Function partialCompactConversation

src/services/compact/compact.ts:801–1140

(
  allMessages: Message[],
  pivotIndex: number,
  context: ToolUseContext,
  cacheSafeParams: CacheSafeParams,
  userFeedback?: string,
  direction: PartialCompactDirection = 'from',
)

 *   Prompt cache is invalidated since the summary precedes the kept messages.
 */
export async function partialCompactConversation(
  allMessages: Message[],
  pivotIndex: number,
  context: ToolUseContext,
  cacheSafeParams: CacheSafeParams,
  userFeedback?: string,
  direction: PartialCompactDirection = 'from',
): Promise<CompactionResult> {
  try {
    const messagesToSummarize =
      direction === 'up_to'
        ? allMessages.slice(0, pivotIndex)
        : allMessages.slice(pivotIndex)
    // 'up_to' must strip old compact boundaries/summaries: for 'up_to',
    // summary_B sits BEFORE kept, so a stale boundary_A in kept wins
    // findLastCompactBoundaryIndex's backward scan and drops summary_B.
    // 'from' keeps them: summary_B sits AFTER kept (backward scan still
    // works), and removing an old summary would lose its covered history.
    const messagesToKeep =
      direction === 'up_to'
        ? allMessages
            .slice(pivotIndex)
            .filter(
              m =>
                m.type !== 'progress' &&
                !isCompactBoundaryMessage(m) &&
                !(m.type === 'user' && m.isCompactSummary),
            )
        : allMessages.slice(0, pivotIndex).filter(m => m.type !== 'progress')

    if (messagesToSummarize.length === 0) {
      throw new Error(
        direction === 'up_to'
          ? 'Nothing to summarize before the selected message.'
          : 'Nothing to summarize after the selected message.',
      )
    }

    const preCompactTokenCount = tokenCountWithEstimation(allMessages)

    context.onCompactProgress?.({
      type: 'hooks_start',
      hookType: 'pre_compact',
    })

    context.setSDKStatus?.('compacting')
    const hookResult = await executePreCompactHooks(
      {
        trigger: 'manual',
        customInstructions: null,
      },
      context.abortController.signal,
    )

    // Merge hook instructions with user feedback
    let customInstructions: string | undefined
    if (hookResult.newCustomInstructions && userFeedback) {
      customInstructions = `${hookResult.newCustomInstructions}\n\nUser context: ${userFeedback}`