fix(coding-agent): compact without provider usage

closes #8328

packages/coding-agent/CHANGELOG.md

@@ -21,6 +21,7 @@
 - Fixed hung pi.dev model catalog requests consuming the entire refresh deadline without retrying ([#8198](https://github.com/earendil-works/pi/issues/8198)).
 - Fixed inherited Xiaomi model catalogs listing shut-down MiMo V2 models in `/model` and `--list-models` ([#8187](https://github.com/earendil-works/pi/issues/8187)).
 - Fixed branch summary entries recording the navigation destination in `fromId` instead of the pre-navigation source leaf.
+- Fixed threshold auto-compaction being skipped when providers omit streaming usage data ([#8328](https://github.com/earendil-works/pi/issues/8328)).
 
 ## [0.84.2] - 2026-08-14
 

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

@@ -2099,17 +2099,20 @@ export class AgentSession {
 		if (assistantMessage.stopReason === "error" || directContextTokens === 0) {
 			const messages = this.agent.state.messages;
 			const estimate = estimateContextTokens(messages);
-			if (estimate.lastUsageIndex === null) return false; // No usage data at all
-			// Verify the usage source is post-compaction. Kept pre-compaction messages
-			// have stale usage reflecting the old (larger) context and would falsely
-			// trigger compaction right after one just finished.
-			const usageMsg = messages[estimate.lastUsageIndex];
-			if (
-				compactionEntry &&
-				usageMsg.role === "assistant" &&
-				(usageMsg as AssistantMessage).timestamp <= new Date(compactionEntry.timestamp).getTime()
-			) {
-				return false;
+			// Without provider usage, estimate.tokens is the pure message-size estimate.
+			// Only usage-backed estimates need the stale pre-compaction check.
+			if (estimate.lastUsageIndex !== null) {
+				// Verify the usage source is post-compaction. Kept pre-compaction messages
+				// have stale usage reflecting the old (larger) context and would falsely
+				// trigger compaction right after one just finished.
+				const usageMsg = messages[estimate.lastUsageIndex];
+				if (
+					compactionEntry &&
+					usageMsg.role === "assistant" &&
+					(usageMsg as AssistantMessage).timestamp <= new Date(compactionEntry.timestamp).getTime()
+				) {
+					return false;
+				}
 			}
 			contextTokens = estimate.tokens;
 		} else {

packages/coding-agent/test/suite/regressions/8328-zero-usage-auto-compaction.test.ts

@@ -0,0 +1,80 @@
+import type { AssistantMessage } from "@earendil-works/pi-ai";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { createHarness, type Harness } from "../harness.ts";
+
+type SessionWithCompactionInternals = {
+	_checkCompaction: (assistantMessage: AssistantMessage) => Promise<boolean>;
+	_runAutoCompaction: (reason: "overflow" | "threshold", willRetry: boolean) => Promise<boolean>;
+};
+
+function createZeroUsageAssistant(harness: Harness): AssistantMessage {
+	const model = harness.getModel();
+	return {
+		role: "assistant",
+		content: [{ type: "text", text: "response" }],
+		api: model.api,
+		provider: model.provider,
+		model: model.id,
+		usage: {
+			input: 0,
+			output: 0,
+			cacheRead: 0,
+			cacheWrite: 0,
+			totalTokens: 0,
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+		},
+		stopReason: "stop",
+		timestamp: Date.now(),
+	};
+}
+
+describe("issue #8328 zero-usage auto-compaction", () => {
+	const harnesses: Harness[] = [];
+
+	afterEach(() => {
+		vi.restoreAllMocks();
+		while (harnesses.length > 0) {
+			harnesses.pop()?.cleanup();
+		}
+	});
+
+	async function createCompactionHarness(): Promise<Harness> {
+		const harness = await createHarness({
+			models: [{ id: "faux-1", contextWindow: 100, maxTokens: 20 }],
+			settings: { compaction: { enabled: true, reserveTokens: 10 } },
+		});
+		harnesses.push(harness);
+		return harness;
+	}
+
+	it("uses the message estimate when no assistant has reported usage", async () => {
+		const harness = await createCompactionHarness();
+		const assistant = createZeroUsageAssistant(harness);
+		harness.session.agent.state.messages = [
+			{ role: "user", content: [{ type: "text", text: "x".repeat(400) }], timestamp: Date.now() - 1 },
+			assistant,
+		];
+		const sessionInternals = harness.session as unknown as SessionWithCompactionInternals;
+		const runAutoCompactionSpy = vi.spyOn(sessionInternals, "_runAutoCompaction").mockResolvedValue(false);
+
+		await sessionInternals._checkCompaction(assistant);
+
+		expect(runAutoCompactionSpy).toHaveBeenCalledOnce();
+		expect(runAutoCompactionSpy).toHaveBeenCalledWith("threshold", false);
+	});
+
+	it("does not compact when the zero-usage message estimate is below the threshold", async () => {
+		const harness = await createCompactionHarness();
+		const assistant = createZeroUsageAssistant(harness);
+		harness.session.agent.state.messages = [
+			{ role: "user", content: [{ type: "text", text: "short" }], timestamp: Date.now() - 1 },
+			assistant,
+		];
+		const sessionInternals = harness.session as unknown as SessionWithCompactionInternals;
+		const runAutoCompactionSpy = vi.spyOn(sessionInternals, "_runAutoCompaction").mockResolvedValue(false);
+
+		await sessionInternals._checkCompaction(assistant);
+
+		expect(runAutoCompactionSpy).not.toHaveBeenCalled();
+	});
+});