chore(ai-chat): guard-пороги в env + единый маркер дегенерации

Три хвоста детектора петель (#444):
- Пороги детектора дегенерации теперь конфигурируются из env (по образцу
  AI_CHAT_FINAL_STEP_LOCKDOWN): AI_CHAT_DEGENERATION_REPEATED_LINES /
  _PERIOD_MAX_LEN / _PERIOD_MIN_REPEATS / _CHECK_STEP. Резолвер читает сырую
  строку (пусто = unset → компилируемый дефолт), требует ≥1, иначе безопасный
  откат к дефолту (0/отриц. сломал бы детектор). Оператор перенастраивает
  анти-babble-guard без редеплоя.
- Единый маркер: при дегенерации live-стрим показывает нейтральное «Response
  stopped.» (клиент не отличает от ручного Stop), а персист-баннер после refetch
  падал в дженерик. Классифицировал OUTPUT_DEGENERATION_ERROR на клиенте под тот
  же заголовок «Response stopped.» + деталь про петлю — live и refetch больше не
  расходятся.
- STEP_LIMIT_NO_ANSWER_MARKER: вместо хардкодной русской строки в content —
  локаль-нейтральная английская (базовая локаль = i18n-ключ этого репо, читаема
  моделью на реплее); не-русские юзеры больше не видят русский текст.

Тесты: env-пороги гоняются против реальных форм повторов с mutation-проверкой
(поднятый checkStep глушит burst; сниженный repeatedLines триггерит короткий
ран); клиентская классификация — новый кейс в error-message.test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/ai-chat/utils/error-message.test.ts

@@ -89,6 +89,23 @@ describe("describeChatError", () => {
     expect(view.title).not.toBe("AI provider not configured");
   });
 
+  it("classifies a token-degeneration abort under the SAME 'Response stopped.' marker the live view shows (#495)", () => {
+    // The exact reason the server persists in metadata.error on a degeneration
+    // abort (ai-chat.service OUTPUT_DEGENERATION_ERROR). Live, this event shows
+    // the neutral "Response stopped." notice; the persisted banner MUST match it
+    // so live and refetch never disagree.
+    const view = describeChatError(
+      "Output degeneration detected (repeated token loop)",
+      t,
+    );
+    expect(view.title).toBe("Response stopped.");
+    expect(view.detail).toBe(
+      "The answer was stopped automatically because the model fell into a repeated output loop.",
+    );
+    // Regression guard: it must NOT fall through to the generic heading.
+    expect(view.title).not.toBe("Something went wrong");
+  });
+
   it("classifies a dropped connection (ECONNRESET) as a lost-connection error", () => {
     expect(
       describeChatError("Cannot connect to API: read ECONNRESET", t).title,

apps/client/src/features/ai-chat/utils/error-message.ts

@@ -77,6 +77,22 @@ export function describeChatError(
     };
   }
 
+  // Our own token-degeneration abort (#444): the server aborts a runaway
+  // repetition loop and persists this exact reason in metadata.error. LIVE, the
+  // same abort surfaces as the neutral "Response stopped." notice (the client
+  // cannot tell it from a manual Stop mid-stream), so the persisted banner must
+  // read the SAME "Response stopped." marker — otherwise the live view and a
+  // later refetch show two different texts for one event. The detail explains the
+  // loop-guard cause without contradicting the shared heading.
+  if (/output degeneration detected|repeated token loop/i.test(msg)) {
+    return {
+      title: t("Response stopped."),
+      detail: t(
+        "The answer was stopped automatically because the model fell into a repeated output loop.",
+      ),
+    };
+  }
+
   if (/"statusCode"\s*:\s*403\b/.test(msg)) {
     return {
       title: t("AI chat is disabled"),

apps/server/src/core/ai-chat/ai-chat.service.ts

@@ -117,9 +117,14 @@ const FINAL_STEP_NUDGE =
 // NO text at all (#444, mitigates the "empty turn" the lockdown used to prevent
 // when the toggle is OFF). Makes the exhausted-without-answer state explicit to
 // the user and, on replay, to the model on the next turn.
+// The persisted content is the app's base locale (en-US) — which is ALSO the
+// i18n key the client localizes through `t()` — instead of a hardcoded Russian
+// string (it used to render Russian for every locale, and fed Russian back to
+// the model on replay). Keep it a plain, model-readable English sentence so the
+// next turn's replay reads cleanly; the client resolves the locale.
 const STEP_LIMIT_NO_ANSWER_MARKER =
-  '(Достигнут лимит шагов — итоговый ответ не сформулирован; работа могла ' +
-  'остаться незавершённой. Напишите «продолжай», чтобы агент продолжил.)';
+  '(Step limit reached — no final answer was produced; the work may be ' +
+  'unfinished. Reply "continue" to let the agent carry on.)';
 
 // Reason recorded in ai_chat_runs.error / the assistant row when the token-
 // degeneration detector (#444) aborts a run. Distinct from a user Stop (no error)

apps/server/src/core/ai-chat/output-degeneration.spec.ts

@@ -9,9 +9,73 @@ import {
   DEGENERATION_CHECK_STEP,
   REPEATED_LINES_THRESHOLD,
   MIN_PERIOD_REPEATS,
+  degenerationThresholds,
 } from './output-degeneration';
 import { AiChatService } from './ai-chat.service';
 
+// Part A (#495 iter10): the detector thresholds are env-tunable. These drive the
+// resolver against real repeat-count shapes and mutation-verify that the env
+// override actually changes the trigger point (not a vacuous read).
+describe('degeneration thresholds are env-configurable', () => {
+  const VARS = [
+    'AI_CHAT_DEGENERATION_REPEATED_LINES',
+    'AI_CHAT_DEGENERATION_PERIOD_MAX_LEN',
+    'AI_CHAT_DEGENERATION_PERIOD_MIN_REPEATS',
+    'AI_CHAT_DEGENERATION_CHECK_STEP',
+  ];
+  const saved: Record<string, string | undefined> = {};
+  beforeEach(() => {
+    for (const v of VARS) saved[v] = process.env[v];
+  });
+  afterEach(() => {
+    for (const v of VARS) {
+      if (saved[v] === undefined) delete process.env[v];
+      else process.env[v] = saved[v];
+    }
+  });
+
+  it('defaults to the compiled constants when unset', () => {
+    for (const v of VARS) delete process.env[v];
+    expect(degenerationThresholds()).toEqual({
+      repeatedLines: REPEATED_LINES_THRESHOLD,
+      maxPeriodLen: 150,
+      minPeriodRepeats: MIN_PERIOD_REPEATS,
+      checkStep: DEGENERATION_CHECK_STEP,
+    });
+  });
+
+  it('falls back to the default on blank / invalid / non-positive values', () => {
+    for (const bad of ['', '  ', 'abc', '0', '-3', '1.5']) {
+      process.env.AI_CHAT_DEGENERATION_REPEATED_LINES = bad;
+      // '1.5' floors to 1 (still ≥1, valid); every other bad value → default.
+      const expected = bad === '1.5' ? 1 : REPEATED_LINES_THRESHOLD;
+      expect(degenerationThresholds().repeatedLines).toBe(expected);
+    }
+  });
+
+  it('a RAISED check-step suppresses a burst the default would have flagged', () => {
+    // A ~3.3KB periodic burst is periodic-degenerate, but shouldCheckDegeneration
+    // is the throttle gate. Default checkStep=2000 arms on it; raising the step
+    // above the burst size means the throttle never re-fires for it.
+    const burstLen = 'loadTools.\n'.repeat(300).length; // ~3300
+    delete process.env.AI_CHAT_DEGENERATION_CHECK_STEP;
+    expect(shouldCheckDegeneration(burstLen, 0)).toBe(true); // default 2000
+    process.env.AI_CHAT_DEGENERATION_CHECK_STEP = String(burstLen + 1);
+    expect(shouldCheckDegeneration(burstLen, 0)).toBe(false); // raised gate
+  });
+
+  it('a LOWERED repeated-lines threshold trips on a shorter identical-line run', () => {
+    // 8 identical lines: below the default 25 (rule 1) and below the periodic
+    // rule's 20 repeats — so isDegenerateOutput is false by default.
+    const shortRun = 'x\n'.repeat(8);
+    delete process.env.AI_CHAT_DEGENERATION_REPEATED_LINES;
+    expect(isDegenerateOutput(shortRun)).toBe(false);
+    // Lower rule 1 to 5 → the 8-line run now trips.
+    process.env.AI_CHAT_DEGENERATION_REPEATED_LINES = '5';
+    expect(isDegenerateOutput(shortRun)).toBe(true);
+  });
+});
+
 // Mock ONLY streamText so we can capture the onChunk/onStepFinish callbacks the
 // service registers and drive them by hand; every other `ai` export the service
 // uses (convertToModelMessages, stepCountIs, …) stays real.

apps/server/src/core/ai-chat/output-degeneration.ts

@@ -23,6 +23,54 @@ export const MAX_PERIOD_LEN = 150;
 /** Rule 2: minimum number of consecutive block repeats to trigger. */
 export const MIN_PERIOD_REPEATS = 20;
 
+/**
+ * Read a positive-integer threshold from an env var, falling back to `fallback`
+ * on unset/blank/invalid/non-positive. Mirrors the `AI_STREAM_PRE_RESPONSE_RETRIES`
+ * resolver in `ai-streaming-fetch.ts`: read the RAW string first so a blank value
+ * is treated as "unset" (→ fallback) rather than coercing to 0. Thresholds must
+ * stay ≥ 1 — a 0/negative would make the detector fire on any text (or never), so
+ * a bad value degrades to the safe compiled default instead. Env-tunable so an
+ * operator can retune the anti-babble guard (#444) without a redeploy, following
+ * the `AI_CHAT_FINAL_STEP_LOCKDOWN` toggle convention.
+ */
+function envThreshold(name: string, fallback: number): number {
+  const rawStr = process.env[name];
+  if (rawStr === undefined || rawStr.trim() === '') return fallback;
+  const raw = Number(rawStr);
+  return Number.isFinite(raw) && raw >= 1 ? Math.floor(raw) : fallback;
+}
+
+/**
+ * Resolve the degeneration-detector thresholds from the environment, each
+ * defaulting to the compiled constant above. Read fresh per call (not cached at
+ * import) so a test — or a runtime env change — takes effect deterministically.
+ */
+export function degenerationThresholds(): {
+  repeatedLines: number;
+  maxPeriodLen: number;
+  minPeriodRepeats: number;
+  checkStep: number;
+} {
+  return {
+    repeatedLines: envThreshold(
+      'AI_CHAT_DEGENERATION_REPEATED_LINES',
+      REPEATED_LINES_THRESHOLD,
+    ),
+    maxPeriodLen: envThreshold(
+      'AI_CHAT_DEGENERATION_PERIOD_MAX_LEN',
+      MAX_PERIOD_LEN,
+    ),
+    minPeriodRepeats: envThreshold(
+      'AI_CHAT_DEGENERATION_PERIOD_MIN_REPEATS',
+      MIN_PERIOD_REPEATS,
+    ),
+    checkStep: envThreshold(
+      'AI_CHAT_DEGENERATION_CHECK_STEP',
+      DEGENERATION_CHECK_STEP,
+    ),
+  };
+}
+
 /**
  * Rule 1 — ≥`REPEATED_LINES_THRESHOLD` consecutive IDENTICAL non-empty lines at
  * the tail. Catches the classic newline-delimited loop ("loadTools.\n" ×N).
@@ -128,7 +176,11 @@ export function hasPeriodicTail(
  * Pure — the caller owns the abort side effect.
  */
 export function isDegenerateOutput(text: string): boolean {
-  return hasRepeatedLineRun(text) || hasPeriodicTail(text);
+  const cfg = degenerationThresholds();
+  return (
+    hasRepeatedLineRun(text, cfg.repeatedLines) ||
+    hasPeriodicTail(text, cfg.maxPeriodLen, cfg.minPeriodRepeats)
+  );
 }
 
 /**
@@ -154,7 +206,7 @@ export function shouldCheckDegeneration(
   textLen: number,
   lastCheckLen: number,
 ): boolean {
-  return textLen - lastCheckLen >= DEGENERATION_CHECK_STEP;
+  return textLen - lastCheckLen >= degenerationThresholds().checkStep;
 }
 
 /**