package mcp

import "golang.org/x/tools/internal/mcp"

The mcp package provides an SDK for writing model context protocol clients and servers.

To get started, create either a Client or Server, and connect it to a peer using a Transport. The diagram below illustrates how this works: 

Client                                                   Server
 ⇅                          (jsonrpc2)                     ⇅
ClientSession ⇄ Client Transport ⇄ Server Transport ⇄ ServerSession

A Client is an MCP client, which can be configured with various client capabilities. Clients may be connected to a Server instance using the Client.Connect method.

Similarly, a Server is an MCP server, which can be configured with various server capabilities. Servers may be connected to one or more Client instances using the Server.Connect method, which creates a ServerSession.

A Transport connects a bidirectional Connection of jsonrpc2 messages. In practice, transports in the MCP spec are are either client transports or server transports. For example, the StdioTransport is a server transport that communicates over stdin/stdout, and its counterpart is a CommandTransport that communicates with a subprocess over its stdin/stdout.

Some transports may hide more complicated details, such as an SSEClientTransport, which reads messages via server-sent events on a hanging GET request, and writes them to a POST endpoint. Users of this SDK may define their own custom Transports by implementing the Transport interface.

Example (ProgressMiddleware)

This middleware function adds a progress token to every outgoing request from the client.

Code:play  

package main

import (
	"context"
	"sync/atomic"

	"golang.org/x/tools/internal/mcp"
)

var nextProgressToken atomic.Int64

// This middleware function adds a progress token to every outgoing request
// from the client.
func main() {
	c := mcp.NewClient("test", "v1", nil)
	c.AddSendingMiddleware(addProgressToken[*mcp.ClientSession])
	_ = c
}

func addProgressToken[S mcp.Session](h mcp.MethodHandler[S]) mcp.MethodHandler[S] {
	return func(ctx context.Context, s S, method string, params mcp.Params) (result mcp.Result, err error) {
		params.GetMeta().ProgressToken = nextProgressToken.Add(1)
		return h(ctx, s, method, params)
	}
}

Index

Examples

Constants 

const (
	LevelDebug     = slog.LevelDebug
	LevelInfo      = slog.LevelInfo
	LevelNotice    = (slog.LevelInfo + slog.LevelWarn) / 2
	LevelWarning   = slog.LevelWarn
	LevelError     = slog.LevelError
	LevelCritical  = slog.LevelError + 4
	LevelAlert     = slog.LevelError + 8
	LevelEmergency = slog.LevelError + 12
)

Logging levels. 

const (
	// The error code to return when a resource isn't found.
	// See https://modelcontextprotocol.io/specification/2025-03-26/server/resources#error-handling
	// However, the code they chose is in the wrong space
	// (see https://github.com/modelcontextprotocol/modelcontextprotocol/issues/509).
	// so we pick a different one, arbitrarily for now (until they fix it).
	// The immediate problem is that jsonprc2 defines -32002 as "server closing".
	CodeResourceNotFound = -31002
	// The error code if the method exists and was called properly, but the peer does not support it.
	CodeUnsupportedMethod = -31001
)

Error codes 

const DefaultPageSize = 1000

Variables 

var ErrConnectionClosed = errors.New("connection closed")

ErrConnectionClosed is returned when sending a message to a connection that is closed or in the process of closing.

Functions

func NewInMemoryTransports 

func NewInMemoryTransports() (*InMemoryTransport, *InMemoryTransport)

NewInMemoryTransports returns two InMemoryTransports that connect to each other.

func ResourceNotFoundError 

func ResourceNotFoundError(uri string) error

ResourceNotFoundError returns an error indicating that a resource being read could not be found.

Types

type Annotations 

type Annotations struct {
	// Describes who the intended customer of this object or data is.
	//
	// It can include multiple entries to indicate content useful for multiple
	// audiences (e.g., `["user", "assistant"]`).
	Audience []Role `json:"audience,omitempty"`
	// Describes how important this data is for operating the server.
	//
	// A value of 1 means "most important," and indicates that the data is
	// effectively required, while 0 means "least important," and indicates that the
	// data is entirely optional.
	Priority float64 `json:"priority,omitempty"`
}

Optional annotations for the client. The client can use annotations to inform how objects are used or displayed

type CallToolParams 

type CallToolParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta      Meta   `json:"_meta,omitempty"`
	Arguments any    `json:"arguments,omitempty"`
	Name      string `json:"name"`
}

func (*CallToolParams) GetMeta 

func (x *CallToolParams) GetMeta() *Meta

type CallToolParamsFor 

type CallToolParamsFor[In any] struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta      Meta   `json:"_meta,omitempty"`
	Arguments In     `json:"arguments,omitempty"`
	Name      string `json:"name"`
}

func (*CallToolParamsFor[In]) GetMeta 

func (x *CallToolParamsFor[In]) GetMeta() *Meta

type CallToolResult 

type CallToolResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta    Meta       `json:"_meta,omitempty"`
	Content []*Content `json:"content"`
	// Whether the tool call ended in an error.
	//
	// If not set, this is assumed to be false (the call was successful).
	IsError bool `json:"isError,omitempty"`
}

The server's response to a tool call.

Any errors that originate from the tool SHOULD be reported inside the result object, with `isError` set to true, _not_ as an MCP protocol-level error response. Otherwise, the LLM would not be able to see that an error occurred and self-correct.

However, any errors in _finding_ the tool, an error indicating that the server does not support tool calls, or any other exceptional conditions, should be reported as an MCP error response.

func (*CallToolResult) GetMeta 

func (x *CallToolResult) GetMeta() *Meta

type CallToolResultFor 

type CallToolResultFor[Out any] struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta    Meta       `json:"_meta,omitempty"`
	Content []*Content `json:"content"`
	// Whether the tool call ended in an error.
	//
	// If not set, this is assumed to be false (the call was successful).
	IsError bool `json:"isError,omitempty"`
}

func (*CallToolResultFor[Out]) GetMeta 

func (x *CallToolResultFor[Out]) GetMeta() *Meta

type CancelledParams 

type CancelledParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An optional string describing the reason for the cancellation. This MAY be
	// logged or presented to the user.
	Reason string `json:"reason,omitempty"`
	// The ID of the request to cancel.
	//
	// This MUST correspond to the ID of a request previously issued in the same
	// direction.
	RequestID any `json:"requestId"`
}

func (*CancelledParams) GetMeta 

func (x *CancelledParams) GetMeta() *Meta

type Client 

type Client struct {
	// contains filtered or unexported fields
}

A Client is an MCP client, which may be connected to an MCP server using the Client.Connect method.

func NewClient 

func NewClient(name, version string, opts *ClientOptions) *Client

NewClient creates a new Client.

Use Client.Connect to connect it to an MCP server.

If non-nil, the provided options configure the Client.

func (*Client) AddReceivingMiddleware 

func (c *Client) AddReceivingMiddleware(middleware ...Middleware[*ClientSession])

AddReceivingMiddleware wraps the current receiving method handler using the provided middleware. Middleware is applied from right to left, so that the first one is executed first.

For example, AddReceivingMiddleware(m1, m2, m3) augments the method handler as m1(m2(m3(handler))).

Receiving middleware is called when a request is received. It is useful for tasks such as authentication, request logging and metrics.

func (*Client) AddRoots 

func (c *Client) AddRoots(roots ...*Root)

AddRoots adds the given roots to the client, replacing any with the same URIs, and notifies any connected servers.

func (*Client) AddSendingMiddleware 

func (c *Client) AddSendingMiddleware(middleware ...Middleware[*ClientSession])

AddSendingMiddleware wraps the current sending method handler using the provided middleware. Middleware is applied from right to left, so that the first one is executed first.

For example, AddSendingMiddleware(m1, m2, m3) augments the method handler as m1(m2(m3(handler))).

Sending middleware is called when a request is sent. It is useful for tasks such as tracing, metrics, and adding progress tokens.

func (*Client) Connect 

func (c *Client) Connect(ctx context.Context, t Transport) (cs *ClientSession, err error)

Connect begins an MCP session by connecting to a server over the given transport, and initializing the session.

Typically, it is the responsibility of the client to close the connection when it is no longer needed. However, if the connection is closed by the server, calls or notifications will return an error wrapping ErrConnectionClosed.

func (*Client) RemoveRoots 

func (c *Client) RemoveRoots(uris ...string)

RemoveRoots removes the roots with the given URIs, and notifies any connected servers if the list has changed. It is not an error to remove a nonexistent root. TODO: notification

type ClientCapabilities 

type ClientCapabilities struct {
	// Experimental, non-standard capabilities that the client supports.
	Experimental map[string]struct {
	} `json:"experimental,omitempty"`
	// Present if the client supports listing roots.
	Roots struct {
		// Whether the client supports notifications for changes to the roots list.
		ListChanged bool `json:"listChanged,omitempty"`
	} `json:"roots,omitempty"`
	// Present if the client supports sampling from an LLM.
	Sampling *SamplingCapabilities `json:"sampling,omitempty"`
}

Capabilities a client may support. Known capabilities are defined here, in this schema, but this is not a closed set: any client can define its own, additional capabilities.

type ClientOptions 

type ClientOptions struct {
	// Handler for sampling.
	// Called when a server calls CreateMessage.
	CreateMessageHandler func(context.Context, *ClientSession, *CreateMessageParams) (*CreateMessageResult, error)
	// Handlers for notifications from the server.
	ToolListChangedHandler      func(context.Context, *ClientSession, *ToolListChangedParams)
	PromptListChangedHandler    func(context.Context, *ClientSession, *PromptListChangedParams)
	ResourceListChangedHandler  func(context.Context, *ClientSession, *ResourceListChangedParams)
	LoggingMessageHandler       func(context.Context, *ClientSession, *LoggingMessageParams)
	ProgressNotificationHandler func(context.Context, *ClientSession, *ProgressNotificationParams)
}

ClientOptions configures the behavior of the client.

type ClientSession 

type ClientSession struct {
	// contains filtered or unexported fields
}

A ClientSession is a logical connection with an MCP server. Its methods can be used to send requests or notifications to the server. Create a session by calling Client.Connect.

Call ClientSession.Close to close the connection, or await client termination with ServerSession.Wait.

func (*ClientSession) CallTool 

func (cs *ClientSession) CallTool(ctx context.Context, params *CallToolParams) (*CallToolResult, error)

CallTool calls the tool with the given name and arguments. The arguments can be any value that marshals into a JSON object.

func (*ClientSession) Close 

func (cs *ClientSession) Close() error

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.

func (*ClientSession) GetPrompt 

func (cs *ClientSession) GetPrompt(ctx context.Context, params *GetPromptParams) (*GetPromptResult, error)

GetPrompt gets a prompt from the server.

func (*ClientSession) ListPrompts 

func (cs *ClientSession) ListPrompts(ctx context.Context, params *ListPromptsParams) (*ListPromptsResult, error)

ListPrompts lists prompts that are currently available on the server.

func (*ClientSession) ListResourceTemplates 

func (cs *ClientSession) ListResourceTemplates(ctx context.Context, params *ListResourceTemplatesParams) (*ListResourceTemplatesResult, error)

ListResourceTemplates lists the resource templates that are currently available on the server.

func (*ClientSession) ListResources 

func (cs *ClientSession) ListResources(ctx context.Context, params *ListResourcesParams) (*ListResourcesResult, error)

ListResources lists the resources that are currently available on the server.

func (*ClientSession) ListTools 

func (cs *ClientSession) ListTools(ctx context.Context, params *ListToolsParams) (*ListToolsResult, error)

ListTools lists tools that are currently available on the server.

func (*ClientSession) NotifyProgress 

func (cs *ClientSession) NotifyProgress(ctx context.Context, params *ProgressNotificationParams) error

NotifyProgress sends a progress notification from the client to the server associated with this session. This can be used if the client is performing a long-running task that was initiated by the server

func (*ClientSession) Ping 

func (cs *ClientSession) Ping(ctx context.Context, params *PingParams) error

Ping makes an MCP "ping" request to the server.

func (*ClientSession) Prompts 

func (cs *ClientSession) Prompts(ctx context.Context, params *ListPromptsParams) iter.Seq2[*Prompt, error]

Prompts provides an iterator for all prompts available on the server, automatically fetching pages and managing cursors. The params argument can set the initial cursor. Iteration stops at the first encountered error, which will be yielded.

func (*ClientSession) ReadResource 

func (cs *ClientSession) ReadResource(ctx context.Context, params *ReadResourceParams) (*ReadResourceResult, error)

ReadResource ask the server to read a resource and return its contents.

func (*ClientSession) ResourceTemplates 

func (cs *ClientSession) ResourceTemplates(ctx context.Context, params *ListResourceTemplatesParams) iter.Seq2[*ResourceTemplate, error]

ResourceTemplates provides an iterator for all resource templates available on the server, automatically fetching pages and managing cursors. The `params` argument can set the initial cursor. Iteration stops at the first encountered error, which will be yielded.

func (*ClientSession) Resources 

func (cs *ClientSession) Resources(ctx context.Context, params *ListResourcesParams) iter.Seq2[*Resource, error]

Resources provides an iterator for all resources available on the server, automatically fetching pages and managing cursors. The params argument can set the initial cursor. Iteration stops at the first encountered error, which will be yielded.

func (*ClientSession) SetLevel 

func (cs *ClientSession) SetLevel(ctx context.Context, params *SetLevelParams) error

func (*ClientSession) Tools 

func (cs *ClientSession) Tools(ctx context.Context, params *ListToolsParams) iter.Seq2[*Tool, error]

Tools provides an iterator for all tools available on the server, automatically fetching pages and managing cursors. The params argument can set the initial cursor. Iteration stops at the first encountered error, which will be yielded.

func (*ClientSession) Wait 

func (cs *ClientSession) Wait() error

Wait waits for the connection to be closed by the server. Generally, clients should be responsible for closing the connection.

type CommandTransport 

type CommandTransport struct {
	// contains filtered or unexported fields
}

A CommandTransport is a Transport that runs a command and communicates with it over stdin/stdout, using newline-delimited JSON.

func NewCommandTransport 

func NewCommandTransport(cmd *exec.Cmd) *CommandTransport

NewCommandTransport returns a CommandTransport that runs the given command and communicates with it over stdin/stdout.

The resulting transport takes ownership of the command, starting it during CommandTransport.Connect, and stopping it when the connection is closed.

func (*CommandTransport) Connect 

func (t *CommandTransport) Connect(ctx context.Context) (Connection, error)

Connect starts the command, and connects to it over stdin/stdout.

type Connection 

type Connection interface {
	Read(context.Context) (JSONRPCMessage, error)
	Write(context.Context, JSONRPCMessage) error
	Close() error // may be called concurrently by both peers
}

A Connection is a logical bidirectional JSON-RPC connection.

type Content 

type Content struct {
	Type        string            `json:"type"`
	Text        string            `json:"text,omitempty"`
	MIMEType    string            `json:"mimeType,omitempty"`
	Data        []byte            `json:"data,omitempty"`
	Resource    *ResourceContents `json:"resource,omitempty"`
	Annotations *Annotations      `json:"annotations,omitempty"`
}

Content is the wire format for content. It represents the protocol types TextContent, ImageContent, AudioContent and EmbeddedResource. Use NewTextContent, NewImageContent, NewAudioContent or NewResourceContent to create one.

The Type field must be one of "text", "image", "audio" or "resource". The constructors above populate this field appropriately. Although at most one of Text, Data, and Resource should be non-zero, consumers of Content use the Type field to determine which value to use; values in the other fields are ignored.

func NewAudioContent 

func NewAudioContent(data []byte, mimeType string) *Content

NewAudioContent creates a Content with audio data.

func NewImageContent 

func NewImageContent(data []byte, mimeType string) *Content

NewImageContent creates a Content with image data.

func NewResourceContent 

func NewResourceContent(resource *ResourceContents) *Content

NewResourceContent creates a Content with an embedded resource.

func NewTextContent 

func NewTextContent(text string) *Content

NewTextContent creates a Content with text.

func (*Content) UnmarshalJSON 

func (c *Content) UnmarshalJSON(data []byte) error

type CreateMessageParams 

type CreateMessageParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// A request to include context from one or more MCP servers (including the
	// caller), to be attached to the prompt. The client MAY ignore this request.
	IncludeContext string `json:"includeContext,omitempty"`
	// The maximum number of tokens to sample, as requested by the server. The
	// client MAY choose to sample fewer tokens than requested.
	MaxTokens int64              `json:"maxTokens"`
	Messages  []*SamplingMessage `json:"messages"`
	// Optional metadata to pass through to the LLM provider. The format of this
	// metadata is provider-specific.
	Metadata struct {
	} `json:"metadata,omitempty"`
	// The server's preferences for which model to select. The client MAY ignore
	// these preferences.
	ModelPreferences *ModelPreferences `json:"modelPreferences,omitempty"`
	StopSequences    []string          `json:"stopSequences,omitempty"`
	// An optional system prompt the server wants to use for sampling. The client
	// MAY modify or omit this prompt.
	SystemPrompt string  `json:"systemPrompt,omitempty"`
	Temperature  float64 `json:"temperature,omitempty"`
}

func (*CreateMessageParams) GetMeta 

func (x *CreateMessageParams) GetMeta() *Meta

type CreateMessageResult 

type CreateMessageResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta    Meta     `json:"_meta,omitempty"`
	Content *Content `json:"content"`
	// The name of the model that generated the message.
	Model string `json:"model"`
	Role  Role   `json:"role"`
	// The reason why sampling stopped, if known.
	StopReason string `json:"stopReason,omitempty"`
}

The client's response to a sampling/create_message request from the server. The client should inform the user before returning the sampled message, to allow them to inspect the response (human in the loop) and decide whether to allow the server to see it.

func (*CreateMessageResult) GetMeta 

func (x *CreateMessageResult) GetMeta() *Meta

type GetPromptParams 

type GetPromptParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// Arguments to use for templating the prompt.
	Arguments map[string]string `json:"arguments,omitempty"`
	// The name of the prompt or prompt template.
	Name string `json:"name"`
}

func (*GetPromptParams) GetMeta 

func (x *GetPromptParams) GetMeta() *Meta

type GetPromptResult 

type GetPromptResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An optional description for the prompt.
	Description string           `json:"description,omitempty"`
	Messages    []*PromptMessage `json:"messages"`
}

The server's response to a prompts/get request from the client.

func (*GetPromptResult) GetMeta 

func (x *GetPromptResult) GetMeta() *Meta

type InMemoryTransport 

type InMemoryTransport struct {
	// contains filtered or unexported fields
}

An InMemoryTransport is a Transport that communicates over an in-memory network connection, using newline-delimited JSON.

func (*InMemoryTransport) Connect 

func (t *InMemoryTransport) Connect(context.Context) (Connection, error)

type InitializeParams 

type InitializeParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta         Meta                `json:"_meta,omitempty"`
	Capabilities *ClientCapabilities `json:"capabilities"`
	ClientInfo   *implementation     `json:"clientInfo"`
	// The latest version of the Model Context Protocol that the client supports.
	// The client MAY decide to support older versions as well.
	ProtocolVersion string `json:"protocolVersion"`
}

func (*InitializeParams) GetMeta 

func (x *InitializeParams) GetMeta() *Meta

type InitializeResult 

type InitializeResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta         Meta                `json:"_meta,omitempty"`
	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
	// tools, resources, etc. It can be thought of like a "hint" to the model. For
	// example, this information MAY be added to the system prompt.
	Instructions string `json:"instructions,omitempty"`
	// The version of the Model Context Protocol that the server wants to use. This
	// may not match the version that the client requested. If the client cannot
	// support this version, it MUST disconnect.
	ProtocolVersion string          `json:"protocolVersion"`
	ServerInfo      *implementation `json:"serverInfo"`
}

After receiving an initialize request from the client, the server sends this response.

func (*InitializeResult) GetMeta 

func (x *InitializeResult) GetMeta() *Meta

type InitializedParams 

type InitializedParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
}

func (*InitializedParams) GetMeta 

func (x *InitializedParams) GetMeta() *Meta

type JSONRPCID 

type JSONRPCID = jsonrpc2.ID

JSONRPCID is a JSON-RPC request ID.

type JSONRPCMessage 

type JSONRPCMessage = jsonrpc2.Message

JSONRPCMessage is a JSON-RPC message.

type JSONRPCRequest 

type JSONRPCRequest = jsonrpc2.Request

JSONRPCRequest is a JSON-RPC request.

type JSONRPCResponse 

type JSONRPCResponse = jsonrpc2.Response

JSONRPCResponse is a JSON-RPC response.

type ListPromptsParams 

type ListPromptsParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An opaque token representing the current pagination position. If provided,
	// the server should return results starting after this cursor.
	Cursor string `json:"cursor,omitempty"`
}

func (*ListPromptsParams) GetMeta 

func (x *ListPromptsParams) GetMeta() *Meta

type ListPromptsResult 

type ListPromptsResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An opaque token representing the pagination position after the last returned
	// result. If present, there may be more results available.
	NextCursor string    `json:"nextCursor,omitempty"`
	Prompts    []*Prompt `json:"prompts"`
}

The server's response to a prompts/list request from the client.

func (*ListPromptsResult) GetMeta 

func (x *ListPromptsResult) GetMeta() *Meta

type ListResourceTemplatesParams 

type ListResourceTemplatesParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An opaque token representing the current pagination position. If provided,
	// the server should return results starting after this cursor.
	Cursor string `json:"cursor,omitempty"`
}

func (*ListResourceTemplatesParams) GetMeta 

func (x *ListResourceTemplatesParams) GetMeta() *Meta

type ListResourceTemplatesResult 

type ListResourceTemplatesResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An opaque token representing the pagination position after the last returned
	// result. If present, there may be more results available.
	NextCursor        string              `json:"nextCursor,omitempty"`
	ResourceTemplates []*ResourceTemplate `json:"resourceTemplates"`
}

The server's response to a resources/templates/list request from the client.

func (*ListResourceTemplatesResult) GetMeta 

func (x *ListResourceTemplatesResult) GetMeta() *Meta

type ListResourcesParams 

type ListResourcesParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An opaque token representing the current pagination position. If provided,
	// the server should return results starting after this cursor.
	Cursor string `json:"cursor,omitempty"`
}

func (*ListResourcesParams) GetMeta 

func (x *ListResourcesParams) GetMeta() *Meta

type ListResourcesResult 

type ListResourcesResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An opaque token representing the pagination position after the last returned
	// result. If present, there may be more results available.
	NextCursor string      `json:"nextCursor,omitempty"`
	Resources  []*Resource `json:"resources"`
}

The server's response to a resources/list request from the client.

func (*ListResourcesResult) GetMeta 

func (x *ListResourcesResult) GetMeta() *Meta

type ListRootsParams 

type ListRootsParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
}

func (*ListRootsParams) GetMeta 

func (x *ListRootsParams) GetMeta() *Meta

type ListRootsResult 

type ListRootsResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta  Meta    `json:"_meta,omitempty"`
	Roots []*Root `json:"roots"`
}

The client's response to a roots/list request from the server. This result contains an array of Root objects, each representing a root directory or file that the server can operate on.

func (*ListRootsResult) GetMeta 

func (x *ListRootsResult) GetMeta() *Meta

type ListToolsParams 

type ListToolsParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An opaque token representing the current pagination position. If provided,
	// the server should return results starting after this cursor.
	Cursor string `json:"cursor,omitempty"`
}

func (*ListToolsParams) GetMeta 

func (x *ListToolsParams) GetMeta() *Meta

type ListToolsResult 

type ListToolsResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An opaque token representing the pagination position after the last returned
	// result. If present, there may be more results available.
	NextCursor string  `json:"nextCursor,omitempty"`
	Tools      []*Tool `json:"tools"`
}

The server's response to a tools/list request from the client.

func (*ListToolsResult) GetMeta 

func (x *ListToolsResult) GetMeta() *Meta

type LoggingHandler 

type LoggingHandler struct {
	// contains filtered or unexported fields
}

A LoggingHandler is a slog.Handler for MCP.

func NewLoggingHandler 

func NewLoggingHandler(ss *ServerSession, opts *LoggingHandlerOptions) *LoggingHandler

NewLoggingHandler creates a LoggingHandler that logs to the given ServerSession using a slog.JSONHandler.

func (*LoggingHandler) Enabled 

func (h *LoggingHandler) Enabled(ctx context.Context, level slog.Level) bool

Enabled implements slog.Handler.Enabled by comparing level to the ServerSession's level.

func (*LoggingHandler) Handle 

func (h *LoggingHandler) Handle(ctx context.Context, r slog.Record) error

Handle implements slog.Handler.Handle by writing the Record to a JSONHandler, then calling [ServerSession.LoggingMesssage] with the result.

func (*LoggingHandler) WithAttrs 

func (h *LoggingHandler) WithAttrs(as []slog.Attr) slog.Handler

WithAttrs implements slog.Handler.WithAttrs.

func (*LoggingHandler) WithGroup 

func (h *LoggingHandler) WithGroup(name string) slog.Handler

WithGroup implements slog.Handler.WithGroup.

type LoggingHandlerOptions 

type LoggingHandlerOptions struct {
	// The value for the "logger" field of logging notifications.
	LoggerName string
	// Limits the rate at which log messages are sent.
	// If zero, there is no rate limiting.
	MinInterval time.Duration
}

LoggingHandlerOptions are options for a LoggingHandler.

type LoggingLevel 

type LoggingLevel string

The severity of a log message.

These map to syslog message severities, as specified in RFC-5424: https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.1

type LoggingMessageParams 

type LoggingMessageParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// The data to be logged, such as a string message or an object. Any JSON
	// serializable type is allowed here.
	Data any `json:"data"`
	// The severity of this log message.
	Level LoggingLevel `json:"level"`
	// An optional name of the logger issuing this message.
	Logger string `json:"logger,omitempty"`
}

func (*LoggingMessageParams) GetMeta 

func (x *LoggingMessageParams) GetMeta() *Meta

type LoggingTransport 

type LoggingTransport struct {
	// contains filtered or unexported fields
}

A LoggingTransport is a Transport that delegates to another transport, writing RPC logs to an io.Writer.

func NewLoggingTransport 

func NewLoggingTransport(delegate Transport, w io.Writer) *LoggingTransport

NewLoggingTransport creates a new LoggingTransport that delegates to the provided transport, writing RPC logs to the provided io.Writer.

func (*LoggingTransport) Connect 

func (t *LoggingTransport) Connect(ctx context.Context) (Connection, error)

Connect connects the underlying transport, returning a Connection that writes logs to the configured destination.

type Meta 

type Meta struct {
	Data map[string]any `json:",omitempty"`
	// For params, the progress token can be nil, a string or an integer.
	// It should be nil for results.
	ProgressToken any `json:"progressToken,omitempty"`
}

func (Meta) MarshalJSON 

func (m Meta) MarshalJSON() ([]byte, error)

func (*Meta) UnmarshalJSON 

func (m *Meta) UnmarshalJSON(data []byte) error

type MethodHandler 

type MethodHandler[S Session] func(ctx context.Context, _ S, method string, params Params) (result Result, err error)

A MethodHandler handles MCP messages. For methods, exactly one of the return values must be nil. For notifications, both must be nil.

type Middleware 

type Middleware[S Session] func(MethodHandler[S]) MethodHandler[S]

Middleware is a function from MethodHandlers to MethodHandlers.

type ModelHint 

type ModelHint struct {
	// A hint for a model name.
	//
	// The client SHOULD treat this as a substring of a model name; for example: -
	// `claude-3-5-sonnet` should match `claude-3-5-sonnet-20241022` - `sonnet`
	// should match `claude-3-5-sonnet-20241022`, `claude-3-sonnet-20240229`, etc. -
	// `claude` should match any Claude model
	//
	// The client MAY also map the string to a different provider's model name or a
	// different model family, as long as it fills a similar niche; for example: -
	// `gemini-1.5-flash` could match `claude-3-haiku-20240307`
	Name string `json:"name,omitempty"`
}

Hints to use for model selection.

Keys not declared here are currently left unspecified by the spec and are up to the client to interpret.

type ModelPreferences 

type ModelPreferences struct {
	// How much to prioritize cost when selecting a model. A value of 0 means cost
	// is not important, while a value of 1 means cost is the most important factor.
	CostPriority float64 `json:"costPriority,omitempty"`
	// Optional hints to use for model selection.
	//
	// If multiple hints are specified, the client MUST evaluate them in order (such
	// that the first match is taken).
	//
	// The client SHOULD prioritize these hints over the numeric priorities, but MAY
	// still use the priorities to select from ambiguous matches.
	Hints []*ModelHint `json:"hints,omitempty"`
	// How much to prioritize intelligence and capabilities when selecting a model.
	// A value of 0 means intelligence is not important, while a value of 1 means
	// intelligence is the most important factor.
	IntelligencePriority float64 `json:"intelligencePriority,omitempty"`
	// How much to prioritize sampling speed (latency) when selecting a model. A
	// value of 0 means speed is not important, while a value of 1 means speed is
	// the most important factor.
	SpeedPriority float64 `json:"speedPriority,omitempty"`
}

The server's preferences for model selection, requested of the client during sampling.

Because LLMs can vary along multiple dimensions, choosing the "best" model is rarely straightforward. Different models excel in different areas—some are faster but less capable, others are more capable but more expensive, and so on. This interface allows servers to express their priorities across multiple dimensions to help clients make an appropriate selection for their use case.

These preferences are always advisory. The client MAY ignore them. It is also up to the client to decide how to interpret these preferences and how to balance them against other considerations.

type Params 

type Params interface {
	// Returns a pointer to the params's Meta field.
	GetMeta() *Meta
}

Params is a parameter (input) type for an MCP call or notification.

type PingParams 

type PingParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
}

func (*PingParams) GetMeta 

func (x *PingParams) GetMeta() *Meta

type ProgressNotificationParams 

type ProgressNotificationParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// An optional message describing the current progress.
	Message string `json:"message,omitempty"`
	// The progress thus far. This should increase every time progress is made, even
	// if the total is unknown.
	Progress float64 `json:"progress"`
	// The progress token which was given in the initial request, used to associate
	// this notification with the request that is proceeding.
	ProgressToken any `json:"progressToken"`
	// Total number of items to process (or total progress required), if known.
	Total float64 `json:"total,omitempty"`
}

func (*ProgressNotificationParams) GetMeta 

func (x *ProgressNotificationParams) GetMeta() *Meta

type Prompt 

type Prompt struct {
	// A list of arguments to use for templating the prompt.
	Arguments []*PromptArgument `json:"arguments,omitempty"`
	// An optional description of what this prompt provides
	Description string `json:"description,omitempty"`
	// The name of the prompt or prompt template.
	Name string `json:"name"`
}

A prompt or prompt template that the server offers.

type PromptArgument 

type PromptArgument struct {
	// A human-readable description of the argument.
	Description string `json:"description,omitempty"`
	// The name of the argument.
	Name string `json:"name"`
	// Whether this argument must be provided.
	Required bool `json:"required,omitempty"`
}

Describes an argument that a prompt can accept.

type PromptHandler 

type PromptHandler func(context.Context, *ServerSession, *GetPromptParams) (*GetPromptResult, error)

A PromptHandler handles a call to prompts/get.

type PromptListChangedParams 

type PromptListChangedParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
}

func (*PromptListChangedParams) GetMeta 

func (x *PromptListChangedParams) GetMeta() *Meta

type PromptMessage 

type PromptMessage struct {
	Content *Content `json:"content"`
	Role    Role     `json:"role"`
}

Describes a message returned as part of a prompt.

This is similar to `SamplingMessage`, but also supports the embedding of resources from the MCP server.

type ReadResourceParams 

type ReadResourceParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// The URI of the resource to read. The URI can use any protocol; it is up to
	// the server how to interpret it.
	URI string `json:"uri"`
}

func (*ReadResourceParams) GetMeta 

func (x *ReadResourceParams) GetMeta() *Meta

type ReadResourceResult 

type ReadResourceResult struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta     Meta                `json:"_meta,omitempty"`
	Contents []*ResourceContents `json:"contents"`
}

The server's response to a resources/read request from the client.

func (*ReadResourceResult) GetMeta 

func (x *ReadResourceResult) GetMeta() *Meta

type Resource 

type Resource struct {
	// Optional annotations for the client.
	Annotations *Annotations `json:"annotations,omitempty"`
	// A description of what this resource represents.
	//
	// This can be used by clients to improve the LLM's understanding of available
	// resources. It can be thought of like a "hint" to the model.
	Description string `json:"description,omitempty"`
	// The MIME type of this resource, if known.
	MIMEType string `json:"mimeType,omitempty"`
	// A human-readable name for this resource.
	//
	// This can be used by clients to populate UI elements.
	Name string `json:"name"`
	// The size of the raw resource content, in bytes (i.e., before base64 encoding
	// or any tokenization), if known.
	//
	// This can be used by Hosts to display file sizes and estimate context window
	// usage.
	Size int64 `json:"size,omitempty"`
	// The URI of this resource.
	URI string `json:"uri"`
}

A known resource that the server is capable of reading.

type ResourceContents 

type ResourceContents struct {
	URI      string `json:"uri"` // resource location; must not be empty
	MIMEType string `json:"mimeType,omitempty"`
	Text     string `json:"text"`
	Blob     []byte `json:"blob,omitempty"` // if nil, then text; else blob
}

A ResourceContents is either a TextResourceContents or a BlobResourceContents. Use NewTextResourceContents or [NextBlobResourceContents] to create one.

func NewBlobResourceContents 

func NewBlobResourceContents(uri, mimeType string, blob []byte) *ResourceContents

NewBlobResourceContents returns a ResourceContents containing a byte slice.

func NewTextResourceContents 

func NewTextResourceContents(uri, mimeType, text string) *ResourceContents

NewTextResourceContents returns a ResourceContents containing text.

func (ResourceContents) MarshalJSON 

func (r ResourceContents) MarshalJSON() ([]byte, error)

type ResourceHandler 

type ResourceHandler func(context.Context, *ServerSession, *ReadResourceParams) (*ReadResourceResult, error)

A ResourceHandler is a function that reads a resource. It will be called when the client calls ClientSession.ReadResource. If it cannot find the resource, it should return the result of calling ResourceNotFoundError.

type ResourceListChangedParams 

type ResourceListChangedParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
}

func (*ResourceListChangedParams) GetMeta 

func (x *ResourceListChangedParams) GetMeta() *Meta

type ResourceTemplate 

type ResourceTemplate struct {
	// Optional annotations for the client.
	Annotations *Annotations `json:"annotations,omitempty"`
	// A description of what this template is for.
	//
	// This can be used by clients to improve the LLM's understanding of available
	// resources. It can be thought of like a "hint" to the model.
	Description string `json:"description,omitempty"`
	// The MIME type for all resources that match this template. This should only be
	// included if all resources matching this template have the same type.
	MIMEType string `json:"mimeType,omitempty"`
	// A human-readable name for the type of resource this template refers to.
	//
	// This can be used by clients to populate UI elements.
	Name string `json:"name"`
	// A URI template (according to RFC 6570) that can be used to construct resource
	// URIs.
	URITemplate string `json:"uriTemplate"`
}

A template description for resources available on the server.

type Result 

type Result interface {
	// Returns a pointer to the result's Meta field.
	GetMeta() *Meta
}

Result is a result of an MCP call.

type Role 

type Role string

The sender or recipient of messages and data in a conversation.

type Root 

type Root struct {
	// An optional name for the root. This can be used to provide a human-readable
	// identifier for the root, which may be useful for display purposes or for
	// referencing the root in other parts of the application.
	Name string `json:"name,omitempty"`
	// The URI identifying the root. This *must* start with file:// for now. This
	// restriction may be relaxed in future versions of the protocol to allow other
	// URI schemes.
	URI string `json:"uri"`
}

Represents a root directory or file that the server can operate on.

type RootsListChangedParams 

type RootsListChangedParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
}

func (*RootsListChangedParams) GetMeta 

func (x *RootsListChangedParams) GetMeta() *Meta

type SSEClientTransport 

type SSEClientTransport struct {
	// contains filtered or unexported fields
}

An SSEClientTransport is a Transport that can communicate with an MCP endpoint serving the SSE transport defined by the 2024-11-05 version of the spec.

https://modelcontextprotocol.io/specification/2024-11-05/basic/transports

func NewSSEClientTransport 

func NewSSEClientTransport(baseURL string, opts *SSEClientTransportOptions) *SSEClientTransport

NewSSEClientTransport returns a new client transport that connects to the SSE server at the provided URL.

NewSSEClientTransport panics if the given URL is invalid.

func (*SSEClientTransport) Connect 

func (c *SSEClientTransport) Connect(ctx context.Context) (Connection, error)

Connect connects through the client endpoint.

type SSEClientTransportOptions 

type SSEClientTransportOptions struct {
	// HTTPClient is the client to use for making HTTP requests. If nil,
	// http.DefaultClient is used.
	HTTPClient *http.Client
}

SSEClientTransportOptions provides options for the NewSSEClientTransport constructor.

type SSEHandler 

type SSEHandler struct {
	// contains filtered or unexported fields
}

SSEHandler is an http.Handler that serves SSE-based MCP sessions as defined by the 2024-11-05 version of the MCP spec.

Example

Code:play  

package main

import (
	"context"
	"fmt"
	"log"
	"net/http"
	"net/http/httptest"

	"golang.org/x/tools/internal/mcp"
)

type AddParams struct {
	X, Y int
}

func Add(ctx context.Context, cc *mcp.ServerSession, params *mcp.CallToolParamsFor[AddParams]) (*mcp.CallToolResultFor[any], error) {
	return &mcp.CallToolResultFor[any]{
		Content: []*mcp.Content{mcp.NewTextContent(fmt.Sprintf("%d", params.Arguments.X+params.Arguments.Y))},
	}, nil
}

func main() {
	server := mcp.NewServer("adder", "v0.0.1", nil)
	server.AddTools(mcp.NewServerTool("add", "add two numbers", Add))

	handler := mcp.NewSSEHandler(func(*http.Request) *mcp.Server { return server })
	httpServer := httptest.NewServer(handler)
	defer httpServer.Close()

	ctx := context.Background()
	transport := mcp.NewSSEClientTransport(httpServer.URL, nil)
	client := mcp.NewClient("test", "v1.0.0", nil)
	cs, err := client.Connect(ctx, transport)
	if err != nil {
		log.Fatal(err)
	}
	defer cs.Close()

	res, err := cs.CallTool(ctx, &mcp.CallToolParams{
		Name:      "add",
		Arguments: map[string]any{"x": 1, "y": 2},
	})
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(res.Content[0].Text)

}

Output:

3

func NewSSEHandler 

func NewSSEHandler(getServer func(request *http.Request) *Server) *SSEHandler

NewSSEHandler returns a new SSEHandler that creates and manages MCP sessions created via incoming HTTP requests.

Sessions are created when the client issues a GET request to the server, which must accept text/event-stream responses (server-sent events). For each such request, a new SSEServerTransport is created with a distinct messages endpoint, and connected to the server returned by getServer. It is up to the user whether getServer returns a distinct Server for each new request, or reuses an existing server.

The SSEHandler also handles requests to the message endpoints, by delegating them to the relevant server transport.

TODO(rfindley): add options.

func (*SSEHandler) ServeHTTP 

func (h *SSEHandler) ServeHTTP(w http.ResponseWriter, req *http.Request)

type SSEServerTransport 

type SSEServerTransport struct {
	// contains filtered or unexported fields
}

A SSEServerTransport is a logical SSE session created through a hanging GET request.

When connected, it returns the following Connection implementation:

func NewSSEServerTransport 

func NewSSEServerTransport(endpoint string, w http.ResponseWriter) *SSEServerTransport

NewSSEServerTransport creates a new SSE transport for the given messages endpoint, and hanging GET response.

Use SSEServerTransport.Connect to initiate the flow of messages.

The transport is itself an http.Handler. It is the caller's responsibility to ensure that the resulting transport serves HTTP requests on the given session endpoint.

Most callers should instead use an SSEHandler, which transparently handles the delegation to SSEServerTransports.

func (*SSEServerTransport) Connect 

func (t *SSEServerTransport) Connect(context.Context) (Connection, error)

Connect sends the 'endpoint' event to the client. See SSEServerTransport for more details on the Connection implementation.

func (*SSEServerTransport) ServeHTTP 

func (t *SSEServerTransport) ServeHTTP(w http.ResponseWriter, req *http.Request)

ServeHTTP handles POST requests to the transport endpoint.

type SamplingCapabilities 

type SamplingCapabilities struct {
}

Present if the client supports sampling from an LLM.

type SamplingMessage 

type SamplingMessage struct {
	Content *Content `json:"content"`
	Role    Role     `json:"role"`
}

Describes a message issued to or received from an LLM API.

type SchemaOption 

type SchemaOption interface {
	// contains filtered or unexported methods
}

A SchemaOption configures a jsonschema.Schema.

func Description 

func Description(desc string) SchemaOption

Description sets the provided schema description.

func Enum 

func Enum(values ...any) SchemaOption

Enum sets the provided values as the "enum" value of the schema.

func Property 

func Property(name string, opts ...SchemaOption) SchemaOption

Property configures the schema for the property of the given name. If there is no such property in the schema, it is created.

func Required 

func Required(v bool) SchemaOption

Required sets whether the associated property is required. It is only valid when used in a Property option: using Required outside of Property panics.

func Schema 

func Schema(schema *jsonschema.Schema) SchemaOption

Schema overrides the inferred schema with a shallow copy of the given schema.

type Server 

type Server struct {
	// contains filtered or unexported fields
}

A Server is an instance of an MCP server.

Servers expose server-side MCP features, which can serve one or more MCP sessions by using [Server.Start] or Server.Run.

Example

Code:play  

package main

import (
	"context"
	"fmt"
	"log"

	"golang.org/x/tools/internal/mcp"
)

type SayHiParams struct {
	Name string `json:"name"`
}

func SayHi(ctx context.Context, cc *mcp.ServerSession, params *mcp.CallToolParamsFor[SayHiParams]) (*mcp.CallToolResultFor[any], error) {
	return &mcp.CallToolResultFor[any]{
		Content: []*mcp.Content{
			mcp.NewTextContent("Hi " + params.Arguments.Name),
		},
	}, nil
}

func main() {
	ctx := context.Background()
	clientTransport, serverTransport := mcp.NewInMemoryTransports()

	server := mcp.NewServer("greeter", "v0.0.1", nil)
	server.AddTools(mcp.NewServerTool("greet", "say hi", SayHi))

	serverSession, err := server.Connect(ctx, serverTransport)
	if err != nil {
		log.Fatal(err)
	}

	client := mcp.NewClient("client", "v0.0.1", nil)
	clientSession, err := client.Connect(ctx, clientTransport)
	if err != nil {
		log.Fatal(err)
	}

	res, err := clientSession.CallTool(ctx, &mcp.CallToolParams{
		Name:      "greet",
		Arguments: map[string]any{"name": "user"},
	})
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(res.Content[0].Text)

	clientSession.Close()
	serverSession.Wait()

}

// createSessions creates and connects an in-memory client and server session for testing purposes.
func createSessions(ctx context.Context) (*mcp.ClientSession, *mcp.ServerSession, *mcp.Server) {
	server := mcp.NewServer("server", "v0.0.1", nil)
	client := mcp.NewClient("client", "v0.0.1", nil)
	serverTransport, clientTransport := mcp.NewInMemoryTransports()
	serverSession, err := server.Connect(ctx, serverTransport)
	if err != nil {
		log.Fatal(err)
	}
	clientSession, err := client.Connect(ctx, clientTransport)
	if err != nil {
		log.Fatal(err)
	}
	return clientSession, serverSession, server
}

Output:

Hi user

func NewServer 

func NewServer(name, version string, opts *ServerOptions) *Server

NewServer creates a new MCP server. The resulting server has no features: add features using Server.AddTools, Server.AddPrompts and Server.AddResources.

The server can be connected to one or more MCP clients using [Server.Start] or Server.Run.

If non-nil, the provided options is used to configure the server.

func (*Server) AddPrompts 

func (s *Server) AddPrompts(prompts ...*ServerPrompt)

AddPrompts adds the given prompts to the server, replacing any with the same names.

func (*Server) AddReceivingMiddleware 

func (s *Server) AddReceivingMiddleware(middleware ...Middleware[*ServerSession])

AddReceivingMiddleware wraps the current receiving method handler using the provided middleware. Middleware is applied from right to left, so that the first one is executed first.

For example, AddReceivingMiddleware(m1, m2, m3) augments the method handler as m1(m2(m3(handler))).

Receiving middleware is called when a request is received. It is useful for tasks such as authentication, request logging and metrics.

func (*Server) AddResourceTemplates 

func (s *Server) AddResourceTemplates(templates ...*ServerResourceTemplate)

AddResourceTemplates adds the given resource templates to the server. If a resource template with the same URI template already exists, it will be replaced. AddResourceTemplates panics if a URI template is invalid or not absolute (has an empty scheme).

func (*Server) AddResources 

func (s *Server) AddResources(resources ...*ServerResource)

AddResources adds the given resources to the server. If a resource with the same URI already exists, it is replaced. AddResources panics if a resource URI is invalid or not absolute (has an empty scheme).

func (*Server) AddSendingMiddleware 

func (s *Server) AddSendingMiddleware(middleware ...Middleware[*ServerSession])

AddSendingMiddleware wraps the current sending method handler using the provided middleware. Middleware is applied from right to left, so that the first one is executed first.

For example, AddSendingMiddleware(m1, m2, m3) augments the method handler as m1(m2(m3(handler))).

Sending middleware is called when a request is sent. It is useful for tasks such as tracing, metrics, and adding progress tokens.

func (*Server) AddTools 

func (s *Server) AddTools(tools ...*ServerTool)

AddTools adds the given tools to the server, replacing any with the same names. The arguments must not be modified after this call.

AddTools panics if errors are detected.

func (*Server) Connect 

func (s *Server) Connect(ctx context.Context, t Transport) (*ServerSession, error)

Connect connects the MCP server over the given transport and starts handling messages.

It returns a connection object that may be used to terminate the connection (with [Connection.Close]), or await client termination (with [Connection.Wait]).

func (*Server) RemovePrompts 

func (s *Server) RemovePrompts(names ...string)

RemovePrompts removes the prompts with the given names. It is not an error to remove a nonexistent prompt.

func (*Server) RemoveResourceTemplates 

func (s *Server) RemoveResourceTemplates(uriTemplates ...string)

RemoveResourceTemplates removes the resource templates with the given URI templates. It is not an error to remove a nonexistent resource.

func (*Server) RemoveResources 

func (s *Server) RemoveResources(uris ...string)

RemoveResources removes the resources with the given URIs. It is not an error to remove a nonexistent resource.

func (*Server) RemoveTools 

func (s *Server) RemoveTools(names ...string)

RemoveTools removes the tools with the given names. It is not an error to remove a nonexistent tool.

func (*Server) Run 

func (s *Server) Run(ctx context.Context, t Transport) error

Run runs the server over the given transport, which must be persistent.

Run blocks until the client terminates the connection.

func (*Server) Sessions 

func (s *Server) Sessions() iter.Seq[*ServerSession]

Sessions returns an iterator that yields the current set of server sessions.

type ServerOptions 

type ServerOptions struct {
	// Optional instructions for connected clients.
	Instructions string
	// If non-nil, called when "notifications/initialized" is received.
	InitializedHandler func(context.Context, *ServerSession, *InitializedParams)
	// PageSize is the maximum number of items to return in a single page for
	// list methods (e.g. ListTools).
	PageSize int
	// If non-nil, called when "notifications/roots/list_changed" is received.
	RootsListChangedHandler func(context.Context, *ServerSession, *RootsListChangedParams)
	// If non-nil, called when "notifications/progress" is received.
	ProgressNotificationHandler func(context.Context, *ServerSession, *ProgressNotificationParams)
}

ServerOptions is used to configure behavior of the server.

type ServerPrompt 

type ServerPrompt struct {
	Prompt  *Prompt
	Handler PromptHandler
}

A Prompt is a prompt definition bound to a prompt handler.

type ServerResource 

type ServerResource struct {
	Resource *Resource
	Handler  ResourceHandler
}

A ServerResource associates a Resource with its handler.

type ServerResourceTemplate 

type ServerResourceTemplate struct {
	ResourceTemplate *ResourceTemplate
	Handler          ResourceHandler
}

A ServerResourceTemplate associates a ResourceTemplate with its handler.

func (*ServerResourceTemplate) Matches 

func (sr *ServerResourceTemplate) Matches(uri string) bool

Matches reports whether the receiver's uri template matches the uri. TODO: use "github.com/yosida95/uritemplate/v3"

type ServerSession 

type ServerSession struct {
	// contains filtered or unexported fields
}

A ServerSession is a logical connection from a single MCP client. Its methods can be used to send requests or notifications to the client. Create a session by calling Server.Connect.

Call ServerSession.Close to close the connection, or await client termination with ServerSession.Wait.

func (*ServerSession) Close 

func (ss *ServerSession) Close() error

Close performs a graceful shutdown of the connection, preventing new requests from being handled, and waiting for ongoing requests to return. Close then terminates the connection.

func (*ServerSession) CreateMessage 

func (ss *ServerSession) CreateMessage(ctx context.Context, params *CreateMessageParams) (*CreateMessageResult, error)

CreateMessage sends a sampling request to the client.

func (*ServerSession) ListRoots 

func (ss *ServerSession) ListRoots(ctx context.Context, params *ListRootsParams) (*ListRootsResult, error)

ListRoots lists the client roots.

func (*ServerSession) LoggingMessage 

func (ss *ServerSession) LoggingMessage(ctx context.Context, params *LoggingMessageParams) error

LoggingMessage sends a logging message to the client. The message is not sent if the client has not called SetLevel, or if its level is below that of the last SetLevel.

TODO(jba): rename to Log or LogMessage. A logging message is the thing that is sent to logging.

func (*ServerSession) NotifyProgress 

func (ss *ServerSession) NotifyProgress(ctx context.Context, params *ProgressNotificationParams) error

NotifyProgress sends a progress notification from the server to the client associated with this session. This is typically used to report on the status of a long-running request that was initiated by the client.

func (*ServerSession) Ping 

func (ss *ServerSession) Ping(ctx context.Context, params *PingParams) error

Ping pings the client.

func (*ServerSession) Wait 

func (ss *ServerSession) Wait() error

Wait waits for the connection to be closed by the client.

type ServerTool 

type ServerTool struct {
	Tool    *Tool
	Handler ToolHandler
	// contains filtered or unexported fields
}

A Tool is a tool definition that is bound to a tool handler.

func NewServerTool 

func NewServerTool[In, Out any](name, description string, handler ToolHandlerFor[In, Out], opts ...ToolOption) *ServerTool

NewServerTool is a helper to make a tool using reflection on the given type parameters. When the tool is called, CallToolParams.Arguments will be of type In.

If provided, variadic ToolOption values may be used to customize the tool.

The input schema for the tool is extracted from the request type for the handler, and used to unmmarshal and validate requests to the handler. This schema may be customized using the Input option.

type Session 

type Session interface {
	*ClientSession | *ServerSession
	// contains filtered or unexported methods
}

A Session is either a ClientSession or a ServerSession.

type SetLevelParams 

type SetLevelParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
	// The level of logging that the client wants to receive from the server. The
	// server should send all logs at this level and higher (i.e., more severe) to
	// the client as notifications/message.
	Level LoggingLevel `json:"level"`
}

func (*SetLevelParams) GetMeta 

func (x *SetLevelParams) GetMeta() *Meta

type StdioTransport 

type StdioTransport struct {
	// contains filtered or unexported fields
}

A StdioTransport is a Transport that communicates over stdin/stdout using newline-delimited JSON.

func NewStdioTransport 

func NewStdioTransport() *StdioTransport

NewStdioTransport constructs a transport that communicates over stdin/stdout.

func (*StdioTransport) Connect 

func (t *StdioTransport) Connect(context.Context) (Connection, error)

type StreamableClientTransport 

type StreamableClientTransport struct {
	// contains filtered or unexported fields
}

A StreamableClientTransport is a Transport that can communicate with an MCP endpoint serving the streamable HTTP transport defined by the 2025-03-26 version of the spec.

TODO(rfindley): support retries and resumption tokens.

func NewStreamableClientTransport 

func NewStreamableClientTransport(url string, opts *StreamableClientTransportOptions) *StreamableClientTransport

NewStreamableClientTransport returns a new client transport that connects to the streamable HTTP server at the provided URL.

func (*StreamableClientTransport) Connect 

func (t *StreamableClientTransport) Connect(ctx context.Context) (Connection, error)

Connect implements the Transport interface.

The resulting Connection writes messages via POST requests to the transport URL with the Mcp-Session-Id header set, and reads messages from hanging requests.

When closed, the connection issues a DELETE request to terminate the logical session.

type StreamableClientTransportOptions 

type StreamableClientTransportOptions struct {
	// HTTPClient is the client to use for making HTTP requests. If nil,
	// http.DefaultClient is used.
	HTTPClient *http.Client
}

StreamableClientTransportOptions provides options for the NewStreamableClientTransport constructor.

type StreamableHTTPHandler 

type StreamableHTTPHandler struct {
	// contains filtered or unexported fields
}

A StreamableHTTPHandler is an http.Handler that serves streamable MCP sessions, as defined by the MCP spec.

func NewStreamableHTTPHandler 

func NewStreamableHTTPHandler(getServer func(*http.Request) *Server, opts *StreamableHTTPOptions) *StreamableHTTPHandler

NewStreamableHTTPHandler returns a new StreamableHTTPHandler.

The getServer function is used to create or look up servers for new sessions. It is OK for getServer to return the same server multiple times.

func (*StreamableHTTPHandler) ServeHTTP 

func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Request)

type StreamableHTTPOptions 

type StreamableHTTPOptions struct {
}

StreamableHTTPOptions is a placeholder options struct for future configuration of the StreamableHTTP handler.

type StreamableServerTransport 

type StreamableServerTransport struct {
	// contains filtered or unexported fields
}

A StreamableServerTransport implements the Transport interface for a single session.

func NewStreamableServerTransport 

func NewStreamableServerTransport(sessionID string) *StreamableServerTransport

NewStreamableServerTransport returns a new StreamableServerTransport with the given session ID.

A StreamableServerTransport implements the server-side of the streamable transport.

TODO(rfindley): consider adding options here, to configure event storage policy.

func (*StreamableServerTransport) Close 

func (t *StreamableServerTransport) Close() error

Close implements the Connection interface.

func (*StreamableServerTransport) Connect 

func (s *StreamableServerTransport) Connect(context.Context) (Connection, error)

Connect implements the Transport interface.

TODO(rfindley): Connect should return a new object.

func (*StreamableServerTransport) Read 

func (t *StreamableServerTransport) Read(ctx context.Context) (JSONRPCMessage, error)

Read implements the Connection interface.

func (*StreamableServerTransport) ServeHTTP 

func (t *StreamableServerTransport) ServeHTTP(w http.ResponseWriter, req *http.Request)

ServeHTTP handles a single HTTP request for the session.

func (*StreamableServerTransport) Write 

func (t *StreamableServerTransport) Write(ctx context.Context, msg JSONRPCMessage) error

Write implements the Connection interface.

type Tool 

type Tool struct {
	// Optional additional tool information.
	Annotations *ToolAnnotations `json:"annotations,omitempty"`
	// A human-readable description of the tool.
	//
	// This can be used by clients to improve the LLM's understanding of available
	// tools. It can be thought of like a "hint" to the model.
	Description string `json:"description,omitempty"`
	// A JSON Schema object defining the expected parameters for the tool.
	InputSchema *jsonschema.Schema `json:"inputSchema"`
	// The name of the tool.
	Name string `json:"name"`
}

Definition for a tool the client can call.

type ToolAnnotations 

type ToolAnnotations struct {
	// If true, the tool may perform destructive updates to its environment. If
	// false, the tool performs only additive updates.
	//
	// (This property is meaningful only when `readOnlyHint == false`)
	//
	// Default: true
	DestructiveHint bool `json:"destructiveHint,omitempty"`
	// If true, calling the tool repeatedly with the same arguments will have no
	// additional effect on the its environment.
	//
	// (This property is meaningful only when `readOnlyHint == false`)
	//
	// Default: false
	IdempotentHint bool `json:"idempotentHint,omitempty"`
	// If true, this tool may interact with an "open world" of external entities. If
	// false, the tool's domain of interaction is closed. For example, the world of
	// a web search tool is open, whereas that of a memory tool is not.
	//
	// Default: true
	OpenWorldHint bool `json:"openWorldHint,omitempty"`
	// If true, the tool does not modify its environment.
	//
	// Default: false
	ReadOnlyHint bool `json:"readOnlyHint,omitempty"`
	// A human-readable title for the tool.
	Title string `json:"title,omitempty"`
}

Additional properties describing a Tool to clients.

NOTE: all properties in ToolAnnotations are **hints**. They are not guaranteed to provide a faithful description of tool behavior (including descriptive properties like `title`).

Clients should never make tool use decisions based on ToolAnnotations received from untrusted servers.

type ToolHandler 

type ToolHandler func(context.Context, *ServerSession, *CallToolParamsFor[map[string]any]) (*CallToolResult, error)

A ToolHandler handles a call to tools/call. [CallToolParams.Arguments] will contain a map[string]any that has been validated against the input schema. Perhaps this should be an alias for ToolHandlerFor[map[string]any, map[string]any].

type ToolHandlerFor 

type ToolHandlerFor[In, Out any] func(context.Context, *ServerSession, *CallToolParamsFor[In]) (*CallToolResultFor[Out], error)

A ToolHandlerFor handles a call to tools/call with typed arguments and results.

type ToolListChangedParams 

type ToolListChangedParams struct {
	// This property is reserved by the protocol to allow clients and servers to
	// attach additional metadata to their responses.
	Meta Meta `json:"_meta,omitempty"`
}

func (*ToolListChangedParams) GetMeta 

func (x *ToolListChangedParams) GetMeta() *Meta

type ToolOption 

type ToolOption interface {
	// contains filtered or unexported methods
}

A ToolOption configures the behavior of a Tool.

func Input 

func Input(opts ...SchemaOption) ToolOption

Input applies the provided SchemaOption configuration to the tool's input schema.

type Transport 

type Transport interface {
	// Connect returns the logical JSON-RPC connection..
	//
	// It is called exactly once by [Server.Connect] or [Client.Connect].
	Connect(ctx context.Context) (Connection, error)
}

A Transport is used to create a bidirectional connection between MCP client and server.

Transports should be used for at most one call to Server.Connect or [Client.Start].

Source Files

client.go cmd.go content.go features.go logging.go mcp.go prompt.go protocol.go resource.go resource_pre_go124.go root.go server.go shared.go sse.go streamable.go tool.go transport.go util.go

Directories

PathSynopsis
internal/mcp/examples
internal/mcp/examples/hello
internal/mcp/examples/sse
internal/mcp/internal
internal/mcp/internal/readme
internal/mcp/internal/readme/client!+
internal/mcp/internal/readme/server!+
internal/mcp/internal/util
internal/mcp/jsonschemaPackage jsonschema is an implementation of the [JSON Schema specification], a JSON-based format for describing the structure of JSON data.
Version
v0.35.0 (latest)
Published
Jul 11, 2025
Platform
js/wasm
Imports
34 packages
Last checked
33 minutes ago –

Tools for package owners.