diff --git a/packages/loopover-engine/src/index.ts b/packages/loopover-engine/src/index.ts
index 6c86cd3948..ecfaa012c3 100644
--- a/packages/loopover-engine/src/index.ts
+++ b/packages/loopover-engine/src/index.ts
@@ -593,6 +593,13 @@ export {
   type ResultChangedFile,
   type ResultsPayload,
 } from "./results-payload.js";
+export {
+  evaluateTenantQuota,
+  type QuotaDimension,
+  type TenantQuota,
+  type TenantQuotaDecision,
+  type TenantUsage,
+} from "./tenant-quota.js";
 export {
   buildProgressSnapshot,
   progressChanged,
diff --git a/packages/loopover-engine/src/tenant-quota.ts b/packages/loopover-engine/src/tenant-quota.ts
new file mode 100644
index 0000000000..182760c32b
--- /dev/null
+++ b/packages/loopover-engine/src/tenant-quota.ts
@@ -0,0 +1,101 @@
+// Per-tenant resource quota evaluation (pure) — #4796, part of the Rent-a-Loop path #4778.
+//
+// Deterministic and side-effect-free: given ONE tenant's already-metered usage and their allocation (the
+// paid/staked quota that #4792's rental ledger resolves), it decides whether the tenant is still within quota
+// and, when not, which resource dimension was exhausted plus a clear, user-facing reason. It reads only the
+// tenant it is handed, so evaluating one tenant can never observe or affect another's state — the isolation the
+// multi-tenant quota model requires. It computes a decision only: it does NOT store usage, meter compute, or
+// stop a loop; that enforcement wiring is a separate, maintainer-owned concern. Every numeric input is
+// normalized first, so a non-finite, fractional, or negative usage/quota can never make a decision NaN,
+// fractional, or negative. Mirrors the governor's pure rate-limit calculator (governor/rate-limit.ts).
+
+/** A tenant's allocation — hard resource caps for the current billing period, from the rental ledger (#4792). */
+export type TenantQuota = {
+  /** Compute-unit ceiling for the period. */
+  computeUnits: number;
+  /** Wall-clock-millisecond ceiling for the period. */
+  wallClockMs: number;
+  /** Maximum loops the tenant may run at once. */
+  maxConcurrentLoops: number;
+};
+
+/** A tenant's already-metered consumption this period — the input, never mutated. */
+export type TenantUsage = {
+  computeUnitsUsed: number;
+  wallClockMsUsed: number;
+  activeLoops: number;
+};
+
+/** The resource dimension a tenant exhausted, in the order they are checked. */
+export type QuotaDimension = "compute" | "time" | "concurrency";
+
+export type TenantQuotaDecision = {
+  /** Whether the tenant is within quota and may consume more / start another loop. */
+  allowed: boolean;
+  /** The first exhausted dimension when blocked, else null. */
+  exceeded: QuotaDimension | null;
+  /** A clear, actionable, user-facing explanation when blocked, else null. */
+  reason: string | null;
+  /** Headroom left in each dimension (0 when exhausted), echoed for callers that render the decision. */
+  remaining: { computeUnits: number; wallClockMs: number; concurrentLoops: number };
+};
+
+// Normalize any numeric input to a non-negative integer (a non-finite or negative value becomes 0), so usage
+// and quota 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;
+}
+
+function quotaReason(dimension: QuotaDimension, cap: number): string {
+  switch (dimension) {
+    case "compute":
+      return `Quota exceeded: you have used all ${cap} compute units in your current allocation. Increase your allocation or wait for the next period before running more.`;
+    case "time":
+      return `Quota exceeded: you have used all ${cap} ms of wall-clock time in your current allocation. Increase your allocation or wait for the next period before running more.`;
+    case "concurrency":
+      return `Quota exceeded: you already have the maximum of ${cap} loops running. Wait for a running loop to finish before starting another.`;
+  }
+}
+
+/**
+ * Decide whether a tenant is within quota. Pure: reads only the given tenant's usage and quota and returns a
+ * decision without mutating anything. Dimensions are checked in a fixed precedence — compute, then time, then
+ * concurrency — and the FIRST exhausted one is reported so the tenant gets a single, clear, actionable message.
+ * A dimension counts as exhausted when usage has reached (>=) its cap, so a tenant that has consumed its entire
+ * allocation is stopped rather than allowed one more over the line. Because it never reads shared or other-tenant
+ * state, one tenant hitting its quota has no effect on another tenant's decision.
+ */
+export function evaluateTenantQuota(usage: TenantUsage, quota: TenantQuota): TenantQuotaDecision {
+  const computeUsed = finiteNonNegativeInt(usage.computeUnitsUsed);
+  const timeUsed = finiteNonNegativeInt(usage.wallClockMsUsed);
+  const loops = finiteNonNegativeInt(usage.activeLoops);
+  const computeCap = finiteNonNegativeInt(quota.computeUnits);
+  const timeCap = finiteNonNegativeInt(quota.wallClockMs);
+  const loopCap = finiteNonNegativeInt(quota.maxConcurrentLoops);
+
+  const remaining = {
+    computeUnits: Math.max(0, computeCap - computeUsed),
+    wallClockMs: Math.max(0, timeCap - timeUsed),
+    concurrentLoops: Math.max(0, loopCap - loops),
+  };
+
+  let exceeded: QuotaDimension | null = null;
+  let cap = 0;
+  if (computeUsed >= computeCap) {
+    exceeded = "compute";
+    cap = computeCap;
+  } else if (timeUsed >= timeCap) {
+    exceeded = "time";
+    cap = timeCap;
+  } else if (loops >= loopCap) {
+    exceeded = "concurrency";
+    cap = loopCap;
+  }
+
+  return {
+    allowed: exceeded === null,
+    exceeded,
+    reason: exceeded === null ? null : quotaReason(exceeded, cap),
+    remaining,
+  };
+}
diff --git a/test/unit/tenant-quota.test.ts b/test/unit/tenant-quota.test.ts
new file mode 100644
index 0000000000..217790b890
--- /dev/null
+++ b/test/unit/tenant-quota.test.ts
@@ -0,0 +1,74 @@
+import { describe, expect, it } from "vitest";
+
+import { evaluateTenantQuota } from "../../packages/loopover-engine/src/tenant-quota";
+
+const QUOTA = { computeUnits: 100, wallClockMs: 60_000, maxConcurrentLoops: 3 };
+
+describe("evaluateTenantQuota (#4796)", () => {
+  it("allows a tenant within every dimension and reports headroom", () => {
+    const d = evaluateTenantQuota({ computeUnitsUsed: 40, wallClockMsUsed: 10_000, activeLoops: 1 }, QUOTA);
+    expect(d.allowed).toBe(true);
+    expect(d.exceeded).toBeNull();
+    expect(d.reason).toBeNull();
+    expect(d.remaining).toEqual({ computeUnits: 60, wallClockMs: 50_000, concurrentLoops: 2 });
+  });
+
+  it("stops a tenant that has exhausted its compute allocation, with a user-facing reason (acceptance 1)", () => {
+    const d = evaluateTenantQuota({ computeUnitsUsed: 100, wallClockMsUsed: 0, activeLoops: 0 }, QUOTA);
+    expect(d.allowed).toBe(false);
+    expect(d.exceeded).toBe("compute");
+    expect(d.reason).toContain("compute units");
+    expect(d.reason).toContain("100");
+    expect(d.remaining.computeUnits).toBe(0);
+  });
+
+  it("reports the time dimension when compute is fine but wall-clock is exhausted", () => {
+    const d = evaluateTenantQuota({ computeUnitsUsed: 10, wallClockMsUsed: 60_000, activeLoops: 0 }, QUOTA);
+    expect(d.exceeded).toBe("time");
+    expect(d.reason).toContain("wall-clock");
+    expect(d.remaining.wallClockMs).toBe(0);
+  });
+
+  it("reports the concurrency dimension when compute and time are fine but max loops are running", () => {
+    const d = evaluateTenantQuota({ computeUnitsUsed: 10, wallClockMsUsed: 1_000, activeLoops: 3 }, QUOTA);
+    expect(d.exceeded).toBe("concurrency");
+    expect(d.reason).toContain("loops running");
+    expect(d.remaining.concurrentLoops).toBe(0);
+  });
+
+  it("checks dimensions in a fixed precedence — compute is reported before time or concurrency", () => {
+    const d = evaluateTenantQuota({ computeUnitsUsed: 200, wallClockMsUsed: 99_999, activeLoops: 9 }, QUOTA);
+    expect(d.exceeded).toBe("compute");
+  });
+
+  it("normalizes non-finite and negative inputs to 0 so a decision is never NaN or negative", () => {
+    const d = evaluateTenantQuota(
+      { computeUnitsUsed: Number.NaN, wallClockMsUsed: -5, activeLoops: Infinity },
+      { computeUnits: 100, wallClockMs: 60_000, maxConcurrentLoops: Number.NaN },
+    );
+    // NaN compute → 0 used (within), -5 time → 0 used (within), Infinity loops → 0 used, NaN loop cap → 0.
+    // activeLoops(0) >= loopCap(0) → concurrency is the first exhausted dimension.
+    expect(d.exceeded).toBe("concurrency");
+    expect(d.remaining.computeUnits).toBe(100);
+    expect(Number.isNaN(d.remaining.wallClockMs)).toBe(false);
+    expect(d.remaining.wallClockMs).toBe(60_000);
+  });
+
+  it("denies a tenant with zero allocation immediately", () => {
+    const d = evaluateTenantQuota(
+      { computeUnitsUsed: 0, wallClockMsUsed: 0, activeLoops: 0 },
+      { computeUnits: 0, wallClockMs: 0, maxConcurrentLoops: 0 },
+    );
+    expect(d.allowed).toBe(false);
+    expect(d.exceeded).toBe("compute");
+  });
+
+  it("isolates tenants — one over quota does not affect another's decision (acceptance 2)", () => {
+    const over = evaluateTenantQuota({ computeUnitsUsed: 100, wallClockMsUsed: 0, activeLoops: 0 }, QUOTA);
+    const under = evaluateTenantQuota({ computeUnitsUsed: 5, wallClockMsUsed: 5_000, activeLoops: 1 }, QUOTA);
+    expect(over.allowed).toBe(false);
+    expect(under.allowed).toBe(true);
+    // Re-evaluating the over-quota tenant does not change the under-quota tenant's independent decision.
+    expect(evaluateTenantQuota({ computeUnitsUsed: 5, wallClockMsUsed: 5_000, activeLoops: 1 }, QUOTA)).toEqual(under);
+  });
+});