mcp: don't persist failed streamable sessions (#605)

If a stateful streamable session fails to initialize, it is unusable.
Avoid allocating resources by closing the session immediately.

Also:
- Fix a bug where InitializeParams is not guarded with its mutex.
- Fix a resource leak where sessions are persisted with session id "",
even though they are unaddressable.
- Add a relevant benchmark, and update tests.

Fixes #578

mcp/server.go

@@ -1145,7 +1145,13 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
 	return handleReceive(ctx, ss, req)
 }
 
-func (ss *ServerSession) InitializeParams() *InitializeParams { return ss.state.InitializeParams }
+// InitializeParams returns the InitializeParams provided during the client's
+// initial connection.
+func (ss *ServerSession) InitializeParams() *InitializeParams {
+	ss.mu.Lock()
+	defer ss.mu.Unlock()
+	return ss.state.InitializeParams
+}
 
 func (ss *ServerSession) initialize(ctx context.Context, params *InitializeParams) (*InitializeResult, error) {
 	if params == nil {

mcp/streamable.go

@@ -324,10 +324,13 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 			logger:       h.opts.Logger,
 		}
 
+		// Sessions without a session ID are also stateless: there's no way to
+		// address them.
+		stateless := h.opts.Stateless || sessionID == ""
 		// To support stateless mode, we initialize the session with a default
 		// state, so that it doesn't reject subsequent requests.
 		var connectOpts *ServerSessionOptions
-		if h.opts.Stateless {
+		if stateless {
 			// Peek at the body to see if it is initialize or initialized.
 			// We want those to be handled as usual.
 			var hasInitialize, hasInitialized bool
@@ -405,7 +408,7 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 			transport: transport,
 		}
 
-		if h.opts.Stateless {
+		if stateless {
 			// Stateless mode: close the session when the request exits.
 			defer session.Close() // close the fake session after handling the request
 		} else {
@@ -424,6 +427,13 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 			h.mu.Lock()
 			h.sessions[transport.SessionID] = sessInfo
 			h.mu.Unlock()
+			defer func() {
+				// If initialization failed, clean up the session (#578).
+				if session.InitializeParams() == nil {
+					// Initialization failed.
+					session.Close()
+				}
+			}()
 		}
 	}
 

mcp/streamable_bench_test.go

@@ -6,9 +6,15 @@ package mcp_test
 
 import (
 	"context"
+	"flag"
+	"log"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"reflect"
+	"runtime"
+	"runtime/pprof"
+	"strings"
 	"testing"
 
 	"github.com/google/jsonschema-go/jsonschema"
@@ -65,3 +71,62 @@ func BenchmarkStreamableServing(b *testing.B) {
 		}
 	}
 }
+
+var streamableHeap = flag.String("streamable_heap", "", "if set, write streamable heap profiles with this prefix")
+
+func BenchmarkStreamableServing_BadSessions(b *testing.B) {
+	server := mcp.NewServer(&mcp.Implementation{Name: "server", Version: "v0.0.1"}, nil)
+
+	handler := mcp.NewStreamableHTTPHandler(func(r *http.Request) *mcp.Server {
+		return server
+	}, &mcp.StreamableHTTPOptions{JSONResponse: true})
+
+	httpServer := httptest.NewServer(handler)
+	defer httpServer.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	if *streamableHeap != "" {
+		writeHeap := func(file string) {
+			// GC a couple times to ensure accurate heap.
+			runtime.GC()
+			runtime.GC()
+			f, err := os.Create(file)
+			if err != nil {
+				log.Fatal("could not create memory profile: ", err)
+			}
+			defer func() {
+				if err := f.Close(); err != nil {
+					b.Errorf("writing heap file %q: %v", file, err)
+				}
+			}()
+			if err := pprof.Lookup("heap").WriteTo(f, 0); err != nil {
+				b.Errorf("could not write heap profile: %v", err)
+			}
+		}
+		writeHeap(*streamableHeap + ".before")
+		defer writeHeap(*streamableHeap + ".after")
+	}
+
+	b.ResetTimer()
+	for range b.N {
+		req, err := http.NewRequestWithContext(ctx, http.MethodPost, httpServer.URL, strings.NewReader("{}"))
+		if err != nil {
+			b.Fatal(err)
+		}
+		req.Header.Add("Accept", "application/json")
+		req.Header.Add("Accept", "text/event-stream")
+		resp, err := http.DefaultClient.Do(req)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if got, want := resp.StatusCode, http.StatusBadRequest; got != want {
+			b.Fatalf("POST got status %d, want %d", got, want)
+		}
+		if got := resp.Header.Get("Mcp-Session-Id"); got != "" {
+			b.Fatalf("POST got unexpected session ID")
+		}
+		resp.Body.Close()
+	}
+}

mcp/streamable_test.go

@@ -690,10 +690,11 @@ func TestStreamableServerTransport(t *testing.T) {
 	}
 
 	tests := []struct {
-		name     string
-		replay   bool // if set, use a MemoryEventStore to enable stream replay
-		tool     func(*testing.T, context.Context, *ServerSession)
-		requests []streamableRequest // http requests
+		name         string
+		replay       bool // if set, use a MemoryEventStore to enable stream replay
+		tool         func(*testing.T, context.Context, *ServerSession)
+		requests     []streamableRequest // http requests
+		wantSessions int                 // number of sessions expected after the test
 	}{
 		{
 			name: "basic",
@@ -707,6 +708,19 @@ func TestStreamableServerTransport(t *testing.T) {
 					wantMessages:   []jsonrpc.Message{resp(2, &CallToolResult{Content: []Content{}}, nil)},
 				},
 			},
+			wantSessions: 1,
+		},
+		{
+			name: "uninitialized",
+			requests: []streamableRequest{
+				{
+					method:             "POST",
+					messages:           []jsonrpc.Message{req(2, "tools/call", &CallToolParams{Name: "tool"})},
+					wantStatusCode:     http.StatusOK,
+					wantBodyContaining: "invalid during session initialization",
+				},
+			},
+			wantSessions: 0,
 		},
 		{
 			name: "accept headers",
@@ -741,6 +755,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					wantMessages:   []jsonrpc.Message{resp(4, &CallToolResult{Content: []Content{}}, nil)},
 				},
 			},
+			wantSessions: 1,
 		},
 		{
 			name: "protocol version headers",
@@ -756,6 +771,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					wantSessionID:      false,        // could be true, but shouldn't matter
 				},
 			},
+			wantSessions: 1,
 		},
 		{
 			name: "batch rejected on 2025-06-18",
@@ -775,6 +791,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					wantBodyContaining: "batch",
 				},
 			},
+			wantSessions: 1,
 		},
 		{
 			name: "batch accepted on 2025-03-26",
@@ -797,6 +814,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					},
 				},
 			},
+			wantSessions: 1,
 		},
 		{
 			name: "tool notification",
@@ -821,6 +839,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					},
 				},
 			},
+			wantSessions: 1,
 		},
 		{
 			name: "tool upcall",
@@ -853,6 +872,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					},
 				},
 			},
+			wantSessions: 1,
 		},
 		{
 			name: "background",
@@ -915,6 +935,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					headers: map[string][]string{"Accept": nil},
 				},
 			},
+			wantSessions: 0, // session deleted
 		},
 		{
 			name: "errors",
@@ -946,6 +967,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					})},
 				},
 			},
+			wantSessions: 0,
 		},
 	}
 
@@ -972,6 +994,9 @@ func TestStreamableServerTransport(t *testing.T) {
 			defer handler.closeAll()
 
 			testStreamableHandler(t, handler, test.requests)
+			if got := len(slices.Collect(server.Sessions())); got != test.wantSessions {
+				t.Errorf("after test, got %d sessions, want %d", got, test.wantSessions)
+			}
 		})
 	}
 }