Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add borhen68/SkillEngine --skill test-driven-developmentgit clone --depth 1 https://github.com/borhen68/SkillEngineWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/borhen68/skillengine/test-driven-development)<a href="https://agentmods.dev/skills/borhen68/skillengine/test-driven-development"><img src="https://agentmods.dev/badge/skills/borhen68/skillengine/test-driven-development/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/borhen68/skillengine/test-driven-development"><img src="https://agentmods.dev/badge/skills/borhen68/skillengine/test-driven-development.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00079 | $0.03961 |
| Opus 5.5 | $0.00032 | $0.01584 |
| Sonnet 5.5 | $0.00016 | $0.00792 |
| Haiku 4.5 | $0.00008 | $0.00396 |
Grade A, and why
test-driven-development scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 407 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Test-Driven Development
Overview
Write a failing test before writing the code that makes it pass. For bug fixes, reproduce the bug with a test before attempting a fix. Tests are proof — "seems right" is not done. A codebase with good tests is an AI agent's superpower; a codebase without tests is a liability.
When to Use
- Implementing any new logic or behavior
- Fixing any bug (the Prove-It Pattern)
- Modifying existing functionality
- Adding edge case handling
- Any change that could break existing behavior
When NOT to use: Pure configuration changes, documentation updates, or static content changes that have no behavioral impact.
Related: For browser-based changes, combine TDD with runtime verification using Chrome DevTools MCP — see the Browser Testing section below.
Iron Rules
These target the failure modes that make AI-written tests worthless. Each rule is absolute — no rationalization in this skill or outside it overrides them.
- Never weaken a test to make it pass. When a test fails, the default assumption is that the code is wrong, not the test. Changing an assertion, widening a tolerance, or deleting a test case requires proof that the test was incorrect — state that proof explicitly before touching the test.
- RED must fail for the right reason. A test failing with
ImportErrororundefined is not a functionproves nothing. Read the failure output: it must show the behavioral gap the test targets (e.g.,expected 'completed', received 'pending'). A setup failure is not RED — fix the setup and re-run. - Never hardcode to the test. If the implementation special-cases test inputs (
if (id === 'test-1')), no behavior was implemented — the test was laundered into the code. Implement the general rule the test exemplifies, then ask: "would this pass for inputs the tests don't mention?" - Evidence or it didn't happen. "All tests pass" must be backed by actual runner output: suite counts, pass/fail numbers, duration. Reporting results from memory is fabrication.
- A silently skipped test is a lie.
.skip,.todo, commenting out, or deleting a test to get a green suite must be declared to the user with a reason — never done silently.
What ships with it
1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 1mo ago First seen · 407 lines · 79 tokens per session scan A ae814e37d63f
test-driven-development is a skill published in the GitHub repository borhen68/SkillEngine (17 stars, last pushed 4mo ago), licensed MIT. It adds 79 tokens to every session and 3,961 once invoked, about $0.0003 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
Other skills, from other repositories
constraint-driven-development
A development method for writing a project's quality standards as a lasting, checkable contract. It can cover areas such as test coverage, security, performance, accessibility, and architecture.
test-driven-development
Instructions for test-driven development, or TDD: writing a failing test first, then the smallest code that makes it pass, followed by cleanup.
test-driven-development
Drives development with tests using the red-green-refactor loop. Use when implementing any logic, fixing any bug, or changing any behavior. Use when you need to prove that code works, when a bug report arrives, or when you're about to modify existing functionality. Use when asked to add tests to a function, fix a bug…
tdd
Test-driven development, from unit to end-to-end tests. Use when the user wants to build features or fix bugs test-first, mentions "red-green-refactor", or wants integration or end-to-end tests (Playwright, Cypress). Don't use to backfill tests on existing code (/refactor) or diagnose an unknown bug (/debug).
browser-testing-with-devtools
A guide for testing web pages and browser-based applications in a real Chrome browser through Chrome DevTools. It checks what is actually rendered and how the page behaves at runtime.
test-driven-development
Red-green-refactor cycle with meaningful coverage. Tests are written before implementation. Coverage is a side effect of good tests, not the goal.