This document discusses the design of a Go SDK for the model context protocol. The golang.org/x/tools/internal/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 from and expanded on the APIs of that prototype, and this document should be considered canonical.
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 where possible tried to align with its design. However, the APIs here diverge in a number of ways in order to keep the official SDK minimal, allow for future spec evolution, and support additional features. We have noted significant differences from mcp-go 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.)
Thank you to everyone who contributes to mcp-go and other Go SDKs. We hope that we can collaborate to leverage all that we've learned about MCP and Go in an official SDK.
These may be obvious, but it's worthwhile to define goals for an official MCP SDK. An official SDK should aim to be:
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, and propose a Go API.
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. We believe that having a single package aids discoverability in package documentation and in the IDE. Furthermore, it avoids arbitrary decisions about package structure that may be rendered inaccurate by future evolution of the spec.
Functionality that is not directly related to MCP (like jsonschema or jsonrpc2) belongs in a separate package.
Therefore, this is the core package layout, assuming github.com/modelcontextprotocol/go-sdk as the module path.
github.com/modelcontextprotocol/go-sdk/mcp: the bulk of the user facing APIgithub.com/modelcontextprotocol/go-sdk/jsonschema: a jsonschema implementation, with validationgithub.com/modelcontextprotocol/go-sdk/internal/jsonrpc2: a fork of x/tools/internal/jsonrpc2_v2The 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 by promoting them from the mcp package using aliases or wrappers.
Difference from mcp-go: Our mcp package includes all the functionality of mcp-go's mcp, client, server and transport packages.
The MCP is defined in terms of client-server communication over bidirectional JSON-RPC message connections. Specifically, version 2025-03-26 of the spec defines two transports:
Additionally, version 2024-11-05 of the spec defined a simpler (yet stateful) HTTP transport:
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 connection, we can use this to model the MCP transport abstraction:
type ( JSONRPCID = jsonrpc2.ID JSONRPCMessage = jsonrpc2.Message JSONRPCRequest = jsonrpc2.Request JSONRPCResponse = jsonrpc2.Response ) // A Transport is used to create a bidirectional connection between MCP client // and server. type Transport interface { Connect(ctx context.Context) (Connection, error) } // A Connection is a bidirectional jsonrpc2 connection. type Connection interface { Read(ctx context.Context) (JSONRPCMessage, error) Write(ctx context.Context, JSONRPCMessage) 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 connection, and nothing more. Connections 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 connection, 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.
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 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 different types for client and server stdio transport.
The CommandTransport is the client side of the stdio transport, and connects by starting a command and streaming JSON-RPC messages over 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) (Connection, 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) (Connection, 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 logical 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. As described below, MCP servers can optionally connect to multiple clients. This allows customization of per-session servers: if the MCP server is stateless, the user can return the same MCP server for each connection. On the other hand, if any per-session customization is required, it is possible by returning a different Server instance for each connection.
// 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 instead 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 returns the following [Connection] 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. // // Most callers should instead use an [SSEHandler], which transparently handles // the delegation to SSEServerTransports. 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 [Connection] implementation. func (*SSEServerTransport) Connect(context.Context) (Connection, error)
The SSE client transport is simpler, and hopefully self-explanatory.
type SSEClientTransport struct { /* ... */ } // SSEClientTransportOptions provides options for the [NewSSEClientTransport] // constructor. type SSEClientTransportOptions struct { // HTTPClient is the client to use for making HTTP requests. If nil, // http.DefaultClient is used. HTTPClient *http.Client } // NewSSEClientTransport returns a new client transport that connects to the // SSE server at the provided URL. func NewSSEClientTransport(url string, opts *SSEClientTransportOptions) (*SSEClientTransport, error) // Connect connects through the client endpoint. func (*SSEClientTransport) Connect(ctx context.Context) (Connection, 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) *StreamableServerTransport func (*StreamableServerTransport) ServeHTTP(w http.ResponseWriter, req *http.Request) func (*StreamableServerTransport) Connect(context.Context) (Connection, error) // The streamable client handles reconnection transparently to the user. type StreamableClientTransport struct { /* ... */ } // StreamableClientTransportOptions provides options for the // [NewStreamableClientTransport] constructor. type StreamableClientTransportOptions struct { // HTTPClient is the client to use for making HTTP requests. If nil, // http.DefaultClient is used. HTTPClient *http.Client } func NewStreamableClientTransport(url string, opts *StreamableClientTransportOptions) *StreamableClientTransport func (*StreamableClientTransport) Connect(context.Context) (Connection, error)
Differences from mcp-go: 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. Users can manage their own sessions with RegisterSession and UnregisterSession. Rather than use a server constructor to get a distinct server for each connection, there is a concept of a “session tool” that overlays tools for a specific session.
Here, we tried to differentiate the concept of a Server, HTTPHandler, and Transport, and provide per-session customization through either the getServer constructor or middleware. Additionally, individual handlers and transports here have a minimal API, and do not expose internal details. (Open question: are we oversimplifying?)
We also provide a couple of transport implementations for special scenarios. An InMemoryTransport can be used when the client and server reside in the same process. A LoggingTransport is a middleware layer that logs RPC logs to a desired location, specified as an io.Writer.
// An InMemoryTransport is a [Transport] that communicates over an in-memory // network connection, using newline-delimited JSON. type InMemoryTransport struct { /* ... */ } // NewInMemoryTransports returns two InMemoryTransports that connect to each // other. func NewInMemoryTransports() (*InMemoryTransport, *InMemoryTransport) // A LoggingTransport is a [Transport] that delegates to another transport, // writing RPC logs to an io.Writer. type LoggingTransport struct { /* ... */ } func NewLoggingTransport(delegate Transport, w io.Writer) *LoggingTransport
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 request 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 or map[string]any, depending on the use case.
For union types, which can't be represented in Go (specifically Content and ResourceContents), 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 ReadResourceParams struct { URI string `json:"uri"` } type CallToolResult struct { Meta Meta `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 *ResourceContents `json:"resource,omitempty"` } // NewTextContent creates a [Content] with text. func NewTextContent(text string) *Content // etc.
The Meta type includes a map[string]any for arbitrary data, and a ProgressToken field.
Differences from mcp-go: these types are largely similar, but our type generator flattens types rather than using struct embedding.
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.
Supporting multiple connections to servers (and from clients) still allows for stateful components, as it is up to the user to decide whether or not to create distinct servers/clients for each connection. For example, if the user wants to create a distinct server for each new connection, they can do so in the getServer factory passed to transport handlers.
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 (*Client) Connect(context.Context, Transport) (*ClientSession, error) func (*Client) Sessions() iter.Seq[*ClientSession] // 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. // For example: ClientSession.ListTools. type Server struct { /* ... */ } func NewServer(name, version string, opts *ServerOptions) *Server func (*Server) Connect(context.Context, Transport) (*ServerSession, error) func (*Server) Sessions() iter.Seq[*ServerSession] // 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. // For example: ServerSession.ListRoots.
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, "greet", map[string]any{"name": "you"}, nil) ... 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.AddTools(mcp.NewServerTool("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 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 sessions. Per-session behavior is implemented though a ‘session tool’ overlay. 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 simpler package documentation.
In our SDK, RPC methods that are defined in the specification take a context and a params pointer as arguments, and return a result pointer and error:
func (*ClientSession) ListTools(context.Context, *ListToolsParams) (*ListToolsResult, error)
Our SDK has a method for every RPC in the spec, their signatures all share this form. We do this, rather than providing more convenient shortcut signatures, to maintain backward compatibility if the spec makes backward-compatible changes such as adding a new property to the request parameters (as in this commit, for example). 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.”
CallTool is the only exception: for convenience when binding to Go argument types, *CallToolParams[TArgs] is generic, with a type parameter providing the Go type of the tool arguments. The spec method accepts a *CallToolParams[json.RawMessage], but we provide a generic helper function. See the section on Tools below for details.
Why do we use params instead of the full JSON-RPC request? As much as possible, we endeavor to hide JSON-RPC details when they are not relevant to the business logic of your client or server. In this case, the additional information in the JSON-RPC request is just the request ID and method name; the request ID is irrelevant, and the method name is implied by the name of the Go method providing the API.
We believe that any change to the spec that would require callers to pass a new a parameter is not backward compatible. Therefore, it will always work to pass nil for any XXXParams argument that isn't currently necessary. For example, it is okay to call Ping like so:
err := session.Ping(ctx, nil)
For convenience, iterator methods handle pagination for the List spec methods automatically, traversing all pages. If Params are supplied, iteration begins from the provided cursor (if present).
func (*ClientSession) Tools(context.Context, *ListToolsParams) iter.Seq2[Tool, error]
func (*ClientSession) Prompts(context.Context, *ListPromptsParams) iter.Seq2[Prompt, error]
func (*ClientSession) Resources(context.Context, *ListResourceParams) iter.Seq2[Resource, error]
func (*ClientSession) ResourceTemplates(context.Context, *ListResourceTemplatesParams) iter.Seq2[ResourceTemplate, error]
We provide a mechanism to add MCP-level middleware on the both the client and server side. Receiving middleware runs after the request has been parsed but before any normal handling. It is analogous to traditional HTTP server middleware. Sending middleware runs after a call to a method but before the request is sent. It is an alternative to transport middleware that exposes MCP types instead of raw JSON-RPC 2.0 messages. It is useful for tracing and setting progress tokens, for example.
// A MethodHandler handles MCP messages. // The params argument is an XXXParams struct pointer, such as *GetPromptParams. // For methods, a MethodHandler must return either an XXResult struct pointer and a nil error, or // nil with a non-nil error. // For notifications, a MethodHandler must return nil, nil. type MethodHandler[S Session] func( ctx context.Context, _ *S, method string, params Params) (result Result, err error) // Middleware is a function from MethodHandlers to MethodHandlers. type Middleware[S Session] func(MethodHandler[S]) MethodHandler[S] // AddMiddleware wraps the client/server's current method handler using the provided // middleware. Middleware is applied from right to left, so that the first one // is executed first. // // For example, AddMiddleware(m1, m2, m3) augments the server method handler as // m1(m2(m3(handler))). func (c *Client) AddSendingMiddleware(middleware ...Middleware[*ClientSession]) func (c *Client) AddReceivingMiddleware(middleware ...Middleware[*ClientSession]) func (s *Server) AddSendingMiddleware(middleware ...Middleware[*ServerSession]) func (s *Server) AddReceivingMiddleware(middleware ...Middleware[*ServerSession])
As an example, this code adds server-side logging:
func withLogging(h mcp.MethodHandler[*ServerSession]) mcp.MethodHandler[*ServerSession]{
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.AddReceivingMiddleware(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.
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 include ID and Method, which can be inferred from the caller. Otherwise, this behavior is similar.
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{}, nil)
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.
A caller can request progress notifications by setting the Meta.ProgressToken field on any request.
type XXXParams struct { // where XXX is each type of call Meta Meta ... } type Meta struct { Data map[string]any ProgressToken any // string or int }
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)
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).
Clients support the MCP Roots feature, 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)
Server sessions 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) }
The Roots method provides a cached iterator of the root set, invalidated when roots change.
func (*ServerSession) Roots(context.Context) (iter.Seq[*Root, error])
Clients that support sampling are created with a CreateMessageHandler option for handling server calls. To perform sampling, a server session calls the spec method CreateMessage.
type ClientOptions struct {
...
CreateMessageHandler func(context.Context, *ClientSession, *CreateMessageParams) (*CreateMessageResult, error)
}
A Tool is a logical MCP tool, generated from the MCP spec, and a ServerTool is a tool bound to a tool handler.
A tool handler accepts CallToolParams and returns a CallToolResult. However, since we want to bind tools to Go input types, it is convenient in associated APIs to make CallToolParams generic, with a type parameter TArgs for the tool argument type. This allows tool APIs to manage the marshalling and unmarshalling of tool inputs for their caller. The bound ServerTool type expects a json.RawMessage for its tool arguments, but the NewServerTool constructor described below provides a mechanism to bind a typed handler.
type CallToolParams[TArgs any] struct {
Meta Meta `json:"_meta,omitempty"`
Arguments TArgs `json:"arguments,omitempty"`
Name string `json:"name"`
}
type Tool struct {
Annotations *ToolAnnotations `json:"annotations,omitempty"`
Description string `json:"description,omitempty"`
InputSchema *jsonschema.Schema `json:"inputSchema"`
Name string `json:"name"`
}
type ToolHandler[TArgs] func(context.Context, *ServerSession, *CallToolParams[TArgs]) (*CallToolResult, error)
type ServerTool struct {
Tool Tool
Handler ToolHandler[json.RawMessage]
}
Add tools to a server with AddTools:
server.AddTools(
mcp.NewServerTool("add", "add numbers", addHandler),
mcp.NewServerTool("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:
metoro-io/mcp-golang)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 NewServerTool 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.
// NewServerTool creates a Tool using reflection on the given handler. func NewServerTool[TArgs any](name, description string, handler ToolHandler[TArgs], opts …ToolOption) *ServerTool type ToolOption interface { /* ... */ }
NewServerTool determines the input schema for a Tool from the TArgs type. 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:
NewServerTool(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:
NewServerTool(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.
Client sessions can call the spec method ListTools or an iterator method Tools to list the available tools, and use spec method CallTool to call tools. Similar to ServerTool.Handler, CallTool expects *CallToolParams[json.RawMessage], but we provide a generic CallTool helper to operate on typed arguments.
func (cs *ClientSession) CallTool(context.Context, *CallToolParams[json.RawMessage]) (*CallToolResult, error)
func CallTool[TArgs any](context.Context, *ClientSession, *CallToolParams[TArgs]) (*CallToolResult, error)
Differences from mcp-go: using variadic options to configure tools was significantly 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 NewServerTool 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).
Use NewServerPrompt to create a prompt. As with tools, prompt argument schemas can be inferred from a struct, or obtained from options.
func NewServerPrompt[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(
NewServerPrompt("code_review", "review code", codeReviewHandler,
Argument("code", Description("the code to review"))))
server.RemovePrompts("code_review")
Client sessions can call the spec method ListPrompts or the iterator method Prompts to list the available prompts, and the spec method GetPrompt to get one.
Differences from mcp-go: We provide a NewServerPrompt 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.
In our design, each resource and resource template is associated with a function that reads it, with this signature:
type ResourceHandler func(context.Context, *ServerSession, *ReadResourceParams) (*ReadResourceResult, error)
The arguments include the ServerSession so the handler can observe the client's roots. The handler should return the resource contents in a ReadResourceResult, calling either NewTextResourceContents or NewBlobResourceContents. If the handler omits the URI or MIME type, the server will populate them from the resource.
The ServerResource and ServerResourceTemplate types hold the association between the resource and its handler:
type ServerResource struct {
Resource Resource
Handler ResourceHandler
}
type ServerResourceTemplate struct {
Template ResourceTemplate
Handler ResourceHandler
}
To add a resource or resource template to a server, users call the AddResources and AddResourceTemplates methods with one or more ServerResources or ServerResourceTemplates. We also provide methods to remove them.
func (*Server) AddResources(...*ServerResource)
func (*Server) AddResourceTemplates(...*ServerResourceTemplate)
func (s *Server) RemoveResources(uris ...string)
func (s *Server) RemoveResourceTemplates(uriTemplates ...string)
The ReadResource method finds a resource or resource template matching the argument URI and calls its associated handler.
To read files from the local filesystem, we recommend using FileResourceHandler to construct a handler:
// FileResourceHandler returns a ResourceHandler 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) FileResourceHandler(dir string) ResourceHandler
Here is an example:
// Safely read "/public/puppies.txt". s.AddResources(&mcp.ServerResource{ Resource: mcp.Resource{URI: "file:///puppies.txt"}, Handler: s.FileReadResourceHandler("/public")})
Server sessions also support the spec methods ListResources and ListResourceTemplates, and the corresponding iterator methods Resources and ResourceTemplates.
Differences from mcp-go: for symmetry with tools and prompts, we use AddResources rather than AddResource. Additionally, the ResourceHandler returns a ReadResourceResult, rather than just its content, for compatibility with future evolution of the spec.
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.
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) // For both resources and resource templates. 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.
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.
MCP specifies a notification for servers to log to clients. Server sessions implement this with the LoggingMessage method. It honors the minimum log level established by the client session's SetLevel call.
As a convenience, we also provide a slog.Handler that allows server authors to write logs with the log/slog package::
// A LoggingHandler is a [slog.Handler] for MCP. type LoggingHandler struct {...} // LoggingHandlerOptions are options for a LoggingHandler. 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 } // NewLoggingHandler creates a [LoggingHandler] that logs to the given [ServerSession] using a // [slog.JSONHandler]. func NewLoggingHandler(ss *ServerSession, opts *LoggingHandlerOptions) *LoggingHandler
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 // sent more frequently than this duration. LoggingInterval time.Duration }
A call to a log method like Info is 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 map 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 Log(ctx, mcp.LevelNotice, "message").
A client that wishes to receive log messages must provide a handler:
type ClientOptions struct {
...
LoggingMessageHandler func(context.Context, *ClientSession, *LoggingMessageParams)
}
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 of the feature 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 clients by automatically handling the underlying pagination logic. See Iterator Methods above.
Differences with mcp-go: the PageSize configuration is set with a configuration field rather than a variadic option. Additionally, this design proposes pagination by default, as this is likely desirable for most servers
While the sections above propose an initial implementation of the Go SDK, MCP is evolving rapidly. SDKs need to keep pace, by implementing changes to the spec, fixing bugs, and accommodating new and emerging use-cases. This section proposes how the SDK project can be managed so that it can change safely and transparently.
Initially, the Go SDK repository will be administered by the Go team and Anthropic, and they will be the Approvers (the set of people able to merge PRs to the SDK). The policies here are also intended to satisfy necessary constraints of the Go team's participation in the project.
The content in this section will also be included in a CONTRIBUTING.md file in the repo root.
The SDK will be hosted under github.com/modelcontextprotocol/go-sdk, MIT license, copyright “Go SDK Authors”. Each Go file in the repository will have a standard copyright header. For example:
// Copyright 2025 The Go MCP SDK Authors. All rights reserved. // Use of this source code is governed by an MIT-style // license that can be found in the LICENSE file.
The SDK will use its GitHub issue tracker for bug tracking, and pull requests for contributions.
Contributions to the SDK will be welcomed, and will be accepted provided they are high quality and consistent with the direction and philosophy of the SDK outlined above. An official SDK must be conservative in the changes it accepts, to defend against compatibility problems, security vulnerabilities, and churn. To avoid being declined, PRs should be associated with open issues, and those issues should either be labeled ‘Help Wanted’, or the PR author should ask on the issue before contributing.
A proposal is an issue that proposes a new API for the SDK, or a change to the signature or behavior of an existing API. Proposals will be labeled with the ‘Proposal’ label, and require an explicit approval before being accepted (applied through the ‘Proposal-Accepted’ label). Proposals will remain open for at least a week to allow discussion before being accepted or declined by an Approver.
Proposals that are straightforward and uncontroversial may be approved based on GitHub discussion. However, proposals that are deemed to be sufficiently unclear or complicated will be deferred to a regular steering meeting (see below).
This process is similar to the Go proposal process, but is necessarily lighter weight to accommodate the greater rate of change expected for the SDK.
On a regular basis, we will host a virtual steering meeting to discuss outstanding proposals and other changes to the SDK. These 1hr meetings and their agenda will be announced in advance, and open to all to join. The meetings will be recorded, and recordings and meeting notes will be made available afterward.
This process is similar to the Go Tools call, though it is expected that meetings will at least initially occur on a more frequent basis (likely biweekly).
Discord (either the public or private Anthropic discord servers) should only be used for logistical coordination or answering questions. Design discussion and decisions should occur in GitHub issues or public steering meetings.
It is important that the SDK avoids bias toward specific integration paths or providers. Therefore, the CONTRIBUTING.md file will include an antitrust policy that outlines terms and practices intended to avoid such bias, or the appearance thereof. (The details of this policy will be determined by Google and Anthropic lawyers).
The SDK will consist of a single Go module, and will be released through versioned Git tags. Accordingly, it will follow semantic versioning.
Up until the v1.0.0 release, the SDK may be unstable and may change in breaking ways. An initial v1.0.0 release will occur when the SDK is deemed by Approvers to be stable, production ready, and sufficiently complete (though some unimplemented features may remain). Subsequent to that release, new APIs will be added in minor versions, and breaking changes will require a v2 release of the module (and therefore should be avoided). All releases will have corresponding release notes in GitHub.
It is desirable that releases occur frequently, and that a v1.0.0 release is achieved as quickly as possible.
If feasible, the SDK will support all versions of the MCP spec. However, if breaking changes to the spec make this infeasible, preference will be given to the most recent version of the MCP spec.
On an ongoing basis, the administrators of the SDK will evaluate whether it is keeping pace with changes to the MCP spec and meeting its goals of openness and transparency. If it is not meeting these goals, either because it exceeds the bandwidth of its current Approvers, or because the processes here are inadequate, these processes will be re-evaluated. At this time, the Approvers set may be expanded to include additional community members, based on their history of strong contribution.