Function normalizeSystemPrompts

packages/ai/src/system-prompts.ts:70–98

(
  // Accept the wide public shape (`SystemPrompt<unknown>`) regardless of the
  // caller's `TMetadata`. Adapters know their own metadata shape; the
  // generic narrows the *output* so adapter code can read `p.metadata.X`
  // without an additional cast.
  prompts: ReadonlyArray<SystemPrompt> | undefined,
)

 * `"undefined"` into the model's system prompt with no signal.
 */
export function normalizeSystemPrompts<TMetadata = unknown>(
  // Accept the wide public shape (`SystemPrompt<unknown>`) regardless of the
  // caller's `TMetadata`. Adapters know their own metadata shape; the
  // generic narrows the *output* so adapter code can read `p.metadata.X`
  // without an additional cast.
  prompts: ReadonlyArray<SystemPrompt> | undefined,
): Array<NormalizedSystemPrompt<TMetadata>> {
  if (!prompts || prompts.length === 0) return []
  return prompts.map((p, i) => {
    if (typeof p === 'string') return { content: p }
    // Defence in depth: TypeScript narrows `p` to the object arm here, but
    // this function is a public API boundary that callers can reach via
    // plain JS or `as any`. Re-validate at runtime so we never stream a
    // literal `"undefined"` into the model.
    const candidate = p as unknown
    if (candidate === null || typeof candidate !== 'object') {
      throw new TypeError(
        `systemPrompts[${i}]: expected a string or { content, metadata? }, got ${candidate === null ? 'null' : typeof candidate}`,
      )
    }
    const { content } = candidate as { content?: unknown }
    if (typeof content !== 'string') {
      throw new TypeError(
        `systemPrompts[${i}]: content must be a string, got ${typeof content}`,
      )
    }
    return p as NormalizedSystemPrompt<TMetadata>
  })
}