diff --git a/src/cli/parse-duration.test.ts b/src/cli/parse-duration.test.ts
index c26bcc114..0c14ac494 100644
--- a/src/cli/parse-duration.test.ts
+++ b/src/cli/parse-duration.test.ts
@@ -26,4 +26,28 @@ describe("parseDurationMs", () => {
   it("supports decimals", () => {
     expect(parseDurationMs("0.5s")).toBe(500);
   });
+
+  it("throws on empty string", () => {
+    expect(() => parseDurationMs("")).toThrow("invalid duration (empty)");
+    expect(() => parseDurationMs("  ")).toThrow("invalid duration (empty)");
+  });
+
+  it("throws on invalid format", () => {
+    expect(() => parseDurationMs("abc")).toThrow("invalid duration: abc");
+    expect(() => parseDurationMs("10x")).toThrow("invalid duration: 10x");
+    expect(() => parseDurationMs("not-a-number")).toThrow("invalid duration: not-a-number");
+  });
+
+  it("throws on negative values", () => {
+    expect(() => parseDurationMs("-5")).toThrow("invalid duration: -5");
+    expect(() => parseDurationMs("-10s")).toThrow("invalid duration: -10s");
+  });
+
+  it("respects custom defaultUnit option", () => {
+    expect(parseDurationMs("10", { defaultUnit: "s" })).toBe(10_000);
+    expect(parseDurationMs("5", { defaultUnit: "m" })).toBe(300_000);
+    expect(parseDurationMs("2", { defaultUnit: "h" })).toBe(7_200_000);
+    expect(parseDurationMs("1", { defaultUnit: "d" })).toBe(86_400_000);
+    expect(parseDurationMs("100", { defaultUnit: "ms" })).toBe(100);
+  });
 });