8. Mapped Types & Index Access

Access nested types with [keyof T] and typeof to derive types from runtime objects.

File: packages/agent/src/types.ts L245

export type AgentMessage = Message | CustomAgentMessages[keyof CustomAgentMessages];

This creates a union of all value types in the CustomAgentMessages interface using indexed access.

Deeply nested access:

File: packages/ai/src/models.ts L32-36

export function getModels<TProvider extends KnownProvider>(provider: TProvider): Model<ModelApi<TProvider, keyof (typeof MODELS)[TProvider]>>[] {
  // ...
}

Chains typeof MODELS → index by TProvider → keyof to get all model IDs for a given provider.

Also see Extract on union content types:

File: packages/agent/src/types.ts L38

export type AgentToolCall = Extract<AssistantMessage["content"][number], { type: "toolCall" }>;

[number] indexes into an array type to get the element type, then Extract filters to the specific union member.