Function mergeUserContentBlocks

src/utils/messages.ts:2600–2647

(
  a: ContentBlockParam[],
  b: ContentBlockParam[],
)

}

export function mergeUserContentBlocks(
  a: ContentBlockParam[],
  b: ContentBlockParam[],
): ContentBlockParam[] {
  // See https://anthropic.slack.com/archives/C06FE2FP0Q2/p1747586370117479 and
  // https://anthropic.slack.com/archives/C0AHK9P0129/p1773159663856279:
  // any sibling after tool_result renders as </function_results>\n\nHuman:<...>
  // on the wire. Repeated mid-conversation, this teaches capy to emit Human: at
  // a bare tail → 3-token empty end_turn. A/B (sai-20260310-161901) validated:
  // smoosh into tool_result.content → 92% → 0%.
  const lastBlock = last(a)
  if (lastBlock?.type !== 'tool_result') {
    return [...a, ...b]
  }

  if (!checkStatsigFeatureGate_CACHED_MAY_BE_STALE('tengu_chair_sermon')) {
    // Legacy (ungated) smoosh: only string-content tool_result + all-text
    // siblings → joined string. Matches pre-universal-smoosh behavior on main.
    // The precondition guarantees smooshIntoToolResult hits its string path
    // (no tool_reference bail, string output shape preserved).
    if (
      typeof lastBlock.content === 'string' &&
      b.every(x => x.type === 'text')
    ) {
      const copy = a.slice()
      copy[copy.length - 1] = smooshIntoToolResult(lastBlock, b)!
      return copy
    }
    return [...a, ...b]
  }

  // Universal smoosh (gated): fold all non-tool_result block types (text,
  // image, document, search_result) into tool_result.content. tool_result
  // blocks stay as siblings (hoisted later by hoistToolResults).
  const toSmoosh = b.filter(x => x.type !== 'tool_result')
  const toolResults = b.filter(x => x.type === 'tool_result')
  if (toSmoosh.length === 0) {
    return [...a, ...b]
  }

  const smooshed = smooshIntoToolResult(lastBlock, toSmoosh)
  if (smooshed === null) {
    // tool_reference constraint — fall back to siblings
    return [...a, ...b]
  }

  return [...a.slice(0, -1), smooshed, ...toolResults]
}

// Sometimes the API returns empty messages (eg. "\n\n"). We need to filter these out,
// otherwise they will give an API error when we send them to the API next time we call query().