oh-my-pi/packages/coding-agent/src/session/compaction/compaction.ts

/**
 * Context compaction for long sessions.
 *
 * Pure functions for compaction logic. The session manager handles I/O,
 * and after compaction the session is reloaded.
 */
import type { AgentMessage } from "@oh-my-pi/pi-agent-core";
import type { AssistantMessage, Model, Usage } from "@oh-my-pi/pi-ai";
import { completeSimple } from "@oh-my-pi/pi-ai";
import { logger } from "@oh-my-pi/pi-utils";
import { renderPromptTemplate } from "../../config/prompt-templates";
import compactionShortSummaryPrompt from "../../prompts/compaction/compaction-short-summary.md" with { type: "text" };
import compactionSummaryPrompt from "../../prompts/compaction/compaction-summary.md" with { type: "text" };
import compactionTurnPrefixPrompt from "../../prompts/compaction/compaction-turn-prefix.md" with { type: "text" };
import compactionUpdateSummaryPrompt from "../../prompts/compaction/compaction-update-summary.md" with { type: "text" };
import { convertToLlm, createBranchSummaryMessage, createCustomMessage } from "../../session/messages";
import type { CompactionEntry, SessionEntry } from "../../session/session-manager";

import {
	computeFileLists,
	createFileOps,
	extractFileOpsFromMessage,
	type FileOperations,
	formatFileOperations,
	SUMMARIZATION_SYSTEM_PROMPT,
	serializeConversation,
} from "./utils";

// ============================================================================
// File Operation Tracking
// ============================================================================

/** Details stored in CompactionEntry.details for file tracking */
export interface CompactionDetails {
	readFiles: string[];
	modifiedFiles: string[];
}

/**
 * Extract file operations from messages and previous compaction entries.
 */
function extractFileOperations(
	messages: AgentMessage[],
	entries: SessionEntry[],
	prevCompactionIndex: number,
): FileOperations {
	const fileOps = createFileOps();

	// Collect from previous compaction's details (if pi-generated)
	if (prevCompactionIndex >= 0) {
		const prevCompaction = entries[prevCompactionIndex] as CompactionEntry;
		if (!prevCompaction.fromExtension && prevCompaction.details) {
			const details = prevCompaction.details as CompactionDetails;
			if (Array.isArray(details.readFiles)) {
				for (const f of details.readFiles) fileOps.read.add(f);
			}
			if (Array.isArray(details.modifiedFiles)) {
				for (const f of details.modifiedFiles) fileOps.edited.add(f);
			}
		}
	}

	// Extract from tool calls in messages
	for (const msg of messages) {
		extractFileOpsFromMessage(msg, fileOps);
	}

	return fileOps;
}

// ============================================================================
// Message Extraction
// ============================================================================

/**
 * Extract AgentMessage from an entry if it produces one.
 * Returns undefined for entries that don't contribute to LLM context.
 */
function getMessageFromEntry(entry: SessionEntry): AgentMessage | undefined {
	if (entry.type === "message") {
		return entry.message;
	}
	if (entry.type === "custom_message") {
		return createCustomMessage(entry.customType, entry.content, entry.display, entry.details, entry.timestamp);
	}
	if (entry.type === "branch_summary") {
		return createBranchSummaryMessage(entry.summary, entry.fromId, entry.timestamp);
	}
	return undefined;
}

/** Result from compact() - SessionManager adds uuid/parentUuid when saving */
export interface CompactionResult<T = unknown> {
	summary: string;
	/** Short PR-style summary for display purposes. */
	shortSummary?: string;
	firstKeptEntryId: string;
	tokensBefore: number;
	/** Hook-specific data (e.g., ArtifactIndex, version markers for structured compaction) */
	details?: T;
	/** Hook-provided data to persist alongside compaction entry. */
	preserveData?: Record<string, unknown>;
}

// ============================================================================
// Types
// ============================================================================

export interface CompactionSettings {
	enabled: boolean;
	reserveTokens: number;
	keepRecentTokens: number;
	autoContinue?: boolean;
	remoteEndpoint?: string;
}

export const DEFAULT_COMPACTION_SETTINGS: CompactionSettings = {
	enabled: true,
	reserveTokens: 16384,
	keepRecentTokens: 20000,
	autoContinue: true,
};

// ============================================================================
// Token calculation
// ============================================================================

/**
 * Calculate total context tokens from usage.
 * Uses the native totalTokens field when available, falls back to computing from components.
 */
export function calculateContextTokens(usage: Usage): number {
	return usage.totalTokens || usage.input + usage.output + usage.cacheRead + usage.cacheWrite;
}

export function calculatePromptTokens(usage: Usage): number {
	const promptTokens = usage.input + usage.cacheRead + usage.cacheWrite;
	if (promptTokens > 0) {
		return promptTokens;
	}
	return calculateContextTokens(usage);
}

/**
 * Get usage from an assistant message if available.
 * Skips aborted and error messages as they don't have valid usage data.
 */
function getAssistantUsage(msg: AgentMessage): Usage | undefined {
	if (msg.role === "assistant" && "usage" in msg) {
		const assistantMsg = msg as AssistantMessage;
		if (assistantMsg.stopReason !== "aborted" && assistantMsg.stopReason !== "error" && assistantMsg.usage) {
			return assistantMsg.usage;
		}
	}
	return undefined;
}

/**
 * Find the last non-aborted assistant message usage from session entries.
 */
export function getLastAssistantUsage(entries: SessionEntry[]): Usage | undefined {
	for (let i = entries.length - 1; i >= 0; i--) {
		const entry = entries[i];
		if (entry.type === "message") {
			const usage = getAssistantUsage(entry.message);
			if (usage) return usage;
		}
	}
	return undefined;
}

/**
 * Check if compaction should trigger based on context usage.
 */
export function shouldCompact(contextTokens: number, contextWindow: number, settings: CompactionSettings): boolean {
	if (!settings.enabled) return false;
	return contextTokens > contextWindow - settings.reserveTokens;
}

// ============================================================================
// Cut point detection
// ============================================================================

/**
 * Estimate token count for a message using chars/4 heuristic.
 * This is conservative (overestimates tokens).
 */
export function estimateTokens(message: AgentMessage): number {
	let chars = 0;

	switch (message.role) {
		case "user": {
			const content = (message as { content: string | Array<{ type: string; text?: string }> }).content;
			if (typeof content === "string") {
				chars = content.length;
			} else if (Array.isArray(content)) {
				for (const block of content) {
					if (block.type === "text" && block.text) {
						chars += block.text.length;
					}
				}
			}
			return Math.ceil(chars / 4);
		}
		case "assistant": {
			const assistant = message as AssistantMessage;
			for (const block of assistant.content) {
				if (block.type === "text") {
					chars += block.text.length;
				} else if (block.type === "thinking") {
					chars += block.thinking.length;
				} else if (block.type === "toolCall") {
					chars += block.name.length + JSON.stringify(block.arguments).length;
				}
			}
			return Math.ceil(chars / 4);
		}
		case "hookMessage":
		case "toolResult": {
			if (typeof message.content === "string") {
				chars = message.content.length;
			} else {
				for (const block of message.content) {
					if (block.type === "text" && block.text) {
						chars += block.text.length;
					}
					if (block.type === "image") {
						chars += 4800; // Estimate images as 4000 chars, or 1200 tokens
					}
				}
			}
			return Math.ceil(chars / 4);
		}
		case "bashExecution": {
			chars = message.command.length + message.output.length;
			return Math.ceil(chars / 4);
		}
		case "branchSummary":
		case "compactionSummary": {
			chars = message.summary.length;
			return Math.ceil(chars / 4);
		}
	}

	return 0;
}

function estimateEntriesTokens(entries: SessionEntry[], startIndex: number, endIndex: number): number {
	let total = 0;
	for (let i = startIndex; i < endIndex; i++) {
		const msg = getMessageFromEntry(entries[i]);
		if (msg) {
			total += estimateTokens(msg);
		}
	}
	return total;
}

/**
 * Find valid cut points: indices of user, assistant, custom, or bashExecution messages.
 * Never cut at tool results (they must follow their tool call).
 * When we cut at an assistant message with tool calls, its tool results follow it
 * and will be kept.
 * BashExecutionMessage is treated like a user message (user-initiated context).
 */
function findValidCutPoints(entries: SessionEntry[], startIndex: number, endIndex: number): number[] {
	const cutPoints: number[] = [];
	for (let i = startIndex; i < endIndex; i++) {
		const entry = entries[i];
		switch (entry.type) {
			case "message": {
				const role = entry.message.role;
				switch (role) {
					case "bashExecution":
					case "hookMessage":
					case "branchSummary":
					case "compactionSummary":
					case "user":
					case "assistant":
						cutPoints.push(i);
						break;
					case "toolResult":
						break;
				}
				break;
			}
			case "thinking_level_change":
			case "model_change":
			case "compaction":
			case "branch_summary":
			case "custom":
			case "custom_message":
			case "label":
		}
		// branch_summary and custom_message are user-role messages, valid cut points
		if (entry.type === "branch_summary" || entry.type === "custom_message") {
			cutPoints.push(i);
		}
	}
	return cutPoints;
}

/**
 * Find the user message (or bashExecution) that starts the turn containing the given entry index.
 * Returns -1 if no turn start found before the index.
 * BashExecutionMessage is treated like a user message for turn boundaries.
 */
export function findTurnStartIndex(entries: SessionEntry[], entryIndex: number, startIndex: number): number {
	for (let i = entryIndex; i >= startIndex; i--) {
		const entry = entries[i];
		// branch_summary and custom_message are user-role messages, can start a turn
		if (entry.type === "branch_summary" || entry.type === "custom_message") {
			return i;
		}
		if (entry.type === "message") {
			const role = entry.message.role;
			if (role === "user" || role === "bashExecution") {
				return i;
			}
		}
	}
	return -1;
}

export interface CutPointResult {
	/** Index of first entry to keep */
	firstKeptEntryIndex: number;
	/** Index of user message that starts the turn being split, or -1 if not splitting */
	turnStartIndex: number;
	/** Whether this cut splits a turn (cut point is not a user message) */
	isSplitTurn: boolean;
}

/**
 * Find the cut point in session entries that keeps approximately `keepRecentTokens`.
 *
 * Algorithm: Walk backwards from newest, accumulating estimated message sizes.
 * Stop when we've accumulated >= keepRecentTokens. Cut at that point.
 *
 * Can cut at user OR assistant messages (never tool results). When cutting at an
 * assistant message with tool calls, its tool results come after and will be kept.
 *
 * Returns CutPointResult with:
 * - firstKeptEntryIndex: the entry index to start keeping from
 * - turnStartIndex: if cutting mid-turn, the user message that started that turn
 * - isSplitTurn: whether we're cutting in the middle of a turn
 *
 * Only considers entries between `startIndex` and `endIndex` (exclusive).
 */
export function findCutPoint(
	entries: SessionEntry[],
	startIndex: number,
	endIndex: number,
	keepRecentTokens: number,
): CutPointResult {
	const cutPoints = findValidCutPoints(entries, startIndex, endIndex);

	if (cutPoints.length === 0) {
		return { firstKeptEntryIndex: startIndex, turnStartIndex: -1, isSplitTurn: false };
	}

	// Walk backwards from newest, accumulating estimated message sizes
	let accumulatedTokens = 0;
	let cutIndex = cutPoints[0]; // Default: keep from first message (not header)

	for (let i = endIndex - 1; i >= startIndex; i--) {
		const entry = entries[i];
		if (entry.type !== "message") continue;

		// Estimate this message's size
		const messageTokens = estimateTokens(entry.message);
		accumulatedTokens += messageTokens;

		// Check if we've exceeded the budget
		if (accumulatedTokens >= keepRecentTokens) {
			// Find the closest valid cut point at or after this entry
			for (let c = 0; c < cutPoints.length; c++) {
				if (cutPoints[c] >= i) {
					cutIndex = cutPoints[c];
					break;
				}
			}
			break;
		}
	}

	// Scan backwards from cutIndex to include any non-message entries (bash, settings, etc.)
	while (cutIndex > startIndex) {
		const prevEntry = entries[cutIndex - 1];
		// Stop at session header or compaction boundaries
		if (prevEntry.type === "compaction") {
			break;
		}
		if (prevEntry.type === "message") {
			// Stop if we hit any message
			break;
		}
		// Include this non-message entry (bash, settings change, etc.)
		cutIndex--;
	}

	// Determine if this is a split turn
	const cutEntry = entries[cutIndex];
	const isUserMessage = cutEntry.type === "message" && cutEntry.message.role === "user";
	const turnStartIndex = isUserMessage ? -1 : findTurnStartIndex(entries, cutIndex, startIndex);

	return {
		firstKeptEntryIndex: cutIndex,
		turnStartIndex,
		isSplitTurn: !isUserMessage && turnStartIndex !== -1,
	};
}

// ============================================================================
// Summarization
// ============================================================================

const SUMMARIZATION_PROMPT = renderPromptTemplate(compactionSummaryPrompt);

const UPDATE_SUMMARIZATION_PROMPT = renderPromptTemplate(compactionUpdateSummaryPrompt);

const SHORT_SUMMARY_PROMPT = renderPromptTemplate(compactionShortSummaryPrompt);

function formatAdditionalContext(context: string[] | undefined): string {
	if (!context || context.length === 0) return "";
	const lines = context.map(line => `- ${line}`).join("\n");
	return `<additional-context>\n${lines}\n</additional-context>\n\n`;
}

interface RemoteCompactionRequest {
	systemPrompt: string;
	prompt: string;
}

interface RemoteCompactionResponse {
	summary: string;
	shortSummary?: string;
}

async function requestRemoteCompaction(
	endpoint: string,
	request: RemoteCompactionRequest,
): Promise<RemoteCompactionResponse> {
	const response = await fetch(endpoint, {
		method: "POST",
		headers: { "content-type": "application/json" },
		body: JSON.stringify(request),
	});

	if (!response.ok) {
		const errorText = await response.text().catch(() => "");
		logger.warn("Remote compaction failed", {
			endpoint,
			status: response.status,
			statusText: response.statusText,
			errorText,
		});
		throw new Error(`Remote compaction failed (${response.status} ${response.statusText})`);
	}

	const data = (await response.json()) as RemoteCompactionResponse | undefined;
	if (!data || typeof data.summary !== "string") {
		throw new Error("Remote compaction response missing summary");
	}

	return data;
}

/**
 * Generate a summary of the conversation using the LLM.
 * If previousSummary is provided, uses the update prompt to merge.
 */
export interface SummaryOptions {
	promptOverride?: string;
	extraContext?: string[];
	remoteEndpoint?: string;
}

export async function generateSummary(
	currentMessages: AgentMessage[],
	model: Model,
	reserveTokens: number,
	apiKey: string,
	signal?: AbortSignal,
	customInstructions?: string,
	previousSummary?: string,
	options?: SummaryOptions,
): Promise<string> {
	const maxTokens = Math.floor(0.8 * reserveTokens);

	// Use update prompt if we have a previous summary, otherwise initial prompt
	let basePrompt = previousSummary ? UPDATE_SUMMARIZATION_PROMPT : SUMMARIZATION_PROMPT;
	if (options?.promptOverride) {
		basePrompt = options.promptOverride;
	}
	if (customInstructions) {
		basePrompt = `${basePrompt}\n\nAdditional focus: ${customInstructions}`;
	}

	// Serialize conversation to text so model doesn't try to continue it
	// Convert to LLM messages first (handles custom types like bashExecution, hookMessage, etc.)
	const llmMessages = convertToLlm(currentMessages);
	const conversationText = serializeConversation(llmMessages);

	// Build the prompt with conversation wrapped in tags
	let promptText = `<conversation>\n${conversationText}\n</conversation>\n\n`;
	if (previousSummary) {
		promptText += `<previous-summary>\n${previousSummary}\n</previous-summary>\n\n`;
	}
	promptText += formatAdditionalContext(options?.extraContext);
	promptText += basePrompt;

	const summarizationMessages = [
		{
			role: "user" as const,
			content: [{ type: "text" as const, text: promptText }],
			timestamp: Date.now(),
		},
	];

	if (options?.remoteEndpoint) {
		const remote = await requestRemoteCompaction(options.remoteEndpoint, {
			systemPrompt: SUMMARIZATION_SYSTEM_PROMPT,
			prompt: promptText,
		});
		return remote.summary;
	}

	const response = await completeSimple(
		model,
		{ systemPrompt: SUMMARIZATION_SYSTEM_PROMPT, messages: summarizationMessages },
		{ maxTokens, signal, apiKey, reasoning: "high" },
	);

	if (response.stopReason === "error") {
		throw new Error(`Summarization failed: ${response.errorMessage || "Unknown error"}`);
	}

	const textContent = response.content
		.filter((c): c is { type: "text"; text: string } => c.type === "text")
		.map(c => c.text)
		.join("\n");

	return textContent;
}

async function generateShortSummary(
	recentMessages: AgentMessage[],
	historySummary: string | undefined,
	model: Model,
	reserveTokens: number,
	apiKey: string,
	signal?: AbortSignal,
	options?: SummaryOptions,
): Promise<string> {
	const maxTokens = Math.min(512, Math.floor(0.2 * reserveTokens));
	const llmMessages = convertToLlm(recentMessages);
	const conversationText = serializeConversation(llmMessages);

	let promptText = `<conversation>\n${conversationText}\n</conversation>\n\n`;
	if (historySummary) {
		promptText += `<previous-summary>\n${historySummary}\n</previous-summary>\n\n`;
	}
	promptText += formatAdditionalContext(options?.extraContext);
	promptText += SHORT_SUMMARY_PROMPT;

	if (options?.remoteEndpoint) {
		const remote = await requestRemoteCompaction(options.remoteEndpoint, {
			systemPrompt: SUMMARIZATION_SYSTEM_PROMPT,
			prompt: promptText,
		});
		return remote.summary;
	}

	const response = await completeSimple(
		model,
		{
			systemPrompt: SUMMARIZATION_SYSTEM_PROMPT,
			messages: [{ role: "user", content: [{ type: "text", text: promptText }], timestamp: Date.now() }],
		},
		{ maxTokens, signal, apiKey, reasoning: "high" },
	);

	if (response.stopReason === "error") {
		throw new Error(`Short summary failed: ${response.errorMessage || "Unknown error"}`);
	}

	return response.content
		.filter((c): c is { type: "text"; text: string } => c.type === "text")
		.map(c => c.text)
		.join("\n");
}

// ============================================================================
// Compaction Preparation (for hooks)
// ============================================================================

export interface CompactionPreparation {
	/** UUID of first entry to keep */
	firstKeptEntryId: string;
	/** Messages that will be summarized and discarded */
	messagesToSummarize: AgentMessage[];
	/** Messages that will be turned into turn prefix summary (if splitting) */
	turnPrefixMessages: AgentMessage[];
	/** Messages kept in full after compaction (recent history) */
	recentMessages: AgentMessage[];
	/** Whether this is a split turn (cut point in middle of turn) */
	isSplitTurn: boolean;
	tokensBefore: number;
	/** Summary from previous compaction, for iterative update */
	previousSummary?: string;
	/** File operations extracted from messagesToSummarize */
	fileOps: FileOperations;
	/** Compaction settions from settings.jsonl	*/
	settings: CompactionSettings;
}

export function prepareCompaction(
	pathEntries: SessionEntry[],
	settings: CompactionSettings,
): CompactionPreparation | undefined {
	if (pathEntries.length > 0 && pathEntries[pathEntries.length - 1].type === "compaction") {
		return undefined;
	}

	let prevCompactionIndex = -1;
	for (let i = pathEntries.length - 1; i >= 0; i--) {
		if (pathEntries[i].type === "compaction") {
			prevCompactionIndex = i;
			break;
		}
	}
	const boundaryStart = prevCompactionIndex + 1;
	const boundaryEnd = pathEntries.length;

	const lastUsage = getLastAssistantUsage(pathEntries);
	const tokensBefore = lastUsage ? calculateContextTokens(lastUsage) : 0;
	let keepRecentTokens = settings.keepRecentTokens;
	if (lastUsage) {
		const estimatedTokens = estimateEntriesTokens(pathEntries, boundaryStart, boundaryEnd);
		const promptTokens = calculatePromptTokens(lastUsage);
		const ratio = estimatedTokens > 0 ? promptTokens / estimatedTokens : 0;
		if (Number.isFinite(ratio) && ratio > 1) {
			keepRecentTokens = Math.max(1, Math.floor(keepRecentTokens / ratio));
		}
	}

	const cutPoint = findCutPoint(pathEntries, boundaryStart, boundaryEnd, keepRecentTokens);

	// Get ID of first kept entry
	const firstKeptEntry = pathEntries[cutPoint.firstKeptEntryIndex];
	if (!firstKeptEntry?.id) {
		return undefined; // Session needs migration
	}
	const firstKeptEntryId = firstKeptEntry.id;

	const historyEnd = cutPoint.isSplitTurn ? cutPoint.turnStartIndex : cutPoint.firstKeptEntryIndex;

	// Messages to summarize (will be discarded after summary)
	const messagesToSummarize: AgentMessage[] = [];
	for (let i = boundaryStart; i < historyEnd; i++) {
		const msg = getMessageFromEntry(pathEntries[i]);
		if (msg) messagesToSummarize.push(msg);
	}

	// Messages for turn prefix summary (if splitting a turn)
	const turnPrefixMessages: AgentMessage[] = [];
	if (cutPoint.isSplitTurn) {
		for (let i = cutPoint.turnStartIndex; i < cutPoint.firstKeptEntryIndex; i++) {
			const msg = getMessageFromEntry(pathEntries[i]);
			if (msg) turnPrefixMessages.push(msg);
		}
	}

	// Messages kept after compaction (recent history)
	const recentMessages: AgentMessage[] = [];
	for (let i = cutPoint.firstKeptEntryIndex; i < boundaryEnd; i++) {
		const msg = getMessageFromEntry(pathEntries[i]);
		if (msg) recentMessages.push(msg);
	}

	// Get previous summary for iterative update
	let previousSummary: string | undefined;
	if (prevCompactionIndex >= 0) {
		const prevCompaction = pathEntries[prevCompactionIndex] as CompactionEntry;
		previousSummary = prevCompaction.summary;
	}

	// Extract file operations from messages and previous compaction
	const fileOps = extractFileOperations(messagesToSummarize, pathEntries, prevCompactionIndex);

	// Also extract file ops from turn prefix if splitting
	if (cutPoint.isSplitTurn) {
		for (const msg of turnPrefixMessages) {
			extractFileOpsFromMessage(msg, fileOps);
		}
	}

	return {
		firstKeptEntryId,
		messagesToSummarize,
		turnPrefixMessages,
		recentMessages,
		isSplitTurn: cutPoint.isSplitTurn,
		tokensBefore,
		previousSummary,
		fileOps,
		settings,
	};
}

// ============================================================================
// Main compaction function
// ============================================================================

const TURN_PREFIX_SUMMARIZATION_PROMPT = renderPromptTemplate(compactionTurnPrefixPrompt);

/**
 * Generate summaries for compaction using prepared data.
 * Returns CompactionResult - SessionManager adds id/parentId when saving.
 *
 * @param preparation - Pre-calculated preparation from prepareCompaction()
 * @param customInstructions - Optional custom focus for the summary
 */
export async function compact(
	preparation: CompactionPreparation,
	model: Model,
	apiKey: string,
	customInstructions?: string,
	signal?: AbortSignal,
	options?: SummaryOptions,
): Promise<CompactionResult> {
	const {
		firstKeptEntryId,
		messagesToSummarize,
		turnPrefixMessages,
		recentMessages,
		isSplitTurn,
		tokensBefore,
		previousSummary,
		fileOps,
		settings,
	} = preparation;

	const summaryOptions: SummaryOptions = {
		promptOverride: options?.promptOverride,
		extraContext: options?.extraContext,
		remoteEndpoint: settings.remoteEndpoint,
	};

	// Generate summaries (can be parallel if both needed) and merge into one
	let summary: string;

	if (isSplitTurn && turnPrefixMessages.length > 0) {
		// Generate both summaries in parallel
		const [historyResult, turnPrefixResult] = await Promise.all([
			messagesToSummarize.length > 0
				? generateSummary(
						messagesToSummarize,
						model,
						settings.reserveTokens,
						apiKey,
						signal,
						customInstructions,
						previousSummary,
						summaryOptions,
					)
				: Promise.resolve("No prior history."),
			generateTurnPrefixSummary(turnPrefixMessages, model, settings.reserveTokens, apiKey, signal),
		]);
		// Merge into single summary
		summary = `${historyResult}\n\n---\n\n**Turn Context (split turn):**\n\n${turnPrefixResult}`;
	} else if (messagesToSummarize.length > 0) {
		// Generate history summary from messages to summarize
		summary = await generateSummary(
			messagesToSummarize,
			model,
			settings.reserveTokens,
			apiKey,
			signal,
			customInstructions,
			previousSummary,
			summaryOptions,
		);
	} else if (previousSummary) {
		// No new messages to summarize, preserve previous summary
		summary = previousSummary;
	} else {
		// No messages and no previous summary
		summary = "No prior history.";
	}

	const shortSummary = await generateShortSummary(
		recentMessages,
		summary,
		model,
		settings.reserveTokens,
		apiKey,
		signal,
		{ extraContext: options?.extraContext, remoteEndpoint: settings.remoteEndpoint },
	);

	// Compute file lists and append to summary
	const { readFiles, modifiedFiles } = computeFileLists(fileOps);
	summary += formatFileOperations(readFiles, modifiedFiles);

	if (!firstKeptEntryId) {
		throw new Error("First kept entry has no ID - session may need migration");
	}

	return {
		summary,
		shortSummary,
		firstKeptEntryId,
		tokensBefore,
		details: { readFiles, modifiedFiles } as CompactionDetails,
	};
}

/**
 * Generate a summary for a turn prefix (when splitting a turn).
 */
async function generateTurnPrefixSummary(
	messages: AgentMessage[],
	model: Model,
	reserveTokens: number,
	apiKey: string,
	signal?: AbortSignal,
): Promise<string> {
	const maxTokens = Math.floor(0.5 * reserveTokens); // Smaller budget for turn prefix

	const llmMessages = convertToLlm(messages);
	const conversationText = serializeConversation(llmMessages);
	const promptText = `<conversation>\n${conversationText}\n</conversation>\n\n${TURN_PREFIX_SUMMARIZATION_PROMPT}`;
	const summarizationMessages = [
		{
			role: "user" as const,
			content: [{ type: "text" as const, text: promptText }],
			timestamp: Date.now(),
		},
	];

	const response = await completeSimple(
		model,
		{ systemPrompt: SUMMARIZATION_SYSTEM_PROMPT, messages: summarizationMessages },
		{ maxTokens, signal, apiKey, reasoning: "high" },
	);

	if (response.stopReason === "error") {
		throw new Error(`Turn prefix summarization failed: ${response.errorMessage || "Unknown error"}`);
	}

	return response.content
		.filter((c): c is { type: "text"; text: string } => c.type === "text")
		.map(c => c.text)
		.join("\n");
}