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fix: add jitter to rate-limit backoff #374

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fix: add jitter to rate-limit backoff #374

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21 changes: 13 additions & 8 deletions lib/request/rate-limit-backoff.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -17,13 +17,15 @@ export interface RateLimitBackoffResult {
const RATE_LIMIT_DEDUP_WINDOW_MS = 2000;
const RATE_LIMIT_STATE_RESET_MS = 120_000;
const MAX_BACKOFF_MS = 60_000;
const RATE_LIMIT_BACKOFF_JITTER_FACTOR = 0.2;

export const RATE_LIMIT_SHORT_RETRY_THRESHOLD_MS = 5000;

interface RateLimitState {
consecutive429: number;
lastAt: number;
quotaKey: string;
lastDelayMs: number;
}

const rateLimitStateByAccountQuota = new Map<string, RateLimitState>();
Expand All @@ -33,6 +35,11 @@ function normalizeDelayMs(value: number | null | undefined, fallback: number): n
return Math.max(0, Math.floor(candidate));
}

function addBackoffJitter(baseMs: number): number {
const jitter = baseMs * RATE_LIMIT_BACKOFF_JITTER_FACTOR * (Math.random() * 2 - 1);
return Math.max(0, Math.floor(baseMs + jitter));
}

function pruneStaleRateLimitState(): void {
const now = Date.now();
for (const [key, state] of rateLimitStateByAccountQuota) {
Expand All @@ -58,13 +65,9 @@ export function getRateLimitBackoff(
const baseDelay = normalizeDelayMs(serverRetryAfterMs, 1000);

if (previous && now - previous.lastAt < RATE_LIMIT_DEDUP_WINDOW_MS) {
const backoffDelay = Math.min(
baseDelay * Math.pow(2, previous.consecutive429 - 1),
MAX_BACKOFF_MS,
);
return {
attempt: previous.consecutive429,
delayMs: Math.max(baseDelay, backoffDelay),
delayMs: previous.lastDelayMs,
isDuplicate: true,
};
}
Expand All @@ -74,16 +77,18 @@ export function getRateLimitBackoff(
? previous.consecutive429 + 1
: 1;

const backoffDelay = Math.min(baseDelay * Math.pow(2, attempt - 1), MAX_BACKOFF_MS);
const jitteredDelay = Math.min(addBackoffJitter(backoffDelay), MAX_BACKOFF_MS);
const delayMs = Math.max(baseDelay, jitteredDelay);
Comment on lines +80 to +82
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@greptile-apps greptile-apps Bot Apr 6, 2026

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P1 negative jitter silently discarded on attempt=1

for attempt=1, backoffDelay === baseDelay. if Math.random() returns a value below 0.5, jitteredDelay < baseDelay, and the Math.max floor restores it to baseDelay — so downward jitter has no effect on the first 429. the effective spread is [baseDelay, baseDelay×1.2] (one-sided upward), not the symmetric ±20% the constant implies.

this is the most common 429 case, so the anti-thundering-herd goal stated in the PR description is only half-realised for first retries. attempt=2+ are unaffected because backoffDelay >> baseDelay and symmetric jitter applies.

if the Math.max floor is intentional (to honour the server-sent Retry-After as a hard minimum), add a comment:

Suggested change
const backoffDelay = Math.min(baseDelay * Math.pow(2, attempt - 1), MAX_BACKOFF_MS);
const jitteredDelay = Math.min(addBackoffJitter(backoffDelay), MAX_BACKOFF_MS);
const delayMs = Math.max(baseDelay, jitteredDelay);
const backoffDelay = Math.min(baseDelay * Math.pow(2, attempt - 1), MAX_BACKOFF_MS);
const jitteredDelay = Math.min(addBackoffJitter(backoffDelay), MAX_BACKOFF_MS);
// Math.max honours server Retry-After as a hard floor; on attempt=1 this makes jitter one-sided (upward only)
const delayMs = Math.max(baseDelay, jitteredDelay);

or remove the floor to get symmetric spread:

const delayMs = Math.max(0, jitteredDelay);
Prompt To Fix With AI 
This is a comment left during a code review.
Path: lib/request/rate-limit-backoff.ts
Line: 80-82

Comment:
**negative jitter silently discarded on attempt=1**

for attempt=1, `backoffDelay === baseDelay`. if `Math.random()` returns a value below `0.5`, `jitteredDelay < baseDelay`, and the `Math.max` floor restores it to `baseDelay` — so downward jitter has no effect on the first 429. the effective spread is `[baseDelay, baseDelay×1.2]` (one-sided upward), not the symmetric `±20%` the constant implies.

this is the most common 429 case, so the anti-thundering-herd goal stated in the PR description is only half-realised for first retries. attempt=2+ are unaffected because `backoffDelay >> baseDelay` and symmetric jitter applies.

if the `Math.max` floor is intentional (to honour the server-sent `Retry-After` as a hard minimum), add a comment:

```suggestion
	const backoffDelay = Math.min(baseDelay * Math.pow(2, attempt - 1), MAX_BACKOFF_MS);
	const jitteredDelay = Math.min(addBackoffJitter(backoffDelay), MAX_BACKOFF_MS);
	// Math.max honours server Retry-After as a hard floor; on attempt=1 this makes jitter one-sided (upward only)
	const delayMs = Math.max(baseDelay, jitteredDelay);
```

or remove the floor to get symmetric spread:

```typescript
const delayMs = Math.max(0, jitteredDelay);
```

How can I resolve this? If you propose a fix, please make it concise.

Fix in Codex

rateLimitStateByAccountQuota.set(stateKey, {
consecutive429: attempt,
lastAt: now,
quotaKey,
lastDelayMs: delayMs,
});

const backoffDelay = Math.min(baseDelay * Math.pow(2, attempt - 1), MAX_BACKOFF_MS);
return {
attempt,
delayMs: Math.max(baseDelay, backoffDelay),
delayMs,
isDuplicate: false,
};
}
Expand Down
20 changes: 20 additions & 0 deletions test/rate-limit-backoff.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -12,10 +12,12 @@ describe("Rate limit backoff", () => {
vi.useFakeTimers();
vi.setSystemTime(new Date(0));
clearRateLimitBackoffState();
vi.spyOn(Math, "random").mockReturnValue(0.5);
});

afterEach(() => {
clearRateLimitBackoffState();
vi.restoreAllMocks();
vi.useRealTimers();
});

Expand All @@ -39,6 +41,24 @@ describe("Rate limit backoff", () => {
expect(second.isDuplicate).toBe(false);
});

it("applies jitter to new backoff windows but keeps duplicate retries stable", () => {
vi.mocked(Math.random).mockReturnValueOnce(1);
const first = getRateLimitBackoff(4, "jitter-test", 1000);
expect(first.delayMs).toBe(1200);

vi.setSystemTime(new Date(1000));
vi.mocked(Math.random).mockReturnValueOnce(0);
const duplicate = getRateLimitBackoff(4, "jitter-test", 1000);
expect(duplicate.delayMs).toBe(1200);
expect(duplicate.isDuplicate).toBe(true);

vi.setSystemTime(new Date(2500));
vi.mocked(Math.random).mockReturnValueOnce(0);
const second = getRateLimitBackoff(4, "jitter-test", 1000);
expect(second.delayMs).toBe(1600);
expect(second.isDuplicate).toBe(false);
});

it("resets after quiet period", () => {
getRateLimitBackoff(0, "codex", 1000);
vi.setSystemTime(new Date(121_000));
Expand Down