diff --git a/src/usage.test.ts b/src/usage.test.ts
new file mode 100644
index 0000000..ab84531
--- /dev/null
+++ b/src/usage.test.ts
@@ -0,0 +1,140 @@
+import { beforeEach, describe, it } from "node:test";
+import assert from "node:assert";
+import request from "supertest";
+import { eventLog } from "./events.js";
+import { createApp } from "./index.js";
+import {
+  pauseState,
+  servicesDisabled,
+  servicesMetadata,
+  servicesStore,
+  usageKey,
+  usageStore,
+} from "./store/state.js";
+
+function assertInvalidRequest(body: unknown): void {
+  if (body === null || typeof body !== "object") {
+    throw new TypeError("expected error response object");
+  }
+  const record = body as Record<string, unknown>;
+  assert.strictEqual(record.error, "invalid_request");
+  assert.strictEqual(typeof record.message, "string");
+  assert.ok((record.message as string).length > 0);
+  assert.strictEqual(typeof record.requestId, "string");
+  assert.ok((record.requestId as string).length > 0);
+}
+
+beforeEach(() => {
+  pauseState.paused = false;
+  usageStore.clear();
+  servicesStore.clear();
+  servicesDisabled.clear();
+  servicesMetadata.clear();
+  eventLog.length = 0;
+});
+
+void describe("usage recording endpoints", () => {
+  void it("records, accumulates, reads back totals, and returns zero for unseen pairs", async () => {
+    const app = createApp();
+
+    const first = await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "agent-ledger", serviceId: "svc-meter", requests: 40 });
+    assert.strictEqual(first.status, 201);
+    assert.deepStrictEqual(first.body, {
+      agent: "agent-ledger",
+      serviceId: "svc-meter",
+      total: 40,
+    });
+
+    const second = await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "agent-ledger", serviceId: "svc-meter", requests: 25 });
+    assert.strictEqual(second.status, 201);
+    assert.strictEqual(second.body.total, 65);
+
+    const readBack = await request(app).get("/api/v1/usage/agent-ledger/svc-meter");
+    assert.strictEqual(readBack.status, 200);
+    assert.deepStrictEqual(readBack.body, {
+      agent: "agent-ledger",
+      serviceId: "svc-meter",
+      total: 65,
+    });
+
+    const unseen = await request(app).get("/api/v1/usage/agent-ledger/never-used");
+    assert.strictEqual(unseen.status, 200);
+    assert.deepStrictEqual(unseen.body, {
+      agent: "agent-ledger",
+      serviceId: "never-used",
+      total: 0,
+    });
+  });
+
+  void it("saturates deterministic totals at Number.MAX_SAFE_INTEGER", async () => {
+    const app = createApp();
+    usageStore.set(usageKey("agent-max", "svc-max"), Number.MAX_SAFE_INTEGER - 2);
+
+    const response = await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "agent-max", serviceId: "svc-max", requests: 10 });
+
+    assert.strictEqual(response.status, 201);
+    assert.strictEqual(response.body.total, Number.MAX_SAFE_INTEGER);
+    assert.strictEqual(
+      usageStore.get(usageKey("agent-max", "svc-max")),
+      Number.MAX_SAFE_INTEGER
+    );
+  });
+
+  void it("refuses disabled services without increasing the usage total", async () => {
+    const app = createApp();
+
+    await request(app)
+      .post("/api/v1/services")
+      .send({ serviceId: "svc-disabled", priceStroops: 10 });
+    const initial = await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "agent-disabled", serviceId: "svc-disabled", requests: 3 });
+    assert.strictEqual(initial.status, 201);
+
+    await request(app)
+      .patch("/api/v1/services/svc-disabled/disabled")
+      .send({ disabled: true });
+    const blocked = await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "agent-disabled", serviceId: "svc-disabled", requests: 4 });
+
+    assert.strictEqual(blocked.status, 409);
+    assert.strictEqual(blocked.body.error, "service_disabled");
+    assert.strictEqual(typeof blocked.body.requestId, "string");
+    assert.strictEqual(usageStore.get(usageKey("agent-disabled", "svc-disabled")), 3);
+  });
+
+  for (const [label, payload] of [
+    ["missing agent", { serviceId: "svc", requests: 1 }],
+    ["non-string agent", { agent: 7, serviceId: "svc", requests: 1 }],
+    ["empty agent", { agent: "", serviceId: "svc", requests: 1 }],
+    ["oversized agent", { agent: "a".repeat(257), serviceId: "svc", requests: 1 }],
+    ["missing serviceId", { agent: "agent", requests: 1 }],
+    ["non-string serviceId", { agent: "agent", serviceId: 8, requests: 1 }],
+    ["empty serviceId", { agent: "agent", serviceId: "", requests: 1 }],
+    [
+      "oversized serviceId",
+      { agent: "agent", serviceId: "s".repeat(129), requests: 1 },
+    ],
+    ["missing requests", { agent: "agent", serviceId: "svc" }],
+    ["zero requests", { agent: "agent", serviceId: "svc", requests: 0 }],
+    ["negative requests", { agent: "agent", serviceId: "svc", requests: -2 }],
+    ["fractional requests", { agent: "agent", serviceId: "svc", requests: 1.5 }],
+    ["non-number requests", { agent: "agent", serviceId: "svc", requests: "1" }],
+  ] as const) {
+    void it(`rejects ${label} with standard invalid_request shape`, async () => {
+      const app = createApp();
+
+      const response = await request(app).post("/api/v1/usage").send(payload);
+
+      assert.strictEqual(response.status, 400);
+      assertInvalidRequest(response.body as unknown);
+    });
+  }
+});