internal/mcp/streamable.go

// Copyright 2025 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package mcp

import (
	"bytes"
	"context"
	"crypto/rand"
	"fmt"
	"io"
	"net/http"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"

	jsonrpc2 "golang.org/x/tools/internal/jsonrpc2_v2"
)

// A StreamableHTTPHandler is an http.Handler that serves streamable MCP
// sessions, as defined by the [MCP spec].
//
// [MCP spec]: https://modelcontextprotocol.io/2025/03/26/streamable-http-transport.html
type StreamableHTTPHandler struct {
	getServer func(*http.Request) *Server

	sessionsMu sync.Mutex
	sessions   map[string]*StreamableServerTransport // keyed by IDs (from Mcp-Session-Id header)
}

// StreamableHTTPOptions is a placeholder options struct for future
// configuration of the StreamableHTTP handler.
type StreamableHTTPOptions struct {
	// TODO(rfindley): support configurable session ID generation and event
	// store, session retention, and event retention.
}

// NewStreamableHTTPHandler returns a new [StreamableHTTPHandler].
//
// The getServer function is used to create or look up servers for new
// sessions. It is OK for getServer to return the same server multiple times.
func NewStreamableHTTPHandler(getServer func(*http.Request) *Server, opts *StreamableHTTPOptions) *StreamableHTTPHandler {
	return &StreamableHTTPHandler{
		getServer: getServer,
		sessions:  make(map[string]*StreamableServerTransport),
	}
}

// closeAll closes all ongoing sessions.
//
// TODO(rfindley): investigate the best API for callers to configure their
// session lifecycle.
//
// Should we allow passing in a session store? That would allow the handler to
// be stateless.
func (h *StreamableHTTPHandler) closeAll() {
	h.sessionsMu.Lock()
	defer h.sessionsMu.Unlock()
	for _, s := range h.sessions {
		s.Close()
	}
	h.sessions = nil
}

func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	// Allow multiple 'Accept' headers.
	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Accept#syntax
	accept := strings.Split(strings.Join(req.Header.Values("Accept"), ","), ",")
	var jsonOK, streamOK bool
	for _, c := range accept {
		switch strings.TrimSpace(c) {
		case "application/json":
			jsonOK = true
		case "text/event-stream":
			streamOK = true
		}
	}

	if req.Method == http.MethodGet {
		if !streamOK {
			http.Error(w, "Accept must contain 'text/event-stream' for GET requests", http.StatusBadRequest)
			return
		}
	} else if !jsonOK || !streamOK {
		http.Error(w, "Accept must contain both 'application/json' and 'text/event-stream'", http.StatusBadRequest)
		return
	}

	var session *StreamableServerTransport
	if id := req.Header.Get("Mcp-Session-Id"); id != "" {
		h.sessionsMu.Lock()
		session, _ = h.sessions[id]
		h.sessionsMu.Unlock()
		if session == nil {
			http.Error(w, "session not found", http.StatusNotFound)
			return
		}
	}

	// TODO(rfindley): simplify the locking so that each request has only one
	// critical section.
	if req.Method == http.MethodDelete {
		if session == nil {
			// => Mcp-Session-Id was not set; else we'd have returned NotFound above.
			http.Error(w, "DELETE requires an Mcp-Session-Id header", http.StatusBadRequest)
			return
		}
		h.sessionsMu.Lock()
		delete(h.sessions, session.id)
		h.sessionsMu.Unlock()
		session.Close()
		w.WriteHeader(http.StatusNoContent)
		return
	}

	switch req.Method {
	case http.MethodPost, http.MethodGet:
	default:
		w.Header().Set("Allow", "GET, POST")
		http.Error(w, "unsupported method", http.StatusMethodNotAllowed)
		return
	}

	if session == nil {
		s := NewStreamableServerTransport(rand.Text())
		server := h.getServer(req)
		// Pass req.Context() here, to allow middleware to add context values.
		// The context is detached in the jsonrpc2 library when handling the
		// long-running stream.
		if _, err := server.Connect(req.Context(), s); err != nil {
			http.Error(w, "failed connection", http.StatusInternalServerError)
			return
		}
		h.sessionsMu.Lock()
		h.sessions[s.id] = s
		h.sessionsMu.Unlock()
		session = s
	}

	session.ServeHTTP(w, req)
}

// NewStreamableServerTransport returns a new [StreamableServerTransport] with
// the given session ID.
//
// A StreamableServerTransport implements the server-side of the streamable
// transport.
//
// TODO(rfindley): consider adding options here, to configure event storage
// policy.
func NewStreamableServerTransport(sessionID string) *StreamableServerTransport {
	return &StreamableServerTransport{
		id:               sessionID,
		incoming:         make(chan JSONRPCMessage, 10),
		done:             make(chan struct{}),
		outgoingMessages: make(map[streamID][]*streamableMsg),
		signals:          make(map[streamID]chan struct{}),
		requestStreams:   make(map[JSONRPCID]streamID),
		streamRequests:   make(map[streamID]map[JSONRPCID]struct{}),
	}
}

// A StreamableServerTransport implements the [Transport] interface for a
// single session.
type StreamableServerTransport struct {
	nextStreamID atomic.Int64 // incrementing next stream ID

	id       string
	incoming chan JSONRPCMessage // messages from the client to the server

	mu sync.Mutex

	// Sessions are closed exactly once.
	isDone bool
	done   chan struct{}

	// Sessions can have multiple logical connections, corresponding to HTTP
	// requests. Additionally, logical sessions may be resumed by subsequent HTTP
	// requests, when the session is terminated unexpectedly.
	//
	// Therefore, we use a logical connection ID to key the connection state, and
	// perform the accounting described below when incoming HTTP requests are
	// handled.
	//
	// The accounting is complicated. It is tempting to merge some of the maps
	// below, but they each have different lifecycles, as indicated by Lifecycle:
	// comments.
	//
	// TODO(rfindley): simplify.

	// outgoingMessages is the collection of outgoingMessages messages, keyed by the logical
	// stream ID where they should be delivered.
	//
	// streamID 0 is used for messages that don't correlate with an incoming
	// request.
	//
	// Lifecycle: outgoingMessages persists for the duration of the session.
	//
	// TODO(rfindley): garbage collect this data. For now, we save all outgoingMessages
	// messages for the lifespan of the transport.
	outgoingMessages map[streamID][]*streamableMsg

	// signals maps a logical stream ID to a 1-buffered channel, owned by an
	// incoming HTTP request, that signals that there are messages available to
	// write into the HTTP response. Signals guarantees that at most one HTTP
	// response can receive messages for a logical stream. After claiming
	// the stream, incoming requests should read from outgoing, to ensure
	// that no new messages are missed.
	//
	// Lifecycle: signals persists for the duration of an HTTP POST or GET
	// request for the given streamID.
	signals map[streamID]chan struct{}

	// requestStreams maps incoming requests to their logical stream ID.
	//
	// Lifecycle: requestStreams persists for the duration of the session.
	//
	// TODO(rfindley): clean up once requests are handled.
	requestStreams map[JSONRPCID]streamID

	// outstandingRequests tracks the set of unanswered incoming RPCs for each logical
	// stream.
	//
	// When the server has responded to each request, the stream should be
	// closed.
	//
	// Lifecycle: outstandingRequests values persist as until the requests have been
	// replied to by the server. Notably, NOT until they are sent to an HTTP
	// response, as delivery is not guaranteed.
	streamRequests map[streamID]map[JSONRPCID]struct{}
}

type streamID int64

// a streamableMsg is an SSE event with an index into its logical stream.
type streamableMsg struct {
	idx   int
	event event
}

// Connect implements the [Transport] interface.
//
// TODO(rfindley): Connect should return a new object.
func (s *StreamableServerTransport) Connect(context.Context) (Connection, error) {
	return s, nil
}

// We track the incoming request ID inside the handler context using
// idContextValue, so that notifications and server->client calls that occur in
// the course of handling incoming requests are correlated with the incoming
// request that caused them, and can be dispatched as server-sent events to the
// correct HTTP request.
//
// Currently, this is implemented in [ServerSession.handle]. This is not ideal,
// because it means that a user of the MCP package couldn't implement the
// streamable transport, as they'd lack this priviledged access.
//
// If we ever wanted to expose this mechanism, we have a few options:
//  1. Make ServerSession an interface, and provide an implementation of
//     ServerSession to handlers that closes over the incoming request ID.
//  2. Expose a 'HandlerTransport' interface that allows transports to provide
//     a handler middleware, so that we don't hard-code this behavior in
//     ServerSession.handle.
//  3. Add a `func ForRequest(context.Context) JSONRPCID` accessor that lets
//     any transport access the incoming request ID.
//
// For now, by giving only the StreamableServerTransport access to the request
// ID, we avoid having to make this API decision.
type idContextKey struct{}

// ServeHTTP handles a single HTTP request for the session.
func (t *StreamableServerTransport) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	switch req.Method {
	case http.MethodGet:
		t.serveGET(w, req)
	case http.MethodPost:
		t.servePOST(w, req)
	default:
		// Should not be reached, as this is checked in StreamableHTTPHandler.ServeHTTP.
		w.Header().Set("Allow", "GET, POST")
		http.Error(w, "unsupported method", http.StatusMethodNotAllowed)
	}
}

func (t *StreamableServerTransport) serveGET(w http.ResponseWriter, req *http.Request) {
	// connID 0 corresponds to the default GET request.
	id, nextIdx := streamID(0), 0
	if len(req.Header.Values("Last-Event-ID")) > 0 {
		eid := req.Header.Get("Last-Event-ID")
		var ok bool
		id, nextIdx, ok = parseEventID(eid)
		if !ok {
			http.Error(w, fmt.Sprintf("malformed Last-Event-ID %q", eid), http.StatusBadRequest)
			return
		}
		nextIdx++
	}

	t.mu.Lock()
	if _, ok := t.signals[id]; ok {
		http.Error(w, "stream ID conflicts with ongoing stream", http.StatusBadRequest)
		t.mu.Unlock()
		return
	}
	signal := make(chan struct{}, 1)
	t.signals[id] = signal
	t.mu.Unlock()

	t.streamResponse(w, req, id, nextIdx, signal)
}

func (t *StreamableServerTransport) servePOST(w http.ResponseWriter, req *http.Request) {
	if len(req.Header.Values("Last-Event-ID")) > 0 {
		http.Error(w, "can't send Last-Event-ID for POST request", http.StatusBadRequest)
		return
	}

	// Read incoming messages.
	body, err := io.ReadAll(req.Body)
	if err != nil {
		http.Error(w, "failed to read body", http.StatusBadRequest)
		return
	}
	if len(body) == 0 {
		http.Error(w, "POST requires a non-empty body", http.StatusBadRequest)
		return
	}
	incoming, _, err := readBatch(body)
	if err != nil {
		http.Error(w, fmt.Sprintf("malformed payload: %v", err), http.StatusBadRequest)
		return
	}
	var requests = make(map[JSONRPCID]struct{})
	for _, msg := range incoming {
		if req, ok := msg.(*JSONRPCRequest); ok && req.ID.IsValid() {
			requests[req.ID] = struct{}{}
		}
	}

	// Update accounting for this request.
	id := streamID(t.nextStreamID.Add(1))
	signal := make(chan struct{}, 1)
	t.mu.Lock()
	if len(requests) > 0 {
		t.streamRequests[id] = make(map[JSONRPCID]struct{})
	}
	for reqID := range requests {
		t.requestStreams[reqID] = id
		t.streamRequests[id][reqID] = struct{}{}
	}
	t.signals[id] = signal
	t.mu.Unlock()

	// Publish incoming messages.
	for _, msg := range incoming {
		t.incoming <- msg
	}

	// TODO(rfindley): consider optimizing for a single incoming request, by
	// responding with application/json when there is only a single message in
	// the response.
	t.streamResponse(w, req, id, 0, signal)
}

func (t *StreamableServerTransport) streamResponse(w http.ResponseWriter, req *http.Request, id streamID, nextIndex int, signal chan struct{}) {
	defer func() {
		t.mu.Lock()
		delete(t.signals, id)
		t.mu.Unlock()
	}()

	// Stream resumption: adjust outgoing index based on what the user says
	// they've received.
	if nextIndex > 0 {
		t.mu.Lock()
		// Clamp nextIndex to outgoing messages.
		outgoing := t.outgoingMessages[id]
		if nextIndex > len(outgoing) {
			nextIndex = len(outgoing)
		}
		t.mu.Unlock()
	}

	w.Header().Set("Mcp-Session-Id", t.id)
	w.Header().Set("Content-Type", "text/event-stream") // Accept checked in [StreamableHTTPHandler]
	w.Header().Set("Cache-Control", "no-cache, no-transform")
	w.Header().Set("Connection", "keep-alive")

	writes := 0
stream:
	for {
		// Send outgoing messages
		t.mu.Lock()
		outgoing := t.outgoingMessages[id][nextIndex:]
		t.mu.Unlock()

		for _, msg := range outgoing {
			if _, err := writeEvent(w, msg.event); err != nil {
				// Connection closed or broken.
				return
			}
			writes++
			nextIndex++
		}

		t.mu.Lock()
		nOutstanding := len(t.streamRequests[id])
		nOutgoing := len(t.outgoingMessages[id])
		t.mu.Unlock()
		// If all requests have been handled and replied to, we can terminate this
		// connection. However, in the case of a sequencing violation from the server
		// (a send on the request context after the request has been handled), we
		// loop until we've written all messages.
		//
		// TODO(rfindley): should we instead refuse to send messages after the last
		// response? Decide, write a test, and change the behavior.
		if nextIndex < nOutgoing {
			continue // more to send
		}
		if req.Method == http.MethodPost && nOutstanding == 0 {
			if writes == 0 {
				// Spec: If the server accepts the input, the server MUST return HTTP
				// status code 202 Accepted with no body.
				w.WriteHeader(http.StatusAccepted)
			}
			return
		}

		select {
		case <-signal:
		case <-t.done:
			if writes == 0 {
				http.Error(w, "session terminated", http.StatusGone)
			}
			break stream
		case <-req.Context().Done():
			if writes == 0 {
				w.WriteHeader(http.StatusNoContent)
			}
			break stream
		}
	}
}

// Event IDs: encode both the logical connection ID and the index, as
// <streamID>_<idx>, to be consistent with the typescript implementation.

// formatEventID returns the event ID to use for the logical connection ID
// streamID and message index idx.
//
// See also [parseEventID].
func formatEventID(sid streamID, idx int) string {
	return fmt.Sprintf("%d_%d", sid, idx)
}

// parseEventID parses a Last-Event-ID value into a logical stream id and
// index.
//
// See also [formatEventID].
func parseEventID(eventID string) (sid streamID, idx int, ok bool) {
	parts := strings.Split(eventID, "_")
	if len(parts) != 2 {
		return 0, 0, false
	}
	stream, err := strconv.ParseInt(parts[0], 10, 64)
	if err != nil || stream < 0 {
		return 0, 0, false
	}
	idx, err = strconv.Atoi(parts[1])
	if err != nil || idx < 0 {
		return 0, 0, false
	}
	return streamID(stream), idx, true
}

// Read implements the [Connection] interface.
func (t *StreamableServerTransport) Read(ctx context.Context) (JSONRPCMessage, error) {
	select {
	case <-ctx.Done():
		return nil, ctx.Err()
	case msg, ok := <-t.incoming:
		if !ok {
			return nil, io.EOF
		}
		return msg, nil
	case <-t.done:
		return nil, io.EOF
	}
}

// Write implements the [Connection] interface.
func (t *StreamableServerTransport) Write(ctx context.Context, msg JSONRPCMessage) error {
	// Find the incoming request that this write relates to, if any.
	var forRequest, replyTo JSONRPCID
	if resp, ok := msg.(*JSONRPCResponse); ok {
		// If the message is a response, it relates to its request (of course).
		forRequest = resp.ID
		replyTo = resp.ID
	} else {
		// Otherwise, we check to see if it request was made in the context of an
		// ongoing request. This may not be the case if the request way made with
		// an unrelated context.
		if v := ctx.Value(idContextKey{}); v != nil {
			forRequest = v.(JSONRPCID)
		}
	}

	// Find the logical connection corresponding to this request.
	//
	// For messages sent outside of a request context, this is the default
	// connection 0.
	var forConn streamID
	if forRequest.IsValid() {
		t.mu.Lock()
		forConn = t.requestStreams[forRequest]
		t.mu.Unlock()
	}

	data, err := jsonrpc2.EncodeMessage(msg)
	if err != nil {
		return err
	}

	t.mu.Lock()
	defer t.mu.Unlock()
	if t.isDone {
		return fmt.Errorf("session is closed") // TODO: should this be EOF?
	}

	if _, ok := t.streamRequests[forConn]; !ok && forConn != 0 {
		// No outstanding requests for this connection, which means it is logically
		// done. This is a sequencing violation from the server, so we should report
		// a side-channel error here. Put the message on the general queue to avoid
		// dropping messages.
		forConn = 0
	}

	idx := len(t.outgoingMessages[forConn])
	t.outgoingMessages[forConn] = append(t.outgoingMessages[forConn], &streamableMsg{
		idx: idx,
		event: event{
			name: "message",
			id:   formatEventID(forConn, idx),
			data: data,
		},
	})
	if replyTo.IsValid() {
		// Once we've put the reply on the queue, it's no longer outstanding.
		delete(t.streamRequests[forConn], replyTo)
		if len(t.streamRequests[forConn]) == 0 {
			delete(t.streamRequests, forConn)
		}
	}

	// Signal work.
	if c, ok := t.signals[forConn]; ok {
		select {
		case c <- struct{}{}:
		default:
		}
	}
	return nil
}

// Close implements the [Connection] interface.
func (t *StreamableServerTransport) Close() error {
	t.mu.Lock()
	defer t.mu.Unlock()
	if !t.isDone {
		t.isDone = true
		close(t.done)
	}
	return nil
}

// A StreamableClientTransport is a [Transport] that can communicate with an MCP
// endpoint serving the streamable HTTP transport defined by the 2025-03-26
// version of the spec.
//
// TODO(rfindley): support retries and resumption tokens.
type StreamableClientTransport struct {
	url  string
	opts StreamableClientTransportOptions
}

// 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
}

// NewStreamableClientTransport returns a new client transport that connects to
// the streamable HTTP server at the provided URL.
func NewStreamableClientTransport(url string, opts *StreamableClientTransportOptions) *StreamableClientTransport {
	t := &StreamableClientTransport{url: url}
	if opts != nil {
		t.opts = *opts
	}
	return t
}

// Connect implements the [Transport] interface.
//
// The resulting [Connection] writes messages via POST requests to the
// transport URL with the Mcp-Session-Id header set, and reads messages from
// hanging requests.
//
// When closed, the connection issues a DELETE request to terminate the logical
// session.
func (t *StreamableClientTransport) Connect(ctx context.Context) (Connection, error) {
	client := t.opts.HTTPClient
	if client == nil {
		client = http.DefaultClient
	}
	return &streamableClientConn{
		url:      t.url,
		client:   client,
		incoming: make(chan []byte, 100),
		done:     make(chan struct{}),
	}, nil
}

type streamableClientConn struct {
	url       string
	sessionID string
	client    *http.Client
	incoming  chan []byte
	done      chan struct{}

	closeOnce sync.Once
	closeErr  error

	mu sync.Mutex
	// bodies map[*http.Response]io.Closer
	err error
}

// Read implements the [Connection] interface.
func (s *streamableClientConn) Read(ctx context.Context) (JSONRPCMessage, error) {
	select {
	case <-ctx.Done():
		return nil, ctx.Err()
	case <-s.done:
		return nil, io.EOF
	case data := <-s.incoming:
		return jsonrpc2.DecodeMessage(data)
	}
}

// Write implements the [Connection] interface.
func (s *streamableClientConn) Write(ctx context.Context, msg JSONRPCMessage) error {
	s.mu.Lock()
	if s.err != nil {
		s.mu.Unlock()
		return s.err
	}

	sessionID := s.sessionID
	if sessionID == "" {
		// Hold lock for the first request.
		defer s.mu.Unlock()
	} else {
		s.mu.Unlock()
	}

	gotSessionID, err := s.postMessage(ctx, sessionID, msg)
	if err != nil {
		if sessionID != "" {
			// unlocked; lock to set err
			s.mu.Lock()
			defer s.mu.Unlock()
		}
		if s.err != nil {
			s.err = err
		}
		return err
	}

	if sessionID == "" {
		// locked
		s.sessionID = gotSessionID
	}

	return nil
}

func (s *streamableClientConn) postMessage(ctx context.Context, sessionID string, msg JSONRPCMessage) (string, error) {
	data, err := jsonrpc2.EncodeMessage(msg)
	if err != nil {
		return "", err
	}

	req, err := http.NewRequestWithContext(ctx, http.MethodPost, s.url, bytes.NewReader(data))
	if err != nil {
		return "", err
	}
	if sessionID != "" {
		req.Header.Set("Mcp-Session-Id", sessionID)
	}
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("Accept", "application/json, text/event-stream")

	resp, err := s.client.Do(req)
	if err != nil {
		return "", err
	}

	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		// TODO: do a best effort read of the body here, and format it in the error.
		resp.Body.Close()
		return "", fmt.Errorf("broken session: %v", resp.Status)
	}

	sessionID = resp.Header.Get("Mcp-Session-Id")
	if resp.Header.Get("Content-Type") == "text/event-stream" {
		go s.handleSSE(resp)
	} else {
		resp.Body.Close()
	}
	return sessionID, nil
}

func (s *streamableClientConn) handleSSE(resp *http.Response) {
	defer resp.Body.Close()

	done := make(chan struct{})
	go func() {
		defer close(done)
		for evt, err := range scanEvents(resp.Body) {
			if err != nil {
				// TODO: surface this error; possibly break the stream
				return
			}
			s.incoming <- evt.data
		}
	}()

	select {
	case <-s.done:
	case <-done:
	}
}

// Close implements the [Connection] interface.
func (s *streamableClientConn) Close() error {
	s.closeOnce.Do(func() {
		close(s.done)

		req, err := http.NewRequest(http.MethodDelete, s.url, nil)
		if err != nil {
			s.closeErr = err
		} else {
			req.Header.Set("Mcp-Session-Id", s.sessionID)
			if _, err := s.client.Do(req); err != nil {
				s.closeErr = err
			}
		}
	})
	return s.closeErr
}