fix: reject duplicate initialize requests (#962)

## Summary
- reject a second `initialize` request on an already initialized server
session
- keep the original `ServerSession.InitializeParams()` instead of
replacing it with later client parameters
- add a raw JSON-RPC regression test for the duplicate-initialize path

Fixes #961.

## To verify
- `go test ./mcp -run TestServerRejectsDuplicateInitialize -count=1`
- `go test ./mcp -count=1`
- `go test ./...`

---------

Co-authored-by: guglielmoc <guglielmoc@google.com>

mcp/server.go

@@ -1488,9 +1488,17 @@ func (ss *ServerSession) initialize(ctx context.Context, params *InitializeParam
 	if params == nil {
 		return nil, fmt.Errorf("%w: \"params\" must be be provided", jsonrpc2.ErrInvalidParams)
 	}
+	var wasInit bool
 	ss.updateState(func(state *ServerSessionState) {
-		state.InitializeParams = params
+		wasInit = state.InitializeParams != nil
+		if !wasInit {
+			state.InitializeParams = params
+		}
 	})
+	if wasInit {
+		ss.server.opts.Logger.Error("duplicate initialize request")
+		return nil, fmt.Errorf("duplicate %q received", methodInitialize)
+	}
 
 	s := ss.server
 	return &InitializeResult{

mcp/server_test.go

@@ -825,6 +825,92 @@ func TestClientRootCapabilities(t *testing.T) {
 	}
 }
 
+func TestServerRejectsDuplicateInitialize(t *testing.T) {
+	ctx := context.Background()
+
+	server := NewServer(&Implementation{Name: "testServer", Version: "v1.0.0"}, nil)
+	cTransport, sTransport := NewInMemoryTransports()
+	ss, err := server.Connect(ctx, sTransport, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ss.Close()
+
+	cConn, err := cTransport.Connect(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cConn.Close()
+
+	firstParams := json.RawMessage(`{
+		"protocolVersion": "2025-11-25",
+		"clientInfo": {"name": "first-client", "version": "1.0.0"}
+	}`)
+	firstReq, err := jsonrpc2.NewCall(jsonrpc2.Int64ID(1), methodInitialize, firstParams)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := cConn.Write(ctx, firstReq); err != nil {
+		t.Fatalf("first initialize write failed: %v", err)
+	}
+	msg, err := cConn.Read(ctx)
+	if err != nil {
+		t.Fatalf("first initialize read failed: %v", err)
+	}
+	resp, ok := msg.(*jsonrpc2.Response)
+	if !ok {
+		t.Fatalf("expected Response, got %T", msg)
+	}
+	if resp.Error != nil {
+		t.Fatalf("first initialize failed: %v", resp.Error)
+	}
+
+	initializedReq, err := jsonrpc2.NewNotification(notificationInitialized, &InitializedParams{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := cConn.Write(ctx, initializedReq); err != nil {
+		t.Fatalf("initialized notification write failed: %v", err)
+	}
+
+	secondParams := json.RawMessage(`{
+		"protocolVersion": "2024-11-05",
+		"clientInfo": {"name": "second-client", "version": "2.0.0"}
+	}`)
+	secondReq, err := jsonrpc2.NewCall(jsonrpc2.Int64ID(2), methodInitialize, secondParams)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := cConn.Write(ctx, secondReq); err != nil {
+		t.Fatalf("second initialize write failed: %v", err)
+	}
+	msg, err = cConn.Read(ctx)
+	if err != nil {
+		t.Fatalf("second initialize read failed: %v", err)
+	}
+	resp, ok = msg.(*jsonrpc2.Response)
+	if !ok {
+		t.Fatalf("expected Response, got %T", msg)
+	}
+	if resp.Error == nil {
+		t.Fatal("second initialize unexpectedly succeeded")
+	}
+	if !strings.Contains(resp.Error.Error(), `duplicate "initialize" received`) {
+		t.Fatalf("second initialize error = %v, want duplicate initialize", resp.Error)
+	}
+
+	got := ss.InitializeParams()
+	if got == nil {
+		t.Fatal("InitializeParams is nil")
+	}
+	if got.ProtocolVersion != "2025-11-25" {
+		t.Fatalf("ProtocolVersion = %q, want first initialize value", got.ProtocolVersion)
+	}
+	if got.ClientInfo == nil || got.ClientInfo.Name != "first-client" {
+		t.Fatalf("ClientInfo = %#v, want first initialize value", got.ClientInfo)
+	}
+}
+
 // TODO: move this to tool_test.go
 func TestToolForSchemas(t *testing.T) {
 	// Validate that toolForErr handles schemas properly.