mcp/server: add initial capability server option

mcp/server.go

@@ -69,6 +69,15 @@ type ServerOptions struct {
 	SubscribeHandler func(context.Context, *SubscribeParams) error
 	// Function called when a client session unsubscribes from a resource.
 	UnsubscribeHandler func(context.Context, *UnsubscribeParams) error
+	// If true, advertises the prompts capability during initialization,
+	// even if no prompts have been registered.
+	HasPrompts bool
+	// If true, advertises the resources capability during initialization,
+	// even if no resources have been registered.
+	HasResources bool
+	// If true, advertises the tools capability during initialization,
+	// even if no tools have been registered.
+	HasTools bool
 }
 
 // NewServer creates a new MCP server. The resulting server has no features:
@@ -230,16 +239,17 @@ func (s *Server) capabilities() *serverCapabilities {
 	defer s.mu.Unlock()
 
 	caps := &serverCapabilities{
+		// TODO(samthanawalla): check for completionHandler before advertising capability.
 		Completions: &completionCapabilities{},
 		Logging:     &loggingCapabilities{},
 	}
-	if s.tools.len() > 0 {
+	if s.opts.HasTools || s.tools.len() > 0 {
 		caps.Tools = &toolCapabilities{ListChanged: true}
 	}
-	if s.prompts.len() > 0 {
+	if s.opts.HasPrompts || s.prompts.len() > 0 {
 		caps.Prompts = &promptCapabilities{ListChanged: true}
 	}
-	if s.resources.len() > 0 || s.resourceTemplates.len() > 0 {
+	if s.opts.HasResources || s.resources.len() > 0 || s.resourceTemplates.len() > 0 {
 		caps.Resources = &resourceCapabilities{ListChanged: true}
 		if s.opts.SubscribeHandler != nil {
 			caps.Resources.Subscribe = true

mcp/server_test.go

@@ -333,6 +333,22 @@ func TestServerCapabilities(t *testing.T) {
 				Tools:       &toolCapabilities{ListChanged: true},
 			},
 		},
+		{
+			name:            "With initial capabilities",
+			configureServer: func(s *Server) {},
+			serverOpts: ServerOptions{
+				HasPrompts:   true,
+				HasResources: true,
+				HasTools:     true,
+			},
+			wantCapabilities: &serverCapabilities{
+				Completions: &completionCapabilities{},
+				Logging:     &loggingCapabilities{},
+				Prompts:     &promptCapabilities{ListChanged: true},
+				Resources:   &resourceCapabilities{ListChanged: true},
+				Tools:       &toolCapabilities{ListChanged: true},
+			},
+		},
 	}
 
 	for _, tc := range testCases {