- 28 predefined error codes with semantic categories
- ErrorCode namespace with helper functions:
  - all(), isValid(), getCategory()
  - isRecoverable(), getHttpStatus(), getJsonRpcCode()
- McpError fluent builder with 15+ factory methods
- ErrorBag collection for multiple errors
- Full TypeScript support with strict types
- Dual ESM/CommonJS builds
- 100% test coverage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>