Getting it into your agent
This one installs as part of its plugin. Adding the marketplace and installing the plugin brings it with everything else the plugin ships.
/plugin marketplace add int2t05/engineering-skills/plugin install engineering-skillsWrote 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/int2t05/engineering-skills/tdd)<a href="https://agentmods.dev/skills/int2t05/engineering-skills/tdd"><img src="https://agentmods.dev/badge/skills/int2t05/engineering-skills/tdd.svg" alt="Measured on agentmods" 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 | $0.00093 | $0.01172 |
| Opus 5 | $0.00046 | $0.00586 |
| Sonnet 5 | $0.00019 | $0.00234 |
| Haiku 4.5 | $0.00009 | $0.00117 |
Grade A, and why
tdd 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 5d 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 — 106 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Test-Driven Development
Write the failing test first. Watch it fail. Write minimal code to pass. Refactor. A test that passes immediately proves nothing — you never saw it catch a bug.
Iron law: No production code without a failing test first. If you wrote code before the test, delete it and start over. "Keep as reference" is testing-after in disguise — delete means delete.
When to use
- Implementing any new feature or behavior
- Fixing a bug (reproduce it with a test first — the Prove-It Pattern)
- Refactoring or changing existing behavior
- Triggers on "tdd", "test-driven", "red green refactor", "测试驱动开发", "红绿重构", "测试驱动"
Not for: generating tests for already-written code (use test-generation); have a spec and want slice-by-slice feature implementation (use implement — it drives TDD within each slice).
Exceptions (confirm with the user): throwaway prototypes, generated code, pure configuration changes with no behavioral impact — for these, confirm with the user before skipping TDD rather than defaulting either way.
Steps
0. Discover the stack first
The cycle is universal; the commands are not. Before the first test, find how
this repo tests: read package.json / pyproject.toml / Cargo.toml /
pom.xml / go.mod, check for ./gradlew, Makefile, CI workflows, and
existing test-file naming. Never assume npm test — use the repo's own command
for every RED, GREEN, and final verification.
1. RED — write one failing test
One behavior, one test, one vertical slice (tracer bullet that responds to what the last cycle taught you). Write the test you wish existed — it describes the desired API, not the implementation. Name it as a specification ("rejects empty email", not "test1").
Agree the seam (public interface under test) before writing. No test at an unconfirmed seam — agreeing seams up front puts testing effort on critical paths instead of every edge case. Use real code over mocks; if you must double a dependency, use the simplest double that works (Real > Fake > Stub > Mock — see references/test-strategy.md). Mocks isolate boundaries, not the thing being tested.
What ships with it
5 files 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.
- 5d ago First seen · 106 lines · 93 tokens per session scan A f95aea52dee9
tdd is a skill published in the GitHub repository int2t05/engineering-skills (3 stars, last pushed 4d ago), licensed MIT. It adds 93 tokens to every session and 1,172 once invoked, about $0.0005 per session on Opus 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 2026-08-31.
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bugfix
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