Function compactConversation

src/services/compact/compact.ts:411–792

(
  messages: Message[],
  context: ToolUseContext,
  cacheSafeParams: CacheSafeParams,
  suppressFollowUpQuestions: boolean,
  customInstructions?: string,
  isAutoCompact: boolean = false,
  recompactionInfo?: RecompactionInfo,
)

 * and preserving recent conversation history.
 */
export async function compactConversation(
  messages: Message[],
  context: ToolUseContext,
  cacheSafeParams: CacheSafeParams,
  suppressFollowUpQuestions: boolean,
  customInstructions?: string,
  isAutoCompact: boolean = false,
  recompactionInfo?: RecompactionInfo,
): Promise<CompactionResult> {
  try {
    if (messages.length === 0) {
      throw new Error(ERROR_MESSAGE_NOT_ENOUGH_MESSAGES)
    }

    const preCompactTokenCount = tokenCountWithEstimation(messages)

    const appState = context.getAppState()
    void logPermissionContextForAnts(appState.toolPermissionContext, 'summary')

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

    // Execute PreCompact hooks
    context.setSDKStatus?.('compacting')
    const hookResult = await executePreCompactHooks(
      {
        trigger: isAutoCompact ? 'auto' : 'manual',
        customInstructions: customInstructions ?? null,
      },
      context.abortController.signal,
    )
    customInstructions = mergeHookInstructions(
      customInstructions,
      hookResult.newCustomInstructions,
    )
    const userDisplayMessage = hookResult.userDisplayMessage

    // Show requesting mode with up arrow and custom message
    context.setStreamMode?.('requesting')
    context.setResponseLength?.(() => 0)
    context.onCompactProgress?.({ type: 'compact_start' })

    // 3P default: true — forked-agent path reuses main conversation's prompt cache.
    // Experiment (Jan 2026) confirmed: false path is 98% cache miss, costs ~0.76% of
    // fleet cache_creation (~38B tok/day), concentrated in ephemeral envs (CCR/GHA/SDK)
    // with cold GB cache and 3P providers where GB is disabled. GB gate kept as kill-switch.
    const promptCacheSharingEnabled = getFeatureValue_CACHED_MAY_BE_STALE(
      'tengu_compact_cache_prefix',
      true,
    )

    const compactPrompt = getCompactPrompt(customInstructions)
    const summaryRequest = createUserMessage({
      content: compactPrompt,
    })