Add ReadonlySessionManager and refactor branch summarization

- Add ReadonlySessionManager interface to session-manager.ts
- Re-export from hooks/index.ts
- Add collectEntriesForBranchSummary() to extract entries for summarization
- Don't stop at compaction boundaries (include their summaries as context)
- Add token budget support to prepareBranchEntries()
- Walk entries newest-to-oldest to prioritize recent context
- Use options object for generateBranchSummary()
- Handle compaction entries as context summaries
- Export new types: CollectEntriesResult, GenerateBranchSummaryOptions

packages/coding-agent/src/core/agent-session.ts

@@ -21,6 +21,7 @@ import { type BashResult, executeBash as executeBashCommand } from "./bash-execu
 import {
 	type CompactionResult,
 	calculateContextTokens,
+	collectEntriesForBranchSummary,
 	compact,
 	generateBranchSummary,
 	prepareCompaction,
@@ -42,7 +43,7 @@ import type {
 } from "./hooks/index.js";
 import type { BashExecutionMessage, HookMessage } from "./messages.js";
 import type { ModelRegistry } from "./model-registry.js";
-import type { BranchSummaryEntry, CompactionEntry, SessionEntry, SessionManager } from "./session-manager.js";
+import type { BranchSummaryEntry, CompactionEntry, SessionManager } from "./session-manager.js";
 import type { SettingsManager, SkillsSettings } from "./settings-manager.js";
 import { expandSlashCommand, type FileSlashCommand } from "./slash-commands.js";
 
@@ -1601,30 +1602,12 @@ export class AgentSession {
 			throw new Error(`Entry ${targetId} not found`);
 		}
 
-		// Find common ancestor (if oldLeafId is null, there's no old path)
-		const oldPath = oldLeafId ? new Set(this.sessionManager.getPath(oldLeafId).map((e) => e.id)) : new Set<string>();
-		const targetPath = this.sessionManager.getPath(targetId);
-		let commonAncestorId: string | null = null;
-		for (const entry of targetPath) {
-			if (oldPath.has(entry.id)) {
-				commonAncestorId = entry.id;
-				break;
-			}
-		}
-
-		// Collect entries to summarize (old leaf back to common ancestor, stop at compaction)
-		const entriesToSummarize: SessionEntry[] = [];
-		if (options.summarize && oldLeafId) {
-			let current: string | null = oldLeafId;
-			while (current && current !== commonAncestorId) {
-				const entry = this.sessionManager.getEntry(current);
-				if (!entry) break;
-				if (entry.type === "compaction") break;
-				entriesToSummarize.push(entry);
-				current = entry.parentId;
-			}
-			entriesToSummarize.reverse(); // Chronological order
-		}
+		// Collect entries to summarize (from old leaf to common ancestor)
+		const { entries: entriesToSummarize, commonAncestorId } = collectEntriesForBranchSummary(
+			this.sessionManager,
+			oldLeafId,
+			targetId,
+		);
 
 		// Prepare event data
 		const preparation: TreePreparation = {
@@ -1667,13 +1650,12 @@ export class AgentSession {
 			if (!apiKey) {
 				throw new Error(`No API key for ${model.provider}`);
 			}
-			const result = await generateBranchSummary(
-				entriesToSummarize,
+			const result = await generateBranchSummary(entriesToSummarize, {
 				model,
 				apiKey,
-				this._branchSummaryAbortController.signal,
-				options.customInstructions,
-			);
+				signal: this._branchSummaryAbortController.signal,
+				customInstructions: options.customInstructions,
+			});
 			this._branchSummaryAbortController = undefined;
 			if (result.aborted) {
 				return { cancelled: true, aborted: true };

packages/coding-agent/src/core/compaction/branch-summarization.ts

@@ -7,7 +7,7 @@
 
 import type { Model } from "@mariozechner/pi-ai";
 import { complete } from "@mariozechner/pi-ai";
-import type { SessionEntry } from "../session-manager.js";
+import type { ReadonlySessionManager, SessionEntry } from "../session-manager.js";
 
 // ============================================================================
 // Types
@@ -26,18 +26,100 @@ export interface FileOperations {
 }
 
 export interface BranchPreparation {
-	/** Messages extracted for summarization */
-	messages: Array<{ role: string; content: string }>;
+	/** Messages extracted for summarization, in chronological order */
+	messages: Array<{ role: string; content: string; tokens: number }>;
 	/** File operations extracted from tool calls */
 	fileOps: FileOperations;
-	/** Previous summaries found in entries */
-	previousSummaries: string[];
+	/** Total tokens in messages */
+	totalTokens: number;
+}
+
+export interface CollectEntriesResult {
+	/** Entries to summarize, in chronological order */
+	entries: SessionEntry[];
+	/** Common ancestor between old and new position, if any */
+	commonAncestorId: string | null;
+}
+
+export interface GenerateBranchSummaryOptions {
+	/** Model to use for summarization */
+	model: Model<any>;
+	/** API key for the model */
+	apiKey: string;
+	/** Abort signal for cancellation */
+	signal: AbortSignal;
+	/** Optional custom instructions for summarization */
+	customInstructions?: string;
+	/** Reserve this fraction of context window for summary (default 0.2) */
+	reserveFraction?: number;
+}
+
+// ============================================================================
+// Entry Collection
+// ============================================================================
+
+/**
+ * Collect entries that should be summarized when navigating from one position to another.
+ *
+ * Walks from oldLeafId back to the common ancestor with targetId, collecting entries
+ * along the way. Does NOT stop at compaction boundaries - those are included and their
+ * summaries become context.
+ *
+ * @param session - Session manager (read-only access)
+ * @param oldLeafId - Current position (where we're navigating from)
+ * @param targetId - Target position (where we're navigating to)
+ * @returns Entries to summarize and the common ancestor
+ */
+export function collectEntriesForBranchSummary(
+	session: ReadonlySessionManager,
+	oldLeafId: string | null,
+	targetId: string,
+): CollectEntriesResult {
+	// If no old position, nothing to summarize
+	if (!oldLeafId) {
+		return { entries: [], commonAncestorId: null };
+	}
+
+	// Find common ancestor
+	const oldPath = new Set(session.getPath(oldLeafId).map((e) => e.id));
+	const targetPath = session.getPath(targetId);
+
+	let commonAncestorId: string | null = null;
+	for (const entry of targetPath) {
+		if (oldPath.has(entry.id)) {
+			commonAncestorId = entry.id;
+			break;
+		}
+	}
+
+	// Collect entries from old leaf back to common ancestor
+	const entries: SessionEntry[] = [];
+	let current: string | null = oldLeafId;
+
+	while (current && current !== commonAncestorId) {
+		const entry = session.getEntry(current);
+		if (!entry) break;
+		entries.push(entry);
+		current = entry.parentId;
+	}
+
+	// Reverse to get chronological order
+	entries.reverse();
+
+	return { entries, commonAncestorId };
 }
 
 // ============================================================================
 // Entry Parsing
 // ============================================================================
 
+/**
+ * Estimate token count for a string using chars/4 heuristic.
+ */
+function estimateStringTokens(text: string): number {
+	return Math.ceil(text.length / 4);
+}
+
 /**
  * Extract text content from any message type.
  */
@@ -84,44 +166,55 @@ function extractFileOpsFromToolCalls(message: any, fileOps: FileOperations): voi
 }
 
 /**
- * Prepare entries for summarization.
+ * Prepare entries for summarization with token budget.
  *
- * Extracts:
- * - Messages (user, assistant text, custom_message)
- * - File operations from tool calls
- * - Previous branch summaries
+ * Walks entries from NEWEST to OLDEST, adding messages until we hit the token budget.
+ * This ensures we keep the most recent context when the branch is too long.
+ *
+ * Handles:
+ * - message (user, assistant) - extracts text, counts tokens
+ * - custom_message - treated as user message
+ * - branch_summary - included as context
+ * - compaction - includes summary as context
  *
  * Skips:
- * - toolResult messages (context already in assistant message)
+ * - toolResult messages (context already in assistant's tool call)
  * - thinking_level_change, model_change, custom, label entries
- * - compaction entries (these are boundaries, shouldn't be in the input)
+ *
+ * @param entries - Entries in chronological order
+ * @param tokenBudget - Maximum tokens to include (0 = no limit)
  */
-export function prepareBranchEntries(entries: SessionEntry[]): BranchPreparation {
-	const messages: Array<{ role: string; content: string }> = [];
+export function prepareBranchEntries(entries: SessionEntry[], tokenBudget: number = 0): BranchPreparation {
+	const messages: Array<{ role: string; content: string; tokens: number }> = [];
 	const fileOps: FileOperations = {
 		read: new Set(),
 		written: new Set(),
 		edited: new Set(),
 	};
-	const previousSummaries: string[] = [];
+	let totalTokens = 0;
+
+	// Walk from newest to oldest to prioritize recent context
+	for (let i = entries.length - 1; i >= 0; i--) {
+		const entry = entries[i];
+		let role: string | undefined;
+		let content: string | undefined;
 
-	for (const entry of entries) {
 		switch (entry.type) {
 			case "message": {
-				const role = entry.message.role;
+				const msgRole = entry.message.role;
 
-				// Skip tool results - the context is in the assistant's tool call
-				if (role === "toolResult") continue;
+				// Skip tool results - context is in assistant's tool call
+				if (msgRole === "toolResult") continue;
 
 				// Extract file ops from assistant tool calls
-				if (role === "assistant") {
+				if (msgRole === "assistant") {
 					extractFileOpsFromToolCalls(entry.message, fileOps);
 				}
 
-				// Extract text content
 				const text = extractMessageText(entry.message);
 				if (text) {
-					messages.push({ role, content: text });
+					role = msgRole;
+					content = text;
 				}
 				break;
 			}
@@ -135,27 +228,56 @@ export function prepareBranchEntries(entries: SessionEntry[]): BranchPreparation
 								.map((c) => c.text)
 								.join("");
 				if (text) {
-					messages.push({ role: "user", content: text });
+					role = "user";
+					content = text;
 				}
 				break;
 			}
 
 			case "branch_summary": {
-				previousSummaries.push(entry.summary);
+				role = "context";
+				content = `[Branch summary: ${entry.summary}]`;
 				break;
 			}
 
-			// Skip these entry types - they don't contribute to conversation content
-			case "compaction":
+			case "compaction": {
+				role = "context";
+				content = `[Session summary: ${entry.summary}]`;
+				break;
+			}
+
+			// Skip these - don't contribute to conversation content
 			case "thinking_level_change":
 			case "model_change":
 			case "custom":
 			case "label":
+				continue;
+		}
+
+		if (role && content) {
+			const tokens = estimateStringTokens(content);
+
+			// Check budget before adding
+			if (tokenBudget > 0 && totalTokens + tokens > tokenBudget) {
+				// If this is a summary entry, try to fit it anyway as it's important context
+				if (entry.type === "compaction" || entry.type === "branch_summary") {
+					// Add truncated version or skip
+					if (totalTokens < tokenBudget * 0.9) {
+						// Still have some room, add it
+						messages.unshift({ role, content, tokens });
+						totalTokens += tokens;
+					}
+				}
+				// Stop - we've hit the budget
 				break;
+			}
+
+			messages.unshift({ role, content, tokens });
+			totalTokens += tokens;
 		}
 	}
 
-	return { messages, fileOps, previousSummaries };
+	return { messages, fileOps, totalTokens };
 }
 
 // ============================================================================
@@ -202,37 +324,27 @@ function formatFileOperations(fileOps: FileOperations): string {
 /**
  * Generate a summary of abandoned branch entries.
  *
- * @param entries - Session entries to summarize
- * @param model - Model to use for summarization
- * @param apiKey - API key for the model
- * @param signal - Abort signal for cancellation
- * @param customInstructions - Optional custom instructions for summarization
+ * @param entries - Session entries to summarize (chronological order)
+ * @param options - Generation options
  */
 export async function generateBranchSummary(
 	entries: SessionEntry[],
-	model: Model<any>,
-	apiKey: string,
-	signal: AbortSignal,
-	customInstructions?: string,
+	options: GenerateBranchSummaryOptions,
 ): Promise<BranchSummaryResult> {
-	const { messages, fileOps, previousSummaries } = prepareBranchEntries(entries);
+	const { model, apiKey, signal, customInstructions, reserveFraction = 0.2 } = options;
+
+	// Calculate token budget (leave room for summary generation)
+	const contextWindow = model.contextWindow || 128000;
+	const tokenBudget = Math.floor(contextWindow * (1 - reserveFraction));
+
+	const { messages, fileOps } = prepareBranchEntries(entries, tokenBudget);
 
 	if (messages.length === 0) {
 		return { summary: "No content to summarize" };
 	}
 
 	// Build conversation text
-	const parts: string[] = [];
-
-	// Include previous summaries as context
-	if (previousSummaries.length > 0) {
-		parts.push(`[Previous context: ${previousSummaries.join(" | ")}]`);
-	}
-
-	// Add conversation
-	parts.push(messages.map((m) => `${m.role}: ${m.content}`).join("\n\n"));
-
-	const conversationText = parts.join("\n\n");
+	const conversationText = messages.map((m) => `${m.role}: ${m.content}`).join("\n\n");
 	const instructions = customInstructions || BRANCH_SUMMARY_PROMPT;
 	const prompt = `${instructions}\n\nConversation:\n${conversationText}`;
 
@@ -248,7 +360,7 @@ export async function generateBranchSummary(
 				},
 			],
 		},
-		{ apiKey, signal, maxTokens: 1024 },
+		{ apiKey, signal, maxTokens: 2048 },
 	);
 
 	// Check if aborted or errored

packages/coding-agent/src/core/hooks/index.ts

@@ -10,3 +10,4 @@ export {
 export { execCommand, HookRunner, type HookErrorListener } from "./runner.js";
 export { wrapToolsWithHooks, wrapToolWithHooks } from "./tool-wrapper.js";
 export type * from "./types.js";
+export type { ReadonlySessionManager } from "../session-manager.js";

packages/coding-agent/src/core/session-manager.ts

@@ -155,6 +155,20 @@ export interface SessionInfo {
 	allMessagesText: string;
 }
 
+/**
+ * Read-only interface for SessionManager.
+ * Used by compaction/summarization utilities that only need to read session data.
+ */
+export interface ReadonlySessionManager {
+	getLeafId(): string | null;
+	getEntry(id: string): SessionEntry | undefined;
+	getPath(fromId?: string): SessionEntry[];
+	getEntries(): SessionEntry[];
+	getChildren(parentId: string): SessionEntry[];
+	getTree(): SessionTreeNode[];
+	getLabel(id: string): string | undefined;
+}
+
 /** Generate a unique short ID (8 hex chars, collision-checked) */
 function generateId(byId: { has(id: string): boolean }): string {
 	for (let i = 0; i < 100; i++) {

packages/coding-agent/src/index.ts

@@ -14,15 +14,18 @@ export { type ApiKeyCredential, type AuthCredential, AuthStorage, type OAuthCred
 export {
 	type BranchPreparation,
 	type BranchSummaryResult,
+	type CollectEntriesResult,
 	type CompactionResult,
 	type CutPointResult,
 	calculateContextTokens,
+	collectEntriesForBranchSummary,
 	compact,
 	DEFAULT_COMPACTION_SETTINGS,
 	estimateTokens,
 	type FileOperations,
 	findCutPoint,
 	findTurnStartIndex,
+	type GenerateBranchSummaryOptions,
 	generateBranchSummary,
 	generateSummary,
 	getLastAssistantUsage,