fix(coding-agent): avoid empty compaction summaries

closes #4811

packages/coding-agent/test/compaction-extensions.test.ts

@@ -98,6 +98,7 @@ describe.skipIf(!API_KEY)("Compaction extensions", () => {
 
 		const sessionManager = SessionManager.create(tempDir);
 		const settingsManager = SettingsManager.create(tempDir, tempDir);
+		settingsManager.applyOverrides({ compaction: { keepRecentTokens: 1 } });
 		const authStorage = AuthStorage.create(join(tempDir, "auth.json"));
 		const modelRegistry = ModelRegistry.create(authStorage);
 

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

@@ -9,7 +9,6 @@ import {
 	calculateContextTokens,
 	compact,
 	DEFAULT_COMPACTION_SETTINGS,
-	estimateContextTokens,
 	findCutPoint,
 	getLastAssistantUsage,
 	prepareCompaction,
@@ -396,7 +395,7 @@ describe("buildSessionContext", () => {
 });
 
 describe("prepareCompaction with previous compaction", () => {
-	it("should preserve kept messages across repeated compactions when they still fit", () => {
+	it("should skip repeated compactions when kept messages still fit", () => {
 		const u1 = createMessageEntry(createUserMessage("user msg 1 (summarized by compaction1)"));
 		const a1 = createMessageEntry(createAssistantMessage("assistant msg 1"));
 		const u2 = createMessageEntry(createUserMessage("user msg 2 - kept by compaction1"));
@@ -408,29 +407,9 @@ describe("prepareCompaction with previous compaction", () => {
 		const a4 = createMessageEntry(createAssistantMessage("assistant msg 4", createMockUsage(8000, 2000)));
 
 		const pathEntries = [u1, a1, u2, a2, u3, a3, compaction1, u4, a4];
-		const contextBefore = buildSessionContext(pathEntries);
 		const preparation = prepareCompaction(pathEntries, DEFAULT_COMPACTION_SETTINGS);
 
-		expect(preparation).toBeDefined();
-		expect(preparation!.firstKeptEntryId).toBe(u2.id);
-		expect(preparation!.previousSummary).toBe("First summary");
-		expect(extractText(preparation!.messagesToSummarize)).not.toContain("First summary");
-		expect(preparation!.tokensBefore).toBe(estimateContextTokens(contextBefore.messages).tokens);
-
-		const compaction2: CompactionEntry = {
-			type: "compaction",
-			id: "compaction2-id",
-			parentId: a4.id,
-			timestamp: new Date().toISOString(),
-			summary: "Second summary",
-			firstKeptEntryId: preparation!.firstKeptEntryId,
-			tokensBefore: preparation!.tokensBefore,
-		};
-		const contextAfter = buildSessionContext([...pathEntries, compaction2]);
-		const contextAfterText = extractText(contextAfter.messages);
-
-		expect(contextAfterText).toContain("user msg 2 - kept by compaction1");
-		expect(contextAfterText).toContain("user msg 3 - kept by compaction1");
+		expect(preparation).toBeUndefined();
 	});
 
 	it("should re-summarize previously kept messages when the recent window moves past them", () => {

packages/coding-agent/test/suite/agent-session-compaction.test.ts

@@ -67,19 +67,20 @@ function useSummaryStreamFn(harness: Harness, summary: string): () => number {
 }
 
 function seedCompactableSession(harness: Harness): void {
+	harness.settingsManager.applyOverrides({ compaction: { keepRecentTokens: 1 } });
 	const now = Date.now();
 	harness.sessionManager.appendMessage({
 		role: "user",
 		content: [{ type: "text", text: "message to compact" }],
 		timestamp: now - 1000,
 	});
-	harness.sessionManager.appendMessage(
-		createAssistant(harness, {
-			stopReason: "stop",
-			totalTokens: 100,
-			timestamp: now - 500,
-		}),
-	);
+	const assistant = createAssistant(harness, {
+		stopReason: "stop",
+		totalTokens: 100,
+		timestamp: now - 500,
+	});
+	assistant.content = [{ type: "text", text: "assistant response to compact" }];
+	harness.sessionManager.appendMessage(assistant);
 	harness.session.agent.state.messages = harness.sessionManager.buildSessionContext().messages;
 }
 
@@ -96,6 +97,7 @@ describe("AgentSession compaction characterization", () => {
 
 	it("manually compacts using an extension-provided summary", async () => {
 		const harness = await createHarness({
+			settings: { compaction: { keepRecentTokens: 1 } },
 			extensionFactories: [
 				(pi) => {
 					pi.on("session_before_compact", async (event) => ({
@@ -145,7 +147,7 @@ describe("AgentSession compaction characterization", () => {
 
 		const result = await harness.session.compact();
 
-		expect(result.summary).toBe("summary from custom stream");
+		expect(result.summary).toContain("summary from custom stream");
 		expect(getStreamCallCount()).toBe(1);
 	});
 
@@ -165,6 +167,7 @@ describe("AgentSession compaction characterization", () => {
 
 	it("cancels in-progress manual compaction when abortCompaction is called", async () => {
 		const harness = await createHarness({
+			settings: { compaction: { keepRecentTokens: 1 } },
 			extensionFactories: [
 				(pi) => {
 					pi.on("session_before_compact", async (event) => {