go-sdk/design/design.md

Go MCP SDK design

This file discusses the design of a Go SDK for the model context protocol. It is intended to seed a GitHub discussion about the official Go MCP SDK.

The github.com/modelcontextprotocol/go-sdk/mcp package contains a prototype that we built to explore the MCP design space. Many of the ideas there are present in this document. However, we have diverged and expanded on the APIs of that prototype, and this document should be considered canonical.

Similarities and differences with mark3labs/mcp-go

The most popular unofficial MCP SDK for Go is mark3labs/mcp-go. As of this writing, it is imported by over 400 packages that span over 200 modules.

We admire mcp-go, and seriously considered simply adopting it as a starting point for this SDK. However, as we looked at doing so, we realized that a significant amount of its API would probably need to change. In some cases, mcp-go has older APIs that predated newer variations--an obvious opportunity for cleanup. In others, it took a batteries-included approach that is probably not viable for an official SDK. In yet others, we simply think there is room for API refinement, and we should take this opportunity to consider our options. Therefore, we wrote this document as though it were proposing a new implementation. Nevertheless, much of the API discussed here originated from or was inspired by mcp-go and other unofficial SDKs, and if the consensus of this discussion is close enough to mcp-go or any other unofficial SDK, we can start from a fork.

Since mcp-go is so influential and popular, we have noted significant differences from its API in the sections below. Although the API here is not compatible with mcp-go, translating between them should be straightforward in most cases. (Later, we will provide a detailed translation guide.)

Requirements

These may be obvious, but it's worthwhile to define goals for an official MCP SDK. An official SDK should aim to be:

• complete: it should be possible to implement every feature of the MCP spec, and these features should conform to all of the semantics described by the spec.
• idiomatic: as much as possible, MCP features should be modeled using features of the Go language and its standard library. Additionally, the SDK should repeat idioms from similar domains.
• robust: the SDK itself should be well tested and reliable, and should enable easy testability for its users.
• future-proof: the SDK should allow for future evolution of the MCP spec, in such a way that we can (as much as possible) avoid incompatible changes to the SDK API.
• extensible: to best serve the previous four concerns, the SDK should be minimal. However, it should admit extensibility using (for example) simple interfaces, middleware, or hooks.

Design considerations

In the sections below, we visit each aspect of the MCP spec, in approximately the order they are presented by the official spec For each, we discuss considerations for the Go implementation. In many cases an API is suggested, though in some there may be open questions.

Foundations

Package layout

In the sections that follow, it is assumed that most of the MCP API lives in a single shared package, the mcp package. This is inconsistent with other MCP SDKs, but is consistent with Go packages like net/http, net/rpc, or google.golang.org/grpc.

Functionality that is not directly related to MCP (like jsonschema or jsonrpc2) belongs in a separate package.

Therefore, this is the package layout. module.path is a placeholder for the final module path of the mcp module

• module.path/mcp: the bulk of the user facing API
• module.path/jsonschema: a jsonschema implementation, with validation
• module.path/internal/jsonrpc2: a fork of x/tools/internal/jsonrpc2_v2

The JSON-RPC implementation is hidden, to avoid tight coupling. As described in the next section, the only aspects of JSON-RPC that need to be exposed in the SDK are the message types, for the purposes of defining custom transports. We can expose these types from the mcp package via aliases or wrappers.

Difference from mcp-go: Our mcp package includes all the functionality of mcp-go's mcp, client, server and transport packages.

jsonrpc2 and Transports

The MCP is defined in terms of client-server communication over bidirectional JSON-RPC message streams. Specifically, version 2025-03-26 of the spec defines two transports:

• stdio: communication with a subprocess over stdin/stdout.
• streamable http: communication over a relatively complicated series of text/event-stream GET and HTTP POST requests.

Additionally, version 2024-11-05 of the spec defined a simpler HTTP transport:

• sse: client issues a hanging GET request and receives messages via text/event-stream, and sends messages via POST to a session endpoint.

Furthermore, the spec states that it must be possible for users to define their own custom transports.

Given the diversity of the transport implementations, they can be challenging to abstract. However, since JSON-RPC requires a bidirectional stream, we can use this to model the MCP transport abstraction:

// A Transport is used to create a bidirectional connection between MCP client
// and server.
type Transport interface {
    Connect(ctx context.Context) (Stream, error)
}

// A Stream is a bidirectional jsonrpc2 Stream.
type Stream interface {
    Read(ctx context.Context) (jsonrpc2.Message, error)
    Write(ctx context.Context, jsonrpc2.Message) error
    Close() error
}

Methods accept a Go Context and return an error, as is idiomatic for APIs that do I/O.

A Transport is something that connects a logical JSON-RPC stream, and nothing more. Streams must be closeable in order to implement client and server shutdown, and therefore conform to the io.Closer interface.

Other SDKs define higher-level transports, with, for example, methods to send a notification or make a call. Those are jsonrpc2 operations on top of the logical stream, and the lower-level interface is easier to implement in most cases, which means it is easier to implement custom transports.

For our prototype, we've used an internal jsonrpc2 package based on the Go language server gopls, which we propose to fork for the MCP SDK. It already handles concerns like client/server connection, request lifecycle, cancellation, and shutdown.

In the MCP Spec, the stdio transport uses newline-delimited JSON to communicate over stdin/stdout. It's possible to model both client side and server side of this communication with a shared type that communicates over an io.ReadWriteCloser. However, for the purposes of future-proofing, we should use a distinct types for both client and server stdio transport.

The CommandTransport is the client side of the stdio transport, and connects by starting a command and binding its jsonrpc2 stream to its stdin/stdout.

// A CommandTransport is a [Transport] that runs a command and communicates
// with it over stdin/stdout, using newline-delimited JSON.
type CommandTransport struct { /* unexported fields */ }

// NewCommandTransport returns a [CommandTransport] that runs the given command
// and communicates with it over stdin/stdout.
func NewCommandTransport(cmd *exec.Command) *CommandTransport

// Connect starts the command, and connects to it over stdin/stdout.
func (*CommandTransport) Connect(ctx context.Context) (Stream, error) {

The StdIOTransport is the server side of the stdio transport, and connects by binding to os.Stdin and os.Stdout.

// A StdIOTransport is a [Transport] that communicates using newline-delimited
// JSON over stdin/stdout.
type StdIOTransport struct { /* unexported fields */ }

func NewStdIOTransport() *StdIOTransport

func (t *StdIOTransport) Connect(context.Context) (Stream, error)

The HTTP transport APIs are even more asymmetrical. Since connections are initiated via HTTP requests, the client developer will create a transport, but the server developer will typically install an HTTP handler. Internally, the HTTP handler will create a transport for each new client connection.

Importantly, since they serve many connections, the HTTP handlers must accept a callback to get an MCP server for each new session.

// SSEHTTPHandler is an http.Handler that serves SSE-based MCP sessions as defined by
// the 2024-11-05 version of the MCP protocol.
type SSEHTTPHandler struct { /* unexported fields */ }

// NewSSEHTTPHandler returns a new [SSEHTTPHandler] that is ready to serve HTTP.
//
// The getServer function is used to bind created servers for new sessions. It
// is OK for getServer to return the same server multiple times.
func NewSSEHTTPHandler(getServer func(request *http.Request) *Server) *SSEHTTPHandler

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

// Close prevents the SSEHTTPHandler from accepting new sessions, closes active
// sessions, and awaits their graceful termination.
func (*SSEHTTPHandler) Close() error

Notably absent are options to hook into low-level request handling for the purposes of authentication or context injection. These concerns are better handled using standard HTTP middleware patterns. For middleware at the level of the MCP protocol, see Middleware below.

By default, the SSE handler creates messages endpoints with the ?sessionId=... query parameter. Users that want more control over the management of sessions and session endpoints may write their own handler, and create SSEServerTransport instances themselves, for incoming GET requests.

// A SSEServerTransport is a logical SSE session created through a hanging GET
// request.
//
// When connected, it it returns the following [Stream] implementation:
//   - Writes are SSE 'message' events to the GET response.
//   - Reads are received from POSTs to the session endpoint, via
//     [SSEServerTransport.ServeHTTP].
//   - Close terminates the hanging GET.
type SSEServerTransport struct { /* ... */ }

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

// ServeHTTP handles POST requests to the transport endpoint.
func (*SSEServerTransport) ServeHTTP(w http.ResponseWriter, req *http.Request)

// Connect sends the 'endpoint' event to the client.
// See [SSEServerTransport] for more details on the [Stream] implementation.
func (*SSEServerTransport) Connect(context.Context) (Stream, error)

The SSE client transport is simpler, and hopefully self-explanatory.

type SSEClientTransport struct { /* ... */ }

// NewSSEClientTransport returns a new client transport that connects to the
// SSE server at the provided URL.
func NewSSEClientTransport(url string) (*SSEClientTransport, error) {

// Connect connects through the client endpoint.
func (*SSEClientTransport) Connect(ctx context.Context) (Stream, error)

The Streamable HTTP transports are similar to the SSE transport, albeit with a more complicated implementation. For brevity, we summarize only the differences from the equivalent SSE types:

// The StreamableHTTPHandler interface is symmetrical to the SSEHTTPHandler.
type StreamableHTTPHandler struct { /* unexported fields */ }
func NewStreamableHTTPHandler(getServer func(request *http.Request) *Server) *StreamableHTTPHandler
func (*StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Request)
func (*StreamableHTTPHandler) Close() error

// Unlike the SSE transport, the streamable transport constructor accepts a
// session ID, not an endpoint, along with the http response for the request
// that created the session. It is the caller's responsibility to delegate
// requests to this session.
type StreamableServerTransport struct { /* ... */ }
func NewStreamableServerTransport(sessionID string, w http.ResponseWriter) *StreamableServerTransport
func (*StreamableServerTransport) ServeHTTP(w http.ResponseWriter, req *http.Request)
func (*StreamableServerTransport) Connect(context.Context) (Stream, error)

// The streamable client handles reconnection transparently to the user.
type StreamableClientTransport struct { /* ... */ }
func NewStreamableClientTransport(url string) *StreamableClientTransport {
func (*StreamableClientTransport) Connect(context.Context) (Stream, error)

Differences from mcp-go: The Go team has a battle-tested JSON-RPC implementation that we use for gopls, our Go LSP server. We are using the new version of this library as part of our MCP SDK. It handles all JSON-RPC 2.0 features, including cancellation.

The Transport interface here is lower-level than that of mcp-go, but serves a similar purpose. We believe the lower-level interface is easier to implement.

In mcp-go, server authors create an MCPServer, populate it with tools, resources and so on, and then wrap it in an SSEServer or StdioServer. These also use session IDs, which are exposed. Users can manage their own sessions with RegisterSession and UnregisterSession.

We find the similarity in names among the three server types to be confusing, and we could not discover any uses of the session methods in the open-source ecosystem. In our design, server authors create a Server, and then connect it to a Transport. An SSEHTTPHandler manages sessions for incoming SSE connections, but does not expose them. HTTP handlers accept a server constructor, rather than Server, to allow for stateful or "per-session" servers.

Individual handlers and transports here have a minimal smaller API, and do not expose internal details. Customization of things like handlers or session management is intended to be implemented with middleware and/or compositional patterns.

Protocol types

Types needed for the protocol are generated from the JSON schema of the MCP spec.

These types will be included in the mcp package, but will be unexported unless they are needed for the user-facing API. Notably, JSON-RPC message types are elided, since they are handled by the jsonrpc2 package and should not be observed by the user.

For user-provided data, we use json.RawMessage, so that marshalling/unmarshalling can be delegated to the business logic of the client or server.

For union types, which can't be represented in Go (specifically Content and Resource), we prefer distinguished unions: struct types with fields corresponding to the union of all properties for union elements.

For brevity, only a few examples are shown here:

type CallToolParams struct {
	Arguments map[string]json.RawMessage `json:"arguments,omitempty"`
	Name      string                     `json:"name"`
}

type CallToolResult struct {
	Meta    map[string]json.RawMessage `json:"_meta,omitempty"`
	Content []Content                  `json:"content"`
	IsError bool                       `json:"isError,omitempty"`
}

// Content is the wire format for content.
//
// The Type field distinguishes the type of the content.
// At most one of Text, MIMEType, Data, and Resource is non-zero.
type Content struct {
	Type     string    `json:"type"`
	Text     string    `json:"text,omitempty"`
	MIMEType string    `json:"mimeType,omitempty"`
	Data     []byte    `json:"data,omitempty"`
	Resource *Resource `json:"resource,omitempty"`
}

// Resource is the wire format for embedded resources.
//
// The URI field describes the resource location. At most one of Text and Blob
// is non-zero.
type Resource struct {
	URI      string  `json:"uri,"`
	MIMEType string  `json:"mimeType,omitempty"`
	Text     string  `json:"text"`
	Blob     []byte `json:"blob"`
}

Differences from mcp-go: these types are largely similar, but our type generation flattens types rather than using struct embedding.

Clients and Servers

Generally speaking, the SDK is used by creating a Client or Server instance, adding features to it, and connecting it to a peer.

However, the SDK must make a non-obvious choice in these APIs: are clients 1:1 with their logical connections? What about servers? Both clients and servers are stateful: users may add or remove roots from clients, and tools, prompts, and resources from servers. Additionally, handlers for these features may themselves be stateful, for example if a tool handler caches state from earlier requests in the session.

We believe that in the common case, any change to a client or server, such as adding a tool, is intended for all its peers. It is therefore more useful to allow multiple connections from a client, and to a server. This is similar to the net/http packages, in which an http.Client and http.Server each may handle multiple unrelated connections. When users add features to a client or server, all connected peers are notified of the change.

Following the terminology of the spec, we call the logical connection between a client and server a "session". There must necessarily be a ClientSession and a ServerSession, corresponding to the APIs available from the client and server perspective, respectively.

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

Sessions are created from either Client or Server using the Connect method.

type Client struct { /* ... */ }
func NewClient(name, version string, opts *ClientOptions) *Client
func (c *Client) Connect(context.Context, Transport) (*ClientSession, error)
// Methods for adding/removing client features are described below.

type ClientOptions struct { /* ... */ } // described below

type ClientSession struct { /* ... */ }
func (*ClientSession) Client() *Client
func (*ClientSession) Close() error
func (*ClientSession) Wait() error
// Methods for calling through the ClientSession are described below.

type Server struct { /* ... */ }
func NewServer(name, version string, opts *ServerOptions) *Server
func (s *Server) Connect(context.Context, Transport) (*ServerSession, error)
// Methods for adding/removing server features are described below.

type ServerOptions struct { /* ... */ } // described below

type ServerSession struct { /* ... */ }
func (*ServerSession) Server() *Server
func (*ServerSession) Close() error
func (*ServerSession) Wait() error
// Methods for calling through the ServerSession are described below.

Here's an example of these APIs from the client side:

client := mcp.NewClient("mcp-client", "v1.0.0", nil)
// Connect to a server over stdin/stdout
transport := mcp.NewCommandTransport(exec.Command("myserver"))
session, err := client.Connect(ctx, transport)
if err != nil { ... }
// Call a tool on the server.
content, err := session.CallTool(ctx, &CallToolParams{
  Name: "greet",
  Arguments: map[string]any{"name": "you"} ,
})
...
return session.Close()

A server that can handle that client call would look like this:

// Create a server with a single tool.
server := mcp.NewServer("greeter", "v1.0.0", nil)
server.AddTool(mcp.NewTool("greet", "say hi", SayHi))
// Run the server over stdin/stdout, until the client disconnects.
transport := mcp.NewStdIOTransport()
session, err := server.Connect(ctx, transport)
...
return session.Wait()

For convenience, we provide Server.Run to handle the common case of running a session until the client disconnects:

func (*Server) Run(context.Context, Transport)

Differences from mcp-go: the Server APIs are very similar to mcp-go, though the association between servers and transports is different. In mcp-go, a single server is bound to what we would call an SSEHTTPHandler, and reused for all client sessions. As discussed above, the transport abstraction here is differentiated from HTTP serving, and the Server.Connect method provides a consistent API for binding to an arbitrary transport. Servers here do not have methods for sending notifications or calls, because they are logically distinct from the ServerSession. In mcp-go, servers are n:1, but there is no abstraction of a server session: sessions are addressed in Server APIs through their sessionID: SendNotificationToAllClients, SendNotificationToClient, SendNotificationToSpecificClient.

The client API here is different, since clients and client sessions are conceptually distinct. The ClientSession is closer to mcp-go's notion of Client.

For both clients and servers, mcp-go uses variadic options to customize behavior, whereas an options struct is used here. We felt that in this case, an options struct would be more readable, and result in cleaner package documentation.

Spec Methods

As we saw above, the ClientSession method for the specification's CallTool RPC takes a context and a params pointer as arguments, and returns a result pointer and error:

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

Our SDK has a method for every RPC in the spec, and their signatures all share this form. To avoid boilerplate, we don't repeat this signature for RPCs defined in the spec; readers may assume it when we mention a "spec method."

Why do we use params instead of the full request? JSON-RPC requests consist of a method name and a set of parameters, and the method is already encoded in the Go method name. Technically, the MCP spec could add a field to a request while preserving backward compatibility, which would break the Go SDK's compatibility. But in the unlikely event that were to happen, we would add that field to the Params struct.

Middleware

We provide a mechanism to add MCP-level middleware, which runs after the request has been parsed, but before any normal handling.

// A Dispatcher dispatches an MCP message to the appropriate handler.
// The params argument will be an XXXParams struct pointer, such as *GetPromptParams.
// The response if err is non-nil should be an XXXResult struct pointer.
type Dispatcher func(ctx context.Context, s *ServerSession, method string, params any) (result any, err error)

// AddDispatchers calls each function from right to left on the previous result, beginning
// with the server's current dispatcher, and installs the result as the new handler.
func (*Server) AddDispatchers(middleware ...func(Handler) Handler))

As an example, this code adds server-side logging:

func withLogging(h mcp.Handler) mcp.Handler {
    return func(ctx context.Context, s *mcp.ServerSession, method string, params any) (res any, err error) {
        log.Printf("request: %s %v", method, params)
        defer func() { log.Printf("response: %v, %v", res, err) }()
        return h(ctx, s , method, params)
    }
}

server.AddDispatchers(withLogging)

Differences from mcp-go: Version 0.26.0 of mcp-go defines 24 server hooks. Each hook consists of a field in the Hooks struct, a Hooks.Add method, and a type for the hook function. These are rarely used. The most common is OnError, which occurs fewer than ten times in open-source code.

Errors

With the exception of tool handler errors, protocol errors are handled transparently as Go errors: errors in server-side feature handlers are propagated as errors from calls from the ClientSession, and vice-versa.

Protocol errors wrap a JSONRPCError type which exposes its underlying error code.

type JSONRPCError struct {
	Code int64           `json:"code"`
	Message string       `json:"message"`
	Data json.RawMessage `json:"data,omitempty"`
}

As described by the spec, tool execution errors are reported in tool results.

Differences from mcp-go: the JSONRPCError type here does not expose details that are irrelevant or can be inferred from the caller (ID and Method). Otherwise, this behavior is similar.

Cancellation

Cancellation is implemented transparently using context cancellation. The user can cancel an operation by cancelling the associated context:

ctx, cancel := context.WithCancel(ctx)
go session.CallTool(ctx, "slow", map[string]any{})
cancel()

When this client call is cancelled, a "notifications/cancelled" notification is sent to the server. However, the client call returns immediately with ctx.Err(): it does not wait for the result from the server.

The server observes a client cancellation as a cancelled context.

Progress handling

A caller can request progress notifications by setting the ProgressToken field on any request.

type ProgressToken any // string or int

type XXXParams struct { // where XXX is each type of call
  ...
  ProgressToken ProgressToken
}

Handlers can notify their peer about progress by calling the NotifyProgress method. The notification is only sent if the peer requested it by providing a progress token.

func (*ClientSession) NotifyProgress(context.Context, *ProgressNotification)
func (*ServerSession) NotifyProgress(context.Context, *ProgressNotification)

Ping / KeepAlive

Both ClientSession and ServerSession expose a Ping method to call "ping" on their peer.

func (c *ClientSession) Ping(ctx context.Context, *PingParams) error
func (c *ServerSession) Ping(ctx context.Context, *PingParams) error

Additionally, client and server sessions can be configured with automatic keepalive behavior. If the KeepAlive option is set to a non-zero duration, it defines an interval for regular "ping" requests. If the peer fails to respond to pings originating from the keepalive check, the session is automatically closed.

type ClientOptions struct {
  ...
  KeepAlive time.Duration
}

type ServerOptions struct {
  ...
  KeepAlive time.Duration
}

Differences from mcp-go: in mcp-go the Ping method is only provided for client, not server, and the keepalive option is only provided for SSE servers (as a variadic option).

Client Features

Roots

Clients support the MCP Roots feature out of the box, including roots-changed notifications. Roots can be added and removed from a Client with AddRoots and RemoveRoots:

// AddRoots adds the given roots to the client,
// replacing any with the same URIs,
// and notifies any connected servers.
func (*Client) AddRoots(roots ...Root)

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

Servers can call the spec method ListRoots to get the roots. If a server installs a RootsChangedHandler, it will be called when the client sends a roots-changed notification, which happens whenever the list of roots changes after a connection has been established.

type ServerOptions {
  ...
  // If non-nil, called when a client sends a roots-changed notification.
  RootsChangedHandler func(context.Context, *ServerSession, *RootsChangedParams)
}

Sampling

Clients that support sampling are created with a CreateMessageHandler option for handling server calls. To perform sampling, a server calls the spec method CreateMessage.

type ClientOptions struct {
  ...
  CreateMessageHandler func(context.Context, *ClientSession, *CreateMessageParams) (*CreateMessageResult, error)
}

Server Features

Tools

A Tool is a logical MCP tool, generated from the MCP spec, and a ServerTool is a tool bound to a tool handler.

type Tool struct {
	Annotations *ToolAnnotations   `json:"annotations,omitempty"`
	Description string             `json:"description,omitempty"`
	InputSchema *jsonschema.Schema `json:"inputSchema"`
	Name string                    `json:"name"`
}

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

type ServerTool struct {
	Tool    Tool
	Handler ToolHandler
}

Add tools to a server with AddTools:

server.AddTools(
  mcp.NewTool("add", "add numbers", addHandler),
  mcp.NewTools("subtract, subtract numbers", subHandler))

Remove them by name with RemoveTools:

server.RemoveTools("add", "subtract")

A tool's input schema, expressed as a JSON Schema, provides a way to validate the tool's input. One of the challenges in defining tools is the need to associate them with a Go function, yet support the arbitrary complexity of JSON Schema. To achieve this, we have seen two primary approaches:

1. Use reflection to generate the tool's input schema from a Go type (à la metoro-io/mcp-golang)
2. Explicitly build the input schema (à la mark3labs/mcp-go).

Both of these have their advantages and disadvantages. Reflection is nice, because it allows you to bind directly to a Go API, and means that the JSON schema of your API is compatible with your Go types by construction. It also means that concerns like parsing and validation can be handled automatically. However, it can become cumbersome to express the full breadth of JSON schema using Go types or struct tags, and sometimes you want to express things that aren’t naturally modeled by Go types, like unions. Explicit schemas are simple and readable, and give the caller full control over their tool definition, but involve significant boilerplate.

We have found that a hybrid model works well, where the initial schema is derived using reflection, but any customization on top of that schema is applied using variadic options. We achieve this using a NewTool helper, which generates the schema from the input type, and wraps the handler to provide parsing and validation. The schema (and potentially other features) can be customized using ToolOptions.

// NewTool creates a Tool using reflection on the given handler.
func NewTool[TInput any](name, description string, handler func(context.Context, *ServerSession, TInput) ([]Content, error), opts …ToolOption) *ServerTool

type ToolOption interface { /* ... */ }

NewTool determines the input schema for a Tool from the struct used in the handler. Each struct field that would be marshaled by encoding/json.Marshal becomes a property of the schema. The property is required unless the field's json tag specifies "omitempty" or "omitzero" (new in Go 1.24). For example, given this struct:

struct {
  Name     string `json:"name"`
  Count    int    `json:"count,omitempty"`
  Choices  []string
  Password []byte `json:"-"`
}

"name" and "Choices" are required, while "count" is optional.

As of this writing, the only ToolOption is Input, which allows customizing the input schema of the tool using schema options. These schema options are recursive, in the sense that they may also be applied to properties.

func Input(...SchemaOption) ToolOption

type Property(name string, opts ...SchemaOption) SchemaOption
type Description(desc string) SchemaOption
// etc.

For example:

NewTool(name, description, handler,
    Input(Property("count", Description("size of the inventory"))))

The most recent JSON Schema spec defines over 40 keywords. Providing them all as options would bloat the API despite the fact that most would be very rarely used. For less common keywords, use the Schema option to set the schema explicitly:

NewTool(name, description, handler,
    Input(Property("Choices", Schema(&jsonschema.Schema{UniqueItems: true}))))

Schemas are validated on the server before the tool handler is called.

Since all the fields of the Tool struct are exported, a Tool can also be created directly with assignment or a struct literal.

Differences from mcp-go: using variadic options to configure tools was signficantly inspired by mcp-go. However, the distinction between ToolOption and SchemaOption allows for recursive application of schema options. For example, that limitation is visible in this code, which must resort to untyped maps to express a nested schema.

Additionally, the NewTool helper provides a means for building a tool from a Go function using reflection, that automatically handles parsing and validation of inputs.

We provide a full JSON Schema implementation for validating tool input schemas against incoming arguments. The jsonschema.Schema type provides exported features for all keywords in the JSON Schema draft2020-12 spec. Tool definers can use it to construct any schema they want, so there is no need to provide options for all of them. When combined with schema inference from input structs, we found that we needed only three options to cover the common cases, instead of mcp-go's 23. For example, we will provide Enum, which occurs 125 times in open source code, but not MinItems, MinLength or MinProperties, which each occur only once (and in an SDK that wraps mcp-go).

For registering tools, we provide only AddTools; mcp-go's SetTools, AddTool, AddSessionTool, and AddSessionTools are deemed unnecessary. (similarly for Delete/Remove).

Prompts

Use NewPrompt to create a prompt. As with tools, prompt argument schemas can be inferred from a struct, or obtained from options.

func NewPrompt[TReq any](name, description string,
  handler func(context.Context, *ServerSession, TReq) (*GetPromptResult, error),
  opts ...PromptOption) *ServerPrompt

Use AddPrompts to add prompts to the server, and RemovePrompts to remove them by name.

type codeReviewArgs struct {
  Code string `json:"code"`
}

func codeReviewHandler(context.Context, *ServerSession, codeReviewArgs) {...}

server.AddPrompts(
  NewPrompt("code_review", "review code", codeReviewHandler,
    Argument("code", Description("the code to review"))))

server.RemovePrompts("code_review")

Clients can call the spec method ListPrompts to list the available prompts and the spec method GetPrompt to get one.

Differences from mcp-go: We provide a NewPrompt helper to bind a prompt handler to a Go function using reflection to derive its arguments. We provide RemovePrompts to remove prompts from the server.

Resources and resource templates

To add a resource or resource template to a server, users call the AddResource and AddResourceTemplate methods, passing the resource or template and a function for reading it:

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

func (*Server) AddResource(*Resource, ReadResourceHandler)
func (*Server) AddResourceTemplate(*ResourceTemplate, ReadResourceHandler)

The Resource is passed to the reader function even though it is redundant (the function could have closed over it) so a single handler can support multiple resources. If the incoming resource matches a template, a Resource argument is constructed from the fields in the ResourceTemplate. The ServerSession argument is there so the reader can observe the client's roots.

To read files from the local filesystem, we recommend using FileReadResourceHandler to construct a handler:

// FileReadResourceHandler returns a ReadResourceHandler that reads paths using dir as a root directory.
// It protects against path traversal attacks.
// It will not read any file that is not in the root set of the client requesting the resource.
func (*Server) FileReadResourceHandler(dir string) ReadResourceHandler

It guards against path traversal attacks and observes the client's roots. Here is an example:

// Safely read "/public/puppies.txt".
s.AddResource(
  &mcp.Resource{URI: "file:///puppies.txt"},
  s.FileReadResourceHandler("/public"))

There are also server methods to remove resources and resource templates.

func (*Server) RemoveResources(uris ...string)
func (*Server) RemoveResourceTemplates(names ...string)

Resource templates don't have unique identifiers, so removing a name will remove all resource templates with that name.

Servers support all of the resource-related spec methods:

• ListResources and ListResourceTemplates for listings.
• ReadResource to get the contents of a resource.
• Subscribe and Unsubscribe to manage subscriptions on resources.

ReadResource checks the incoming URI against the server's list of resources and resource templates to make sure it matches one of them, then returns the result of calling the associated reader function.

Subscriptions

ClientSessions can manage change notifications on particular resources:

func (*ClientSession) Subscribe(context.Context, *SubscribeParams) error
func (*ClientSession) Unsubscribe(context.Context, *UnsubscribeParams) error

The server does not implement resource subscriptions. It passes along subscription requests to the user, and supplies a method to notify clients of changes. It tracks which sessions have subscribed to which resources so the user doesn't have to.

If a server author wants to support resource subscriptions, they must provide handlers to be called when clients subscribe and unsubscribe. It is an error to provide only one of these handlers.

type ServerOptions struct {
  ...
  // Function called when a client session subscribes to a resource.
  SubscribeHandler func(context.Context, *SubscribeParams) error
  // Function called when a client session unsubscribes from a resource.
  UnsubscribeHandler func(context.Context, *UnsubscribeParams) error
}

User code should call ResourceUpdated when a subscribed resource changes.

func (*Server) ResourceUpdated(context.Context, *ResourceUpdatedNotification) error

The server routes these notifications to the server sessions that subscribed to the resource.

ListChanged notifications

When a list of tools, prompts or resources changes as the result of an AddXXX or RemoveXXX call, the server informs all its connected clients by sending the corresponding type of notification. A client will receive these notifications if it was created with the corresponding option:

type ClientOptions struct {
  ...
  ToolListChangedHandler func(context.Context, *ClientSession, *ToolListChangedParams)
  PromptListChangedHandler func(context.Context, *ClientSession, *PromptListChangedParams)
  ResourceListChangedHandler func(context.Context, *ClientSession, *ResourceListChangedParams)
}

Differences from mcp-go: mcp-go instead provides a general OnNotification handler. For type-safety, and to hide JSON RPC details, we provide feature-specific handlers here.

Completion

Clients call the spec method Complete to request completions. Servers automatically handle these requests based on their collections of prompts and resources.

Differences from mcp-go: the client API is similar. mcp-go has not yet defined its server-side behavior.

Logging

Server-to-client logging is configured with ServerOptions:

type ServerOptions {
  ...
  // The value for the "logger" field of the notification.
  LoggerName string
  // Log notifications to a single ClientSession will not be
  // send more frequently than this duration.
  LogInterval time.Duration
}

ServerSessions have access to a slog.Logger that writes to the client. A call to a log method like Infois translated to a LoggingMessageNotification as follows:

• The attributes and the message populate the "data" property with the output of a slog.JSONHandler: The result is always a JSON object, with the key "msg" for the message.

• If the LoggerName server option is set, it populates the "logger" property.

• The standard slog levels Info, Debug, Warn and Error map to the corresponding levels in the MCP spec. The other spec levels will be mapped to integers between the slog levels. For example, "notice" is level 2 because it is between "warning" (slog value 4) and "info" (slog value 0). The mcp package defines consts for these levels. To log at the "notice" level, a handler would call session.Log(ctx, mcp.LevelNotice, "message").

A client that wishes to receive log messages must provide a handler:

type ClientOptions struct {
  ...
  LogMessageHandler func(context.Context, *ClientSession, *LoggingMessageParams)
}

Pagination

Servers initiate pagination for ListTools, ListPrompts, ListResources, and ListResourceTemplates, dictating the page size and providing a NextCursor field in the Result if more pages exist. The SDK implements keyset pagination, using the unique ID as the key for a stable sort order and encoding the cursor as an opaque string.

For server implementations, the page size for the list operation may be configured via the ServerOptions.PageSize field. PageSize must be a non-negative integer. If zero, a sensible default is used.

type ServerOptions {
  ...
  PageSize int
}

Client requests for List methods include an optional Cursor field for pagination. Server responses for List methods include a NextCursor field if more pages exist.

In addition to the List methods, the SDK provides an iterator method for each list operation. This simplifies pagination for cients by automatically handling the underlying pagination logic.

For example, we if we have a List method like this:

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

We will also provide an iterator method like this:

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