diff --git a/packages/gittensory-engine/src/governor/rate-limit.ts b/packages/gittensory-engine/src/governor/rate-limit.ts
new file mode 100644
index 0000000000..3cff8f961d
--- /dev/null
+++ b/packages/gittensory-engine/src/governor/rate-limit.ts
@@ -0,0 +1,101 @@
+// Governor local rate-limit + jittered backoff (pure).
+// Deterministic, side-effect-free bucket math for the local Governor. Given a rolling-window bucket and a
+// clock reading it decides whether an event is allowed and, when blocked, how long to wait; and it computes a
+// jittered exponential backoff from an INJECTED random source (never Math.random) so it stays fully unit-
+// testable. This module computes numbers only — it does NOT store state, schedule, or gate any write action;
+// that enforcement wiring is a separate, maintainer-owned concern. The vocabulary mirrors the server-side
+// `RateLimitConfig`/`RateLimitDecision` in src/auth/rate-limit.ts (that one is a Cloudflare Durable Object and
+// is not reusable in the fully-local miner), but this variant is millisecond-based and state-free.
+
+export type LocalRateLimitConfig = {
+  /** Maximum number of events permitted within one window. */
+  limit: number;
+  /** Rolling window length in milliseconds. */
+  windowMs: number;
+};
+
+export type LocalRateBucket = {
+  /** Events already counted in the current window. */
+  count: number;
+  /** Start of the current window as a millisecond epoch. */
+  windowStartMs: number;
+};
+
+export type LocalRateLimitDecision = {
+  /** Whether an event at `nowMs` is permitted under the bucket + config. */
+  allowed: boolean;
+  /** The configured limit, echoed for callers that render the decision. */
+  limit: number;
+  /** Events still permitted in the effective window AFTER this one (0 when blocked). */
+  remaining: number;
+  /** When the effective window resets, as a millisecond epoch. */
+  resetAtMs: number;
+  /** Milliseconds to wait before the next permitted attempt (0 when allowed). */
+  retryAfterMs: number;
+};
+
+// Cap the backoff exponent so `2 ** attempt` cannot overflow into Infinity for a pathological attempt count;
+// beyond this the delay is already saturated at its ceiling anyway.
+const MAX_BACKOFF_EXPONENT = 30;
+
+// Normalize any numeric input to a non-negative integer (a non-finite or negative value becomes 0), so counts,
+// limits, and window lengths can never make a decision NaN, fractional, or negative.
+function finiteNonNegativeInt(value: number): number {
+  return Number.isFinite(value) ? Math.max(0, Math.floor(value)) : 0;
+}
+
+/**
+ * Decide whether an event at `nowMs` is allowed for a rolling-window bucket. Pure: it reads the bucket and
+ * clock and returns a decision without mutating anything. A window that has fully elapsed is treated as reset,
+ * so a stale bucket at its limit is permitted again in the new window. Every numeric input is normalized first,
+ * so a non-finite, fractional, or negative count/limit/window can never produce a NaN or negative decision.
+ */
+export function evaluateLocalRateLimit(
+  bucket: LocalRateBucket,
+  config: LocalRateLimitConfig,
+  nowMs: number,
+): LocalRateLimitDecision {
+  const limit = finiteNonNegativeInt(config.limit);
+  const windowMs = finiteNonNegativeInt(config.windowMs);
+  const count = finiteNonNegativeInt(bucket.count);
+  const windowStartMs = Number.isFinite(bucket.windowStartMs) ? bucket.windowStartMs : 0;
+  const now = Number.isFinite(nowMs) ? nowMs : 0;
+
+  const windowElapsed = now - windowStartMs >= windowMs;
+  const effectiveCount = windowElapsed ? 0 : count;
+  const effectiveWindowStart = windowElapsed ? now : windowStartMs;
+  const resetAtMs = effectiveWindowStart + windowMs;
+
+  const allowed = effectiveCount < limit;
+  const remaining = allowed ? limit - effectiveCount - 1 : 0;
+  const retryAfterMs = allowed ? 0 : Math.max(0, resetAtMs - now);
+
+  return { allowed, limit, remaining, resetAtMs, retryAfterMs };
+}
+
+/**
+ * Compute a jittered exponential backoff in milliseconds for a retry `attempt` (0-based). The exponential base
+ * is `baseMs * 2 ** attempt` (attempt clamped to a non-negative, bounded range), scaled by a multiplicative
+ * jitter factor drawn from `randomFn` (expected to return a value in [0, 1), like `Math.random`, but injected
+ * so tests stay deterministic): the factor lands in [0.5, 1.5), and the final delay is that product rounded to
+ * the nearest integer (so at the top of the band a delay may round up to the `1.5 * base` value). The result is
+ * always a non-negative integer.
+ */
+export function jitteredBackoffMs(baseMs: number, attempt: number, randomFn: () => number): number {
+  // Normalize non-finite inputs so the result can never be NaN or Infinity — it is always a non-negative integer,
+  // matching this function's documented contract. A non-finite base is treated as 0; a NaN attempt means no
+  // backoff growth, while a huge or Infinity attempt saturates at the capped exponent.
+  const safeBase = Number.isFinite(baseMs) ? Math.max(0, baseMs) : 0;
+  const exponent = Number.isNaN(attempt) ? 0 : Math.min(MAX_BACKOFF_EXPONENT, Math.max(0, Math.floor(attempt)));
+  const exponential = safeBase * 2 ** exponent;
+  // Clamp the random draw into [0, 1) so an out-of-contract source cannot push the factor outside [0.5, 1.5).
+  // A non-finite draw (e.g. NaN) is treated as 0 so the delay never becomes NaN.
+  const rawDraw = randomFn();
+  const draw = Number.isFinite(rawDraw) ? Math.min(0.999999, Math.max(0, rawDraw)) : 0;
+  const jitterFactor = 0.5 + draw;
+  // Round to an integer AND guard finiteness on the return itself: a fractional base yields a rounded integer,
+  // and an extreme (but finite) base that overflows the multiplication to Infinity falls back to a finite max —
+  // so the result is always a non-negative integer for any input, per the documented contract.
+  const rawDelay = exponential * jitterFactor;
+  return Number.isFinite(rawDelay) ? Math.max(0, Math.round(rawDelay)) : Number.MAX_SAFE_INTEGER;
+}
diff --git a/packages/gittensory-engine/src/index.ts b/packages/gittensory-engine/src/index.ts
index c6458fbc46..6d380e9d34 100644
--- a/packages/gittensory-engine/src/index.ts
+++ b/packages/gittensory-engine/src/index.ts
@@ -9,3 +9,4 @@ export {
   rankOpportunities,
   type OpportunityRankInput,
 } from "./opportunity-ranker.js";
+export * from "./governor/rate-limit.js";
diff --git a/test/unit/governor-rate-limit.test.ts b/test/unit/governor-rate-limit.test.ts
new file mode 100644
index 0000000000..4711c457d5
--- /dev/null
+++ b/test/unit/governor-rate-limit.test.ts
@@ -0,0 +1,144 @@
+import { describe, expect, it } from "vitest";
+import { evaluateLocalRateLimit, jitteredBackoffMs, type LocalRateBucket, type LocalRateLimitConfig } from "../../packages/gittensory-engine/src/governor/rate-limit";
+
+const config: LocalRateLimitConfig = { limit: 10, windowMs: 60_000 };
+
+describe("evaluateLocalRateLimit", () => {
+  it("permits an event when the bucket is under the limit", () => {
+    const bucket: LocalRateBucket = { count: 3, windowStartMs: 1_000 };
+    const d = evaluateLocalRateLimit(bucket, config, 5_000);
+    expect(d.allowed).toBe(true);
+    expect(d.remaining).toBe(6); // 10 - 3 - 1
+    expect(d.retryAfterMs).toBe(0);
+    expect(d.resetAtMs).toBe(61_000); // windowStart 1000 + 60000
+    expect(d.limit).toBe(10);
+  });
+
+  it("blocks and reports retry timing when the bucket is at the limit", () => {
+    const bucket: LocalRateBucket = { count: 10, windowStartMs: 1_000 };
+    const d = evaluateLocalRateLimit(bucket, config, 5_000);
+    expect(d.allowed).toBe(false);
+    expect(d.remaining).toBe(0);
+    expect(d.resetAtMs).toBe(61_000);
+    expect(d.retryAfterMs).toBe(56_000); // 61000 - 5000
+  });
+
+  it("treats a fully elapsed window as reset, permitting a previously maxed bucket", () => {
+    const bucket: LocalRateBucket = { count: 10, windowStartMs: 1_000 };
+    const d = evaluateLocalRateLimit(bucket, config, 61_000); // exactly one window later → elapsed (>=)
+    expect(d.allowed).toBe(true);
+    expect(d.remaining).toBe(9); // fresh window: 10 - 0 - 1
+    expect(d.resetAtMs).toBe(121_000); // new window starts at now
+    expect(d.retryAfterMs).toBe(0);
+  });
+
+  it("permits within a window that has not yet elapsed", () => {
+    const bucket: LocalRateBucket = { count: 10, windowStartMs: 1_000 };
+    const d = evaluateLocalRateLimit(bucket, config, 60_999); // 1ms before elapse → still blocked window
+    expect(d.allowed).toBe(false);
+    expect(d.retryAfterMs).toBe(1); // 61000 - 60999
+  });
+
+  it("clamps a negative stored count to zero", () => {
+    const bucket: LocalRateBucket = { count: -5, windowStartMs: 1_000 };
+    const d = evaluateLocalRateLimit(bucket, config, 5_000);
+    expect(d.allowed).toBe(true);
+    expect(d.remaining).toBe(9); // treated as count 0
+  });
+
+  it("blocks when the stored count exceeds the limit", () => {
+    const bucket: LocalRateBucket = { count: 15, windowStartMs: 1_000 };
+    const d = evaluateLocalRateLimit(bucket, config, 5_000);
+    expect(d.allowed).toBe(false);
+    expect(d.remaining).toBe(0);
+  });
+
+  it("floors a fractional stored count instead of leaking a fractional remaining", () => {
+    const d = evaluateLocalRateLimit({ count: 3.9, windowStartMs: 1_000 }, config, 5_000);
+    expect(d.remaining).toBe(6); // 3.9 floored to 3 → 10 - 3 - 1
+    expect(Number.isInteger(d.remaining)).toBe(true);
+  });
+
+  it("normalizes non-finite numeric inputs so the decision is never NaN or negative", () => {
+    const d = evaluateLocalRateLimit({ count: NaN, windowStartMs: NaN }, { limit: NaN, windowMs: NaN }, NaN);
+    for (const value of [d.limit, d.remaining, d.resetAtMs, d.retryAfterMs]) {
+      expect(Number.isFinite(value)).toBe(true);
+      expect(value).toBeGreaterThanOrEqual(0);
+    }
+    expect(typeof d.allowed).toBe("boolean");
+  });
+
+  it("clamps a non-positive window to a defined always-reset decision", () => {
+    // windowMs -10 clamps to 0 → the window is always elapsed → a fresh window opens at now, permitting the event.
+    const d = evaluateLocalRateLimit({ count: 0, windowStartMs: 1_000 }, { limit: 5, windowMs: -10 }, 5_000);
+    expect(d).toEqual({ allowed: true, limit: 5, remaining: 4, resetAtMs: 5_000, retryAfterMs: 0 });
+  });
+});
+
+describe("jitteredBackoffMs", () => {
+  it("returns the base delay at attempt 0 with a mid jitter draw", () => {
+    expect(jitteredBackoffMs(100, 0, () => 0.5)).toBe(100); // 100 * 2^0 * (0.5 + 0.5)
+  });
+
+  it("grows exponentially with the attempt count", () => {
+    expect(jitteredBackoffMs(100, 3, () => 0.5)).toBe(800); // 100 * 2^3 * 1.0
+  });
+
+  it("keeps the jitter factor within the [0.5, 1.5) band across a swept attempt range", () => {
+    for (let attempt = 0; attempt <= 12; attempt++) {
+      const exp = 100 * 2 ** attempt;
+      const low = jitteredBackoffMs(100, attempt, () => 0);
+      const high = jitteredBackoffMs(100, attempt, () => 0.999999);
+      expect(low).toBe(Math.round(exp * 0.5));
+      expect(high).toBeGreaterThanOrEqual(Math.round(exp * 0.5));
+      expect(high).toBeLessThan(Math.round(exp * 1.5) + 1);
+    }
+  });
+
+  it("clamps a negative attempt to zero", () => {
+    expect(jitteredBackoffMs(100, -4, () => 0.5)).toBe(100);
+  });
+
+  it("floors a fractional attempt", () => {
+    expect(jitteredBackoffMs(100, 2.9, () => 0.5)).toBe(400); // floor(2.9) = 2 → 2^2
+  });
+
+  it("caps the exponent so a pathological attempt cannot overflow to Infinity", () => {
+    const capped = jitteredBackoffMs(1, 1000, () => 0.5);
+    expect(Number.isFinite(capped)).toBe(true);
+    expect(capped).toBe(Math.round(2 ** 30)); // 1 * 2^30 * 1.0
+  });
+
+  it("treats a negative base as zero", () => {
+    expect(jitteredBackoffMs(-100, 5, () => 0.9)).toBe(0);
+  });
+
+  it("clamps an out-of-contract random draw into the band", () => {
+    expect(jitteredBackoffMs(100, 0, () => 5)).toBe(Math.round(100 * (0.5 + 0.999999))); // draw clamped to 0.999999
+    expect(jitteredBackoffMs(100, 0, () => -3)).toBe(50); // draw clamped to 0 → factor 0.5
+  });
+
+  it("treats a non-finite random draw as zero so the delay never becomes NaN", () => {
+    expect(jitteredBackoffMs(100, 0, () => NaN)).toBe(50); // NaN draw → 0 → factor 0.5
+    expect(jitteredBackoffMs(100, 2, () => Number.POSITIVE_INFINITY)).toBe(200); // Infinity draw → 0 → 400 * 0.5
+  });
+
+  it("treats a non-finite base or attempt as zero so the result stays a non-negative integer", () => {
+    expect(jitteredBackoffMs(NaN, 0, () => 0.5)).toBe(0); // base NaN → 0
+    expect(jitteredBackoffMs(Number.POSITIVE_INFINITY, 0, () => 0.5)).toBe(0); // base Infinity → 0
+    expect(jitteredBackoffMs(100, NaN, () => 0.5)).toBe(100); // attempt NaN → exponent 0 → 100
+    expect(jitteredBackoffMs(100, Number.POSITIVE_INFINITY, () => 0.5)).toBe(Math.round(2 ** 30 * 100)); // attempt ∞ → capped exponent
+  });
+
+  it("rounds a fractional base to an integer delay", () => {
+    expect(jitteredBackoffMs(0.1, 0, () => 0.5)).toBe(0); // round(0.1 * 1) = 0
+    expect(jitteredBackoffMs(1.9, 0, () => 0.5)).toBe(2); // round(1.9 * 1) = 2
+    expect(Number.isInteger(jitteredBackoffMs(2.5, 3, () => 0.7))).toBe(true);
+  });
+
+  it("clamps an overflowing result to a finite integer instead of returning Infinity", () => {
+    const result = jitteredBackoffMs(Number.MAX_VALUE, 30, () => 0.5); // MAX_VALUE * 2^30 overflows to Infinity
+    expect(Number.isFinite(result)).toBe(true);
+    expect(result).toBe(Number.MAX_SAFE_INTEGER);
+  });
+});