go-sdk/design/design.md

# Go MCP SDK design

This file discusses the design of a Go SDK for the [model context
protocol](https://modelcontextprotocol.io/specification/2025-03-26). It is
intended to seed a GitHub discussion about the official Go MCP SDK, and so
approaches each design aspect from first principles. Of course, there is
significant prior art in various unofficial SDKs, along with other practical
constraints. Nevertheless, if we can first agree on the most natural way to
model the MCP spec, we can then discuss the shortest path to get there.

# 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](https://modelcontextprotocol.io/specification/2025-03-26)
For each, we discuss considerations for the Go implementation. In many cases an
API is suggested, though in some there many be open questions.

<!--

Instructions: for each section below:
 - Summarize the spec.
 - If applicable, reference prior art or alternatives.
 - If possible, propose a Go API, and justify why it meets the requirements
   above.
-->

## 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/mcp/protocol`: generated types for the MCP spec.
- `module.path/jsonschema`: a jsonschema implementation, with validation
- `module.path/internal/jsonrpc2`: a fork of x/tools/internal/jsonrpc2_v2

For now, this layout assumes we want to separate the 'protocol' types from the
'mcp' package, since they won't be needed by most users. It is unclear whether
this is worthwhile.

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.

### 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:

- ****: 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](https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#custom-transports) 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:

```go
// 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
}
```

Specifically, a `Transport` is something that connects a logical JSON-RPC
stream, and nothing more (methods accept a Go `Context` and return an `error`,
as is idiomatic for APIs that do I/O). 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 or middleware.

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.

```go
// 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`.

```go
// 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.

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

// NewSSEHandler returns a new [SSEHandler] 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 NewSSEHandler(getServer func(request *http.Request) *Server) *SSEHandler

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

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

Notably absent are options to hook into the request handling for the purposes
of authentication or context injection. These concerns are better handled using
standard HTTP middleware patterns.

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.

```go
// 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.

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

// 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 NewSSEClientTransport(url string) *SSEClientTransport {

// 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:

```go
// The StreamableHandler interface is symmetrical to the SSEHandler.
type StreamableHandler struct { /* unexported fields */ }
func NewStreamableHandler(getServer func(request *http.Request) *Server) *StreamableHandler
func (*StreamableHandler) ServeHTTP(w http.ResponseWriter, req *http.Request)
func (*StreamableHandler) 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)
```

### Protocol types

As described in the section on package layout above, the `protocol` package
will contain definitions of types referenced by the MCP spec that are needed
for the SDK. JSON-RPC message types are elided, since they are handled by the
`jsonrpc2` package and should not be observed by the user. The user interacts
only with the params/result types relevant to MCP operations.

For user-provided data, 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.

These types will be auto-generated from the [JSON schema of the MCP
spec](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2025-03-26/schema.json).
For brevity, only a few examples are shown here:

```go
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     string    `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     *string `json:"blob"`
}
```

### Clients and Servers

<!--
TODO: discuss the construction of new clients and servers, and connecting
them to a transport. Pay particular attention to the 1:1 nature of binding.
-->

### Errors

<!-- TODO: a brief section discussing how errors are handled. -->

### Cancellation

<!-- TODO: a brief section discussing how cancellation is handled. -->

### Progress handling

<!-- TODO: a brief section discussing how progress is handled. -->

### Ping / Keepalive

<!--
TODO: discuss the implementation of 'ping', as well as APIs for
parameterizing automatic keepalive.
-->

## 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 roots to the client's list of roots.
// If the list changes, the client notifies the server.
// If a root does not begin with a valid URI schema such as "https://" or "file://",
// it is intepreted as a directory path on the local filesystem.
func (*Client) AddRoots(roots ...string)

// RemoveRoots removes the given roots from the client's list, and notifies
// the server if the list has changed.
// It is not an error to remove a nonexistent root.
func (*Client) RemoveRoots(roots ...string)
```

Servers can call `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.
```
func (*Server) ListRoots(context.Context, *ListRootsParams) (*ListRootsResult, error)

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

### Sampling

Clients that support sampling are created with a `CreateMessageHandler` option for handling server
calls.
To perform sampling, a server calls `CreateMessage`.
```
type ClientOptions struct {
  ...
  CreateMessageHandler func(context.Context, *CreateMessageParams) (*CreateMessageResult, error)
}

func (*Server) CreateMessage(context.Context, *CreateMessageParams) (*CreateMessageResult, error)
```

## Server Features

### Tools

Add tools to a server with `AddTools`:
```
server.AddTools(
  mcp.NewTool("add", "add numbers", addHandler),
  mcp.NewTools("subtract, subtract numbers", subHandler))
```
Remove them with `RemoveTools`:
```
  server.RemoveTools("add", "subtract")
```

We provide a convenient and type-safe way to construct a Tool:

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

The `TReq` type is typically a struct, and we use reflection on the struct to
determine the names and types of the tool's input. `ToolOption`s allow further
customization of a Tool's input schema.
Since all the fields of the Tool struct are exported, a Tool can also be created
directly with assignment or a struct literal.

A tool's input schema, expressed as a [JSON Schema](https://json-schema.org),
provides a way to validate the tool's input.

We chose a hybrid a approach to specifying the schema, combining reflection
and variadic options. We found that this makes the common cases easy (sometimes
free!) to express and keeps the API small.  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. Our approach
also guarantees that the input schema is compatible with tool parameters, by
construction.

`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.

The struct provides the names, types and required status of the properties.
Other JSON Schema keywords can be specified by passing options to `NewTool`:
```
NewTool(name, description, handler,
    Input(Property("count", Description("size of the inventory"))))
```

For less common keywords, use the `Schema` option:
```
NewTool(name, description, handler,
    Input(Property("Choices", Schema(&jsonschema.Schema{UniqueItems: true}))))
```

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

### Prompts

<!--
TODO(rfindley):
NewPrompt
Server.AddPrompts
Server.RemovePrompts
-->

### Resources

<!--
TODO:
NewResource
Server.AddResources
Server.RemoveResources
-->

### Completion

<!-- TODO: needs design -->

### Logging

Servers have access to a `slog.Logger` that writes to the client. A call to
a log method like `Info`is translated to a `LoggingMessageNotification` as
follows:

- An attribute with key "logger" is used to populate the "logger" field of the notification.

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

- 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 server would call `Log(ctx, mcp.LevelNotice, "message")`.

### Pagination

<!-- TODO: needs design -->

## Differences with mark3labs/mcp-go

The most popular MCP module for Go is [mark3labs/mcp-go](https://pkg.go.dev/github.com/
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 reconsider. Therefore,
we wrote this SDK design from the perspective of 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.

Although our API is not compatible with mcp-go's, translating between them should be
straightforward in most cases.
(Later, we will provide a detailed translation guide.)

### Packages

As we mentioned above, we decided to put most of the API into a single package.
Our `mcp` package includes all the functionality of mcp-go's `mcp`, `client`,
`server` and `transport` packages, but is smaller than the `mcp` package alone.

### Typed tool inputs

We provide a way to supply a struct as the input type of a Tool, as described
in [JSON Schema](#JSON_Schema), above.
The tool handler receives a value of this struct instead of a `map[string]any`,
so it doesn't need to parse its input parameters. Also, we infer the input schema
from the struct, avoiding the need to specify the name, type and required status of
parameters.

### Schema validation

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 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).

Moreover, our options can be used to build nested schemas, while
mcp-go's work only at top level. That limitation is visible in
[this code](https://github.com/DCjanus/dida365-mcp-server/blob/master/cmd/mcp/tools.go#L315),
which must resort to untyped maps to express a nested schema:
```
mcp.WithArray("items",
  mcp.Description("Checklist items of the task"),
  mcp.Items(map[string]any{
   "type": "object",
   "properties": map[string]any{
    "id": map[string]any{
     "type":        "string",
     "description": "Unique identifier of the checklist item",
    },
    "status": map[string]any{
     "type":        "number",
     "description": "Status of the checklist item (0: normal, 1: completed)",
     "enum":        []float64{0, 1},
     },
     ...
```

### JSON-RPC implementation

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.

### Hooks

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.

All of the hooks run before or after the server processes a message,
so instead we provide a single way to intercept this message handling, using
two exported names instead of 72:
```
// A Handler handles an MCP message call.
type Handler func(ctx context.Context, c *ServerConnection, method string, params any) (response any, err error)


// AddMiddleware calls each middleware function from right to left on the previous result, beginning
// with the server's current handler, and installs the result as the new handler.
func (*Server) AddMiddleware(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, c *mcp.ServerConnection, 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, c , method, params)
    }
}

server.AddMiddleware(withLogging)
```

### Options

In Go, the two most common ways to provide options to a function are option structs (for example,
https://pkg.go.dev/net/http#PushOptions) and
variadic option functions. mcp-go uses option functions exclusively. For example,
the `server.NewMCPServer` function has ten associated functions to provide options.
Our API uses both, depending on the context. We use function options for
constructing tools, where they are most convenient. In most other places, we
prefer structs because they have a smaller API footprint and are less verbose.

### Servers

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 `SSEHandler` manages sessions for
incoming SSE connections, but does not expose them.