Function tokenCountWithEstimation

src/utils/tokens.ts:251–292

(messages: readonly Message[])

 * so every interleaved tool_result is included in the rough estimate.
 */
export function tokenCountWithEstimation(messages: readonly Message[]): number {
  let i = messages.length - 1
  while (i >= 0) {
    const message = messages[i]
    const usage = message ? getTokenUsage(message) : undefined
    if (message && usage) {
      // Walk back past any earlier sibling records split from the same API
      // response (same message.id) so interleaved tool_results between them
      // are included in the estimation slice.
      const responseId = getAssistantMessageId(message)
      if (responseId) {
        let j = i - 1
        while (j >= 0) {
          const prior = messages[j]
          const priorId = prior ? getAssistantMessageId(prior) : undefined
          if (priorId === responseId) {
            // Earlier split of the same API response — anchor here instead.
            i = j
          } else if (priorId !== undefined) {
            // Hit a different API response — stop walking.
            break
          }
          // priorId === undefined: a user/tool_result/attachment message,
          // possibly interleaved between splits — keep walking.
          j--
        }
      }
      return (
        getTokenCountFromUsage(usage) +
        roughTokenCountEstimationForMessages(
          messages.slice(i + 1) as Parameters<
            typeof roughTokenCountEstimationForMessages
          >[0],
        )
      )
    }
    i--
  }
  return roughTokenCountEstimationForMessages(
    messages as Parameters<typeof roughTokenCountEstimationForMessages>[0],
  )
}