Function truncateConversation

src/core/context-management/index.ts:93–161

(messages: ApiMessage[], fracToRemove: number, taskId: string)

 * @returns {TruncationResult} Object containing the tagged messages, truncation ID, and count of messages removed.
 */
export function truncateConversation(messages: ApiMessage[], fracToRemove: number, taskId: string): TruncationResult {
	TelemetryService.instance.captureSlidingWindowTruncation(taskId)

	const truncationId = crypto.randomUUID()

	// Filter to only visible messages (those not already truncated)
	// We need to track original indices to correctly tag messages in the full array
	const visibleIndices: number[] = []
	messages.forEach((msg, index) => {
		if (!msg.truncationParent && !msg.isTruncationMarker) {
			visibleIndices.push(index)
		}
	})

	// Calculate how many visible messages to truncate (excluding first visible message)
	const visibleCount = visibleIndices.length
	const rawMessagesToRemove = Math.floor((visibleCount - 1) * fracToRemove)
	const messagesToRemove = rawMessagesToRemove - (rawMessagesToRemove % 2)

	if (messagesToRemove <= 0) {
		// Nothing to truncate
		return {
			messages,
			truncationId,
			messagesRemoved: 0,
		}
	}

	// Get the indices of visible messages to truncate (skip first visible, take next N)
	const indicesToTruncate = new Set(visibleIndices.slice(1, messagesToRemove + 1))

	// Tag messages that are being "truncated" (hidden from API calls)
	const taggedMessages = messages.map((msg, index) => {
		if (indicesToTruncate.has(index)) {
			return { ...msg, truncationParent: truncationId }
		}
		return msg
	})

	// Find the actual boundary - the index right after the last truncated message
	const lastTruncatedVisibleIndex = visibleIndices[messagesToRemove] // Last visible message being truncated
	// If all visible messages except the first are truncated, insert marker at the end
	const firstKeptVisibleIndex = visibleIndices[messagesToRemove + 1] ?? taggedMessages.length

	// Insert truncation marker at the actual boundary (between last truncated and first kept)
	const firstKeptTs = messages[firstKeptVisibleIndex]?.ts ?? Date.now()
	const truncationMarker: ApiMessage = {
		role: "user",
		content: `[Sliding window truncation: ${messagesToRemove} messages hidden to reduce context]`,
		ts: firstKeptTs - 1,
		isTruncationMarker: true,
		truncationId,
	}

	// Insert marker at the boundary position
	// Find where to insert: right before the first kept visible message
	const insertPosition = firstKeptVisibleIndex
	const result = [