feat(mcp): export initialized result in client session (#320)

Export ServerCapabilities and related subtypes and update protocol,
server, and tests to use exported types. Add InitializeResult method on
ClientSession for accessing the initialize result from sessions.

Fixes: https://github.com/modelcontextprotocol/go-sdk/issues/166

Co-authored-by: Shashank Pachava <spachava753@gmail.com>

mcp/client.go

@@ -177,6 +177,12 @@ type clientSessionState struct {
 	InitializeResult *InitializeResult
 }
 
+func (cs *ClientSession) InitializeResult() *InitializeResult { return cs.state.InitializeResult }
+
+func (cs *ClientSession) setConn(c Connection) {
+	cs.mcpConn = c
+}
+
 func (cs *ClientSession) ID() string {
 	if c, ok := cs.mcpConn.(hasSessionID); ok {
 		return c.SessionID()

mcp/protocol.go

@@ -338,7 +338,7 @@ 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
@@ -971,19 +971,19 @@ type Implementation struct {
 }
 
 // Present if the server supports argument autocompletion suggestions.
-type completionCapabilities struct{}
+type CompletionCapabilities struct{}
 
 // Present if the server supports sending log messages to the client.
-type loggingCapabilities struct{}
+type LoggingCapabilities struct{}
 
 // 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"`
 }
 
 // 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.
@@ -993,23 +993,23 @@ type resourceCapabilities struct {
 // 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"`
 }
 
 // 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

@@ -242,27 +242,27 @@ 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{
-		Logging: &loggingCapabilities{},
+	caps := &ServerCapabilities{
+		Logging: &LoggingCapabilities{},
 	}
 	if s.opts.HasTools || s.tools.len() > 0 {
-		caps.Tools = &toolCapabilities{ListChanged: true}
+		caps.Tools = &ToolCapabilities{ListChanged: true}
 	}
 	if s.opts.HasPrompts || s.prompts.len() > 0 {
-		caps.Prompts = &promptCapabilities{ListChanged: true}
+		caps.Prompts = &PromptCapabilities{ListChanged: true}
 	}
 	if s.opts.HasResources || 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
 		}
 	}
 	if s.opts.CompletionHandler != nil {
-		caps.Completions = &completionCapabilities{}
+		caps.Completions = &CompletionCapabilities{}
 	}
 	return caps
 }

mcp/server_test.go

@@ -236,13 +236,13 @@ 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{
-				Logging: &loggingCapabilities{},
+			wantCapabilities: &ServerCapabilities{
+				Logging: &LoggingCapabilities{},
 			},
 		},
 		{
@@ -250,9 +250,9 @@ func TestServerCapabilities(t *testing.T) {
 			configureServer: func(s *Server) {
 				s.AddPrompt(&Prompt{Name: "p"}, nil)
 			},
-			wantCapabilities: &serverCapabilities{
-				Logging: &loggingCapabilities{},
-				Prompts: &promptCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Logging: &LoggingCapabilities{},
+				Prompts: &PromptCapabilities{ListChanged: true},
 			},
 		},
 		{
@@ -260,9 +260,9 @@ func TestServerCapabilities(t *testing.T) {
 			configureServer: func(s *Server) {
 				s.AddResource(&Resource{URI: "file:///r"}, nil)
 			},
-			wantCapabilities: &serverCapabilities{
-				Logging:   &loggingCapabilities{},
-				Resources: &resourceCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Logging:   &LoggingCapabilities{},
+				Resources: &ResourceCapabilities{ListChanged: true},
 			},
 		},
 		{
@@ -270,9 +270,9 @@ func TestServerCapabilities(t *testing.T) {
 			configureServer: func(s *Server) {
 				s.AddResourceTemplate(&ResourceTemplate{URITemplate: "file:///rt"}, nil)
 			},
-			wantCapabilities: &serverCapabilities{
-				Logging:   &loggingCapabilities{},
-				Resources: &resourceCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Logging:   &LoggingCapabilities{},
+				Resources: &ResourceCapabilities{ListChanged: true},
 			},
 		},
 		{
@@ -288,9 +288,9 @@ func TestServerCapabilities(t *testing.T) {
 					return nil
 				},
 			},
-			wantCapabilities: &serverCapabilities{
-				Logging:   &loggingCapabilities{},
-				Resources: &resourceCapabilities{ListChanged: true, Subscribe: true},
+			wantCapabilities: &ServerCapabilities{
+				Logging:   &LoggingCapabilities{},
+				Resources: &ResourceCapabilities{ListChanged: true, Subscribe: true},
 			},
 		},
 		{
@@ -298,9 +298,9 @@ func TestServerCapabilities(t *testing.T) {
 			configureServer: func(s *Server) {
 				s.AddTool(tool, nil)
 			},
-			wantCapabilities: &serverCapabilities{
-				Logging: &loggingCapabilities{},
-				Tools:   &toolCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Logging: &LoggingCapabilities{},
+				Tools:   &ToolCapabilities{ListChanged: true},
 			},
 		},
 		{
@@ -311,9 +311,9 @@ func TestServerCapabilities(t *testing.T) {
 					return nil, nil
 				},
 			},
-			wantCapabilities: &serverCapabilities{
-				Logging:     &loggingCapabilities{},
-				Completions: &completionCapabilities{},
+			wantCapabilities: &ServerCapabilities{
+				Logging:     &LoggingCapabilities{},
+				Completions: &CompletionCapabilities{},
 			},
 		},
 		{
@@ -335,12 +335,12 @@ func TestServerCapabilities(t *testing.T) {
 					return nil, 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},
 			},
 		},
 		{
@@ -351,11 +351,11 @@ func TestServerCapabilities(t *testing.T) {
 				HasResources: true,
 				HasTools:     true,
 			},
-			wantCapabilities: &serverCapabilities{
-				Logging:   &loggingCapabilities{},
-				Prompts:   &promptCapabilities{ListChanged: true},
-				Resources: &resourceCapabilities{ListChanged: true},
-				Tools:     &toolCapabilities{ListChanged: true},
+			wantCapabilities: &ServerCapabilities{
+				Logging:   &LoggingCapabilities{},
+				Prompts:   &PromptCapabilities{ListChanged: true},
+				Resources: &ResourceCapabilities{ListChanged: true},
+				Tools:     &ToolCapabilities{ListChanged: true},
 			},
 		},
 	}

mcp/streamable_test.go

@@ -409,9 +409,9 @@ func TestStreamableServerTransport(t *testing.T) {
 	// Predefined steps, to avoid repetition below.
 	initReq := req(1, methodInitialize, &InitializeParams{})
 	initResp := resp(1, &InitializeResult{
-		Capabilities: &serverCapabilities{
-			Logging: &loggingCapabilities{},
-			Tools:   &toolCapabilities{ListChanged: true},
+		Capabilities: &ServerCapabilities{
+			Logging: &LoggingCapabilities{},
+			Tools:   &ToolCapabilities{ListChanged: true},
 		},
 		ProtocolVersion: latestProtocolVersion,
 		ServerInfo:      &Implementation{Name: "testServer", Version: "v1.0.0"},
@@ -891,9 +891,10 @@ func TestStreamableClientTransportApplicationJSON(t *testing.T) {
 		}
 	}
 	initResult := &InitializeResult{
-		Capabilities: &serverCapabilities{
-			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"},