19. Accessor Properties in Interfaces

TypeScript interfaces can define getter/setter signatures, not just data properties.

File: packages/agent/src/types.ts L253-277

export interface AgentState {
  systemPrompt: string;
  model: Model<any>;
  thinkingLevel: ThinkingLevel;

  /** Assigning a new array copies the top-level array. */
  set tools(tools: AgentTool<any>[]);
  get tools(): AgentTool<any>[];

  /** Assigning a new array copies the top-level array. */
  set messages(messages: AgentMessage[]);
  get messages(): AgentMessage[];

  /** Read-only properties */
  readonly isStreaming: boolean;
  readonly streamingMessage?: AgentMessage;
  readonly pendingToolCalls: ReadonlySet<string>;
}

set/get in an interface means implementing classes can intercept assignment (e.g., to copy arrays)
readonly fields cannot be assigned from outside
ReadonlySet<string> prevents external mutation of the set