Function truncateHeadForPTLRetry

src/services/compact/compact.ts:243–291

(
  messages: Message[],
  ptlResponse: AssistantMessage,
)

 * that wasn't migrated in bfdb472f's unification.
 */
export function truncateHeadForPTLRetry(
  messages: Message[],
  ptlResponse: AssistantMessage,
): Message[] | null {
  // Strip our own synthetic marker from a previous retry before grouping.
  // Otherwise it becomes its own group 0 and the 20% fallback stalls
  // (drops only the marker, re-adds it, zero progress on retry 2+).
  const input =
    messages[0]?.type === 'user' &&
    messages[0].isMeta &&
    messages[0].message.content === PTL_RETRY_MARKER
      ? messages.slice(1)
      : messages

  const groups = groupMessagesByApiRound(input)
  if (groups.length < 2) return null

  const tokenGap = getPromptTooLongTokenGap(ptlResponse)
  let dropCount: number
  if (tokenGap !== undefined) {
    let acc = 0
    dropCount = 0
    for (const g of groups) {
      acc += roughTokenCountEstimationForMessages(g)
      dropCount++
      if (acc >= tokenGap) break
    }
  } else {
    dropCount = Math.max(1, Math.floor(groups.length * 0.2))
  }

  // Keep at least one group so there's something to summarize.
  dropCount = Math.min(dropCount, groups.length - 1)
  if (dropCount < 1) return null

  const sliced = groups.slice(dropCount).flat()
  // groupMessagesByApiRound puts the preamble in group 0 and starts every
  // subsequent group with an assistant message. Dropping group 0 leaves an
  // assistant-first sequence which the API rejects (first message must be
  // role=user). Prepend a synthetic user marker — ensureToolResultPairing
  // already handles any orphaned tool_results this creates.
  if (sliced[0]?.type === 'assistant') {
    return [
      createUserMessage({ content: PTL_RETRY_MARKER, isMeta: true }),
      ...sliced,
    ]
  }
  return sliced
}

export const ERROR_MESSAGE_PROMPT_TOO_LONG =
  'Conversation too long. Press esc twice to go up a few messages and try again.'