mcp: allow requests before notifications/initialized

Upon a closer reading of the spec, it is OK for the server to response
to client requests before it has received notifications/initialized (as
long as it has received initialize). This effectively rolls back the fix
from #225.

Some hooks are left for enforcing strictness around server->client
requests prior to initialized. These will be revisited in subsequent
CLs.

For #395

Author: findleyr

mcp/mcp_test.go

@@ -1415,7 +1415,7 @@ func TestElicitationCapabilityDeclaration(t *testing.T) {
 			RequestedSchema: &jsonschema.Schema{Type: "object"},
 		})
 		if err != nil {
-			t.Errorf("elicitation should work when capability is declared, got error: %v", err)
+			t.Fatalf("elicitation should work when capability is declared, got error: %v", err)
 		}
 		if result.Action != "cancel" {
 			t.Errorf("got action %q, want %q", result.Action, "cancel")

mcp/server.go

@@ -844,6 +844,33 @@ func (ss *ServerSession) updateState(mut func(*ServerSessionState)) {
 	}
 }
 
+// hasInitialized reports whether the server has received the initialized
+// notification.
+//
+// TODO(findleyr): use this to prevent change notifications.
+func (ss *ServerSession) hasInitialized() bool {
+	ss.mu.Lock()
+	defer ss.mu.Unlock()
+	return ss.state.InitializedParams != nil
+}
+
+// checkInitialized returns a formatted error if the server has not yet
+// received the initialized notification.
+func (ss *ServerSession) checkInitialized(method string) error {
+	if !ss.hasInitialized() {
+		// TODO(rfindley): enable this check.
+		// Right now is is flaky, because server tests don't await the initialized notification.
+		// Perhaps requests should simply block until they have received the initialized notification
+
+		// if strings.HasPrefix(method, "notifications/") {
+		// 	return fmt.Errorf("must not send %q before %q is received", method, notificationInitialized)
+		// } else {
+		// 	return fmt.Errorf("cannot call %q before %q is received", method, notificationInitialized)
+		// }
+	}
+	return nil
+}
+
 func (ss *ServerSession) ID() string {
 	if c, ok := ss.mcpConn.(hasSessionID); ok {
 		return c.SessionID()
@@ -859,11 +886,17 @@ func (ss *ServerSession) Ping(ctx context.Context, params *PingParams) error {
 
 // ListRoots lists the client roots.
 func (ss *ServerSession) ListRoots(ctx context.Context, params *ListRootsParams) (*ListRootsResult, error) {
+	if err := ss.checkInitialized(methodListRoots); err != nil {
+		return nil, err
+	}
 	return handleSend[*ListRootsResult](ctx, methodListRoots, newServerRequest(ss, orZero[Params](params)))
 }
 
 // CreateMessage sends a sampling request to the client.
 func (ss *ServerSession) CreateMessage(ctx context.Context, params *CreateMessageParams) (*CreateMessageResult, error) {
+	if err := ss.checkInitialized(methodCreateMessage); err != nil {
+		return nil, err
+	}
 	if params == nil {
 		params = &CreateMessageParams{Messages: []*SamplingMessage{}}
 	}
@@ -877,6 +910,9 @@ func (ss *ServerSession) CreateMessage(ctx context.Context, params *CreateMessag
 
 // Elicit sends an elicitation request to the client asking for user input.
 func (ss *ServerSession) Elicit(ctx context.Context, params *ElicitParams) (*ElicitResult, error) {
+	if err := ss.checkInitialized(methodElicit); err != nil {
+		return nil, err
+	}
 	return handleSend[*ElicitResult](ctx, methodElicit, newServerRequest(ss, orZero[Params](params)))
 }
 
@@ -978,7 +1014,7 @@ func (ss *ServerSession) getConn() *jsonrpc2.Connection { return ss.conn }
 // handle invokes the method described by the given JSON RPC request.
 func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any, error) {
 	ss.mu.Lock()
-	initialized := ss.state.InitializedParams != nil
+	initialized := ss.state.InitializeParams != nil
 	ss.mu.Unlock()
 
 	// From the spec:

mcp/shared.go

@@ -346,9 +346,12 @@ func notifySessions[S Session, P Params](sessions []S, method string, params P)
 	if sessions == nil {
 		return
 	}
-	// TODO: make this timeout configurable, or call Notify asynchronously.
+	// TODO: make this timeout configurable, or call handleNotify asynchronously.
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
+
+	// TODO: there's a potential spec violation here, when the feature list
+	// changes before the session (client or server) is initialized.
 	for _, s := range sessions {
 		req := newRequest(s, params)
 		if err := handleNotify(ctx, method, req); err != nil {

mcp/testdata/conformance/server/lifecycle.txtar

@@ -5,6 +5,7 @@ See also modelcontextprotocol/go-sdk#225.
 
 -- client --
 { "jsonrpc":"2.0", "method": "notifications/initialized" }
+{ "jsonrpc": "2.0", "id": 2, "method": "tools/list" }
 {
   "jsonrpc": "2.0",
   "id": 1,
@@ -21,6 +22,14 @@ See also modelcontextprotocol/go-sdk#225.
 { "jsonrpc": "2.0", "id": 3, "method": "tools/list" }
 
 -- server --
+{
+	"jsonrpc": "2.0",
+	"id": 2,
+	"error": {
+		"code": 0,
+		"message": "method \"tools/list\" is invalid during session initialization"
+	}
+}
 {
 	"jsonrpc": "2.0",
 	"id": 1,
@@ -43,9 +52,8 @@ See also modelcontextprotocol/go-sdk#225.
 {
 	"jsonrpc": "2.0",
 	"id": 2,
-	"error": {
-		"code": 0,
-		"message": "method \"tools/list\" is invalid during session initialization"
+	"result": {
+		"tools": []
 	}
 }
 {