Function buildForkedMessages

src/tools/AgentTool/forkSubagent.ts:107–169

(
  directive: string,
  assistantMessage: AssistantMessage,
)

 * Only the final text block differs per child, maximizing cache hits.
 */
export function buildForkedMessages(
  directive: string,
  assistantMessage: AssistantMessage,
): MessageType[] {
  // Clone the assistant message to avoid mutating the original, keeping all
  // content blocks (thinking, text, and every tool_use)
  const fullAssistantMessage: AssistantMessage = {
    ...assistantMessage,
    uuid: randomUUID(),
    message: {
      ...assistantMessage.message,
      content: [...assistantMessage.message.content],
    },
  }

  // Collect all tool_use blocks from the assistant message
  const toolUseBlocks = assistantMessage.message.content.filter(
    (block): block is BetaToolUseBlock => block.type === 'tool_use',
  )

  if (toolUseBlocks.length === 0) {
    logForDebugging(
      `No tool_use blocks found in assistant message for fork directive: ${directive.slice(0, 50)}...`,
      { level: 'error' },
    )
    return [
      createUserMessage({
        content: [
          { type: 'text' as const, text: buildChildMessage(directive) },
        ],
      }),
    ]
  }

  // Build tool_result blocks for every tool_use, all with identical placeholder text
  const toolResultBlocks = toolUseBlocks.map(block => ({
    type: 'tool_result' as const,
    tool_use_id: block.id,
    content: [
      {
        type: 'text' as const,
        text: FORK_PLACEHOLDER_RESULT,
      },
    ],
  }))

  // Build a single user message: all placeholder tool_results + the per-child directive
  // TODO(smoosh): this text sibling creates a [tool_result, text] pattern on the wire
  // (renders as </function_results>\n\nHuman:<text>). One-off per-child construction,
  // not a repeated teacher, so low-priority. If we ever care, use smooshIntoToolResult
  // from src/utils/messages.ts to fold the directive into the last tool_result.content.
  const toolResultMessage = createUserMessage({
    content: [
      ...toolResultBlocks,
      {
        type: 'text' as const,
        text: buildChildMessage(directive),
      },