feat(coding-agent): support compaction routing sessions

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

@@ -1873,6 +1873,7 @@ export class AgentSession {
 					env,
 					this.settingsManager.getRetrySettings(),
 					this._summarizationRetryCallbacks({ source: "compaction", reason: "manual" }),
+					undefined, // sessionId
 				);
 				summary = result.summary;
 				firstKeptEntryId = result.firstKeptEntryId;
@@ -2161,6 +2162,7 @@ export class AgentSession {
 					env,
 					this.settingsManager.getRetrySettings(),
 					this._summarizationRetryCallbacks({ source: "compaction", reason }),
+					undefined, // sessionId
 				);
 				summary = compactResult.summary;
 				firstKeptEntryId = compactResult.firstKeptEntryId;

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

@@ -544,8 +544,9 @@ function createSummarizationOptions(
 	env: Record<string, string> | undefined,
 	signal: AbortSignal | undefined,
 	thinkingLevel: ThinkingLevel | undefined,
+	sessionId: string | undefined,
 ): SimpleStreamOptions {
-	const options: SimpleStreamOptions = { maxTokens, signal, apiKey, headers, env };
+	const options: SimpleStreamOptions = { maxTokens, signal, apiKey, headers, env, sessionId };
 	if (model.reasoning && thinkingLevel && thinkingLevel !== "off") {
 		options.reasoning = thinkingLevel;
 	}
@@ -567,11 +568,12 @@ export async function completeSummarization(
 	retry?: RetryPolicy,
 	callbacks?: RetryCallbacks,
 ): Promise<AssistantMessage> {
-	// Summaries are standalone requests, so isolate routing and avoid cache writes that cannot be reused.
+	// Avoid cache writes for one-off summaries. Reuse caller-supplied routing when available;
+	// callers without a session ID, including branch summaries, receive a fresh routing ID.
 	const requestOptions: SimpleStreamOptions = {
 		...options,
 		cacheRetention: "none",
-		sessionId: uuidv7(),
+		sessionId: options.sessionId ?? uuidv7(),
 	};
 	const produce = async (): Promise<AssistantMessage> =>
 		streamFn
@@ -598,6 +600,7 @@ export async function generateSummary(
 	env?: Record<string, string>,
 	retry?: RetryPolicy,
 	callbacks?: RetryCallbacks,
+	sessionId?: string,
 ): Promise<string> {
 	return (
 		await generateSummaryWithUsage(
@@ -614,6 +617,7 @@ export async function generateSummary(
 			env,
 			retry,
 			callbacks,
+			sessionId,
 		)
 	).text;
 }
@@ -633,6 +637,7 @@ export async function generateSummaryWithUsage(
 	env?: Record<string, string>,
 	retry?: RetryPolicy,
 	callbacks?: RetryCallbacks,
+	sessionId?: string,
 ): Promise<{ text: string; usage: Usage }> {
 	const maxTokens = Math.min(
 		Math.floor(0.8 * reserveTokens),
@@ -665,7 +670,16 @@ export async function generateSummaryWithUsage(
 		},
 	];
 
-	const completionOptions = createSummarizationOptions(model, maxTokens, apiKey, headers, env, signal, thinkingLevel);
+	const completionOptions = createSummarizationOptions(
+		model,
+		maxTokens,
+		apiKey,
+		headers,
+		env,
+		signal,
+		thinkingLevel,
+		sessionId,
+	);
 
 	const response = await completeSummarization(
 		model,
@@ -813,6 +827,7 @@ Be concise. Focus on what's needed to understand the kept suffix.`;
  *
  * @param preparation - Pre-calculated preparation from prepareCompaction()
  * @param customInstructions - Optional custom focus for the summary
+ * @param sessionId - Optional routing session ID forwarded without enabling prompt caching
  */
 export async function compact(
 	preparation: CompactionPreparation,
@@ -826,6 +841,7 @@ export async function compact(
 	env?: Record<string, string>,
 	retry?: RetryPolicy,
 	callbacks?: RetryCallbacks,
+	sessionId?: string,
 ): Promise<CompactionResult> {
 	const {
 		firstKeptEntryId,
@@ -860,6 +876,7 @@ export async function compact(
 				env,
 				retry,
 				callbacks,
+				sessionId,
 			);
 			historyText = historyResult.text;
 			historyUsage = historyResult.usage;
@@ -876,6 +893,7 @@ export async function compact(
 			streamFn,
 			retry,
 			callbacks,
+			sessionId,
 		);
 		// Merge into single summary
 		summary = `${historyText}\n\n---\n\n**Turn Context (split turn):**\n\n${turnPrefixResult.text}`;
@@ -896,6 +914,7 @@ export async function compact(
 			env,
 			retry,
 			callbacks,
+			sessionId,
 		);
 		summary = result.text;
 		summaryUsage = result.usage;
@@ -933,6 +952,7 @@ async function generateTurnPrefixSummary(
 	streamFn?: StreamFn,
 	retry?: RetryPolicy,
 	callbacks?: RetryCallbacks,
+	sessionId?: string,
 ): Promise<{ text: string; usage: Usage }> {
 	const maxTokens = Math.min(
 		Math.floor(0.5 * reserveTokens),
@@ -952,7 +972,7 @@ async function generateTurnPrefixSummary(
 	const response = await completeSummarization(
 		model,
 		{ systemPrompt: SUMMARIZATION_SYSTEM_PROMPT, messages: summarizationMessages },
-		createSummarizationOptions(model, maxTokens, apiKey, headers, env, signal, thinkingLevel),
+		createSummarizationOptions(model, maxTokens, apiKey, headers, env, signal, thinkingLevel, sessionId),
 		streamFn,
 		retry,
 		callbacks,

packages/coding-agent/test/compaction-summary-reasoning.test.ts

@@ -4,6 +4,7 @@ import { beforeEach, describe, expect, it, vi } from "vitest";
 import {
 	type CompactionPreparation,
 	compact,
+	completeSummarization,
 	generateSummary,
 	generateSummaryWithUsage,
 } from "../src/core/compaction/index.ts";
@@ -102,6 +103,19 @@ describe("generateSummary reasoning options", () => {
 		expect(sessionIds[0]).not.toBe(sessionIds[1]);
 	});
 
+	it("honors a caller-supplied routing session without prompt caching", async () => {
+		await completeSummarization(
+			createModel(false),
+			{ systemPrompt: "Summarize", messages: [] },
+			{ sessionId: "current-routing-session", cacheRetention: "long" },
+		);
+
+		expect(completeSimpleMock.mock.calls[0][2]).toMatchObject({
+			sessionId: "current-routing-session",
+			cacheRetention: "none",
+		});
+	});
+
 	it("does not set reasoning when thinking is off", async () => {
 		await generateSummary(
 			messages,