Function uiMessageToModelMessages

packages/ai/src/activities/chat/messages.ts:156–172

(
  uiMessage: UIMessage,
)

 * @returns An array of ModelMessages preserving part ordering
 */
export function uiMessageToModelMessages(
  uiMessage: UIMessage,
): Array<ModelMessage> {
  // Skip system messages - they're handled via systemPrompts, not ModelMessages
  if (uiMessage.role === 'system') {
    return []
  }

  // For non-assistant messages (user), use the simpler path since they
  // don't have tool calls or tool results to interleave
  if (uiMessage.role !== 'assistant') {
    return [buildUserOrToolMessage(uiMessage)]
  }

  // For assistant messages, walk parts in order to preserve interleaving
  return buildAssistantMessages(uiMessage)
}

/**
 * Build a single ModelMessage for user messages (simple path).