mcp: don't close session when keepalive ping returns method-not-found (#900)

Since ping is optional per the MCP spec, a peer that doesn't implement
it will respond with a -32601 (method not found) error. Previously,
startKeepalive treated all ping errors identically, closing the session
even when the peer was perfectly healthy.

Now, a method-not-found response stops the keepalive goroutine without
closing the session.

Fixes #897

mcp/mcp_test.go

@@ -25,6 +25,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/jsonschema-go/jsonschema"
+	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
 	"github.com/modelcontextprotocol/go-sdk/jsonrpc"
 )
 
@@ -938,6 +939,61 @@ func TestKeepAlive(t *testing.T) {
 	})
 }
 
+func TestKeepAliveMethodNotFound(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		ctx := context.Background()
+
+		ct, st := NewInMemoryTransports()
+
+		// Server that rejects ping with method-not-found, simulating a
+		// server that does not implement the optional ping method.
+		s := NewServer(testImpl, nil)
+		AddTool(s, greetTool(), sayHi)
+		s.AddReceivingMiddleware(func(next MethodHandler) MethodHandler {
+			return func(ctx context.Context, method string, req Request) (Result, error) {
+				if method == "ping" {
+					return nil, jsonrpc2.ErrMethodNotFound
+				}
+				return next(ctx, method, req)
+			}
+		})
+		ss, err := s.Connect(ctx, st, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer ss.Close()
+
+		clientOpts := &ClientOptions{
+			KeepAlive: 50 * time.Millisecond,
+		}
+		c := NewClient(testImpl, clientOpts)
+		cs, err := c.Connect(ctx, ct, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer cs.Close()
+
+		// Advance past several keepalive cycles.
+		time.Sleep(200 * time.Millisecond)
+
+		// The session should still be alive despite the server not
+		// supporting ping.
+		result, err := cs.CallTool(ctx, &CallToolParams{
+			Name:      "greet",
+			Arguments: map[string]any{"Name": "user"},
+		})
+		if err != nil {
+			t.Fatalf("call failed after keepalive with method-not-found: %v", err)
+		}
+		if len(result.Content) == 0 {
+			t.Fatal("expected content in result")
+		}
+		if textContent, ok := result.Content[0].(*TextContent); !ok || textContent.Text != "hi user" {
+			t.Fatalf("unexpected result: %v", result.Content[0])
+		}
+	})
+}
+
 func TestElicitationUnsupportedMethod(t *testing.T) {
 	ctx := context.Background()
 	ct, st := NewInMemoryTransports()

mcp/shared.go

@@ -14,6 +14,7 @@ package mcp
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log/slog"
 	"net/http"
@@ -605,6 +606,10 @@ func startKeepalive(session keepaliveSession, interval time.Duration, cancelPtr
 				err := session.Ping(pingCtx, nil)
 				pingCancel()
 				if err != nil {
+					if errors.Is(err, jsonrpc2.ErrMethodNotFound) {
+						// Peer doesn't support ping, stop the keepalive process.
+						return
+					}
 					// Ping failed; log it before closing the session so the
 					// failure is observable to operators. See #218.
 					logger.Error("keepalive ping failed; closing session", "error", err)