Improve navigateTree API

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

@@ -1972,11 +1972,13 @@ export class AgentSession {
 	 * @param targetId The entry ID to navigate to
 	 * @param options.summarize Whether user wants to summarize abandoned branch
 	 * @param options.customInstructions Custom instructions for summarizer
+	 * @param options.replaceInstructions If true, customInstructions replaces the default prompt
+	 * @param options.label Label to attach to the branch summary entry
 	 * @returns Result with editorText (if user message) and cancelled status
 	 */
 	async navigateTree(
 		targetId: string,
-		options: { summarize?: boolean; customInstructions?: string } = {},
+		options: { summarize?: boolean; customInstructions?: string; replaceInstructions?: boolean; label?: string } = {},
 	): Promise<{ editorText?: string; cancelled: boolean; aborted?: boolean; summaryEntry?: BranchSummaryEntry }> {
 		const oldLeafId = this.sessionManager.getLeafId();
 
@@ -2002,13 +2004,20 @@ export class AgentSession {
 			targetId,
 		);
 
-		// Prepare event data
+		// Prepare event data - mutable so extensions can override
+		let customInstructions = options.customInstructions;
+		let replaceInstructions = options.replaceInstructions;
+		let label = options.label;
+
 		const preparation: TreePreparation = {
 			targetId,
 			oldLeafId,
 			commonAncestorId,
 			entriesToSummarize,
 			userWantsSummary: options.summarize ?? false,
+			customInstructions,
+			replaceInstructions,
+			label,
 		};
 
 		// Set up abort controller for summarization
@@ -2032,6 +2041,17 @@ export class AgentSession {
 				extensionSummary = result.summary;
 				fromExtension = true;
 			}
+
+			// Allow extensions to override instructions and label
+			if (result?.customInstructions !== undefined) {
+				customInstructions = result.customInstructions;
+			}
+			if (result?.replaceInstructions !== undefined) {
+				replaceInstructions = result.replaceInstructions;
+			}
+			if (result?.label !== undefined) {
+				label = result.label;
+			}
 		}
 
 		// Run default summarizer if needed
@@ -2048,7 +2068,8 @@ export class AgentSession {
 				model,
 				apiKey,
 				signal: this._branchSummaryAbortController.signal,
-				customInstructions: options.customInstructions,
+				customInstructions,
+				replaceInstructions,
 				reserveTokens: branchSummarySettings.reserveTokens,
 			});
 			this._branchSummaryAbortController = undefined;
@@ -2098,6 +2119,11 @@ export class AgentSession {
 			// Create summary at target position (can be null for root)
 			const summaryId = this.sessionManager.branchWithSummary(newLeafId, summaryText, summaryDetails, fromExtension);
 			summaryEntry = this.sessionManager.getEntry(summaryId) as BranchSummaryEntry;
+
+			// Attach label to the summary entry
+			if (label) {
+				this.sessionManager.appendLabelChange(summaryId, label);
+			}
 		} else if (newLeafId === null) {
 			// No summary, navigating to root - reset leaf
 			this.sessionManager.resetLeaf();
@@ -2106,6 +2132,11 @@ export class AgentSession {
 			this.sessionManager.branch(newLeafId);
 		}
 
+		// Attach label to target entry when not summarizing (no summary entry to label)
+		if (label && !summaryText) {
+			this.sessionManager.appendLabelChange(targetId, label);
+		}
+
 		// Update agent state
 		const sessionContext = this.sessionManager.buildSessionContext();
 		this.agent.replaceMessages(sessionContext.messages);

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

@@ -71,6 +71,8 @@ export interface GenerateBranchSummaryOptions {
 	signal: AbortSignal;
 	/** Optional custom instructions for summarization */
 	customInstructions?: string;
+	/** If true, customInstructions replaces the default prompt instead of being appended */
+	replaceInstructions?: boolean;
 	/** Tokens reserved for prompt + LLM response (default 16384) */
 	reserveTokens?: number;
 }
@@ -279,7 +281,7 @@ export async function generateBranchSummary(
 	entries: SessionEntry[],
 	options: GenerateBranchSummaryOptions,
 ): Promise<BranchSummaryResult> {
-	const { model, apiKey, signal, customInstructions, reserveTokens = 16384 } = options;
+	const { model, apiKey, signal, customInstructions, replaceInstructions, reserveTokens = 16384 } = options;
 
 	// Token budget = context window minus reserved space for prompt + response
 	const contextWindow = model.contextWindow || 128000;
@@ -297,9 +299,14 @@ export async function generateBranchSummary(
 	const conversationText = serializeConversation(llmMessages);
 
 	// Build prompt
-	const instructions = customInstructions
-		? `${BRANCH_SUMMARY_PROMPT}\n\nAdditional focus: ${customInstructions}`
-		: BRANCH_SUMMARY_PROMPT;
+	let instructions: string;
+	if (replaceInstructions && customInstructions) {
+		instructions = customInstructions;
+	} else if (customInstructions) {
+		instructions = `${BRANCH_SUMMARY_PROMPT}\n\nAdditional focus: ${customInstructions}`;
+	} else {
+		instructions = BRANCH_SUMMARY_PROMPT;
+	}
 	const promptText = `<conversation>\n${conversationText}\n</conversation>\n\n${instructions}`;
 
 	const summarizationMessages = [

packages/coding-agent/src/core/extensions/runner.ts

@@ -57,7 +57,7 @@ export type ForkHandler = (entryId: string) => Promise<{ cancelled: boolean }>;
 
 export type NavigateTreeHandler = (
 	targetId: string,
-	options?: { summarize?: boolean },
+	options?: { summarize?: boolean; customInstructions?: string; replaceInstructions?: boolean; label?: string },
 ) => Promise<{ cancelled: boolean }>;
 
 export type ShutdownHandler = () => void;

packages/coding-agent/src/core/extensions/types.ts

@@ -344,6 +344,12 @@ export interface TreePreparation {
 	commonAncestorId: string | null;
 	entriesToSummarize: SessionEntry[];
 	userWantsSummary: boolean;
+	/** Custom instructions for summarization */
+	customInstructions?: string;
+	/** If true, customInstructions replaces the default prompt instead of being appended */
+	replaceInstructions?: boolean;
+	/** Label to attach to the branch summary entry */
+	label?: string;
 }
 
 /** Fired before navigating in the session tree (can be cancelled) */
@@ -633,6 +639,12 @@ export interface SessionBeforeTreeResult {
 		summary: string;
 		details?: unknown;
 	};
+	/** Override custom instructions for summarization */
+	customInstructions?: string;
+	/** Override whether customInstructions replaces the default prompt */
+	replaceInstructions?: boolean;
+	/** Override label to attach to the branch summary entry */
+	label?: string;
 }
 
 // ============================================================================
@@ -917,7 +929,10 @@ export interface ExtensionCommandContextActions {
 		setup?: (sessionManager: SessionManager) => Promise<void>;
 	}) => Promise<{ cancelled: boolean }>;
 	fork: (entryId: string) => Promise<{ cancelled: boolean }>;
-	navigateTree: (targetId: string, options?: { summarize?: boolean }) => Promise<{ cancelled: boolean }>;
+	navigateTree: (
+		targetId: string,
+		options?: { summarize?: boolean; customInstructions?: string; replaceInstructions?: boolean; label?: string },
+	) => Promise<{ cancelled: boolean }>;
 }
 
 /**

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -717,7 +717,12 @@ export class InteractiveMode {
 					return { cancelled: false };
 				},
 				navigateTree: async (targetId, options) => {
-					const result = await this.session.navigateTree(targetId, { summarize: options?.summarize });
+					const result = await this.session.navigateTree(targetId, {
+						summarize: options?.summarize,
+						customInstructions: options?.customInstructions,
+						replaceInstructions: options?.replaceInstructions,
+						label: options?.label,
+					});
 					if (result.cancelled) {
 						return { cancelled: true };
 					}