diff --git a/test/cost.test.ts b/test/cost.test.ts
new file mode 100644
index 0000000..7b18681
--- /dev/null
+++ b/test/cost.test.ts
@@ -0,0 +1,77 @@
+import { describe, expect, test } from "bun:test";
+
+import { JEV_USD_PER_MTOK, newMeter, addJevCost, addUsd } from "../src/cost";
+
+test("JEV_USD_PER_MTOK is the published rate", () => {
+  expect(JEV_USD_PER_MTOK).toBe(0.042);
+});
+
+test("newMeter starts at zero", () => {
+  expect(newMeter()).toEqual({ usd: 0 });
+});
+
+describe("addJevCost", () => {
+  test("adds the correct amount for a token count", () => {
+    const m = newMeter();
+    addJevCost(m, 1_000_000);
+    expect(m.usd).toBeCloseTo(JEV_USD_PER_MTOK, 10);
+  });
+
+  test("accumulates across calls", () => {
+    const m = newMeter();
+    addJevCost(m, 500_000);
+    addJevCost(m, 500_000);
+    expect(m.usd).toBeCloseTo(JEV_USD_PER_MTOK, 10);
+  });
+
+  test("ignores NaN, Infinity, negative, zero, and non-numbers", () => {
+    const m = newMeter();
+    for (const bad of [NaN, Infinity, -Infinity, -1, 0, "abc", null, undefined, false]) {
+      addJevCost(m, bad);
+    }
+    expect(m.usd).toBe(0);
+  });
+
+  test("ignores undefined meter", () => {
+    expect(() => addJevCost(undefined, 1000)).not.toThrow();
+  });
+
+  test("accepts numeric strings", () => {
+    const m = newMeter();
+    addJevCost(m, "1000000");
+    expect(m.usd).toBeCloseTo(JEV_USD_PER_MTOK, 10);
+  });
+});
+
+describe("addUsd", () => {
+  test("adds the exact amount", () => {
+    const m = newMeter();
+    addUsd(m, 0.05);
+    expect(m.usd).toBe(0.05);
+  });
+
+  test("accumulates across calls", () => {
+    const m = newMeter();
+    addUsd(m, 0.01);
+    addUsd(m, 0.02);
+    expect(m.usd).toBeCloseTo(0.03, 10);
+  });
+
+  test("ignores NaN, Infinity, negative, zero, and non-numbers", () => {
+    const m = newMeter();
+    for (const bad of [NaN, Infinity, -Infinity, -1, 0, "nope", null, undefined, false]) {
+      addUsd(m, bad);
+    }
+    expect(m.usd).toBe(0);
+  });
+
+  test("ignores undefined meter", () => {
+    expect(() => addUsd(undefined, 1)).not.toThrow();
+  });
+
+  test("accepts numeric strings", () => {
+    const m = newMeter();
+    addUsd(m, "0.25");
+    expect(m.usd).toBe(0.25);
+  });
+});