mcp: Allow registration of custom JSON-RPC methods (#956)

## Description

This PR introduces support for custom (non-standard) JSON-RPC methods in
the MCP Go SDK.
This feature enables servers to handle arbitrary JSON-RPC methods and
clients to invoke them, all while maintaining full type safety and
integrating seamlessly with the SDK's existing middleware and session
management.

- Server-Side Registration: Added AddReceivingCustomMethod to allow
servers to register handlers for custom methods. Incoming requests are
automatically unmarshaled into user-defined parameter structs.
- Client-Side Calling: Added AddSendingCustomMethod which returns a
strongly-typed caller function that clients can use to invoke custom
methods and receive strongly-typed results.
- Helper Structs: Provided ParamsBase and ResultBase for users to embed
in their custom structs. This allows them to easily satisfy the SDK's
internal Params and Result interfaces.

Fixes #954

Author: guglielmo-san

examples/server/custom-method/main.go

@@ -0,0 +1,110 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by the license
+// that can be found in the LICENSE file.
+
+// The custom-method example demonstrates registering and calling a custom
+// JSON-RPC method that is not part of the standard MCP spec.
+//
+// The server registers a "latin/translate" method that translates simple
+// English phrases into Latin. A client connects over an in-memory transport,
+// calls the custom method, and prints the result.
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"strings"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+type TranslateParams struct {
+	mcp.ParamsBase
+	Text string `json:"text"`
+}
+
+type TranslateResult struct {
+	mcp.ResultBase
+	Latin string `json:"latin"`
+}
+
+var translations = map[string]string{
+	"hello":                    "salve",
+	"goodbye":                  "vale",
+	"thank you":                "gratias tibi ago",
+	"how are you":              "quid agis",
+	"good morning":             "bonum mane",
+	"good night":               "bonam noctem",
+	"friend":                   "amicus",
+	"water":                    "aqua",
+	"love":                     "amor",
+	"war":                      "bellum",
+	"peace":                    "pax",
+	"truth":                    "veritas",
+	"light":                    "lux",
+	"time":                     "tempus",
+	"life":                     "vita",
+	"death":                    "mors",
+	"star":                     "stella",
+	"earth":                    "terra",
+	"sea":                      "mare",
+	"the die is cast":          "alea iacta est",
+	"i came i saw i conquered": "veni vidi vici",
+	"seize the day":            "carpe diem",
+}
+
+func main() {
+	ctx := context.Background()
+
+	server := mcp.NewServer(&mcp.Implementation{Name: "latin-server", Version: "v1.0.0"}, nil)
+
+	if err := mcp.AddReceivingCustomMethod(server, "latin/translate",
+		func(ctx context.Context, ss *mcp.ServerSession, params *TranslateParams) (*TranslateResult, error) {
+			key := strings.ToLower(strings.TrimSpace(params.Text))
+			latin, ok := translations[key]
+			if !ok {
+				latin = fmt.Sprintf("[unknown: %q — try: %s]", params.Text, knownPhrases())
+			}
+			return &TranslateResult{Latin: latin}, nil
+		}); err != nil {
+		log.Fatal(err)
+	}
+
+	ct, st := mcp.NewInMemoryTransports()
+
+	ss, err := server.Connect(ctx, st, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer ss.Close()
+
+	client := mcp.NewClient(&mcp.Implementation{Name: "latin-client", Version: "v1.0.0"}, nil)
+	if err := mcp.AddSendingCustomMethod[*TranslateParams, *TranslateResult](client, "latin/translate"); err != nil {
+		log.Fatal(err)
+	}
+
+	cs, err := client.Connect(ctx, ct, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer cs.Close()
+
+	phrases := []string{"Hello", "Seize the day", "Peace", "Truth", "I came I saw I conquered"}
+	for _, phrase := range phrases {
+		result, err := mcp.CallCustomMethod[*TranslateParams, *TranslateResult](
+			ctx, cs, "latin/translate", &TranslateParams{Text: phrase})
+		if err != nil {
+			log.Fatalf("translate %q: %v", phrase, err)
+		}
+		fmt.Printf("%-35s → %s\n", phrase, result.Latin)
+	}
+}
+
+func knownPhrases() string {
+	phrases := make([]string, 0, len(translations))
+	for k := range translations {
+		phrases = append(phrases, fmt.Sprintf("%q", k))
+	}
+	return strings.Join(phrases, ", ")
+}

mcp/client.go

@@ -10,6 +10,8 @@ import (
 	"fmt"
 	"iter"
 	"log/slog"
+	"maps"
+	"reflect"
 	"slices"
 	"strings"
 	"sync"
@@ -32,6 +34,11 @@ type Client struct {
 	sessions                []*ClientSession
 	sendingMethodHandler_   MethodHandler
 	receivingMethodHandler_ MethodHandler
+	// sendMethods is the list of methods this client may send to a
+	// server: it always contains the standard server methods (from
+	// serverMethodInfos) plus any custom methods registered via
+	// [AddSendingCustomMethod].
+	sendMethods map[string]methodInfo
 }
 
 // NewClient creates a new [Client].
@@ -58,12 +65,16 @@ func NewClient(impl *Implementation, options *ClientOptions) *Client {
 		opts.Logger = ensureLogger(nil)
 	}
 
+	sendMethods := make(map[string]methodInfo, len(serverMethodInfos))
+	maps.Copy(sendMethods, serverMethodInfos)
+
 	c := &Client{
 		impl:                    impl,
 		opts:                    opts,
 		roots:                   newFeatureSet(func(r *Root) string { return r.URI }),
 		sendingMethodHandler_:   defaultSendingMethodHandler,
 		receivingMethodHandler_: defaultReceivingMethodHandler[*ClientSession],
+		sendMethods:             sendMethods,
 	}
 	if opts.MultiRoundTrip == nil || !opts.MultiRoundTrip.Disabled {
 		c.AddSendingMiddleware(clientMultiRoundTripMiddleware())
@@ -504,7 +515,7 @@ func injectRequestMeta[T any, P interface {
 	Params
 }](cs *ClientSession, params P) P {
 	res := cs.state.InitializeResult
-	if params.isNil() {
+	if params == nil {
 		params = new(T)
 	}
 	m := params.GetMeta()
@@ -1153,7 +1164,9 @@ var clientMethodInfos = map[string]methodInfo{
 }
 
 func (cs *ClientSession) sendingMethodInfos() map[string]methodInfo {
-	return serverMethodInfos
+	cs.client.mu.Lock()
+	defer cs.client.mu.Unlock()
+	return cs.client.sendMethods
 }
 
 func (cs *ClientSession) receivingMethodInfos() map[string]methodInfo {
@@ -1597,3 +1610,67 @@ func paginate[P listParams, R listResult[T], T any](ctx context.Context, params
 		}
 	}
 }
+
+// AddSendingCustomMethod registers a custom JSON-RPC method
+// that the client may send to the server.
+//
+// Registration is decoupled from invocation: extensions typically call this
+// during setup, while the actual call site uses [CallCustomMethod].
+//
+//	if err := mcp.AddSendingCustomMethod[*SearchParams, *SearchResult](c, "acme/search"); err != nil {
+//	    return err
+//	}
+//	// ... later, anywhere a *ClientSession is available:
+//	result, err := mcp.CallCustomMethod[*SearchParams, *SearchResult](
+//	    ctx, cs, "acme/search", &SearchParams{Query: "hello"})
+//
+// AddSendingCustomMethod returns an error if method is the name of a standard
+// MCP method. Registering the same method twice replaces the previous
+// registration.
+func AddSendingCustomMethod[P paramsPtr[PT], R Result, PT any](
+	c *Client,
+	method string,
+) error {
+	if _, ok := serverMethodInfos[method]; ok {
+		return fmt.Errorf("mcp: AddSendingCustomMethod: %q shadows a standard MCP method", method)
+	}
+
+	mi := methodInfo{
+		newResult: func() Result {
+			return reflect.New(reflect.TypeFor[R]().Elem()).Interface().(R)
+		},
+	}
+
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.sendMethods[method] = mi
+	return nil
+}
+
+// CallCustomMethod sends a custom (non-standard) JSON-RPC method to the
+// server and decodes the response into R.
+//
+// The method must have been registered on the session's client via
+// [AddSendingCustomMethod].
+func CallCustomMethod[P paramsPtr[PT], R Result, PT any](
+	ctx context.Context,
+	cs *ClientSession,
+	method string,
+	params P,
+) (R, error) {
+	c := cs.client
+	c.mu.Lock()
+	_, ok := c.sendMethods[method]
+	c.mu.Unlock()
+	if !ok {
+		var zero R
+		return zero, fmt.Errorf("mcp: CallCustomMethod: %q is not registered; call AddSendingCustomMethod first", method)
+	}
+	if cs.usesNewProtocol() {
+		params = injectRequestMeta(cs, params)
+	}
+	return handleSend[R](ctx, method, &ClientRequest[P]{
+		Session: cs,
+		Params:  params,
+	})
+}

mcp/mcp_test.go

@@ -3233,3 +3233,145 @@ func TestSubscriptionsListen_DisconnectScrubsMaps(t *testing.T) {
 		time.Sleep(10 * time.Millisecond)
 	}
 }
+
+func TestCustomMethods(t *testing.T) {
+	type searchParams struct {
+		ParamsBase
+		Query string `json:"query"`
+		Limit int    `json:"limit,omitempty"`
+	}
+
+	type searchResult struct {
+		ResultBase
+		Hits  []string `json:"hits"`
+		Total int      `json:"total"`
+	}
+
+	callCustom := func(ctx context.Context, conn *jsonrpc2.Connection, method string, params, result any) error {
+		return conn.Call(ctx, method, params).Await(ctx, result)
+	}
+
+	ctx := context.Background()
+	s := NewServer(testImpl, nil)
+
+	if err := AddReceivingCustomMethod(s, "acme/search", func(ctx context.Context, ss *ServerSession, params *searchParams) (*searchResult, error) {
+		hits := []string{"result for " + params.Query}
+		return &searchResult{
+			Hits:  hits,
+			Total: len(hits),
+		}, nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	ct, st := NewInMemoryTransports()
+	ss, err := s.Connect(ctx, st, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = ss.Close() })
+
+	c := NewClient(testImpl, nil)
+	if err := AddSendingCustomMethod[*searchParams, *searchResult](c, "acme/search"); err != nil {
+		t.Fatal(err)
+	}
+
+	cs, err := c.Connect(ctx, ct, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = cs.Close() })
+
+	// Test raw JSON-RPC call.
+	var result1 searchResult
+	if err := callCustom(ctx, cs.getConn(), "acme/search", &searchParams{Query: "hello", Limit: 10}, &result1); err != nil {
+		t.Fatal(err)
+	}
+	if len(result1.Hits) != 1 || result1.Hits[0] != "result for hello" {
+		t.Errorf("raw call: unexpected hits: %v", result1.Hits)
+	}
+	if result1.Total != 1 {
+		t.Errorf("raw call: unexpected total: %d", result1.Total)
+	}
+
+	// Test the typed CallCustomMethod helper.
+	result2, err := CallCustomMethod[*searchParams, *searchResult](
+		ctx, cs, "acme/search", &searchParams{Query: "world"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(result2.Hits) != 1 || result2.Hits[0] != "result for world" {
+		t.Errorf("CallCustomMethod: unexpected hits: %v", result2.Hits)
+	}
+	if result2.Total != 1 {
+		t.Errorf("CallCustomMethod: unexpected total: %d", result2.Total)
+	}
+
+	// CallCustomMethod must reject methods that were never registered.
+	if _, err := CallCustomMethod[*searchParams, *searchResult](
+		ctx, cs, "acme/unregistered", &searchParams{Query: "x"}); err == nil {
+		t.Error("CallCustomMethod: expected error for unregistered method, got nil")
+	}
+}
+
+func TestAddCustomMethodRejectsStandardMethods(t *testing.T) {
+	type params struct{ ParamsBase }
+	type result struct{ ResultBase }
+
+	t.Run("server", func(t *testing.T) {
+		s := NewServer(testImpl, nil)
+		err := AddReceivingCustomMethod(s, "tools/call",
+			func(ctx context.Context, ss *ServerSession, p *params) (*result, error) {
+				return &result{}, nil
+			})
+		if err == nil {
+			t.Fatal("AddReceivingCustomMethod: expected error when shadowing a standard method, got nil")
+		}
+	})
+
+	t.Run("client", func(t *testing.T) {
+		c := NewClient(testImpl, nil)
+		if err := AddSendingCustomMethod[*params, *result](c, "tools/call"); err == nil {
+			t.Fatal("AddSendingCustomMethod: expected error when shadowing a standard method, got nil")
+		}
+	})
+}
+
+// TestCallCustomMethodTypedNilParams exercises the typed-nil params path.
+// User param types embed ParamsBase, so the inherited isNil forwarder would
+// dereference a typed-nil outer if injectRequestMeta were called with it.
+// CallCustomMethod must allocate a fresh value before the meta-injection step.
+func TestCallCustomMethodTypedNilParams(t *testing.T) {
+	type pingParams struct{ ParamsBase }
+	type pingResult struct{ ResultBase }
+
+	ctx := context.Background()
+	s := NewServer(testImpl, nil)
+	if err := AddReceivingCustomMethod(s, "acme/ping",
+		func(ctx context.Context, ss *ServerSession, p *pingParams) (*pingResult, error) {
+			return &pingResult{}, nil
+		}); err != nil {
+		t.Fatal(err)
+	}
+	ct, st := NewInMemoryTransports()
+	ss, err := s.Connect(ctx, st, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = ss.Close() })
+
+	c := NewClient(testImpl, nil)
+	if err := AddSendingCustomMethod[*pingParams, *pingResult](c, "acme/ping"); err != nil {
+		t.Fatal(err)
+	}
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = cs.Close() })
+
+	var typedNil *pingParams
+	if _, err := CallCustomMethod[*pingParams, *pingResult](ctx, cs, "acme/ping", typedNil); err != nil {
+		t.Fatalf("CallCustomMethod with typed-nil params: %v", err)
+	}
+}