Function smooshSystemReminderSiblings

src/utils/messages.ts:1835–1873

* Final pass: smoosh any ` `-prefixed text siblings into the * last tool_result of the same user message. Catches siblings from: * - PreToolUse hook additionalContext (Gap F: attachment between assistant and * tool_result → standalone push → mergeUserMessages → hoist → sibling)

(
  messages: (UserMessage | AssistantMessage)[],
)

 * Idempotent. Pure function of shape.
 */
function smooshSystemReminderSiblings(
  messages: (UserMessage | AssistantMessage)[],
): (UserMessage | AssistantMessage)[] {
  return messages.map(msg => {
    if (msg.type !== 'user') return msg
    const content = msg.message.content
    if (!Array.isArray(content)) return msg

    const hasToolResult = content.some(b => b.type === 'tool_result')
    if (!hasToolResult) return msg

    const srText: TextBlockParam[] = []
    const kept: ContentBlockParam[] = []
    for (const b of content) {
      if (b.type === 'text' && b.text.startsWith('<system-reminder>')) {
        srText.push(b)
      } else {
        kept.push(b)
      }
    }
    if (srText.length === 0) return msg

    // Smoosh into the LAST tool_result (positionally adjacent in rendered prompt)
    const lastTrIdx = kept.findLastIndex(b => b.type === 'tool_result')
    const lastTr = kept[lastTrIdx] as ToolResultBlockParam
    const smooshed = smooshIntoToolResult(lastTr, srText)
    if (smooshed === null) return msg // tool_ref constraint — leave alone

    const newContent = [
      ...kept.slice(0, lastTrIdx),
      smooshed,
      ...kept.slice(lastTrIdx + 1),
    ]
    return {
      ...msg,
      message: { ...msg.message, content: newContent },
    }
  })
}

/**
 * Strip non-text blocks from is_error tool_results — the API rejects the