diff --git a/README.md b/README.md
index 337ee4f..806ffd3 100644
--- a/README.md
+++ b/README.md
@@ -74,6 +74,23 @@ agentpay-backend/
   stroops, `priceStroops`, `billedStroops`, `/api/v1/billing/*`, and why
   `POST /api/v1/settle` drains backend counters without moving funds.
 
+## Query Parameters
+
+Numeric query parameters are parsed defensively. Malformed, `NaN`, infinite, or
+missing numeric values fall back to the endpoint default, while values below or
+above the supported range are clamped:
+
+| Endpoint family                       | Parameter | Default | Range            |
+| ------------------------------------- | --------- | ------- | ---------------- |
+| `GET /api/v1/agents`                  | `limit`   | `200`   | `1` to `1000`    |
+| `GET /api/v1/services`                | `limit`   | `200`   | `1` to `1000`    |
+| `GET /api/v1/services/:id/agents/top` | `limit`   | `10`    | `1` to `100`     |
+| `GET /api/v1/events`                  | `limit`   | `100`   | `1` to event cap |
+| `GET /api/v1/events`                  | `since`   | `0`     | `0` or higher    |
+
+This keeps bad input such as `?limit=abc` or `?since=abc` from propagating
+`NaN` into slices or timestamp filters.
+
 ## Quickstart
 
 Start a local backend on `http://localhost:3001` with the checked-in
diff --git a/src/query-params.test.ts b/src/query-params.test.ts
new file mode 100644
index 0000000..294041c
--- /dev/null
+++ b/src/query-params.test.ts
@@ -0,0 +1,72 @@
+import assert from "node:assert";
+import { describe, it } from "node:test";
+import request from "supertest";
+import { app } from "./index.js";
+import { parseIntParam } from "./queryParams.js";
+
+void describe("numeric query parameter parsing", () => {
+  void it("falls back and clamps numeric query parameters", () => {
+    const options = { defaultValue: 10, min: 1, max: 100 };
+
+    assert.strictEqual(parseIntParam(undefined, options), 10);
+    assert.strictEqual(parseIntParam("abc", options), 10);
+    assert.strictEqual(parseIntParam("NaN", options), 10);
+    assert.strictEqual(parseIntParam("Infinity", options), 10);
+    assert.strictEqual(parseIntParam("0", options), 1);
+    assert.strictEqual(parseIntParam("-5", options), 1);
+    assert.strictEqual(parseIntParam("250", options), 100);
+    assert.strictEqual(parseIntParam("3.7", options), 3);
+  });
+
+  void it("falls back to the default limit on agent lists", async () => {
+    await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "qp-agent-a", serviceId: "qp-service", requests: 1 });
+    await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "qp-agent-b", serviceId: "qp-service", requests: 1 });
+
+    const fallback = await request(app).get("/api/v1/agents?limit=abc");
+    const clamped = await request(app).get("/api/v1/agents?limit=0");
+
+    assert.strictEqual(fallback.status, 200);
+    assert.ok(fallback.body.agents.includes("qp-agent-a"));
+    assert.ok(fallback.body.agents.includes("qp-agent-b"));
+    assert.strictEqual(clamped.body.agents.length, 1);
+  });
+
+  void it("falls back to the default limit on top service-agent lists", async () => {
+    await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "qp-top-a", serviceId: "qp-top-service", requests: 5 });
+    await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "qp-top-b", serviceId: "qp-top-service", requests: 3 });
+
+    const fallback = await request(app).get(
+      "/api/v1/services/qp-top-service/agents/top?limit=abc"
+    );
+
+    assert.strictEqual(fallback.status, 200);
+    assert.deepStrictEqual(
+      fallback.body.items.map((item: { agent: string }) => item.agent),
+      ["qp-top-a", "qp-top-b"]
+    );
+  });
+
+  void it("falls back to since=0 instead of hiding every event", async () => {
+    await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "qp-event-agent", serviceId: "qp-event-service", requests: 1 });
+
+    const res = await request(app).get("/api/v1/events?since=abc&limit=100");
+
+    assert.strictEqual(res.status, 200);
+    assert.ok(
+      res.body.items.some(
+        (item: { type: string; payload?: { agent?: string } }) =>
+          item.type === "usage.recorded" && item.payload?.agent === "qp-event-agent"
+      )
+    );
+  });
+});
diff --git a/src/queryParams.ts b/src/queryParams.ts
new file mode 100644
index 0000000..8dbee73
--- /dev/null
+++ b/src/queryParams.ts
@@ -0,0 +1,23 @@
+type IntParamOptions = {
+  defaultValue: number;
+  min: number;
+  max: number;
+};
+
+/**
+ * Parses integer query params with fallback and bounded output.
+ */
+export function parseIntParam(
+  value: unknown,
+  { defaultValue, min, max }: IntParamOptions
+): number {
+  const raw = Array.isArray(value) ? value[0] : value;
+  if (typeof raw !== "string" && typeof raw !== "number") return defaultValue;
+  if (raw === "") return defaultValue;
+
+  const parsed = Number(raw);
+  if (!Number.isFinite(parsed)) return defaultValue;
+
+  const integer = Math.trunc(parsed);
+  return Math.min(max, Math.max(min, integer));
+}
diff --git a/src/routes/events.ts b/src/routes/events.ts
index b173993..0d467b3 100644
--- a/src/routes/events.ts
+++ b/src/routes/events.ts
@@ -1,5 +1,6 @@
 import { Router, type Request, type Response } from "express";
 import { EVENT_LOG_CAP, eventLog } from "../events.js";
+import { parseIntParam } from "../queryParams.js";
 
 /**
  * Builds read-only audit-event routes.
@@ -14,12 +15,17 @@ export function createEventsRouter(): Router {
   });
 
   router.get("/api/v1/events", (req: Request, res: Response) => {
-    const since = Number((req.query.since as string) ?? 0);
+    const since = parseIntParam(req.query.since, {
+      defaultValue: 0,
+      min: 0,
+      max: Number.MAX_SAFE_INTEGER,
+    });
     const type = typeof req.query.type === "string" ? req.query.type : undefined;
-    const limit = Math.min(
-      EVENT_LOG_CAP,
-      Math.max(1, Number((req.query.limit as string) ?? 100))
-    );
+    const limit = parseIntParam(req.query.limit, {
+      defaultValue: 100,
+      min: 1,
+      max: EVENT_LOG_CAP,
+    });
     const items = eventLog
       .filter((e) => e.ts >= since && (type === undefined || e.type === type))
       .slice(-limit);
diff --git a/src/routes/services.ts b/src/routes/services.ts
index c606e5c..af4f945 100644
--- a/src/routes/services.ts
+++ b/src/routes/services.ts
@@ -1,5 +1,6 @@
 import { createHash } from "node:crypto";
 import { Router, type Request, type Response } from "express";
+import { parseIntParam } from "../queryParams.js";
 import {
   servicesDisabled,
   servicesMetadata,
@@ -127,10 +128,11 @@ export function createServicesRouter(): Router {
     "/api/v1/services/:serviceId/agents/top",
     (req: Request, res: Response) => {
       const { serviceId } = req.params;
-      const limit = Math.min(
-        100,
-        Math.max(1, Number((req.query.limit as string) ?? 10))
-      );
+      const limit = parseIntParam(req.query.limit, {
+        defaultValue: 10,
+        min: 1,
+        max: 100,
+      });
       const suffix = `::${serviceId}`;
       const items: { agent: string; total: number }[] = [];
       for (const [key, total] of usageStore.entries()) {
@@ -292,10 +294,11 @@ export function createServicesRouter(): Router {
   router.get("/api/v1/services", (req: Request, res: Response) => {
     const prefix = typeof req.query.prefix === "string" ? req.query.prefix : "";
     const q = typeof req.query.q === "string" ? req.query.q.toLowerCase() : "";
-    const limit = Math.min(
-      1000,
-      Math.max(1, Number((req.query.limit as string) ?? 200))
-    );
+    const limit = parseIntParam(req.query.limit, {
+      defaultValue: 200,
+      min: 1,
+      max: 1000,
+    });
     const services: ServiceReadShape[] = [];
     for (const [serviceId, meta] of servicesStore.entries()) {
       if (prefix && !serviceId.startsWith(prefix)) continue;
diff --git a/src/routes/usage.ts b/src/routes/usage.ts
index cd28a50..15459e0 100644
--- a/src/routes/usage.ts
+++ b/src/routes/usage.ts
@@ -1,5 +1,6 @@
 import { Router, type Request, type Response } from "express";
 import { recordEvent } from "../events.js";
+import { parseIntParam } from "../queryParams.js";
 import {
   servicesDisabled,
   servicesStore,
@@ -212,10 +213,11 @@ export function createUsageRouter(): Router {
   });
 
   router.get("/api/v1/agents", (req: Request, res: Response) => {
-    const limit = Math.min(
-      1000,
-      Math.max(1, Number((req.query.limit as string) ?? 200))
-    );
+    const limit = parseIntParam(req.query.limit, {
+      defaultValue: 200,
+      min: 1,
+      max: 1000,
+    });
     const seen = new Set<string>();
     for (const key of usageStore.keys()) seen.add(key.split("::")[0]);
     const agents = Array.from(seen).slice(0, limit);