Function buildAssistantMessages

packages/ai/src/activities/chat/messages.ts:229–388

* Build ModelMessages for an assistant UIMessage, preserving the * sequential interleaving of text, tool calls, and tool results. * * Walks parts in order. Text and tool-call parts accumulate into the * current "segment". When a tool-result part is encountered, the * current segment is flushed

(uiMessage: UIMessage)

 * result is emitted as a tool message.
 */
function buildAssistantMessages(uiMessage: UIMessage): Array<ModelMessage> {
  const messageList: Array<ModelMessage> = []
  let current = createSegment()
  let pendingThinking: Array<{ content: string; signature?: string }> = []

  // Track emitted tool result IDs to avoid duplicates.
  // A tool call can have BOTH an explicit tool-result part AND an output
  // field on the tool-call part. We only want one per tool call ID.
  const emittedToolResultIds = new Set<string>()

  function flushSegment(): void {
    const content = collapseContentParts(current.contentParts)
    const hasContent = content !== null
    const hasToolCalls = current.toolCalls.length > 0

    if (hasContent || hasToolCalls) {
      messageList.push({
        role: 'assistant',
        content,
        ...(hasToolCalls && { toolCalls: current.toolCalls }),
        ...(pendingThinking.length > 0 && { thinking: pendingThinking }),
      })
      pendingThinking = []
    }
    current = createSegment()
  }

  for (const part of uiMessage.parts) {
    switch (part.type) {
      case 'text':
      case 'image':
      case 'audio':
      case 'video':
      case 'document':
        current.contentParts.push(part)
        break

      case 'tool-call':
        if (isToolCallIncluded(part)) {
          current.toolCalls.push({
            id: part.id,
            type: 'function' as const,
            function: {
              name: part.name,
              arguments: part.arguments,
            },
            ...(part.metadata !== undefined && { metadata: part.metadata }),
          })
        }
        break

      case 'tool-result':
        // Flush the current assistant segment before emitting the tool result
        flushSegment()

        // Emit the tool result
        if (
          (part.state === 'complete' || part.state === 'error') &&