fix(coding-agent): avoid empty compaction summaries

closes #4811

packages/coding-agent/CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Fixed
 
+- Fixed compaction to refuse sessions with no eligible messages instead of producing empty summaries ([#4811](https://github.com/earendil-works/pi/issues/4811)).
 - Fixed successful overflow-triggered auto-compaction to avoid retrying completed assistant responses ([#5720](https://github.com/earendil-works/pi/issues/5720)).
 
 ## [0.79.7] - 2026-06-18

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

@@ -1892,19 +1892,10 @@ export class AgentSession {
 	 */
 	private async _runAutoCompaction(reason: "overflow" | "threshold", willRetry: boolean): Promise<boolean> {
 		const settings = this.settingsManager.getCompactionSettings();
-
-		this._emit({ type: "compaction_start", reason });
-		this._autoCompactionAbortController = new AbortController();
+		let started = false;
 
 		try {
 			if (!this.model) {
-				this._emit({
-					type: "compaction_end",
-					reason,
-					result: undefined,
-					aborted: false,
-					willRetry: false,
-				});
 				return false;
 			}
 
@@ -1914,13 +1905,6 @@ export class AgentSession {
 			if (this.agent.streamFn === streamSimple) {
 				const authResult = await this._modelRegistry.getApiKeyAndHeaders(this.model);
 				if (!authResult.ok || !authResult.apiKey) {
-					this._emit({
-						type: "compaction_end",
-						reason,
-						result: undefined,
-						aborted: false,
-						willRetry: false,
-					});
 					return false;
 				}
 				apiKey = authResult.apiKey;
@@ -1934,16 +1918,13 @@ export class AgentSession {
 
 			const preparation = prepareCompaction(pathEntries, settings);
 			if (!preparation) {
-				this._emit({
-					type: "compaction_end",
-					reason,
-					result: undefined,
-					aborted: false,
-					willRetry: false,
-				});
 				return false;
 			}
 
+			this._emit({ type: "compaction_start", reason });
+			this._autoCompactionAbortController = new AbortController();
+			started = true;
+
 			let extensionCompaction: CompactionResult | undefined;
 			let fromExtension = false;
 
@@ -2054,17 +2035,19 @@ export class AgentSession {
 			return this.agent.hasQueuedMessages();
 		} catch (error) {
 			const errorMessage = error instanceof Error ? error.message : "compaction failed";
-			this._emit({
-				type: "compaction_end",
-				reason,
-				result: undefined,
-				aborted: false,
-				willRetry: false,
-				errorMessage:
-					reason === "overflow"
-						? `Context overflow recovery failed: ${errorMessage}`
-						: `Auto-compaction failed: ${errorMessage}`,
-			});
+			if (started) {
+				this._emit({
+					type: "compaction_end",
+					reason,
+					result: undefined,
+					aborted: false,
+					willRetry: false,
+					errorMessage:
+						reason === "overflow"
+							? `Context overflow recovery failed: ${errorMessage}`
+							: `Auto-compaction failed: ${errorMessage}`,
+				});
+			}
 			return false;
 		} finally {
 			this._autoCompactionAbortController = undefined;

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

@@ -698,6 +698,10 @@ export function prepareCompaction(
 		}
 	}
 
+	if (messagesToSummarize.length === 0 && turnPrefixMessages.length === 0) {
+		return undefined;
+	}
+
 	// Extract file operations from messages and previous compaction
 	const fileOps = extractFileOperations(messagesToSummarize, pathEntries, prevCompactionIndex);
 

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

@@ -3700,7 +3700,6 @@ export class InteractiveMode {
 	showError(errorMessage: string): void {
 		this.chatContainer.addChild(new Spacer(1));
 		this.chatContainer.addChild(new Text(theme.fg("error", `Error: ${errorMessage}`), 1, 0));
-		this.chatContainer.addChild(new Spacer(1));
 		this.ui.requestRender();
 	}
 
@@ -5695,14 +5694,6 @@ export class InteractiveMode {
 	}
 
 	private async handleCompactCommand(customInstructions?: string): Promise<void> {
-		const entries = this.sessionManager.getEntries();
-		const messageCount = entries.filter((e) => e.type === "message").length;
-
-		if (messageCount < 2) {
-			this.showWarning("Nothing to compact (no messages yet)");
-			return;
-		}
-
 		if (this.loadingAnimation) {
 			this.loadingAnimation.stop();
 			this.loadingAnimation = undefined;

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) => {