Function modelMessageToUIMessage

packages/ai/src/activities/chat/messages.ts:402–463

(
  modelMessage: ModelMessage,
  id?: string,
)

 * @returns A UIMessage with parts
 */
export function modelMessageToUIMessage(
  modelMessage: ModelMessage,
  id?: string,
): UIMessage {
  const parts: Array<MessagePart> = []

  if (modelMessage.role === 'assistant' && modelMessage.thinking?.length) {
    for (const thinking of modelMessage.thinking) {
      if (!thinking.content) continue
      parts.push({
        type: 'thinking',
        content: thinking.content,
        ...(thinking.signature && { signature: thinking.signature }),
      })
    }
  }

  // Handle tool results (when role is "tool") - only produce tool-result part,
  // not a text part (the content IS the tool result, not display text)
  if (modelMessage.role === 'tool' && modelMessage.toolCallId) {
    parts.push({
      type: 'tool-result',
      toolCallId: modelMessage.toolCallId,
      content: getTextContent(modelMessage.content),
      state: 'complete',
    })
  } else if (Array.isArray(modelMessage.content)) {
    // Multimodal content - preserve all content parts as MessageParts
    for (const part of modelMessage.content) {
      parts.push(part)
    }
  } else {
    // String or null content
    const textContent = getTextContent(modelMessage.content)
    if (textContent) {
      parts.push({
        type: 'text',
        content: textContent,
      })
    }
  }

  // Handle tool calls
  if (modelMessage.toolCalls && modelMessage.toolCalls.length > 0) {
    for (const toolCall of modelMessage.toolCalls) {
      parts.push({
        type: 'tool-call',
        id: toolCall.id,
        name: toolCall.function.name,
        arguments: toolCall.function.arguments,
        state: 'input-complete', // Model messages have complete arguments
        ...(toolCall.metadata !== undefined && { metadata: toolCall.metadata }),
      })
    }
  }

  return {
    id: id || generateMessageId(),