Message: UserMessage, AssistantMessage, ToolResultMessage) and custom app messages (CustomAgentMessages). AgentMessage extends via declaration merging (packages/agent/src/types.ts). AssistantMessage includes content array (TextContent, ThinkingContent, ToolCall), usage, stopReason, errorMessage, timestamp. ToolResultMessage includes toolCallId, toolName, content (text/image), details, isError.
Related: Primitives index, Chat stream event