. The value type is
+// [LoggingLevel]; an empty string means the request opted out of log messages.
+type logLevelContextKey struct{}
+
 // Log sends a log message to the client.
-// The message is not sent if the client has not called SetLevel, or if its level
-// is below that of the last SetLevel.
+//
+// For new-protocol (>= 2026-06-30) requests, the level is taken from the
+// originating request's `_meta` field (SEP-2575); an absent or empty value
+// suppresses the message per spec. For old-protocol requests, the level is
+// taken from the session state set via `logging/setLevel`.
 func (ss *ServerSession) Log(ctx context.Context, params *LoggingMessageParams) error {
-	ss.mu.Lock()
-	logLevel := ss.state.LogLevel
-	ss.mu.Unlock()
+	logLevel, ok := ctx.Value(logLevelContextKey{}).(LoggingLevel)
+	if !ok {
+		ss.mu.Lock()
+		logLevel = ss.state.LogLevel
+		ss.mu.Unlock()
+	}
 	if logLevel == "" {
 		// The spec is unclear, but seems to imply that no log messages are sent until the client
 		// sets the level.
@@ -1498,7 +1513,7 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
 	}
 
 	switch req.Method {
-	case methodInitialize, methodPing, notificationInitialized:
+	case methodInitialize, methodPing, notificationInitialized, methodSetLevel:
 		if validatedMeta.usesNewProtocol {
 			ss.server.opts.Logger.Error("method removed in the new protocol", "method", req.Method)
 			return nil, &jsonrpc.Error{
@@ -1533,6 +1548,10 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
 	// server->client calls and notifications to the incoming request from which
 	// they originated. See [idContextKey] for details.
 	ctx = context.WithValue(ctx, idContextKey{}, req.ID)
+	// For new-protocol requests, propagate the per-request log level.
+	if validatedMeta.usesNewProtocol {
+		ctx = context.WithValue(ctx, logLevelContextKey{}, validatedMeta.logLevel)
+	}
 	return handleReceive(ctx, ss, req)
 }
 

mcp/shared.go

@@ -498,6 +498,7 @@ func extractRequestMeta(rawParams json.RawMessage) Meta {
 type validatedMeta struct {
 	usesNewProtocol  bool
 	initializeParams *InitializeParams
+	logLevel         LoggingLevel
 }
 
 // validateRequestMeta inspects a JSON-RPC request to detect whether it follows
@@ -518,8 +519,7 @@ func validateRequestMeta(req *jsonrpc.Request) (*validatedMeta, error) {
 	if !ok {
 		return &validatedMeta{usesNewProtocol: false, initializeParams: nil}, nil
 	}
-	// Notifications do not carry full client identity. In new protocol, only cancel notification
-	// is allowed in STDIO.
+	// Notifications do not carry full client identity.
 	if !req.IsCall() {
 		return &validatedMeta{usesNewProtocol: true, initializeParams: nil}, nil
 	}
@@ -537,11 +537,12 @@ func validateRequestMeta(req *jsonrpc.Request) (*validatedMeta, error) {
 			Message: fmt.Sprintf("missing or invalid _meta field %q", MetaKeyClientCapabilities),
 		}
 	}
+	logLevel, _ := decodeMetaValue[LoggingLevel](meta, MetaKeyLogLevel)
 	return &validatedMeta{usesNewProtocol: true, initializeParams: &InitializeParams{
 		ProtocolVersion: protocolVersion,
 		Capabilities:    capabilities,
 		ClientInfo:      clientInfo,
-	}}, nil
+	}, logLevel: logLevel}, nil
 }
 
 // A Request is a method request with parameters and additional information, such as the session.

mcp/shared_test.go

@@ -21,6 +21,7 @@ func TestValidateRequestMeta(t *testing.T) {
 		isNotification  bool
 		params          any
 		wantUsesNew     bool
+		wantLogLevel    LoggingLevel
 		wantErrContains string
 	}{
 		{
@@ -101,6 +102,35 @@ func TestValidateRequestMeta(t *testing.T) {
 			params:      json.RawMessage(`{"_meta": "not an object", "name": "x"}`),
 			wantUsesNew: false,
 		},
+		{
+			name:   "new protocol with logLevel",
+			method: methodCallTool,
+			params: map[string]any{
+				"_meta": map[string]any{
+					MetaKeyProtocolVersion:    protocolVersion20260630,
+					MetaKeyClientInfo:         map[string]any{"name": "c", "version": "1"},
+					MetaKeyClientCapabilities: map[string]any{},
+					MetaKeyLogLevel:           "warning",
+				},
+				"name": "x",
+			},
+			wantUsesNew:  true,
+			wantLogLevel: "warning",
+		},
+		{
+			name:   "new protocol without logLevel",
+			method: methodCallTool,
+			params: map[string]any{
+				"_meta": map[string]any{
+					MetaKeyProtocolVersion:    protocolVersion20260630,
+					MetaKeyClientInfo:         map[string]any{"name": "c", "version": "1"},
+					MetaKeyClientCapabilities: map[string]any{},
+				},
+				"name": "x",
+			},
+			wantUsesNew:  true,
+			wantLogLevel: "",
+		},
 	}
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
@@ -127,6 +157,9 @@ func TestValidateRequestMeta(t *testing.T) {
 			if usesNew != tc.wantUsesNew {
 				t.Errorf("usesNewProtocol = %v, want %v", usesNew, tc.wantUsesNew)
 			}
+			if vmeta != nil && vmeta.logLevel != tc.wantLogLevel {
+				t.Errorf("logLevel = %q, want %q", vmeta.logLevel, tc.wantLogLevel)
+			}
 			if tc.wantErrContains == "" {
 				if err != nil {
 					t.Errorf("unexpected error: %v", err)

mcp/streamable.go

@@ -441,7 +441,9 @@ func (h *StreamableHTTPHandler) ephemeralConnectOpts(req *http.Request) (opts *S
 	if !hasInitialized && !usesNewProtocol {
 		state.InitializedParams = new(InitializedParams)
 	}
-	state.LogLevel = "info"
+	if !usesNewProtocol {
+		state.LogLevel = "info"
+	}
 	return &ServerSessionOptions{
 		State: state,
 	}, usesNewProtocol, nil