Add sentinel errors

Author: semistrict

server/elicitation.go

@@ -2,24 +2,31 @@ package server
 
 import (
 	"context"
-	"fmt"
+	"errors"
 
 	"github.com/mark3labs/mcp-go/mcp"
 )
 
+var (
+	// ErrNoActiveSession is returned when there is no active session in the context
+	ErrNoActiveSession = errors.New("no active session")
+	// ErrElicitationNotSupported is returned when the session does not support elicitation
+	ErrElicitationNotSupported = errors.New("session does not support elicitation")
+)
+
 // RequestElicitation sends an elicitation request to the client.
 // The client must have declared elicitation capability during initialization.
 // The session must implement SessionWithElicitation to support this operation.
 func (s *MCPServer) RequestElicitation(ctx context.Context, request mcp.ElicitationRequest) (*mcp.ElicitationResult, error) {
 	session := ClientSessionFromContext(ctx)
 	if session == nil {
-		return nil, fmt.Errorf("no active session")
+		return nil, ErrNoActiveSession
 	}
 
 	// Check if the session supports elicitation requests
 	if elicitationSession, ok := session.(SessionWithElicitation); ok {
 		return elicitationSession.RequestElicitation(ctx, request)
 	}
 
-	return nil, fmt.Errorf("session does not support elicitation")
+	return nil, ErrElicitationNotSupported
 }

server/elicitation_test.go

@@ -2,6 +2,7 @@ package server
 
 import (
 	"context"
+	"errors"
 	"testing"
 
 	"github.com/mark3labs/mcp-go/mcp"
@@ -75,9 +76,8 @@ func TestMCPServer_RequestElicitation_NoSession(t *testing.T) {
 		t.Error("expected error when no session available")
 	}
 
-	expectedError := "no active session"
-	if err.Error() != expectedError {
-		t.Errorf("expected error %q, got %q", expectedError, err.Error())
+	if !errors.Is(err, ErrNoActiveSession) {
+		t.Errorf("expected ErrNoActiveSession, got %v", err)
 	}
 }
 
@@ -105,9 +105,8 @@ func TestMCPServer_RequestElicitation_SessionDoesNotSupportElicitation(t *testin
 		t.Error("expected error when session doesn't support elicitation")
 	}
 
-	expectedError := "session does not support elicitation"
-	if err.Error() != expectedError {
-		t.Errorf("expected error %q, got %q", expectedError, err.Error())
+	if !errors.Is(err, ErrElicitationNotSupported) {
+		t.Errorf("expected ErrElicitationNotSupported, got %v", err)
 	}
 }
 
@@ -176,7 +175,7 @@ func TestRequestElicitation(t *testing.T) {
 		name          string
 		session       ClientSession
 		request       mcp.ElicitationRequest
-		expectedError string
+		expectedError error
 		expectedType  mcp.ElicitationResponseType
 	}{
 		{
@@ -234,7 +233,7 @@ func TestRequestElicitation(t *testing.T) {
 					RequestedSchema: map[string]any{"type": "object"},
 				},
 			},
-			expectedError: "session does not support elicitation",
+			expectedError: ErrElicitationNotSupported,
 		},
 	}
 
@@ -245,9 +244,9 @@ func TestRequestElicitation(t *testing.T) {
 
 			result, err := server.RequestElicitation(ctx, tt.request)
 
-			if tt.expectedError != "" {
+			if tt.expectedError != nil {
 				require.Error(t, err)
-				assert.Contains(t, err.Error(), tt.expectedError)
+				assert.True(t, errors.Is(err, tt.expectedError), "expected %v, got %v", tt.expectedError, err)
 				return
 			}