feat(mcp): expose server capabilities to client

.gitignore

@@ -0,0 +1,27 @@
+### Go.AllowList template
+# Allowlisting gitignore template for GO projects prevents us
+# from adding various unwanted local files, such as generated
+# files, developer configurations or IDE-specific files etc.
+#
+# Recommended: Go.AllowList.gitignore
+
+# Ignore everything
+*
+
+# But not these files...
+!/.gitignore
+
+!*.go
+!go.*
+
+!*.sh
+
+!README.md
+!CONTRIBUTING.md
+!LICENSE
+
+!design/**
+!.github/**
+
+# ...even if they are in subdirectories
+!*/

mcp/client.go

@@ -174,6 +174,59 @@ func (cs *ClientSession) ID() string {
 	return cs.mcpConn.SessionID()
 }
 
+// ServerCapabilities returns a copy of the server capabilities obtained during initialization.
+// If the session has not been initialized or capabilities are not available, it returns nil.
+func (cs *ClientSession) ServerCapabilities() *ServerCapabilities {
+	if cs.initializeResult == nil || cs.initializeResult.Capabilities == nil {
+		return nil
+	}
+
+	// Create a copy of the capabilities
+	caps := &ServerCapabilities{}
+
+	// Copy experimental capabilities
+	if cs.initializeResult.Capabilities.Experimental != nil {
+		caps.Experimental = make(map[string]struct{})
+		for k, v := range cs.initializeResult.Capabilities.Experimental {
+			caps.Experimental[k] = v
+		}
+	}
+
+	// Copy completion capabilities
+	if cs.initializeResult.Capabilities.Completions != nil {
+		caps.Completions = &CompletionCapabilities{}
+	}
+
+	// Copy logging capabilities
+	if cs.initializeResult.Capabilities.Logging != nil {
+		caps.Logging = &LoggingCapabilities{}
+	}
+
+	// Copy prompt capabilities
+	if cs.initializeResult.Capabilities.Prompts != nil {
+		caps.Prompts = &PromptCapabilities{
+			ListChanged: cs.initializeResult.Capabilities.Prompts.ListChanged,
+		}
+	}
+
+	// Copy resource capabilities
+	if cs.initializeResult.Capabilities.Resources != nil {
+		caps.Resources = &ResourceCapabilities{
+			ListChanged: cs.initializeResult.Capabilities.Resources.ListChanged,
+			Subscribe:   cs.initializeResult.Capabilities.Resources.Subscribe,
+		}
+	}
+
+	// Copy tool capabilities
+	if cs.initializeResult.Capabilities.Tools != nil {
+		caps.Tools = &ToolCapabilities{
+			ListChanged: cs.initializeResult.Capabilities.Tools.ListChanged,
+		}
+	}
+
+	return caps
+}
+
 // Close performs a graceful close of the connection, preventing new requests
 // from being handled, and waiting for ongoing requests to return. Close then
 // terminates the connection.

mcp/client_test.go

@@ -190,3 +190,54 @@ func TestClientPaginateVariousPageSizes(t *testing.T) {
 		})
 	}
 }
+
+func TestClientSession_ServerCapabilities_ReturnsACopy(t *testing.T) {
+	// Test that modifying the returned capabilities doesn't affect the original
+	session := &ClientSession{
+		initializeResult: &InitializeResult{
+			Capabilities: &ServerCapabilities{
+				Experimental: map[string]struct{}{
+					"original": {},
+				},
+				Prompts: &PromptCapabilities{
+					ListChanged: true,
+				},
+				Resources: &ResourceCapabilities{
+					ListChanged: true,
+					Subscribe:   true,
+				},
+			},
+		},
+	}
+
+	// Get the capabilities
+	caps := session.ServerCapabilities()
+	if caps == nil {
+		t.Fatal("expected non-nil capabilities")
+	}
+
+	// Modify the returned capabilities
+	caps.Experimental["new_key"] = struct{}{}
+	caps.Prompts.ListChanged = false
+	caps.Resources.Subscribe = false
+
+	// Get capabilities again to verify the original is unchanged
+	caps2 := session.ServerCapabilities()
+	if caps2 == nil {
+		t.Fatal("expected non-nil capabilities")
+	}
+
+	// Verify original values are unchanged
+	if _, exists := caps2.Experimental["new_key"]; exists {
+		t.Error("experimental capabilities were not properly copied")
+	}
+	if _, exists := caps2.Experimental["original"]; !exists {
+		t.Error("original experimental capability was lost")
+	}
+	if caps2.Prompts.ListChanged != true {
+		t.Error("prompts ListChanged was modified in original")
+	}
+	if caps2.Resources.Subscribe != true {
+		t.Error("resources Subscribe was modified in original")
+	}
+}

mcp/protocol.go

@@ -307,7 +307,7 @@ type InitializeResult struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta         `json:"_meta,omitempty"`
-	Capabilities *serverCapabilities `json:"capabilities"`
+	Capabilities *ServerCapabilities `json:"capabilities"`
 	// Instructions describing how to use the server and its features.
 	//
 	// This can be used by clients to improve the LLM's understanding of available
@@ -907,46 +907,51 @@ type Implementation struct {
 	Version string `json:"version"`
 }
 
+// CompletionCapabilities represents server capabilities for argument autocompletion suggestions.
 // Present if the server supports argument autocompletion suggestions.
-type completionCapabilities struct{}
+type CompletionCapabilities struct{}
 
+// LoggingCapabilities represents server capabilities for sending log messages to the client.
 // Present if the server supports sending log messages to the client.
-type loggingCapabilities struct{}
+type LoggingCapabilities struct{}
 
+// PromptCapabilities represents server capabilities for prompt templates.
 // Present if the server offers any prompt templates.
-type promptCapabilities struct {
+type PromptCapabilities struct {
 	// Whether this server supports notifications for changes to the prompt list.
 	ListChanged bool `json:"listChanged,omitempty"`
 }
 
+// ResourceCapabilities represents server capabilities for resources.
 // Present if the server offers any resources to read.
-type resourceCapabilities struct {
+type ResourceCapabilities struct {
 	// Whether this server supports notifications for changes to the resource list.
 	ListChanged bool `json:"listChanged,omitempty"`
 	// Whether this server supports subscribing to resource updates.
 	Subscribe bool `json:"subscribe,omitempty"`
 }
 
-// Capabilities that a server may support. Known capabilities are defined here,
+// ServerCapabilities represents the capabilities that a server may support. Known capabilities are defined here,
 // in this schema, but this is not a closed set: any server can define its own,
 // additional capabilities.
-type serverCapabilities struct {
+type ServerCapabilities struct {
 	// Present if the server supports argument autocompletion suggestions.
-	Completions *completionCapabilities `json:"completions,omitempty"`
+	Completions *CompletionCapabilities `json:"completions,omitempty"`
 	// Experimental, non-standard capabilities that the server supports.
 	Experimental map[string]struct{} `json:"experimental,omitempty"`
 	// Present if the server supports sending log messages to the client.
-	Logging *loggingCapabilities `json:"logging,omitempty"`
+	Logging *LoggingCapabilities `json:"logging,omitempty"`
 	// Present if the server offers any prompt templates.
-	Prompts *promptCapabilities `json:"prompts,omitempty"`
+	Prompts *PromptCapabilities `json:"prompts,omitempty"`
 	// Present if the server offers any resources to read.
-	Resources *resourceCapabilities `json:"resources,omitempty"`
+	Resources *ResourceCapabilities `json:"resources,omitempty"`
 	// Present if the server offers any tools to call.
-	Tools *toolCapabilities `json:"tools,omitempty"`
+	Tools *ToolCapabilities `json:"tools,omitempty"`
 }
 
+// ToolCapabilities represents server capabilities for tools.
 // Present if the server offers any tools to call.
-type toolCapabilities struct {
+type ToolCapabilities struct {
 	// Whether this server supports notifications for changes to the tool list.
 	ListChanged bool `json:"listChanged,omitempty"`
 }

mcp/server.go

@@ -225,22 +225,22 @@ func (s *Server) RemoveResourceTemplates(uriTemplates ...string) {
 		func() bool { return s.resourceTemplates.remove(uriTemplates...) })
 }
 
-func (s *Server) capabilities() *serverCapabilities {
+func (s *Server) capabilities() *ServerCapabilities {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
-	caps := &serverCapabilities{
-		Completions: &completionCapabilities{},
-		Logging:     &loggingCapabilities{},
+	caps := &ServerCapabilities{
+		Completions: &CompletionCapabilities{},
+		Logging:     &LoggingCapabilities{},
 	}
 	if s.tools.len() > 0 {
-		caps.Tools = &toolCapabilities{ListChanged: true}
+		caps.Tools = &ToolCapabilities{ListChanged: true}
 	}
 	if s.prompts.len() > 0 {
-		caps.Prompts = &promptCapabilities{ListChanged: true}
+		caps.Prompts = &PromptCapabilities{ListChanged: true}
 	}
 	if s.resources.len() > 0 || s.resourceTemplates.len() > 0 {
-		caps.Resources = &resourceCapabilities{ListChanged: true}
+		caps.Resources = &ResourceCapabilities{ListChanged: true}
 		if s.opts.SubscribeHandler != nil {
 			caps.Resources.Subscribe = true
 		}

mcp/server_test.go

@@ -236,47 +236,47 @@ func TestServerCapabilities(t *testing.T) {
 		name             string
 		configureServer  func(s *Server)
 		serverOpts       ServerOptions
-		wantCapabilities *serverCapabilities
+		wantCapabilities *ServerCapabilities
 	}{
 		{
 			name:            "No capabilities",
 			configureServer: func(s *Server) {},
-			wantCapabilities: &serverCapabilities{
-				Completions: &completionCapabilities{},
-				Logging:     &loggingCapabilities{},
+			wantCapabilities: &ServerCapabilities{
+				Completions: &CompletionCapabilities{},
+				Logging:     &LoggingCapabilities{},
 			},
 		},
 		{
 			name: "With prompts",
 			configureServer: func(s *Server) {
 				s.AddPrompt(&Prompt{Name: "p"}, nil)
 			},
-			wantCapabilities: &serverCapabilities{
-				Completions: &completionCapabilities{},
-				Logging:     &loggingCapabilities{},
-				Prompts:     &promptCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Completions: &CompletionCapabilities{},
+				Logging:     &LoggingCapabilities{},
+				Prompts:     &PromptCapabilities{ListChanged: true},
 			},
 		},
 		{
 			name: "With resources",
 			configureServer: func(s *Server) {
 				s.AddResource(&Resource{URI: "file:///r"}, nil)
 			},
-			wantCapabilities: &serverCapabilities{
-				Completions: &completionCapabilities{},
-				Logging:     &loggingCapabilities{},
-				Resources:   &resourceCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Completions: &CompletionCapabilities{},
+				Logging:     &LoggingCapabilities{},
+				Resources:   &ResourceCapabilities{ListChanged: true},
 			},
 		},
 		{
 			name: "With resource templates",
 			configureServer: func(s *Server) {
 				s.AddResourceTemplate(&ResourceTemplate{URITemplate: "file:///rt"}, nil)
 			},
-			wantCapabilities: &serverCapabilities{
-				Completions: &completionCapabilities{},
-				Logging:     &loggingCapabilities{},
-				Resources:   &resourceCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Completions: &CompletionCapabilities{},
+				Logging:     &LoggingCapabilities{},
+				Resources:   &ResourceCapabilities{ListChanged: true},
 			},
 		},
 		{
@@ -292,21 +292,21 @@ func TestServerCapabilities(t *testing.T) {
 					return nil
 				},
 			},
-			wantCapabilities: &serverCapabilities{
-				Completions: &completionCapabilities{},
-				Logging:     &loggingCapabilities{},
-				Resources:   &resourceCapabilities{ListChanged: true, Subscribe: true},
+			wantCapabilities: &ServerCapabilities{
+				Completions: &CompletionCapabilities{},
+				Logging:     &LoggingCapabilities{},
+				Resources:   &ResourceCapabilities{ListChanged: true, Subscribe: true},
 			},
 		},
 		{
 			name: "With tools",
 			configureServer: func(s *Server) {
 				s.AddTool(tool, nil)
 			},
-			wantCapabilities: &serverCapabilities{
-				Completions: &completionCapabilities{},
-				Logging:     &loggingCapabilities{},
-				Tools:       &toolCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Completions: &CompletionCapabilities{},
+				Logging:     &LoggingCapabilities{},
+				Tools:       &ToolCapabilities{ListChanged: true},
 			},
 		},
 		{
@@ -325,12 +325,12 @@ func TestServerCapabilities(t *testing.T) {
 					return nil
 				},
 			},
-			wantCapabilities: &serverCapabilities{
-				Completions: &completionCapabilities{},
-				Logging:     &loggingCapabilities{},
-				Prompts:     &promptCapabilities{ListChanged: true},
-				Resources:   &resourceCapabilities{ListChanged: true, Subscribe: true},
-				Tools:       &toolCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Completions: &CompletionCapabilities{},
+				Logging:     &LoggingCapabilities{},
+				Prompts:     &PromptCapabilities{ListChanged: true},
+				Resources:   &ResourceCapabilities{ListChanged: true, Subscribe: true},
+				Tools:       &ToolCapabilities{ListChanged: true},
 			},
 		},
 	}

mcp/streamable_test.go

@@ -281,10 +281,10 @@ func TestStreamableServerTransport(t *testing.T) {
 	// Predefined steps, to avoid repetition below.
 	initReq := req(1, "initialize", &InitializeParams{})
 	initResp := resp(1, &InitializeResult{
-		Capabilities: &serverCapabilities{
-			Completions: &completionCapabilities{},
-			Logging:     &loggingCapabilities{},
-			Tools:       &toolCapabilities{ListChanged: true},
+		Capabilities: &ServerCapabilities{
+			Completions: &CompletionCapabilities{},
+			Logging:     &LoggingCapabilities{},
+			Tools:       &ToolCapabilities{ListChanged: true},
 		},
 		ProtocolVersion: latestProtocolVersion,
 		ServerInfo:      &Implementation{Name: "testServer", Version: "v1.0.0"},