feat(compaction): fold to a token-capped tail, not recent turns

e2e/compaction-read-paging.test.ts

@@ -41,20 +41,18 @@ function logLines(count: number): string {
 async function openFoldSession(): Promise<E2ESession> {
   const session = await openE2ESession({
     permissionGate: e2ePermissionGate(),
-    // A tiny tail budget keeps the fold honest: the default 7500-token
+    // A tiny tail budget keeps the fold honest: the default 2500-token
     // budget would absorb this small scenario into the live tail and the
     // compactor would correctly no-op.
-    compactionShape: { tailBudgetTokens: 10 },
-    // Echo the summarized turns verbatim: the spine must exist, and this
-    // scenario asserts on the kept live bodies, not the summary text.
-    compactionCompletion: async (turns) =>
-      turns
-        .flatMap((turn) =>
-          turn.content.flatMap((block) =>
-            block.type === "text" ? [block.text] : [],
-          ),
-        )
-        .join("\n"),
+    // Budget fits the paged read windows in the live tail but not the bulky
+    // pad turns, so the fold still fires.
+    compactionShape: { tailBudgetTokens: 500 },
+    // A short handoff, not a prompt echo: complete() receives the summarizer
+    // prompt turns, and a 4k echo gets truncated off the verify-repair tail
+    // so the fold aborts. This scenario asserts on kept live bodies, not the
+    // summary text.
+    compactionCompletion: async () =>
+      "Goal: page through var/log/big.log. Next: keep reading remaining windows.",
   });
   seedFile(session, "var/log/big.log", `${logLines(80)}\n`);
   return session;
@@ -78,7 +76,7 @@ async function lastRequestBody(session: E2ESession): Promise<string> {
 async function padTurns(session: E2ESession, count: number): Promise<void> {
   for (let i = 0; i < count; i++) {
     session.harness.scenario.replyOnce("anthropic", {
-      text: `Acknowledged ${i}.`,
+      text: `Acknowledged ${i}. ${"pad".repeat(800)}`,
       headUsage: LOW_USAGE,
     });
     await runUntilDone(session, `Pad turn ${i}.`);

evals/compaction/fixtures.ts

@@ -11,7 +11,7 @@ export const BASELINE = {
   syntheticLowInput: 100,
   syntheticTriggerInput: 200000,
   outputTokens: 1,
-  keepRecentTurns: 6,
+  tailBudgetTokens: 2500,
   summaryMaxChars: 4000,
   wallTimeoutMs: 30000,
 } as const;

src/agent/chat-event-subscribers.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, test } from "bun:test";
 import {
   CHAT_TASKS_CHANGED_EVENT,
   CHAT_TOOLS_ACTIVATE_EVENT,
+  COMPACTION_FOLD_NONCONVERGED_EVENT,
 } from "./director.js";
 import { handleChatDirectorEvent } from "./chat-event-subscribers.js";
 import type { Task } from "./tasks.js";
@@ -119,4 +120,28 @@ describe("handleChatDirectorEvent", () => {
     expect(dispatched).toBe(false);
     expect(log.calls).toEqual([]);
   });
+
+  test("dispatches a fold-nonconverged notice", () => {
+    const seen: string[] = [];
+    const log = makeLog();
+    const handled = handleChatDirectorEvent(
+      {
+        type: COMPACTION_FOLD_NONCONVERGED_EVENT,
+        data: { notice: "Context fold did not reduce occupancy." },
+      },
+      {
+        onTasksChanged: () => {
+          throw new Error("unexpected tasks-changed dispatch");
+        },
+        onToolsActivate: () => {
+          throw new Error("unexpected tools-activate dispatch");
+        },
+        onFoldNonConverged: (notice) => seen.push(notice),
+      },
+      log.log,
+    );
+    expect(handled).toBe(true);
+    expect(seen).toEqual(["Context fold did not reduce occupancy."]);
+    expect(log.calls).toEqual([]);
+  });
 });

src/agent/chat-event-subscribers.ts

@@ -10,14 +10,17 @@ import { type } from "arktype";
 import {
   CHAT_TASKS_CHANGED_EVENT,
   CHAT_TOOLS_ACTIVATE_EVENT,
+  COMPACTION_FOLD_NONCONVERGED_EVENT,
   ChatTasksChangedDataSchema,
   ChatToolsActivateDataSchema,
+  CompactionFoldNonConvergedDataSchema,
 } from "./director.js";
 import type { Task } from "./tasks.js";
 
 export interface ChatDirectorEventHandlers {
   onTasksChanged: (tasks: Task[]) => void;
   onToolsActivate: (names: string[]) => void;
+  onFoldNonConverged?: (notice: string) => void;
 }
 
 export type ChatDirectorEventDebugLog = (
@@ -58,5 +61,17 @@ export function handleChatDirectorEvent(
     handlers.onToolsActivate(parsed.names);
     return true;
   }
+  if (event.type === COMPACTION_FOLD_NONCONVERGED_EVENT) {
+    const parsed = CompactionFoldNonConvergedDataSchema(event.data);
+    if (parsed instanceof type.errors) {
+      logDebug(
+        "chat fold-nonconverged event dropped invalid payload: {error}",
+        { error: parsed.summary },
+      );
+      return true;
+    }
+    handlers.onFoldNonConverged?.(parsed.notice);
+    return true;
+  }
   return false;
 }