mcp: add resource subscriptions (#138)

This CL adds the ability for clients to subscribe and receive updates
for resources as described in
https://modelcontextprotocol.io/specification/2025-06-18/server/resources#subscriptions

Fixes: #23

design/design.md

@@ -748,13 +748,26 @@ Server sessions also support the spec methods `ListResources` and `ListResourceT
 
 #### Subscriptions
 
-ClientSessions can manage change notifications on particular resources:
+##### Client-Side Usage
+
+Use the Subscribe and Unsubscribe methods on a ClientSession to start or stop receiving updates for a specific resource.
 
 ```go
 func (*ClientSession) Subscribe(context.Context, *SubscribeParams) error
 func (*ClientSession) Unsubscribe(context.Context, *UnsubscribeParams) error
 ```
 
+To process incoming update notifications, you must provide a ResourceUpdatedHandler in your ClientOptions. The SDK calls this function automatically whenever the server sends a notification for a resource you're subscribed to.
+
+```go
+type ClientOptions struct {
+  ...
+  ResourceUpdatedHandler func(context.Context, *ClientSession, *ResourceUpdatedNotificationParams)
+}
+```
+
+##### Server-Side Implementation
+
 The server does not implement resource subscriptions. It passes along subscription requests to the user, and supplies a method to notify clients of changes. It tracks which sessions have subscribed to which resources so the user doesn't have to.
 
 If a server author wants to support resource subscriptions, they must provide handlers to be called when clients subscribe and unsubscribe. It is an error to provide only one of these handlers.
@@ -772,7 +785,7 @@ type ServerOptions struct {
 User code should call `ResourceUpdated` when a subscribed resource changes.
 
 ```go
-func (*Server) ResourceUpdated(context.Context, *ResourceUpdatedNotification) error
+func (*Server) ResourceUpdated(context.Context, *ResourceUpdatedNotificationParams) error
 ```
 
 The server routes these notifications to the server sessions that subscribed to the resource.

mcp/client.go

@@ -60,6 +60,7 @@ type ClientOptions struct {
 	ToolListChangedHandler      func(context.Context, *ClientSession, *ToolListChangedParams)
 	PromptListChangedHandler    func(context.Context, *ClientSession, *PromptListChangedParams)
 	ResourceListChangedHandler  func(context.Context, *ClientSession, *ResourceListChangedParams)
+	ResourceUpdatedHandler      func(context.Context, *ClientSession, *ResourceUpdatedNotificationParams)
 	LoggingMessageHandler       func(context.Context, *ClientSession, *LoggingMessageParams)
 	ProgressNotificationHandler func(context.Context, *ClientSession, *ProgressNotificationParams)
 	// If non-zero, defines an interval for regular "ping" requests.
@@ -293,6 +294,7 @@ var clientMethodInfos = map[string]methodInfo{
 	notificationToolListChanged:     newMethodInfo(clientMethod((*Client).callToolChangedHandler)),
 	notificationPromptListChanged:   newMethodInfo(clientMethod((*Client).callPromptChangedHandler)),
 	notificationResourceListChanged: newMethodInfo(clientMethod((*Client).callResourceChangedHandler)),
+	notificationResourceUpdated:     newMethodInfo(clientMethod((*Client).callResourceUpdatedHandler)),
 	notificationLoggingMessage:      newMethodInfo(clientMethod((*Client).callLoggingHandler)),
 	notificationProgress:            newMethodInfo(sessionMethod((*ClientSession).callProgressNotificationHandler)),
 }
@@ -386,6 +388,20 @@ func (cs *ClientSession) Complete(ctx context.Context, params *CompleteParams) (
 	return handleSend[*CompleteResult](ctx, cs, methodComplete, orZero[Params](params))
 }
 
+// Subscribe sends a "resources/subscribe" request to the server, asking for
+// notifications when the specified resource changes.
+func (cs *ClientSession) Subscribe(ctx context.Context, params *SubscribeParams) error {
+	_, err := handleSend[*emptyResult](ctx, cs, methodSubscribe, orZero[Params](params))
+	return err
+}
+
+// Unsubscribe sends a "resources/unsubscribe" request to the server, cancelling
+// a previous subscription.
+func (cs *ClientSession) Unsubscribe(ctx context.Context, params *UnsubscribeParams) error {
+	_, err := handleSend[*emptyResult](ctx, cs, methodUnsubscribe, orZero[Params](params))
+	return err
+}
+
 func (c *Client) callToolChangedHandler(ctx context.Context, s *ClientSession, params *ToolListChangedParams) (Result, error) {
 	return callNotificationHandler(ctx, c.opts.ToolListChangedHandler, s, params)
 }
@@ -398,6 +414,10 @@ func (c *Client) callResourceChangedHandler(ctx context.Context, s *ClientSessio
 	return callNotificationHandler(ctx, c.opts.ResourceListChangedHandler, s, params)
 }
 
+func (c *Client) callResourceUpdatedHandler(ctx context.Context, s *ClientSession, params *ResourceUpdatedNotificationParams) (Result, error) {
+	return callNotificationHandler(ctx, c.opts.ResourceUpdatedHandler, s, params)
+}
+
 func (c *Client) callLoggingHandler(ctx context.Context, cs *ClientSession, params *LoggingMessageParams) (Result, error) {
 	if h := c.opts.LoggingMessageHandler; h != nil {
 		h(ctx, cs, params)

mcp/mcp_test.go

@@ -60,7 +60,7 @@ func TestEndToEnd(t *testing.T) {
 
 	// Channels to check if notification callbacks happened.
 	notificationChans := map[string]chan int{}
-	for _, name := range []string{"initialized", "roots", "tools", "prompts", "resources", "progress_server", "progress_client"} {
+	for _, name := range []string{"initialized", "roots", "tools", "prompts", "resources", "progress_server", "progress_client", "resource_updated", "subscribe", "unsubscribe"} {
 		notificationChans[name] = make(chan int, 1)
 	}
 	waitForNotification := func(t *testing.T, name string) {
@@ -78,6 +78,14 @@ func TestEndToEnd(t *testing.T) {
 		ProgressNotificationHandler: func(context.Context, *ServerSession, *ProgressNotificationParams) {
 			notificationChans["progress_server"] <- 0
 		},
+		SubscribeHandler: func(context.Context, *SubscribeParams) error {
+			notificationChans["subscribe"] <- 0
+			return nil
+		},
+		UnsubscribeHandler: func(context.Context, *UnsubscribeParams) error {
+			notificationChans["unsubscribe"] <- 0
+			return nil
+		},
 	}
 	s := NewServer(testImpl, sopts)
 	AddTool(s, &Tool{
@@ -128,6 +136,9 @@ func TestEndToEnd(t *testing.T) {
 		ProgressNotificationHandler: func(context.Context, *ClientSession, *ProgressNotificationParams) {
 			notificationChans["progress_client"] <- 0
 		},
+		ResourceUpdatedHandler: func(context.Context, *ClientSession, *ResourceUpdatedNotificationParams) {
+			notificationChans["resource_updated"] <- 0
+		},
 	}
 	c := NewClient(testImpl, opts)
 	rootAbs, err := filepath.Abs(filepath.FromSlash("testdata/files"))
@@ -421,6 +432,37 @@ func TestEndToEnd(t *testing.T) {
 		waitForNotification(t, "progress_server")
 	})
 
+	t.Run("resource_subscriptions", func(t *testing.T) {
+		err := cs.Subscribe(ctx, &SubscribeParams{
+			URI: "test",
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		waitForNotification(t, "subscribe")
+		s.ResourceUpdated(ctx, &ResourceUpdatedNotificationParams{
+			URI: "test",
+		})
+		waitForNotification(t, "resource_updated")
+		err = cs.Unsubscribe(ctx, &UnsubscribeParams{
+			URI: "test",
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		waitForNotification(t, "unsubscribe")
+
+		// Verify the client does not receive the update after unsubscribing.
+		s.ResourceUpdated(ctx, &ResourceUpdatedNotificationParams{
+			URI: "test",
+		})
+		select {
+		case <-notificationChans["resource_updated"]:
+			t.Fatalf("resource updated after unsubscription")
+		case <-time.After(time.Second):
+		}
+	})
+
 	// Disconnect.
 	cs.Close()
 	clientWG.Wait()

mcp/protocol.go

@@ -859,6 +859,38 @@ type ToolListChangedParams struct {
 func (x *ToolListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ToolListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
+// Sent from the client to request resources/updated notifications from the
+// server whenever a particular resource changes.
+type SubscribeParams struct {
+	// This property is reserved by the protocol to allow clients and servers to
+	// attach additional metadata to their responses.
+	Meta `json:"_meta,omitempty"`
+	// The URI of the resource to subscribe to.
+	URI string `json:"uri"`
+}
+
+// Sent from the client to request cancellation of resources/updated
+// notifications from the server. This should follow a previous
+// resources/subscribe request.
+type UnsubscribeParams struct {
+	// This property is reserved by the protocol to allow clients and servers to
+	// attach additional metadata to their responses.
+	Meta `json:"_meta,omitempty"`
+	// The URI of the resource to unsubscribe from.
+	URI string `json:"uri"`
+}
+
+// A notification from the server to the client, informing it that a resource
+// has changed and may need to be read again. This should only be sent if the
+// client previously sent a resources/subscribe request.
+type ResourceUpdatedNotificationParams struct {
+	// This property is reserved by the protocol to allow clients and servers to
+	// attach additional metadata to their responses.
+	Meta `json:"_meta,omitempty"`
+	// The URI of the resource that has been updated. This might be a sub-resource of the one that the client actually subscribed to.
+	URI string `json:"uri"`
+}
+
 // TODO(jba): add CompleteRequest and related types.
 
 // TODO(jba): add ElicitRequest and related types.

mcp/server.go

@@ -13,6 +13,7 @@ import (
 	"fmt"
 	"iter"
 	"log"
+	"maps"
 	"net/url"
 	"path/filepath"
 	"slices"
@@ -43,6 +44,7 @@ type Server struct {
 	sessions                []*ServerSession
 	sendingMethodHandler_   MethodHandler[*ServerSession]
 	receivingMethodHandler_ MethodHandler[*ServerSession]
+	resourceSubscriptions   map[string]map[*ServerSession]bool // uri -> session -> bool
 }
 
 // ServerOptions is used to configure behavior of the server.
@@ -64,6 +66,10 @@ type ServerOptions struct {
 	// If the peer fails to respond to pings originating from the keepalive check,
 	// the session is automatically closed.
 	KeepAlive time.Duration
+	// Function called when a client session subscribes to a resource.
+	SubscribeHandler func(context.Context, *SubscribeParams) error
+	// Function called when a client session unsubscribes from a resource.
+	UnsubscribeHandler func(context.Context, *UnsubscribeParams) error
 }
 
 // NewServer creates a new MCP server. The resulting server has no features:
@@ -89,7 +95,12 @@ func NewServer(impl *Implementation, opts *ServerOptions) *Server {
 	if opts.PageSize == 0 {
 		opts.PageSize = DefaultPageSize
 	}
-
+	if opts.SubscribeHandler != nil && opts.UnsubscribeHandler == nil {
+		panic("SubscribeHandler requires UnsubscribeHandler")
+	}
+	if opts.UnsubscribeHandler != nil && opts.SubscribeHandler == nil {
+		panic("UnsubscribeHandler requires SubscribeHandler")
+	}
 	return &Server{
 		impl:                    impl,
 		opts:                    *opts,
@@ -99,6 +110,7 @@ func NewServer(impl *Implementation, opts *ServerOptions) *Server {
 		resourceTemplates:       newFeatureSet(func(t *serverResourceTemplate) string { return t.resourceTemplate.URITemplate }),
 		sendingMethodHandler_:   defaultSendingMethodHandler[*ServerSession],
 		receivingMethodHandler_: defaultReceivingMethodHandler[*ServerSession],
+		resourceSubscriptions:   make(map[string]map[*ServerSession]bool),
 	}
 }
 
@@ -225,6 +237,9 @@ func (s *Server) capabilities() *serverCapabilities {
 	}
 	if s.resources.len() > 0 || s.resourceTemplates.len() > 0 {
 		caps.Resources = &resourceCapabilities{ListChanged: true}
+		if s.opts.SubscribeHandler != nil {
+			caps.Resources.Subscribe = true
+		}
 	}
 	return caps
 }
@@ -428,6 +443,57 @@ func fileResourceHandler(dir string) ResourceHandler {
 	}
 }
 
+// ResourceUpdated sends a notification to all clients that have subscribed to the
+// resource specified in params. This method is the primary way for a
+// server author to signal that a resource has changed.
+func (s *Server) ResourceUpdated(ctx context.Context, params *ResourceUpdatedNotificationParams) error {
+	s.mu.Lock()
+	subscribedSessions := s.resourceSubscriptions[params.URI]
+	sessions := slices.Collect(maps.Keys(subscribedSessions))
+	s.mu.Unlock()
+	notifySessions(sessions, notificationResourceUpdated, params)
+	return nil
+}
+
+func (s *Server) subscribe(ctx context.Context, ss *ServerSession, params *SubscribeParams) (*emptyResult, error) {
+	if s.opts.SubscribeHandler == nil {
+		return nil, fmt.Errorf("%w: server does not support resource subscriptions", jsonrpc2.ErrMethodNotFound)
+	}
+	if err := s.opts.SubscribeHandler(ctx, params); err != nil {
+		return nil, err
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.resourceSubscriptions[params.URI] == nil {
+		s.resourceSubscriptions[params.URI] = make(map[*ServerSession]bool)
+	}
+	s.resourceSubscriptions[params.URI][ss] = true
+
+	return &emptyResult{}, nil
+}
+
+func (s *Server) unsubscribe(ctx context.Context, ss *ServerSession, params *UnsubscribeParams) (*emptyResult, error) {
+	if s.opts.UnsubscribeHandler == nil {
+		return nil, jsonrpc2.ErrMethodNotFound
+	}
+
+	if err := s.opts.UnsubscribeHandler(ctx, params); err != nil {
+		return nil, err
+	}
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if subscribedSessions, ok := s.resourceSubscriptions[params.URI]; ok {
+		delete(subscribedSessions, ss)
+		if len(subscribedSessions) == 0 {
+			delete(s.resourceSubscriptions, params.URI)
+		}
+	}
+
+	return &emptyResult{}, nil
+}
+
 // Run runs the server over the given transport, which must be persistent.
 //
 // Run blocks until the client terminates the connection or the provided
@@ -475,6 +541,10 @@ func (s *Server) disconnect(cc *ServerSession) {
 	s.sessions = slices.DeleteFunc(s.sessions, func(cc2 *ServerSession) bool {
 		return cc2 == cc
 	})
+
+	for _, subscribedSessions := range s.resourceSubscriptions {
+		delete(subscribedSessions, cc)
+	}
 }
 
 // Connect connects the MCP server over the given transport and starts handling
@@ -616,6 +686,8 @@ var serverMethodInfos = map[string]methodInfo{
 	methodListResourceTemplates:  newMethodInfo(serverMethod((*Server).listResourceTemplates)),
 	methodReadResource:           newMethodInfo(serverMethod((*Server).readResource)),
 	methodSetLevel:               newMethodInfo(sessionMethod((*ServerSession).setLevel)),
+	methodSubscribe:              newMethodInfo(serverMethod((*Server).subscribe)),
+	methodUnsubscribe:            newMethodInfo(serverMethod((*Server).unsubscribe)),
 	notificationInitialized:      newMethodInfo(serverMethod((*Server).callInitializedHandler)),
 	notificationRootsListChanged: newMethodInfo(serverMethod((*Server).callRootsListChangedHandler)),
 	notificationProgress:         newMethodInfo(sessionMethod((*ServerSession).callProgressNotificationHandler)),

mcp/server_test.go

@@ -5,6 +5,7 @@
 package mcp
 
 import (
+	"context"
 	"log"
 	"slices"
 	"testing"
@@ -232,6 +233,7 @@ func TestServerCapabilities(t *testing.T) {
 	testCases := []struct {
 		name             string
 		configureServer  func(s *Server)
+		serverOpts       ServerOptions
 		wantCapabilities *serverCapabilities
 	}{
 		{
@@ -275,6 +277,25 @@ func TestServerCapabilities(t *testing.T) {
 				Resources:   &resourceCapabilities{ListChanged: true},
 			},
 		},
+		{
+			name: "With resource subscriptions",
+			configureServer: func(s *Server) {
+				s.AddResourceTemplate(&ResourceTemplate{URITemplate: "file:///rt"}, nil)
+			},
+			serverOpts: ServerOptions{
+				SubscribeHandler: func(ctx context.Context, sp *SubscribeParams) error {
+					return nil
+				},
+				UnsubscribeHandler: func(ctx context.Context, up *UnsubscribeParams) error {
+					return nil
+				},
+			},
+			wantCapabilities: &serverCapabilities{
+				Completions: &completionCapabilities{},
+				Logging:     &loggingCapabilities{},
+				Resources:   &resourceCapabilities{ListChanged: true, Subscribe: true},
+			},
+		},
 		{
 			name: "With tools",
 			configureServer: func(s *Server) {
@@ -294,11 +315,19 @@ func TestServerCapabilities(t *testing.T) {
 				s.AddResourceTemplate(&ResourceTemplate{URITemplate: "file:///rt"}, nil)
 				s.AddTool(&Tool{Name: "t"}, nil)
 			},
+			serverOpts: ServerOptions{
+				SubscribeHandler: func(ctx context.Context, sp *SubscribeParams) error {
+					return nil
+				},
+				UnsubscribeHandler: func(ctx context.Context, up *UnsubscribeParams) error {
+					return nil
+				},
+			},
 			wantCapabilities: &serverCapabilities{
 				Completions: &completionCapabilities{},
 				Logging:     &loggingCapabilities{},
 				Prompts:     &promptCapabilities{ListChanged: true},
-				Resources:   &resourceCapabilities{ListChanged: true},
+				Resources:   &resourceCapabilities{ListChanged: true, Subscribe: true},
 				Tools:       &toolCapabilities{ListChanged: true},
 			},
 		},
@@ -306,7 +335,7 @@ func TestServerCapabilities(t *testing.T) {
 
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			server := NewServer(testImpl, nil)
+			server := NewServer(testImpl, &tc.serverOpts)
 			tc.configureServer(server)
 			gotCapabilities := server.capabilities()
 			if diff := cmp.Diff(tc.wantCapabilities, gotCapabilities); diff != "" {