fix: preserve assistant usage snapshots during compaction cleanup

src/agents/pi-embedded-runner.sanitize-session-history.test.ts

@@ -14,6 +14,7 @@ import {
   sanitizeWithOpenAIResponses,
   TEST_SESSION_ID,
 } from "./pi-embedded-runner.sanitize-session-history.test-harness.js";
+import { makeZeroUsageSnapshot } from "./usage.js";
 
 vi.mock("./pi-embedded-helpers.js", async () => ({
   ...(await vi.importActual("./pi-embedded-helpers.js")),
@@ -210,7 +211,7 @@ describe("sanitizeSessionHistory", () => {
       | (AgentMessage & { usage?: unknown })
       | undefined;
     expect(staleAssistant).toBeDefined();
-    expect(staleAssistant?.usage).toBeUndefined();
+    expect(staleAssistant?.usage).toEqual(makeZeroUsageSnapshot());
   });
 
   it("preserves fresh assistant usage snapshots created after latest compaction summary", async () => {
@@ -264,7 +265,7 @@ describe("sanitizeSessionHistory", () => {
       AgentMessage & { usage?: unknown }
     >;
     expect(assistants).toHaveLength(2);
-    expect(assistants[0]?.usage).toBeUndefined();
+    expect(assistants[0]?.usage).toEqual(makeZeroUsageSnapshot());
     expect(assistants[1]?.usage).toBeDefined();
   });
 
@@ -306,7 +307,7 @@ describe("sanitizeSessionHistory", () => {
     const assistant = result.find((message) => message.role === "assistant") as
       | (AgentMessage & { usage?: unknown })
       | undefined;
-    expect(assistant?.usage).toBeUndefined();
+    expect(assistant?.usage).toEqual(makeZeroUsageSnapshot());
   });
 
   it("keeps fresh usage after compaction timestamp in summary-first ordering", async () => {
@@ -368,7 +369,7 @@ describe("sanitizeSessionHistory", () => {
     const freshAssistant = assistants.find((message) =>
       JSON.stringify(message.content).includes("fresh answer"),
     );
-    expect(keptAssistant?.usage).toBeUndefined();
+    expect(keptAssistant?.usage).toEqual(makeZeroUsageSnapshot());
     expect(freshAssistant?.usage).toBeDefined();
   });
 

src/agents/pi-embedded-runner/google.ts

@@ -24,6 +24,7 @@ import {
 } from "../session-transcript-repair.js";
 import type { TranscriptPolicy } from "../transcript-policy.js";
 import { resolveTranscriptPolicy } from "../transcript-policy.js";
+import { makeZeroUsageSnapshot } from "../usage.js";
 import { log } from "./logger.js";
 import { dropThinkingBlocks } from "./thinking.js";
 import { describeUnknownError } from "./utils.js";
@@ -186,9 +187,13 @@ function stripStaleAssistantUsageBeforeLatestCompaction(messages: AgentMessage[]
       continue;
     }
 
+    // pi-coding-agent expects assistant usage to always be present during context
+    // accounting. Keep stale snapshots structurally valid, but zeroed out.
     const candidateRecord = candidate as unknown as Record<string, unknown>;
-    const { usage: _droppedUsage, ...rest } = candidateRecord;
-    out[i] = rest as unknown as AgentMessage;
+    out[i] = {
+      ...candidateRecord,
+      usage: makeZeroUsageSnapshot(),
+    } as unknown as AgentMessage;
     touched = true;
   }
   return touched ? out : messages;

src/agents/pi-embedded-subscribe.handlers.compaction.ts

@@ -2,6 +2,7 @@ import type { AgentEvent } from "@mariozechner/pi-agent-core";
 import { emitAgentEvent } from "../infra/agent-events.js";
 import { getGlobalHookRunner } from "../plugins/hook-runner-global.js";
 import type { EmbeddedPiSubscribeContext } from "./pi-embedded-subscribe.handlers.types.js";
+import { makeZeroUsageSnapshot } from "./usage.js";
 
 export function handleAutoCompactionStart(ctx: EmbeddedPiSubscribeContext) {
   ctx.state.compactionInFlight = true;
@@ -96,9 +97,8 @@ function clearStaleAssistantUsageOnSessionMessages(ctx: EmbeddedPiSubscribeConte
     if (candidate.role !== "assistant") {
       continue;
     }
-    if (!("usage" in candidate)) {
-      continue;
-    }
-    delete (candidate as { usage?: unknown }).usage;
+    // pi-coding-agent expects assistant usage to exist when computing context usage.
+    // Reset stale snapshots to zeros instead of deleting the field.
+    candidate.usage = makeZeroUsageSnapshot();
   }
 }

src/agents/pi-embedded-subscribe.subscribe-embedded-pi-session.splits-long-single-line-fenced-blocks-reopen.test.ts

@@ -6,6 +6,7 @@ import {
   expectFencedChunks,
 } from "./pi-embedded-subscribe.e2e-harness.js";
 import { subscribeEmbeddedPiSession } from "./pi-embedded-subscribe.js";
+import { makeZeroUsageSnapshot } from "./usage.js";
 
 type SessionEventHandler = (evt: unknown) => void;
 
@@ -115,4 +116,40 @@ describe("subscribeEmbeddedPiSession", () => {
     expect(resolved).toBe(true);
     expect(subscription.isCompacting()).toBe(false);
   });
+
+  it("resets assistant usage to a zero snapshot after compaction without retry", () => {
+    const listeners: SessionEventHandler[] = [];
+    const session = {
+      messages: [
+        {
+          role: "assistant",
+          content: [{ type: "text", text: "old" }],
+          usage: {
+            input: 120,
+            output: 30,
+            cacheRead: 5,
+            cacheWrite: 0,
+            totalTokens: 155,
+            cost: { input: 0.001, output: 0.002, cacheRead: 0, cacheWrite: 0, total: 0.003 },
+          },
+        },
+      ],
+      subscribe: (listener: SessionEventHandler) => {
+        listeners.push(listener);
+        return () => {};
+      },
+    } as unknown as Parameters<typeof subscribeEmbeddedPiSession>[0]["session"];
+
+    subscribeEmbeddedPiSession({
+      session,
+      runId: "run-3",
+    });
+
+    for (const listener of listeners) {
+      listener({ type: "auto_compaction_end", willRetry: false });
+    }
+
+    const usage = (session.messages?.[0] as { usage?: unknown } | undefined)?.usage;
+    expect(usage).toEqual(makeZeroUsageSnapshot());
+  });
 });

src/agents/usage.ts

@@ -34,6 +34,38 @@ export type NormalizedUsage = {
   total?: number;
 };
 
+export type AssistantUsageSnapshot = {
+  input: number;
+  output: number;
+  cacheRead: number;
+  cacheWrite: number;
+  totalTokens: number;
+  cost: {
+    input: number;
+    output: number;
+    cacheRead: number;
+    cacheWrite: number;
+    total: number;
+  };
+};
+
+export function makeZeroUsageSnapshot(): AssistantUsageSnapshot {
+  return {
+    input: 0,
+    output: 0,
+    cacheRead: 0,
+    cacheWrite: 0,
+    totalTokens: 0,
+    cost: {
+      input: 0,
+      output: 0,
+      cacheRead: 0,
+      cacheWrite: 0,
+      total: 0,
+    },
+  };
+}
+
 const asFiniteNumber = (value: unknown): number | undefined => {
   if (typeof value !== "number") {
     return undefined;