internal/mcp: implement logging

Provide a ServerSession method that directly implements the spec
method.

Also provide a custom slog.Handler that writes a notification on each
log call. It uses a slog.JSONHandler to format the message as JSON, then
adds the additional fields of LoggingMessageParams.

I decided that the direct method should handle levels, since that is
in the spec, but that rate-limiting is an extra feature, so is only
provided in the slog handler.

Change-Id: I1190ca124e992e61fb633ac52ee3492c8e6d99c9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/674498
Auto-Submit: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

design/design.md

@@ -823,6 +823,27 @@ Clients call the spec method `Complete` to request completions. Servers automati
 
 ### Logging
 
+MCP specifies a notification for servers to log to clients. Server sessions implement this with the `LoggingMessage` method. It honors the minimum log level established by the client session's `SetLevel` call.
+
+As a convenience, we also provide a `slog.Handler` that allows server authors to write logs with the `log/slog` package::
+```go
+// A LoggingHandler is a [slog.Handler] for MCP.
+type LoggingHandler struct {...}
+
+// LoggingHandlerOptions are options for a LoggingHandler.
+type LoggingHandlerOptions struct {
+	// The value for the "logger" field of logging notifications.
+	LoggerName string
+	// Limits the rate at which log messages are sent.
+	// If zero, there is no rate limiting.
+	MinInterval time.Duration
+}
+
+// NewLoggingHandler creates a [LoggingHandler] that logs to the given [ServerSession] using a
+// [slog.JSONHandler].
+func NewLoggingHandler(ss *ServerSession, opts *LoggingHandlerOptions) *LoggingHandler
+```
+
 Server-to-client logging is configured with `ServerOptions`:
 
 ```go
@@ -832,24 +853,24 @@ type ServerOptions {
   LoggerName string
   // Log notifications to a single ClientSession will not be
   // sent more frequently than this duration.
-  LogInterval time.Duration
+  LoggingInterval time.Duration
 }
 ```
 
-Server sessions have a field `Logger` holding a `slog.Logger` that writes to the client session. A call to a log method like `Info` is translated to a `LoggingMessageNotification` as follows:
+A call to a log method like `Info` is translated to a `LoggingMessageNotification` as follows:
 
 - The attributes and the message populate the "data" property with the output of a `slog.JSONHandler`: The result is always a JSON object, with the key "msg" for the message.
 
 - If the `LoggerName` server option is set, it populates the "logger" property.
 
-- The standard slog levels `Info`, `Debug`, `Warn` and `Error` map to the corresponding levels in the MCP spec. The other spec levels map to integers between the slog levels. For example, "notice" is level 2 because it is between "warning" (slog value 4) and "info" (slog value 0). The `mcp` package defines consts for these levels. To log at the "notice" level, a handler would call `session.Logger.Log(ctx, mcp.LevelNotice, "message")`.
+- The standard slog levels `Info`, `Debug`, `Warn` and `Error` map to the corresponding levels in the MCP spec. The other spec levels map to integers between the slog levels. For example, "notice" is level 2 because it is between "warning" (slog value 4) and "info" (slog value 0). The `mcp` package defines consts for these levels. To log at the "notice" level, a handler would call `Log(ctx, mcp.LevelNotice, "message")`.
 
 A client that wishes to receive log messages must provide a handler:
 
 ```go
 type ClientOptions struct {
   ...
-  LogMessageHandler func(context.Context, *ClientSession, *LoggingMessageParams)
+  LoggingMessageHandler func(context.Context, *ClientSession, *LoggingMessageParams)
 }
 ```
 

mcp/client.go

@@ -53,6 +53,7 @@ type ClientOptions struct {
 	ToolListChangedHandler     func(context.Context, *ClientSession, *ToolListChangedParams)
 	PromptListChangedHandler   func(context.Context, *ClientSession, *PromptListChangedParams)
 	ResourceListChangedHandler func(context.Context, *ClientSession, *ResourceListChangedParams)
+	LoggingMessageHandler      func(context.Context, *ClientSession, *LoggingMessageParams)
 }
 
 // bind implements the binder[*ClientSession] interface, so that Clients can
@@ -208,6 +209,7 @@ var clientMethodInfos = map[string]methodInfo[ClientSession]{
 	notificationToolListChanged:     newMethodInfo(clientMethod((*Client).callToolChangedHandler)),
 	notificationPromptListChanged:   newMethodInfo(clientMethod((*Client).callPromptChangedHandler)),
 	notificationResourceListChanged: newMethodInfo(clientMethod((*Client).callResourceChangedHandler)),
+	notificationLoggingMessage:      newMethodInfo(clientMethod((*Client).callLoggingHandler)),
 }
 
 var _ session[ClientSession] = (*ClientSession)(nil)
@@ -273,6 +275,10 @@ func (c *ClientSession) CallTool(ctx context.Context, name string, args map[stri
 	return standardCall[CallToolResult](ctx, c.conn, methodCallTool, params)
 }
 
+func (c *ClientSession) SetLevel(ctx context.Context, params *SetLevelParams) error {
+	return call(ctx, c.conn, methodSetLevel, params, nil)
+}
+
 // NOTE: the following struct should consist of all fields of callToolParams except name and arguments.
 
 // CallToolOptions contains options to [ClientSession.CallTool].
@@ -302,6 +308,13 @@ func (c *Client) callResourceChangedHandler(ctx context.Context, s *ClientSessio
 	return callNotificationHandler(ctx, c.opts.ResourceListChangedHandler, s, params)
 }
 
+func (c *Client) callLoggingHandler(ctx context.Context, cs *ClientSession, params *LoggingMessageParams) (any, error) {
+	if h := c.opts.LoggingMessageHandler; h != nil {
+		h(ctx, cs, params)
+	}
+	return nil, nil
+}
+
 func standardCall[TRes, TParams any](ctx context.Context, conn *jsonrpc2.Connection, method string, params TParams) (*TRes, error) {
 	var result TRes
 	if err := call(ctx, conn, method, params, &result); err != nil {

mcp/generate.go

@@ -19,6 +19,7 @@ import (
 	"go/format"
 	"io"
 	"log"
+	"maps"
 	"net/http"
 	"os"
 	"reflect"
@@ -110,7 +111,18 @@ var declarations = config{
 		Name:   "-",
 		Fields: config{"Params": {Name: "ListToolsParams"}},
 	},
-	"ListToolsResult":  {},
+	"ListToolsResult":     {},
+	"loggingCapabilities": {Substitute: "struct{}"},
+	"LoggingLevel":        {},
+	"LoggingMessageNotification": {
+		Name: "-",
+		Fields: config{
+			"Params": {
+				Name:   "LoggingMessageParams",
+				Fields: config{"Data": {Substitute: "any"}},
+			},
+		},
+	},
 	"ModelHint":        {},
 	"ModelPreferences": {},
 	"PingRequest": {
@@ -153,8 +165,13 @@ var declarations = config{
 			"Prompts":   {Name: "promptCapabilities"},
 			"Resources": {Name: "resourceCapabilities"},
 			"Tools":     {Name: "toolCapabilities"},
+			"Logging":   {Name: "loggingCapabilities"},
 		},
 	},
+	"SetLevelRequest": {
+		Name:   "-",
+		Fields: config{"Params": {Name: "SetLevelParams"}},
+	},
 	"Tool": {
 		Fields: config{"InputSchema": {Substitute: "*jsonschema.Schema"}},
 	},
@@ -220,7 +237,7 @@ import (
 	}
 	// Write out method names.
 	fmt.Fprintln(buf, `const (`)
-	for name, s := range schema.Definitions {
+	for _, name := range slices.Sorted(maps.Keys(schema.Definitions)) {
 		prefix := "method"
 		method, found := strings.CutSuffix(name, "Request")
 		if !found {
@@ -228,7 +245,7 @@ import (
 			method, found = strings.CutSuffix(name, "Notification")
 		}
 		if found {
-			if ms, ok := s.Properties["method"]; ok {
+			if ms, ok := schema.Definitions[name].Properties["method"]; ok {
 				if c := ms.Const; c != nil {
 					fmt.Fprintf(buf, "%s%s = %q\n", prefix, method, *c)
 				}
@@ -395,9 +412,9 @@ func writeType(w io.Writer, config *typeConfig, def *jsonschema.Schema, named ma
 					fieldTypeSchema = rs
 				}
 				needPointer := isStruct(fieldTypeSchema)
-				// Special case: there are no sampling capabilities defined, but
-				// we want it to be a struct for future expansion.
-				if !needPointer && name == "sampling" {
+				// Special case: there are no sampling or logging capabilities defined,
+				// but we want them to be structs for future expansion.
+				if !needPointer && (name == "sampling" || name == "logging") {
 					needPointer = true
 				}
 				if config != nil && config.Fields[export] != nil {

mcp/logging.go

@@ -0,0 +1,189 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package mcp
+
+import (
+	"bytes"
+	"cmp"
+	"context"
+	"encoding/json"
+	"log/slog"
+	"sync"
+	"time"
+)
+
+// Logging levels.
+const (
+	LevelDebug     = slog.LevelDebug
+	LevelInfo      = slog.LevelInfo
+	LevelNotice    = (slog.LevelInfo + slog.LevelWarn) / 2
+	LevelWarning   = slog.LevelWarn
+	LevelError     = slog.LevelError
+	LevelCritical  = slog.LevelError + 4
+	LevelAlert     = slog.LevelError + 8
+	LevelEmergency = slog.LevelError + 12
+)
+
+var slogToMCP = map[slog.Level]LoggingLevel{
+	LevelDebug:     "debug",
+	LevelInfo:      "info",
+	LevelNotice:    "notice",
+	LevelWarning:   "warning",
+	LevelError:     "error",
+	LevelCritical:  "critical",
+	LevelAlert:     "alert",
+	LevelEmergency: "emergency",
+}
+
+var mcpToSlog = make(map[LoggingLevel]slog.Level)
+
+func init() {
+	for sl, ml := range slogToMCP {
+		mcpToSlog[ml] = sl
+	}
+}
+
+func slogLevelToMCP(sl slog.Level) LoggingLevel {
+	if ml, ok := slogToMCP[sl]; ok {
+		return ml
+	}
+	return "debug" // for lack of a better idea
+}
+
+func mcpLevelToSlog(ll LoggingLevel) slog.Level {
+	if sl, ok := mcpToSlog[ll]; ok {
+		return sl
+	}
+	// TODO: is there a better default?
+	return LevelDebug
+}
+
+// compareLevels behaves like [cmp.Compare] for [LoggingLevel]s.
+func compareLevels(l1, l2 LoggingLevel) int {
+	return cmp.Compare(mcpLevelToSlog(l1), mcpLevelToSlog(l2))
+}
+
+// LoggingHandlerOptions are options for a LoggingHandler.
+type LoggingHandlerOptions struct {
+	// The value for the "logger" field of logging notifications.
+	LoggerName string
+	// Limits the rate at which log messages are sent.
+	// If zero, there is no rate limiting.
+	MinInterval time.Duration
+}
+
+// A LoggingHandler is a [slog.Handler] for MCP.
+type LoggingHandler struct {
+	opts LoggingHandlerOptions
+	ss   *ServerSession
+	// Ensures that the buffer reset is atomic with the write (see Handle).
+	// A pointer so that clones share the mutex. See
+	// https://github.com/golang/example/blob/master/slog-handler-guide/README.md#getting-the-mutex-right.
+	mu              *sync.Mutex
+	lastMessageSent time.Time // for rate-limiting
+	buf             *bytes.Buffer
+	handler         slog.Handler
+}
+
+// NewLoggingHandler creates a [LoggingHandler] that logs to the given [ServerSession] using a
+// [slog.JSONHandler].
+func NewLoggingHandler(ss *ServerSession, opts *LoggingHandlerOptions) *LoggingHandler {
+	var buf bytes.Buffer
+	jsonHandler := slog.NewJSONHandler(&buf, &slog.HandlerOptions{
+		ReplaceAttr: func(_ []string, a slog.Attr) slog.Attr {
+			// Remove level: it appears in LoggingMessageParams.
+			if a.Key == slog.LevelKey {
+				return slog.Attr{}
+			}
+			return a
+		},
+	})
+	lh := &LoggingHandler{
+		ss:      ss,
+		mu:      new(sync.Mutex),
+		buf:     &buf,
+		handler: jsonHandler,
+	}
+	if opts != nil {
+		lh.opts = *opts
+	}
+	return lh
+}
+
+// Enabled implements [slog.Handler.Enabled] by comparing level to the [ServerSession]'s level.
+func (h *LoggingHandler) Enabled(ctx context.Context, level slog.Level) bool {
+	// This is also checked in ServerSession.LoggingMessage, so checking it here
+	// is just an optimization that skips building the JSON.
+	h.ss.mu.Lock()
+	mcpLevel := h.ss.logLevel
+	h.ss.mu.Unlock()
+	return level >= mcpLevelToSlog(mcpLevel)
+}
+
+// WithAttrs implements [slog.Handler.WithAttrs].
+func (h *LoggingHandler) WithAttrs(as []slog.Attr) slog.Handler {
+	h2 := *h
+	h2.handler = h.handler.WithAttrs(as)
+	return &h2
+}
+
+// WithGroup implements [slog.Handler.WithGroup].
+func (h *LoggingHandler) WithGroup(name string) slog.Handler {
+	h2 := *h
+	h2.handler = h.handler.WithGroup(name)
+	return &h2
+}
+
+// Handle implements [slog.Handler.Handle] by writing the Record to a JSONHandler,
+// then calling [ServerSession.LoggingMesssage] with the result.
+func (h *LoggingHandler) Handle(ctx context.Context, r slog.Record) error {
+	err := h.handle(ctx, r)
+	// TODO(jba): find a way to surface the error.
+	// The return value will probably be ignored.
+	return err
+}
+
+func (h *LoggingHandler) handle(ctx context.Context, r slog.Record) error {
+	// Observe the rate limit.
+	// TODO(jba): use golang.org/x/time/rate. (We can't here because it would require adding
+	// golang.org/x/time to the go.mod file.)
+	h.mu.Lock()
+	skip := time.Since(h.lastMessageSent) < h.opts.MinInterval
+	h.mu.Unlock()
+	if skip {
+		return nil
+	}
+
+	var err error
+	// Make the buffer reset atomic with the record write.
+	// We are careful here in the unlikely event that the handler panics.
+	// We don't want to hold the lock for the entire function, because Notify is
+	// an I/O operation.
+	// This can result in out-of-order delivery.
+	func() {
+		h.mu.Lock()
+		defer h.mu.Unlock()
+		h.buf.Reset()
+		err = h.handler.Handle(ctx, r)
+	}()
+	if err != nil {
+		return err
+	}
+
+	h.mu.Lock()
+	h.lastMessageSent = time.Now()
+	h.mu.Unlock()
+
+	params := &LoggingMessageParams{
+		Logger: h.opts.LoggerName,
+		Level:  slogLevelToMCP(r.Level),
+		Data:   json.RawMessage(h.buf.Bytes()),
+	}
+	// We pass the argument context to Notify, even though slog.Handler.Handle's
+	// documentation says not to.
+	// In this case logging is a service to clients, not a means for debugging the
+	// server, so we want to cancel the log message.
+	return h.ss.LoggingMessage(ctx, params)
+}

mcp/mcp_test.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log/slog"
 	"path/filepath"
 	"runtime"
 	"slices"
@@ -104,6 +105,7 @@ func TestEndToEnd(t *testing.T) {
 		clientWG.Done()
 	}()
 
+	loggingMessages := make(chan *LoggingMessageParams, 100) // big enough for all logging
 	opts := &ClientOptions{
 		CreateMessageHandler: func(context.Context, *ClientSession, *CreateMessageParams) (*CreateMessageResult, error) {
 			return &CreateMessageResult{Model: "aModel"}, nil
@@ -111,6 +113,9 @@ func TestEndToEnd(t *testing.T) {
 		ToolListChangedHandler:     func(context.Context, *ClientSession, *ToolListChangedParams) { notificationChans["tools"] <- 0 },
 		PromptListChangedHandler:   func(context.Context, *ClientSession, *PromptListChangedParams) { notificationChans["prompts"] <- 0 },
 		ResourceListChangedHandler: func(context.Context, *ClientSession, *ResourceListChangedParams) { notificationChans["resources"] <- 0 },
+		LoggingMessageHandler: func(_ context.Context, _ *ClientSession, lm *LoggingMessageParams) {
+			loggingMessages <- lm
+		},
 	}
 	c := NewClient("testClient", "v1.0.0", opts)
 	rootAbs, err := filepath.Abs(filepath.FromSlash("testdata/files"))
@@ -319,6 +324,85 @@ func TestEndToEnd(t *testing.T) {
 			t.Errorf("got %q, want %q", g, w)
 		}
 	})
+	t.Run("logging", func(t *testing.T) {
+		want := []*LoggingMessageParams{
+			{
+				Logger: "test",
+				Level:  "warning",
+				Data: map[string]any{
+					"msg":     "first",
+					"name":    "Pat",
+					"logtest": true,
+				},
+			},
+			{
+				Logger: "test",
+				Level:  "alert",
+				Data: map[string]any{
+					"msg":     "second",
+					"count":   2.0,
+					"logtest": true,
+				},
+			},
+		}
+
+		check := func(t *testing.T) {
+			t.Helper()
+			var got []*LoggingMessageParams
+			// Read messages from this test until we've seen all we expect.
+			for len(got) < len(want) {
+				select {
+				case p := <-loggingMessages:
+					// Ignore logging from other tests.
+					if m, ok := p.Data.(map[string]any); ok && m["logtest"] != nil {
+						delete(m, "time") // remove time because it changes
+						got = append(got, p)
+					}
+				case <-time.After(time.Second):
+					t.Fatal("timed out waiting for log messages")
+				}
+			}
+			if diff := cmp.Diff(want, got); diff != "" {
+				t.Errorf("mismatch (-want, +got):\n%s", diff)
+			}
+		}
+
+		t.Run("direct", func(t *testing.T) { // Use the LoggingMessage method directly.
+
+			mustLog := func(level LoggingLevel, data any) {
+				t.Helper()
+				if err := ss.LoggingMessage(ctx, &LoggingMessageParams{
+					Logger: "test",
+					Level:  level,
+					Data:   data,
+				}); err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			// Nothing should be logged until the client sets a level.
+			mustLog("info", "before")
+			if err := cs.SetLevel(ctx, &SetLevelParams{Level: "warning"}); err != nil {
+				t.Fatal(err)
+			}
+			mustLog("warning", want[0].Data)
+			mustLog("debug", "nope")    // below the level
+			mustLog("info", "negative") // below the level
+			mustLog("alert", want[1].Data)
+			check(t)
+		})
+
+		t.Run("handler", func(t *testing.T) { // Use the slog handler.
+			// We can't check the "before SetLevel" behavior because it's already been set.
+			// Not a big deal: that check is in LoggingMessage anyway.
+			logger := slog.New(NewLoggingHandler(ss, &LoggingHandlerOptions{LoggerName: "test"}))
+			logger.Warn("first", "name", "Pat", "logtest", true)
+			logger.Debug("nope")    // below the level
+			logger.Info("negative") // below the level
+			logger.Log(ctx, LevelAlert, "second", "count", 2, "logtest", true)
+			check(t)
+		})
+	})
 
 	// Disconnect.
 	cs.Close()

mcp/protocol.go

@@ -241,6 +241,22 @@ type ListToolsResult struct {
 	Tools      []*Tool `json:"tools"`
 }
 
+// The severity of a log message.
+//
+// These map to syslog message severities, as specified in RFC-5424:
+// https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.1
+type LoggingLevel string
+
+type LoggingMessageParams struct {
+	// The data to be logged, such as a string message or an object. Any JSON
+	// serializable type is allowed here.
+	Data any `json:"data"`
+	// The severity of this log message.
+	Level LoggingLevel `json:"level"`
+	// An optional name of the logger issuing this message.
+	Logger string `json:"logger,omitempty"`
+}
+
 // Hints to use for model selection.
 //
 // Keys not declared here are currently left unspecified by the spec and are up
@@ -326,8 +342,7 @@ type PromptArgument struct {
 type PromptListChangedParams struct {
 	// This parameter name is reserved by MCP to allow clients and servers to attach
 	// additional metadata to their notifications.
-	Meta struct {
-	} `json:"_meta,omitempty"`
+	Meta map[string]json.RawMessage `json:"_meta,omitempty"`
 }
 
 // Describes a message returned as part of a prompt.
@@ -381,8 +396,7 @@ type Resource struct {
 type ResourceListChangedParams struct {
 	// This parameter name is reserved by MCP to allow clients and servers to attach
 	// additional metadata to their notifications.
-	Meta struct {
-	} `json:"_meta,omitempty"`
+	Meta map[string]json.RawMessage `json:"_meta,omitempty"`
 }
 
 // The sender or recipient of messages and data in a conversation.
@@ -403,8 +417,7 @@ type Root struct {
 type RootsListChangedParams struct {
 	// This parameter name is reserved by MCP to allow clients and servers to attach
 	// additional metadata to their notifications.
-	Meta struct {
-	} `json:"_meta,omitempty"`
+	Meta map[string]json.RawMessage `json:"_meta,omitempty"`
 }
 
 // Present if the client supports sampling from an LLM.
@@ -417,6 +430,13 @@ type SamplingMessage struct {
 	Role    Role     `json:"role"`
 }
 
+type SetLevelParams struct {
+	// The level of logging that the client wants to receive from the server. The
+	// server should send all logs at this level and higher (i.e., more severe) to
+	// the client as notifications/message.
+	Level LoggingLevel `json:"level"`
+}
+
 // Definition for a tool the client can call.
 type Tool struct {
 	// Optional additional tool information.
@@ -472,8 +492,7 @@ type ToolAnnotations struct {
 type ToolListChangedParams struct {
 	// This parameter name is reserved by MCP to allow clients and servers to attach
 	// additional metadata to their notifications.
-	Meta struct {
-	} `json:"_meta,omitempty"`
+	Meta map[string]json.RawMessage `json:"_meta,omitempty"`
 }
 
 // Describes the name and version of an MCP implementation.
@@ -482,6 +501,10 @@ type implementation struct {
 	Version string `json:"version"`
 }
 
+// Present if the server supports sending log messages to the client.
+type loggingCapabilities struct {
+}
+
 // Present if the server offers any prompt templates.
 type promptCapabilities struct {
 	// Whether this server supports notifications for changes to the prompt list.
@@ -507,8 +530,7 @@ type serverCapabilities struct {
 	Experimental map[string]struct {
 	} `json:"experimental,omitempty"`
 	// Present if the server supports sending log messages to the client.
-	Logging struct {
-	} `json:"logging,omitempty"`
+	Logging *loggingCapabilities `json:"logging,omitempty"`
 	// Present if the server offers any prompt templates.
 	Prompts *promptCapabilities `json:"prompts,omitempty"`
 	// Present if the server offers any resources to read.
@@ -524,28 +546,28 @@ type toolCapabilities struct {
 }
 
 const (
-	methodCreateMessage             = "sampling/createMessage"
-	notificationToolListChanged     = "notifications/tools/list_changed"
-	notificationResourceListChanged = "notifications/resources/list_changed"
-	methodListPrompts               = "prompts/list"
-	notificationPromptListChanged   = "notifications/prompts/list_changed"
-	notificationResourceUpdated     = "notifications/resources/updated"
-	notificationCancelled           = "notifications/cancelled"
-	methodSetLevel                  = "logging/setLevel"
-	methodInitialize                = "initialize"
-	methodListRoots                 = "roots/list"
-	notificationProgress            = "notifications/progress"
-	methodGetPrompt                 = "prompts/get"
-	methodListResourceTemplates     = "resources/templates/list"
-	methodPing                      = "ping"
-	methodComplete                  = "completion/complete"
-	methodListResources             = "resources/list"
-	notificationInitialized         = "notifications/initialized"
 	methodCallTool                  = "tools/call"
-	notificationLoggingMessage      = "notifications/message"
-	methodReadResource              = "resources/read"
+	notificationCancelled           = "notifications/cancelled"
+	methodComplete                  = "completion/complete"
+	methodCreateMessage             = "sampling/createMessage"
+	methodGetPrompt                 = "prompts/get"
+	methodInitialize                = "initialize"
+	notificationInitialized         = "notifications/initialized"
+	methodListPrompts               = "prompts/list"
+	methodListResourceTemplates     = "resources/templates/list"
+	methodListResources             = "resources/list"
+	methodListRoots                 = "roots/list"
 	methodListTools                 = "tools/list"
+	notificationLoggingMessage      = "notifications/message"
+	methodPing                      = "ping"
+	notificationProgress            = "notifications/progress"
+	notificationPromptListChanged   = "notifications/prompts/list_changed"
+	methodReadResource              = "resources/read"
+	notificationResourceListChanged = "notifications/resources/list_changed"
+	notificationResourceUpdated     = "notifications/resources/updated"
 	notificationRootsListChanged    = "notifications/roots/list_changed"
+	methodSetLevel                  = "logging/setLevel"
 	methodSubscribe                 = "resources/subscribe"
+	notificationToolListChanged     = "notifications/tools/list_changed"
 	methodUnsubscribe               = "resources/unsubscribe"
 )

mcp/server.go

@@ -299,11 +299,11 @@ func (s *Server) Run(ctx context.Context, t Transport) error {
 // bind implements the binder[*ServerSession] interface, so that Servers can
 // be connected using [connect].
 func (s *Server) bind(conn *jsonrpc2.Connection) *ServerSession {
-	cc := &ServerSession{conn: conn, server: s}
+	ss := &ServerSession{conn: conn, server: s}
 	s.mu.Lock()
-	s.sessions = append(s.sessions, cc)
+	s.sessions = append(s.sessions, ss)
 	s.mu.Unlock()
-	return cc
+	return ss
 }
 
 // disconnect implements the binder[*ServerSession] interface, so that
@@ -341,17 +341,17 @@ func (s *Server) callRootsListChangedHandler(ctx context.Context, ss *ServerSess
 // Call [ServerSession.Close] to close the connection, or await client
 // termination with [ServerSession.Wait].
 type ServerSession struct {
-	server *Server
-	conn   *jsonrpc2.Connection
-
+	server           *Server
+	conn             *jsonrpc2.Connection
 	mu               sync.Mutex
+	logLevel         LoggingLevel
 	initializeParams *InitializeParams
 	initialized      bool
 }
 
 // Ping pings the client.
 func (ss *ServerSession) Ping(ctx context.Context, _ *PingParams) error {
-	return call(ctx, ss.conn, "ping", (*PingParams)(nil), nil)
+	return call(ctx, ss.conn, methodPing, (*PingParams)(nil), nil)
 }
 
 // ListRoots lists the client roots.
@@ -364,6 +364,25 @@ func (ss *ServerSession) CreateMessage(ctx context.Context, params *CreateMessag
 	return standardCall[CreateMessageResult](ctx, ss.conn, methodCreateMessage, params)
 }
 
+// LoggingMessage sends a logging 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.
+func (ss *ServerSession) LoggingMessage(ctx context.Context, params *LoggingMessageParams) error {
+	ss.mu.Lock()
+	logLevel := ss.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.
+		// TODO(jba): read other SDKs, possibly file an issue.
+		return nil
+	}
+	if compareLevels(params.Level, logLevel) < 0 {
+		return nil
+	}
+	return ss.conn.Notify(ctx, notificationLoggingMessage, params)
+}
+
 // AddMiddleware wraps the server's current method handler using the provided
 // middleware. Middleware is applied from right to left, so that the first one
 // is executed first.
@@ -386,6 +405,7 @@ var serverMethodInfos = map[string]methodInfo[ServerSession]{
 	methodCallTool:               newMethodInfo(serverMethod((*Server).callTool)),
 	methodListResources:          newMethodInfo(serverMethod((*Server).listResources)),
 	methodReadResource:           newMethodInfo(serverMethod((*Server).readResource)),
+	methodSetLevel:               newMethodInfo(sessionMethod((*ServerSession).setLevel)),
 	notificationInitialized:      newMethodInfo(serverMethod((*Server).callInitializedHandler)),
 	notificationRootsListChanged: newMethodInfo(serverMethod((*Server).callRootsListChangedHandler)),
 }
@@ -457,6 +477,7 @@ func (ss *ServerSession) initialize(ctx context.Context, params *InitializeParam
 			Resources: &resourceCapabilities{
 				ListChanged: true,
 			},
+			Logging: &loggingCapabilities{},
 		},
 		Instructions: ss.server.opts.Instructions,
 		ServerInfo: &implementation{
@@ -470,6 +491,13 @@ func (ss *ServerSession) ping(context.Context, *PingParams) (struct{}, error) {
 	return struct{}{}, nil
 }
 
+func (ss *ServerSession) setLevel(_ context.Context, params *SetLevelParams) (struct{}, error) {
+	ss.mu.Lock()
+	defer ss.mu.Unlock()
+	ss.logLevel = params.Level
+	return struct{}{}, nil
+}
+
 // Close performs a graceful shutdown of the connection, preventing new
 // requests from being handled, and waiting for ongoing requests to return.
 // Close then terminates the connection.