1. Union & Literal Types | Abhimanyu Aryan

String literal union types restrict a value to a known set of strings while allowing extensibility via (string & {}).

File: packages/ai/src/types.ts L5-17

export type KnownApi =
  | "openai-completions"
  | "mistral-conversations"
  | "openai-responses"
  | "anthropic-messages"
  | "bedrock-converse-stream"
  | "google-generative-ai";

// Allow known strings with autocomplete, but also accept arbitrary strings
export type Api = KnownApi | (string & {});

The (string & {}) trick preserves autocomplete for known values while accepting any string at runtime.

Also see ThinkingLevel and StopReason:

File: packages/ai/src/types.ts L45, L192

export type ThinkingLevel = "minimal" | "low" | "medium" | "high" | "xhigh";
export type StopReason = "stop" | "length" | "toolUse" | "error" | "aborted";