diff --git a/packages/coding-agent/src/core/agent-session.ts b/packages/coding-agent/src/core/agent-session.ts
index f335a707..978fb1b8 100644
--- a/packages/coding-agent/src/core/agent-session.ts
+++ b/packages/coding-agent/src/core/agent-session.ts
@@ -22,9 +22,10 @@ import {
 	type CompactionResult,
 	calculateContextTokens,
 	compact,
+	generateBranchSummary,
 	prepareCompaction,
 	shouldCompact,
-} from "./compaction.js";
+} from "./compaction/index.js";
 import type { LoadedCustomTool, SessionEvent as ToolSessionEvent } from "./custom-tools/index.js";
 import { exportSessionToHtml } from "./export-html.js";
 import type {
@@ -1661,10 +1662,17 @@ export class AgentSession {
 		// Run default summarizer if needed
 		let summaryText: string | undefined;
 		if (options.summarize && entriesToSummarize.length > 0 && !hookSummary) {
-			const result = await this._generateBranchSummary(
+			const model = this.model!;
+			const apiKey = await this._modelRegistry.getApiKey(model);
+			if (!apiKey) {
+				throw new Error(`No API key for ${model.provider}`);
+			}
+			const result = await generateBranchSummary(
 				entriesToSummarize,
-				options.customInstructions,
+				model,
+				apiKey,
 				this._branchSummaryAbortController.signal,
+				options.customInstructions,
 			);
 			this._branchSummaryAbortController = undefined;
 			if (result.aborted) {
@@ -1738,104 +1746,6 @@ export class AgentSession {
 		return { editorText, cancelled: false, summaryEntry };
 	}
 
-	/**
-	 * Generate a summary of abandoned branch entries.
-	 */
-	private async _generateBranchSummary(
-		entries: SessionEntry[],
-		customInstructions: string | undefined,
-		signal: AbortSignal,
-	): Promise<{ summary?: string; aborted?: boolean; error?: string }> {
-		// Convert entries to messages for summarization
-		const messages: Array<{ role: string; content: string }> = [];
-		for (const entry of entries) {
-			if (entry.type === "message") {
-				const text = this._extractMessageText(entry.message);
-				if (text) {
-					messages.push({ role: entry.message.role, content: text });
-				}
-			} else if (entry.type === "custom_message") {
-				const text =
-					typeof entry.content === "string"
-						? entry.content
-						: entry.content
-								.filter((c): c is { type: "text"; text: string } => c.type === "text")
-								.map((c) => c.text)
-								.join("");
-				if (text) {
-					messages.push({ role: "user", content: text });
-				}
-			} else if (entry.type === "branch_summary") {
-				messages.push({ role: "system", content: `[Previous branch summary: ${entry.summary}]` });
-			}
-		}
-
-		if (messages.length === 0) {
-			return { summary: "No content to summarize" };
-		}
-
-		// Build prompt for summarization
-		const conversationText = messages.map((m) => `${m.role}: ${m.content}`).join("\n\n");
-		const instructions = customInstructions
-			? `${customInstructions}\n\n`
-			: "Summarize this conversation branch concisely, capturing key decisions, actions taken, and outcomes.\n\n";
-
-		const prompt = `${instructions}Conversation:\n${conversationText}`;
-
-		// Get API key for current model (model is checked in navigateTree before calling this)
-		const model = this.model!;
-		const apiKey = await this._modelRegistry.getApiKey(model);
-		if (!apiKey) {
-			throw new Error(`No API key for ${model.provider}`);
-		}
-
-		// Call LLM for summarization
-		const { complete } = await import("@mariozechner/pi-ai");
-		const response = await complete(
-			model,
-			{
-				messages: [
-					{
-						role: "user",
-						content: [{ type: "text", text: prompt }],
-						timestamp: Date.now(),
-					},
-				],
-			},
-			{ apiKey, signal, maxTokens: 1024 },
-		);
-
-		// Check if aborted or errored
-		if (response.stopReason === "aborted") {
-			return { aborted: true };
-		}
-		if (response.stopReason === "error") {
-			return { error: response.errorMessage || "Summarization failed" };
-		}
-
-		const summary = response.content
-			.filter((c): c is { type: "text"; text: string } => c.type === "text")
-			.map((c) => c.text)
-			.join("\n");
-
-		return { summary: summary || "No summary generated" };
-	}
-
-	/**
-	 * Extract text content from any message type.
-	 */
-	private _extractMessageText(message: any): string {
-		if (!message.content) return "";
-		if (typeof message.content === "string") return message.content;
-		if (Array.isArray(message.content)) {
-			return message.content
-				.filter((c: any) => c.type === "text")
-				.map((c: any) => c.text)
-				.join("");
-		}
-		return "";
-	}
-
 	/**
 	 * Get all user messages from session for branch selector.
 	 */
diff --git a/packages/coding-agent/src/core/compaction/branch-summarization.ts b/packages/coding-agent/src/core/compaction/branch-summarization.ts
new file mode 100644
index 00000000..7ac097f7
--- /dev/null
+++ b/packages/coding-agent/src/core/compaction/branch-summarization.ts
@@ -0,0 +1,115 @@
+/**
+ * Branch summarization for tree navigation.
+ *
+ * When navigating to a different point in the session tree, this generates
+ * a summary of the branch being left so context isn't lost.
+ */
+
+import type { Model } from "@mariozechner/pi-ai";
+import { complete } from "@mariozechner/pi-ai";
+import type { SessionEntry } from "../session-manager.js";
+
+const DEFAULT_INSTRUCTIONS =
+	"Summarize this conversation branch concisely, capturing key decisions, actions taken, and outcomes.";
+
+export interface BranchSummaryResult {
+	summary?: string;
+	aborted?: boolean;
+	error?: string;
+}
+
+/**
+ * Extract text content from any message type.
+ */
+function extractMessageText(message: any): string {
+	if (!message.content) return "";
+	if (typeof message.content === "string") return message.content;
+	if (Array.isArray(message.content)) {
+		return message.content
+			.filter((c: any) => c.type === "text")
+			.map((c: any) => c.text)
+			.join("");
+	}
+	return "";
+}
+
+/**
+ * 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
+ */
+export async function generateBranchSummary(
+	entries: SessionEntry[],
+	model: Model<any>,
+	apiKey: string,
+	signal: AbortSignal,
+	customInstructions?: string,
+): Promise<BranchSummaryResult> {
+	// Convert entries to messages for summarization
+	const messages: Array<{ role: string; content: string }> = [];
+
+	for (const entry of entries) {
+		if (entry.type === "message") {
+			const text = extractMessageText(entry.message);
+			if (text) {
+				messages.push({ role: entry.message.role, content: text });
+			}
+		} else if (entry.type === "custom_message") {
+			const text =
+				typeof entry.content === "string"
+					? entry.content
+					: entry.content
+							.filter((c): c is { type: "text"; text: string } => c.type === "text")
+							.map((c) => c.text)
+							.join("");
+			if (text) {
+				messages.push({ role: "user", content: text });
+			}
+		} else if (entry.type === "branch_summary") {
+			messages.push({ role: "system", content: `[Previous branch summary: ${entry.summary}]` });
+		}
+	}
+
+	if (messages.length === 0) {
+		return { summary: "No content to summarize" };
+	}
+
+	// Build prompt for summarization
+	const conversationText = messages.map((m) => `${m.role}: ${m.content}`).join("\n\n");
+	const instructions = customInstructions ? `${customInstructions}\n\n` : `${DEFAULT_INSTRUCTIONS}\n\n`;
+	const prompt = `${instructions}Conversation:\n${conversationText}`;
+
+	// Call LLM for summarization
+	const response = await complete(
+		model,
+		{
+			messages: [
+				{
+					role: "user",
+					content: [{ type: "text", text: prompt }],
+					timestamp: Date.now(),
+				},
+			],
+		},
+		{ apiKey, signal, maxTokens: 1024 },
+	);
+
+	// Check if aborted or errored
+	if (response.stopReason === "aborted") {
+		return { aborted: true };
+	}
+	if (response.stopReason === "error") {
+		return { error: response.errorMessage || "Summarization failed" };
+	}
+
+	const summary = response.content
+		.filter((c): c is { type: "text"; text: string } => c.type === "text")
+		.map((c) => c.text)
+		.join("\n");
+
+	return { summary: summary || "No summary generated" };
+}
diff --git a/packages/coding-agent/src/core/compaction.ts b/packages/coding-agent/src/core/compaction/compaction.ts
similarity index 99%
rename from packages/coding-agent/src/core/compaction.ts
rename to packages/coding-agent/src/core/compaction/compaction.ts
index 35d933a3..61e4b950 100644
--- a/packages/coding-agent/src/core/compaction.ts
+++ b/packages/coding-agent/src/core/compaction/compaction.ts
@@ -8,8 +8,8 @@
 import type { AgentMessage } from "@mariozechner/pi-agent-core";
 import type { AssistantMessage, Model, Usage } from "@mariozechner/pi-ai";
 import { complete } from "@mariozechner/pi-ai";
-import { convertToLlm, createBranchSummaryMessage, createHookMessage } from "./messages.js";
-import type { CompactionEntry, SessionEntry } from "./session-manager.js";
+import { convertToLlm, createBranchSummaryMessage, createHookMessage } from "../messages.js";
+import type { CompactionEntry, SessionEntry } from "../session-manager.js";
 
 /**
  * Extract AgentMessage from an entry if it produces one.
diff --git a/packages/coding-agent/src/core/compaction/index.ts b/packages/coding-agent/src/core/compaction/index.ts
new file mode 100644
index 00000000..4f8ad306
--- /dev/null
+++ b/packages/coding-agent/src/core/compaction/index.ts
@@ -0,0 +1,6 @@
+/**
+ * Compaction and summarization utilities.
+ */
+
+export * from "./branch-summarization.js";
+export * from "./compaction.js";
diff --git a/packages/coding-agent/src/core/hooks/types.ts b/packages/coding-agent/src/core/hooks/types.ts
index e3d20e31..e8dd689e 100644
--- a/packages/coding-agent/src/core/hooks/types.ts
+++ b/packages/coding-agent/src/core/hooks/types.ts
@@ -9,7 +9,7 @@ import type { AgentMessage } from "@mariozechner/pi-agent-core";
 import type { ImageContent, Message, Model, TextContent, ToolResultMessage } from "@mariozechner/pi-ai";
 import type { Component } from "@mariozechner/pi-tui";
 import type { Theme } from "../../modes/interactive/theme/theme.js";
-import type { CompactionPreparation, CompactionResult } from "../compaction.js";
+import type { CompactionPreparation, CompactionResult } from "../compaction/index.js";
 import type { ExecOptions, ExecResult } from "../exec.js";
 import type { HookMessage } from "../messages.js";
 import type { ModelRegistry } from "../model-registry.js";
diff --git a/packages/coding-agent/src/core/index.ts b/packages/coding-agent/src/core/index.ts
index 4b75e1cd..b25df418 100644
--- a/packages/coding-agent/src/core/index.ts
+++ b/packages/coding-agent/src/core/index.ts
@@ -12,7 +12,7 @@ export {
 	type SessionStats,
 } from "./agent-session.js";
 export { type BashExecutorOptions, type BashResult, executeBash } from "./bash-executor.js";
-export type { CompactionResult } from "./compaction.js";
+export type { CompactionResult } from "./compaction/index.js";
 export {
 	type CustomAgentTool,
 	type CustomToolFactory,
diff --git a/packages/coding-agent/src/index.ts b/packages/coding-agent/src/index.ts
index 1e563429..1ae08511 100644
--- a/packages/coding-agent/src/index.ts
+++ b/packages/coding-agent/src/index.ts
@@ -12,6 +12,7 @@ export {
 export { type ApiKeyCredential, type AuthCredential, AuthStorage, type OAuthCredential } from "./core/auth-storage.js";
 // Compaction
 export {
+	type BranchSummaryResult,
 	type CompactionResult,
 	type CutPointResult,
 	calculateContextTokens,
@@ -20,10 +21,11 @@ export {
 	estimateTokens,
 	findCutPoint,
 	findTurnStartIndex,
+	generateBranchSummary,
 	generateSummary,
 	getLastAssistantUsage,
 	shouldCompact,
-} from "./core/compaction.js";
+} from "./core/compaction/index.js";
 // Custom tools
 export type {
 	AgentToolUpdateCallback,
diff --git a/packages/coding-agent/src/modes/rpc/rpc-client.ts b/packages/coding-agent/src/modes/rpc/rpc-client.ts
index f8d8f213..0249ca11 100644
--- a/packages/coding-agent/src/modes/rpc/rpc-client.ts
+++ b/packages/coding-agent/src/modes/rpc/rpc-client.ts
@@ -10,7 +10,7 @@ import type { AgentEvent, AgentMessage, ThinkingLevel } from "@mariozechner/pi-a
 import type { ImageContent } from "@mariozechner/pi-ai";
 import type { SessionStats } from "../../core/agent-session.js";
 import type { BashResult } from "../../core/bash-executor.js";
-import type { CompactionResult } from "../../core/compaction.js";
+import type { CompactionResult } from "../../core/compaction/index.js";
 import type { RpcCommand, RpcResponse, RpcSessionState } from "./rpc-types.js";
 
 // ============================================================================
diff --git a/packages/coding-agent/src/modes/rpc/rpc-types.ts b/packages/coding-agent/src/modes/rpc/rpc-types.ts
index d4f7a73a..5feead90 100644
--- a/packages/coding-agent/src/modes/rpc/rpc-types.ts
+++ b/packages/coding-agent/src/modes/rpc/rpc-types.ts
@@ -9,7 +9,7 @@ import type { AgentMessage, ThinkingLevel } from "@mariozechner/pi-agent-core";
 import type { ImageContent, Model } from "@mariozechner/pi-ai";
 import type { SessionStats } from "../../core/agent-session.js";
 import type { BashResult } from "../../core/bash-executor.js";
-import type { CompactionResult } from "../../core/compaction.js";
+import type { CompactionResult } from "../../core/compaction/index.js";
 
 // ============================================================================
 // RPC Commands (stdin)
diff --git a/packages/coding-agent/test/compaction.test.ts b/packages/coding-agent/test/compaction.test.ts
index 331c3b1b..0468a582 100644
--- a/packages/coding-agent/test/compaction.test.ts
+++ b/packages/coding-agent/test/compaction.test.ts
@@ -12,7 +12,7 @@ import {
 	findCutPoint,
 	getLastAssistantUsage,
 	shouldCompact,
-} from "../src/core/compaction.js";
+} from "../src/core/compaction/index.js";
 import {
 	buildSessionContext,
 	type CompactionEntry,