Keep compacted prompt prefixes byte-stable across passes

CHANGELOG.md

@@ -27,6 +27,13 @@ parallel copies under `docs/` or `scripts/notes/`. At cut time: rename
   closures count against `maxAnchorTurns`. The LLM summary is workflow-aware
   and skips degenerate assistant text.
 
+- **Prefix-stable summaries and growth hysteresis.** Existing compacted user
+  turns stay byte-identical across later passes; new folds become later summary
+  turns with an assistant spacer so the prompt prefix can stay in the KV cache.
+  After a compact that remains over the high watermark, the governor waits for
+  usage to grow by 10% of the window before re-arming. Overflow recovery still
+  compacts immediately.
+
 ### Plugins
 
 - **`run_shell` no longer defaults to a 15s timeout.** Omitted timeout arms no

docs/ARCHITECTURE.md

@@ -173,11 +173,11 @@ The agent maintains an optional **`manage_tasks`** list (create/update via the h
 
 #### Context compaction (the compaction governor)
 
-When a cycle's input tokens cross a threshold, the director compacts the inference-facing history (the full run is always retained in the context store). The threshold is **model-aware** — roughly 60% of the active model's real context window — so small-window models compact early enough to avoid provider context-overflow while large-window models do not compact prematurely. The governor covers three cases:
+When a cycle's input tokens cross a threshold, the director compacts the inference-facing history (the full run is always retained in the context store). The threshold is **model-aware** — roughly 60% of the active model's real context window — so small-window models compact early enough to avoid provider context-overflow while large-window models do not compact prematurely. The compacted prefix is **append-only across passes**: the existing compacted user turn stays byte-identical; new folds become later summary turns with an assistant spacer between them so the prompt head can remain in the provider KV cache. The governor covers three cases:
 
-- **Threshold at a tool pause** — Once over threshold, the follow-up `infer` after a tool batch is swapped for a `compact` cycle, and inference resumes via a host continuation message.
+- **Threshold at a tool pause** — Once over threshold, the follow-up `infer` after a tool batch is swapped for a `compact` cycle, and inference resumes via a host continuation message. After a compact that remains over the high watermark, the governor uses **growth hysteresis** (wait for usage to grow by ~10% of the window) instead of re-arming on every cycle; dropping under 60% is not required.
 - **Idle (end-of-turn)** — An interactive turn can end with a reply and then sit idle with no tool batch to intercept; the governor requests a continuation at that pause and compacts when it arrives. An operator message that races the continuation still compacts first, then re-enters inference to answer it.
-- **Overflow recovery** — A `context_overflow` inference error would otherwise become a terminal error reply; the governor compacts and retries instead, bounded so a history the compactor cannot shrink does not loop forever.
+- **Overflow recovery** — A `context_overflow` inference error would otherwise become a terminal error reply; the governor compacts and retries instead, bounded so a history the compactor cannot shrink does not loop forever. Overflow ignores hysteresis for the compact itself.
 
 The compaction control flow is shaped by a reactor invariant: a `compact` action runs in its own cycle (it cannot be paired with `infer`), and **the reactor delivers no event after a compact cycle**. A director that simply emitted `compact` in place of the follow-up `infer` would leave the loop idle forever — the cause of an earlier stall. Instead the governor, after emitting `compact`, self-delivers a content-less inbound message (a host-supplied `requestContinuation` callback). That message adds no turn (`createInboundTurn` returns `null` for empty content) but re-enters the loop, where the director issues the follow-up `infer` against the freshly truncated history.
 

src/agent/compaction.test.ts

@@ -7,7 +7,7 @@ import type {
   TokenUsage,
 } from "@intx/types/runtime";
 import { createCompactionGovernor } from "./compaction.js";
-import { compactionThresholdFor } from "../provider/context-window.js";
+import { compactionResumeDeltaFor, compactionThresholdFor } from "../provider/context-window.js";
 import { COMPACTOR_KEEP_RECENT_TURNS, compactorNoOpFloor } from "../session/compactor.js";
 
 const capabilities = {
@@ -86,6 +86,7 @@ function overflowError(): ReactorInboundEvent {
 }
 
 const overThreshold = compactionThresholdFor("m") + 1;
+const resumeDelta = compactionResumeDeltaFor("m");
 const inferAction: ReactorAction[] = [{ type: "infer" }];
 const tenTurns = turnsOfLength(10, 1);
 const threeTurns = turnsOfLength(3, 1);
@@ -335,4 +336,60 @@ describe("compaction governor", () => {
     // arming decision trusted reported usage, so it is not re-checked here.
     expect(governor.interceptActions(toolDone(), inferAction, capabilities)).toBeNull();
   });
+
+  test("does not re-arm after a compact that remains over the high watermark", () => {
+    const governor = createCompactionGovernor(() => {});
+    governor.noteInferenceDone(inferenceDone(overThreshold), tenTurns);
+    expect(governor.interceptActions(toolDone(), inferAction, capabilities)).not.toBeNull();
+
+    // Post-compact snapshot is still over high; growth hysteresis must hold
+    // the next arm until usage grows by resumeDelta.
+    governor.noteInferenceDone(inferenceDone(overThreshold), tenTurns);
+    expect(governor.interceptActions(toolDone(), inferAction, capabilities)).toBeNull();
+  });
+
+  test("re-arms after usage grows by the resume delta past the last compact", () => {
+    const governor = createCompactionGovernor(() => {});
+    governor.noteInferenceDone(inferenceDone(overThreshold), tenTurns);
+    expect(governor.interceptActions(toolDone(), inferAction, capabilities)).not.toBeNull();
+
+    governor.noteInferenceDone(inferenceDone(overThreshold), tenTurns);
+    expect(governor.interceptActions(toolDone(), inferAction, capabilities)).toBeNull();
+
+    governor.noteInferenceDone(inferenceDone(overThreshold + resumeDelta), tenTurns);
+    const actions = governor.interceptActions(toolDone(), inferAction, capabilities);
+    expect(actions).not.toBeNull();
+    expect(actions?.some((a) => a.type === "compact")).toBe(true);
+  });
+
+  test("clears hysteresis once usage drops under the high watermark", () => {
+    const governor = createCompactionGovernor(() => {});
+    governor.noteInferenceDone(inferenceDone(overThreshold), tenTurns);
+    expect(governor.interceptActions(toolDone(), inferAction, capabilities)).not.toBeNull();
+
+    governor.noteInferenceDone(inferenceDone(overThreshold), tenTurns);
+    expect(governor.interceptActions(toolDone(), inferAction, capabilities)).toBeNull();
+
+    governor.noteInferenceDone(inferenceDone(1000), tenTurns);
+    expect(governor.interceptActions(toolDone(), inferAction, capabilities)).toBeNull();
+
+    // Next crossing of high arms immediately — no growth delta required.
+    governor.noteInferenceDone(inferenceDone(overThreshold), tenTurns);
+    const actions = governor.interceptActions(toolDone(), inferAction, capabilities);
+    expect(actions).not.toBeNull();
+    expect(actions?.some((a) => a.type === "compact")).toBe(true);
+  });
+
+  test("overflow still compact while hysteresis blocks the proactive path", () => {
+    const governor = createCompactionGovernor(() => {});
+    governor.noteInferenceDone(inferenceDone(overThreshold), tenTurns);
+    expect(governor.interceptActions(toolDone(), inferAction, capabilities)).not.toBeNull();
+
+    governor.noteInferenceDone(inferenceDone(overThreshold), tenTurns);
+    expect(governor.interceptActions(toolDone(), inferAction, capabilities)).toBeNull();
+
+    const actions = governor.interceptOverflow(overflowError(), capabilities);
+    expect(actions).not.toBeNull();
+    expect(actions?.some((a) => a.type === "compact")).toBe(true);
+  });
 });

src/agent/compaction.ts

@@ -5,7 +5,11 @@ import type {
   ReactorInboundEvent,
   ToolDefinition,
 } from "@intx/types/runtime";
-import { compactionThresholdFor, contextTokensFromUsage } from "../provider/context-window.js";
+import {
+  compactionResumeDeltaFor,
+  compactionThresholdFor,
+  contextTokensFromUsage,
+} from "../provider/context-window.js";
 import { COMPACTOR_KEEP_RECENT_TURNS, compactorNoOpFloor } from "../session/compactor.js";
 import { createContextEstimate, estimateOverheadTokens } from "./context-estimate.js";
 import { onTurnBoundary } from "./reactor-events.js";
@@ -46,6 +50,12 @@ export function createCompactionGovernor(
   // inference cycles (see interceptActions) where the event carries no model.
   let lastModel: string | undefined;
   let turnCount = 0;
+  // Growth hysteresis after a compact that remained over the high watermark:
+  // snapshot the post-compact infer's usage, then do not re-arm until usage
+  // grows by resumeDelta. Cleared once usage drops back to or under high.
+  // Overflow recovery ignores this and arms regardless.
+  let tokensAtLastCompact: number | undefined;
+  let awaitingPostCompactMeasurement = false;
 
   // Running local estimate of the turns we send, plus the fixed system-prompt
   // and tool-schema overhead every request carries. Providers that omit usage
@@ -63,7 +73,17 @@ export function createCompactionGovernor(
   }
 
   function isOverThreshold(contextTokens: number): boolean {
-    return contextTokens > compactionThresholdFor(lastModel) && turnCount > MIN_TURNS_TO_COMPACT;
+    if (turnCount <= MIN_TURNS_TO_COMPACT) return false;
+    const high = compactionThresholdFor(lastModel);
+    if (contextTokens <= high) return false;
+    if (tokensAtLastCompact !== undefined) {
+      return contextTokens >= tokensAtLastCompact + compactionResumeDeltaFor(lastModel);
+    }
+    return true;
+  }
+
+  function noteCompactIssued(): void {
+    awaitingPostCompactMeasurement = true;
   }
 
   function noteInferenceDone(
@@ -77,6 +97,15 @@ export function createCompactionGovernor(
     const reportedTokens = contextTokensFromUsage(event.usage);
     usingEstimate = reportedTokens <= 0;
     const contextTokens = usingEstimate ? estimate.tokens : reportedTokens;
+    // Snapshot on the first inference.done after a compact (the post-compact
+    // infer), not at intercept time — intercept has no fresh usage.
+    if (awaitingPostCompactMeasurement) {
+      tokensAtLastCompact = contextTokens;
+      awaitingPostCompactMeasurement = false;
+    }
+    if (contextTokens <= compactionThresholdFor(lastModel)) {
+      tokensAtLastCompact = undefined;
+    }
     // Assign, don't OR: an under-threshold follow-up must disarm a sticky
     // pending left from an earlier over-threshold turn (e.g. after the
     // provider reports real usage that lands below the threshold).
@@ -107,6 +136,7 @@ export function createCompactionGovernor(
     if (!actions.some((a) => a.type === "infer")) return null;
     pending = false;
     postCompactInfer = true;
+    noteCompactIssued();
     requestContinuation?.();
     return [
       ...actions.filter((a) => a.type !== "infer"),
@@ -143,6 +173,7 @@ export function createCompactionGovernor(
       postCompactInfer = true;
       requestContinuation?.();
     }
+    noteCompactIssued();
     return [capabilities.compact(COMPACTOR_NAME, "context-threshold")];
   }
 
@@ -161,6 +192,7 @@ export function createCompactionGovernor(
     overflowRecoveries++;
     pending = false;
     postCompactInfer = true;
+    noteCompactIssued();
     requestContinuation();
     return [capabilities.compact(COMPACTOR_NAME, "context-overflow")];
   }

src/context-compactor.test.ts

@@ -6,9 +6,17 @@ import {
   formatPlan,
   classifyTaskBoundary,
   buildLLMTurnSummary,
+  COMPACTED_PREFIX,
+  COMPACT_SPACER_TEXT,
   type SessionMetadata,
 } from "./session/compactor.js";
-import type { ConversationTurn, ReactorState, StrategyContext } from "@intx/types/runtime";
+import { createModelSummarizer } from "./session/summarizer.js";
+import type {
+  ConversationTurn,
+  InferenceSource,
+  ReactorState,
+  StrategyContext,
+} from "@intx/types/runtime";
 
 const mockStrategyCtx: StrategyContext = {
   state: {} as ReactorState,
@@ -577,6 +585,104 @@ describe("createPruningCompactor — summarize receives the workflow context (CL
   });
 });
 
+describe("createPruningCompactor — prefix-stable summaries (CL-6914)", () => {
+  function firstText(turn: ConversationTurn): string {
+    const block = turn.content.find((b) => b.type === "text");
+    return block !== undefined && block.type === "text" ? block.text : "";
+  }
+
+  function compactedTurns(output: ConversationTurn[]): ConversationTurn[] {
+    return output.filter((t) => firstText(t).startsWith(COMPACTED_PREFIX));
+  }
+
+  function grow(base: ConversationTurn[], count: number, label: string): ConversationTurn[] {
+    const extra: ConversationTurn[] = [];
+    for (let i = 0; i < count; i++) {
+      extra.push(
+        makeTurn({
+          role: i % 2 === 0 ? "user" : "assistant",
+          content: [{ type: "text", text: `${label} ${i}` }],
+        }),
+      );
+    }
+    return [...base, ...extra];
+  }
+
+  test("second apply leaves output[0] bytes identical and appends a later summary", async () => {
+    const compactor = createPruningCompactor({ keepRecentTurns: 2, summaryMaxChars: 500 });
+    const turns = grow([], 16, "round1");
+    const output1 = (await compactor.apply(turns, mockStrategyCtx)).output;
+    expect(firstText(output1[0]!)).toContain(COMPACTED_PREFIX);
+
+    const output2 = (await compactor.apply(grow(output1, 16, "round2"), mockStrategyCtx)).output;
+
+    expect(firstText(output2[0]!)).toBe(firstText(output1[0]!));
+    expect(output2[0]).toBe(output1[0]);
+    const summaries = compactedTurns(output2);
+    expect(summaries.length).toBeGreaterThanOrEqual(2);
+    expect(output2.indexOf(summaries[1]!)).toBeGreaterThan(0);
+    expect(hasConsecutiveSameRole(output2)).toBe(false);
+    expect(
+      output2.some((t) => t.role === "assistant" && firstText(t) === COMPACT_SPACER_TEXT),
+    ).toBe(true);
+  });
+
+  test("empty-fold keep-set returns the input unchanged", async () => {
+    const compactor = createPruningCompactor({
+      keepRecentTurns: 1,
+      maxAnchorTurns: 8,
+      summaryMaxChars: 500,
+    });
+    const turns: ConversationTurn[] = [
+      makeTurn({ role: "user", content: [{ type: "text", text: "the initiating task" }] }),
+      makeTurn({
+        role: "assistant",
+        content: [
+          { type: "tool_call", id: "c1", name: "edit_file", arguments: { path: "src/a.ts" } },
+        ],
+      }),
+      makeTurn({ role: "user", content: [{ type: "text", text: "recent" }] }),
+    ];
+    const result = await compactor.apply(turns, mockStrategyCtx);
+    expect(result.output).toBe(turns);
+    expect(result.record.reason).toBe("no compaction needed");
+  });
+
+  test("failing then succeeding summarizer does not rewrite output[0]", async () => {
+    const source: InferenceSource = {
+      id: "test",
+      provider: "openai",
+      model: "test-model",
+      baseURL: "http://localhost:1",
+      apiKey: "k",
+    };
+    let calls = 0;
+    const summarize = createModelSummarizer({
+      getSource: () => source,
+      complete: async () => {
+        calls++;
+        if (calls === 1) throw new Error("model unreachable");
+        return "UNIQUE_SUCCESS_SUMMARY";
+      },
+    });
+    const compactor = createPruningCompactor({
+      keepRecentTurns: 2,
+      summaryMaxChars: 500,
+      summarize,
+    });
+    const turns = grow([], 16, "fail");
+    const output1 = (await compactor.apply(turns, mockStrategyCtx)).output;
+    expect(firstText(output1[0]!)).toContain("Turns compacted:");
+    expect(firstText(output1[0]!)).not.toContain("UNIQUE_SUCCESS_SUMMARY");
+    expect(firstText(output1[0]!)).toContain("Model summary unavailable");
+
+    const output2 = (await compactor.apply(grow(output1, 16, "ok"), mockStrategyCtx)).output;
+    expect(firstText(output2[0]!)).toBe(firstText(output1[0]!));
+    expect(allText(output2)).toContain("UNIQUE_SUCCESS_SUMMARY");
+    expect(hasConsecutiveSameRole(output2)).toBe(false);
+  });
+});
+
 describe("buildContextEnvelope", () => {
   test("includes active task label", () => {
     const result = buildContextEnvelope({

src/provider/context-window.ts

@@ -78,6 +78,11 @@ export function contextWindowFor(model: string): number {
 // warning threshold so the color shift matches when compaction starts.
 export const COMPACTION_WINDOW_FRACTION = 0.6;
 
+// After a compact that remains over the high watermark, the governor waits for
+// usage to grow by this fraction of the window before re-arming. Growth
+// hysteresis, not a low watermark: dropping under 60% is not required.
+export const COMPACTION_RESUME_FRACTION = 0.1;
+
 // Status-bar meter turns danger at this fraction of the window — past
 // compaction and approaching hard overflow at 1.0. Inclusive integer bands
 // keep 80 in warning and start danger at 81.
@@ -102,3 +107,9 @@ export function compactionThresholdFor(model: string | undefined): number {
   const window = model !== undefined ? contextWindowFor(model) : DEFAULT_CONTEXT_WINDOW;
   return Math.floor(window * COMPACTION_WINDOW_FRACTION);
 }
+
+/** Tokens of growth past the last post-compact measurement before re-arming. */
+export function compactionResumeDeltaFor(model: string | undefined): number {
+  const window = model !== undefined ? contextWindowFor(model) : DEFAULT_CONTEXT_WINDOW;
+  return Math.floor(window * COMPACTION_RESUME_FRACTION);
+}