diff --git a/src/billing.test.ts b/src/billing.test.ts
new file mode 100644
index 0000000..d05aec6
--- /dev/null
+++ b/src/billing.test.ts
@@ -0,0 +1,194 @@
+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 {
+  apiKeyStore,
+  pauseState,
+  rateBuckets,
+  servicesDisabled,
+  servicesMetadata,
+  servicesStore,
+  usageKey,
+  usageStore,
+  webhookStore,
+} from "./store/state.js";
+
+function resetState(): void {
+  eventLog.length = 0;
+  pauseState.paused = false;
+  apiKeyStore.clear();
+  rateBuckets.clear();
+  servicesDisabled.clear();
+  servicesMetadata.clear();
+  servicesStore.clear();
+  usageStore.clear();
+  webhookStore.clear();
+}
+
+beforeEach(resetState);
+
+void describe("billing and settlement endpoints", () => {
+  void it("quotes registered, zero-price, and unregistered services", async () => {
+    const app = createApp();
+    servicesStore.set("svc-priced", { priceStroops: 25 });
+    servicesStore.set("svc-free", { priceStroops: 0 });
+    usageStore.set(usageKey("agent-billing", "svc-priced"), 8);
+    usageStore.set(usageKey("agent-billing", "svc-free"), 9);
+    usageStore.set(usageKey("agent-billing", "svc-unregistered"), 7);
+
+    const priced = await request(app).get("/api/v1/billing/agent-billing/svc-priced");
+    assert.strictEqual(priced.status, 200);
+    assert.deepStrictEqual(priced.body, {
+      agent: "agent-billing",
+      serviceId: "svc-priced",
+      requests: 8,
+      priceStroops: 25,
+      billedStroops: 200,
+    });
+
+    const free = await request(app).get("/api/v1/billing/agent-billing/svc-free");
+    assert.strictEqual(free.status, 200);
+    assert.deepStrictEqual(free.body, {
+      agent: "agent-billing",
+      serviceId: "svc-free",
+      requests: 9,
+      priceStroops: 0,
+      billedStroops: 0,
+    });
+
+    const unregistered = await request(app).get(
+      "/api/v1/billing/agent-billing/svc-unregistered"
+    );
+    assert.strictEqual(unregistered.status, 200);
+    assert.deepStrictEqual(unregistered.body, {
+      agent: "agent-billing",
+      serviceId: "svc-unregistered",
+      requests: 7,
+      priceStroops: 0,
+      billedStroops: 0,
+    });
+  });
+
+  void it("sums billing totals across multiple agents and services", async () => {
+    const app = createApp();
+    servicesStore.set("svc-alpha", { priceStroops: 10 });
+    servicesStore.set("svc-beta", { priceStroops: 3 });
+    servicesStore.set("svc-large", { priceStroops: 2 });
+    usageStore.set(usageKey("agent-a", "svc-alpha"), 2);
+    usageStore.set(usageKey("agent-b", "svc-alpha"), 4);
+    usageStore.set(usageKey("agent-c", "svc-beta"), 5);
+    usageStore.set(usageKey("agent-d", "svc-large"), 100_000);
+    usageStore.set(usageKey("agent-e", "svc-missing"), 99);
+
+    const total = await request(app).get("/api/v1/billing/total");
+
+    assert.strictEqual(total.status, 200);
+    assert.strictEqual(total.body.totalStroops, 200_075);
+  });
+
+  void it("settles a pair, drains the accumulator, and emits a usage.settled event", async () => {
+    const app = createApp();
+    servicesStore.set("svc-settle", { priceStroops: 50 });
+
+    const usage = await request(app)
+      .post("/api/v1/usage")
+      .send({ agent: "agent-settle", serviceId: "svc-settle", requests: 6 });
+    assert.strictEqual(usage.status, 201);
+
+    const settle = await request(app)
+      .post("/api/v1/settle")
+      .send({ agent: "agent-settle", serviceId: "svc-settle" });
+
+    assert.strictEqual(settle.status, 200);
+    assert.deepStrictEqual(settle.body, {
+      agent: "agent-settle",
+      serviceId: "svc-settle",
+      requests: 6,
+      priceStroops: 50,
+      billedStroops: 300,
+    });
+
+    const drained = await request(app).get("/api/v1/usage/agent-settle/svc-settle");
+    assert.strictEqual(drained.status, 200);
+    assert.strictEqual(drained.body.total, 0);
+
+    const events = await request(app).get("/api/v1/events?type=usage.settled");
+    assert.strictEqual(events.status, 200);
+    assert.strictEqual(events.body.items.length, 1);
+    assert.strictEqual(events.body.items[0].type, "usage.settled");
+    assert.deepStrictEqual(events.body.items[0].payload, {
+      agent: "agent-settle",
+      serviceId: "svc-settle",
+      requests: 6,
+      billedStroops: 300,
+    });
+  });
+
+  void it("bills zero on a second settle after the first drain", async () => {
+    const app = createApp();
+    servicesStore.set("svc-double", { priceStroops: 12 });
+    usageStore.set(usageKey("agent-double", "svc-double"), 4);
+
+    const first = await request(app)
+      .post("/api/v1/settle")
+      .send({ agent: "agent-double", serviceId: "svc-double" });
+    assert.strictEqual(first.status, 200);
+    assert.strictEqual(first.body.billedStroops, 48);
+
+    const second = await request(app)
+      .post("/api/v1/settle")
+      .send({ agent: "agent-double", serviceId: "svc-double" });
+    assert.strictEqual(second.status, 200);
+    assert.strictEqual(second.body.requests, 0);
+    assert.strictEqual(second.body.billedStroops, 0);
+  });
+
+  void it("rejects missing and invalid settle identifiers with requestId", async () => {
+    const app = createApp();
+    const cases: { label: string; payload: Record<string, unknown> }[] = [
+      { label: "missing agent", payload: { serviceId: "svc-invalid" } },
+      { label: "missing serviceId", payload: { agent: "agent-invalid" } },
+      {
+        label: "non-string agent",
+        payload: { agent: 42, serviceId: "svc-invalid" },
+      },
+      {
+        label: "non-string serviceId",
+        payload: { agent: "agent-invalid", serviceId: 42 },
+      },
+    ];
+
+    for (const { label, payload } of cases) {
+      const requestId = `settle-validation-${label.replace(/[^a-z]/g, "-")}`;
+      const res = await request(app)
+        .post("/api/v1/settle")
+        .set("X-Request-Id", requestId)
+        .send(payload);
+
+      assert.strictEqual(res.status, 400, label);
+      assert.strictEqual(res.body.error, "invalid_request", label);
+      assert.strictEqual(res.body.requestId, requestId, label);
+      assert.strictEqual(typeof res.body.message, "string", label);
+    }
+  });
+
+  void it("does not settle or drain while the backend is paused", async () => {
+    const app = createApp();
+    servicesStore.set("svc-paused", { priceStroops: 20 });
+    usageStore.set(usageKey("agent-paused", "svc-paused"), 3);
+
+    const paused = await request(app).post("/api/v1/admin/pause");
+    assert.strictEqual(paused.status, 200);
+    assert.strictEqual(paused.body.paused, true);
+
+    const blocked = await request(app)
+      .post("/api/v1/settle")
+      .send({ agent: "agent-paused", serviceId: "svc-paused" });
+
+    assert.strictEqual(blocked.status, 503);
+    assert.strictEqual(blocked.body.error, "service_paused");
+    assert.strictEqual(usageStore.get(usageKey("agent-paused", "svc-paused")), 3);
+  });
+});