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 youdotcom-oss/agent-skills --skill tddgit clone --depth 1 https://github.com/youdotcom-oss/agent-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/youdotcom-oss/agent-skills/tdd)<a href="https://agentmods.dev/skills/youdotcom-oss/agent-skills/tdd"><img src="https://agentmods.dev/badge/skills/youdotcom-oss/agent-skills/tdd/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/youdotcom-oss/agent-skills/tdd"><img src="https://agentmods.dev/badge/skills/youdotcom-oss/agent-skills/tdd.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 | $0.00063 | $0.00635 |
| Opus 5.5 | $0.00025 | $0.00254 |
| Sonnet 5.5 | $0.00013 | $0.00127 |
| Haiku 4.5 | $0.00006 | $0.00064 |
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 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.
Copies of this mod
1 near-identical copy found in the catalogue:
- tdd — 88% identical, 7 lines differ
How it starts
The opening of the file, as written. The whole thing — 67 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Test-Driven Development
Core Rule
Work in vertical red-green-refactor slices:
- RED: write one test for one observable behavior.
- GREEN: implement the smallest production change that makes that test pass.
- REFACTOR: clean up only while tests are green.
Do not write a batch of imagined tests before implementation. Each new test should respond to what the last cycle taught you about the actual behavior and interface.
Plaited Defaults
- Use
bun testfor targeted tests andbun --bun tsc --noEmitfor the type gate. - Use
test, notit, and organize withdescribe. - Prefer real dependencies and real runtime boundaries over mocks.
- Mock only external boundaries such as remote APIs, time, randomness, or rare filesystem cases.
- Avoid conditional assertions: assert the branch first, then assert branch-specific values.
- Test both branches for conditionals, fallbacks, and error paths.
- Exercise public interfaces and runtime contracts rather than private helpers.
- Follow applicable repo skills and
AGENTS.mdrules for the touched area.
When Starting Work
Explore the codebase first. If the public interface, expected behavior, or risk boundary is discoverable from code, tests, or docs, use that evidence instead of asking the user.
Ask one concise question only when the decision cannot be discovered and a reasonable assumption would be risky. Include your recommended answer.
Before writing production code, identify:
- the public interface or runtime boundary under test
- the first observable behavior to prove
- the targeted test command for the cycle
- the broader validation needed before handoff
Cycle Discipline
For each behavior:
- Add or update exactly one focused test.
- Run the targeted test and confirm it fails for the intended reason.
- Implement only enough code to pass.
- Run the same targeted test and confirm it passes.
- Refactor if useful, then rerun the affected tests.
If the test passes before production code changes, it is not a valid RED signal. Tighten the test, choose a different behavior, or explain why existing coverage already proves the behavior.
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.
- 1mo ago First seen · 67 lines · 63 tokens per session scan A 4a36f55baf4b
tdd is a skill published in the GitHub repository youdotcom-oss/agent-skills (94 stars, last pushed today), licensed MIT. It adds 63 tokens to every session and 635 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.
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Test-driven development - write failing test first, then minimal code. Use before implementing any feature or bugfix.
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Write thorough tests following TDD and BDD principles.
sumo-qa-implementing-with-tdd
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