Test-Driven Prompting — Failing Tests Are the Best Spec

1. RED:    You write a failing test (or several).
2. PROMPT: Paste the test. Ask the AI to make it pass without breaking others.
3. AGENT:  Edits source, runs the test suite, iterates until green.
4. GREEN:  All tests pass. You read the diff.
5. REFACTOR: "Improve readability without changing behavior. Tests must stay green."
6. COMMIT: Test + implementation as one atomic change.

// Step 1 — write the failing tests yourself
import { describe, it, expect } from "vitest";
import { parseDuration } from "./parse-duration";

describe("parseDuration", () => {
  it("parses simple seconds", () => {
    expect(parseDuration("30s")).toBe(30);
  });
  it("parses minutes", () => {
    expect(parseDuration("5m")).toBe(300);
  });
  it("parses combined", () => {
    expect(parseDuration("1h30m")).toBe(5400);
  });
  it("throws on bad input", () => {
    expect(() => parseDuration("banana")).toThrow();
  });
  it("returns 0 for empty string", () => {
    expect(parseDuration("")).toBe(0);
  });
});

# Step 2 — the prompt

"Make all tests in parse-duration.test.ts pass without breaking
  any other test in the suite. Run `pnpm test` after each edit.
  Show me the final implementation only."

# Claude Code, Cursor Agent, and Codex CLI will all do this end-to-end
# without further intervention.

# DANGEROUS PROMPT:
"Write the function and the tests for it."

# What you'll get: tests written to match the implementation,
# not the spec. Tests pass. Code may still be wrong.
# The tests are tautologies.

# SAFE PROMPT:
"Here are the tests. Here is the function signature.
  Implement the function so all tests pass. Do not add or modify
  any test."