diff --git a/packages/typescript/test/cost-boundaries.test.ts b/packages/typescript/test/cost-boundaries.test.ts
new file mode 100644
index 0000000..4b357af
--- /dev/null
+++ b/packages/typescript/test/cost-boundaries.test.ts
@@ -0,0 +1,15 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import {CostObservation} from "../src/index.js";
+
+test("cost accepts numeric zero and explicit numeric strings", () => {
+  for (const amount of [0, "0", 1.25, "1.25"]) {
+    assert.equal(new CostObservation(amount, "USD", "provider").asEventValue().amount, Number(amount));
+  }
+});
+
+test("cost refuses values that Number would silently coerce", () => {
+  for (const amount of [true, false, null, [], [1], {}, "", " ", "\t\n"]) {
+    assert.throws(() => new CostObservation(amount as number, "USD", "provider").asEventValue(), /finite non-negative/);
+  }
+});