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 agentmods add skills/stchiotis/library-first-engineering/lfe-tddnpx skills add StChiotis/Library-First-Engineering --skill lfe-tddgit clone --depth 1 https://github.com/StChiotis/Library-First-EngineeringWrote 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/stchiotis/library-first-engineering/lfe-tdd)<a href="https://agentmods.dev/skills/stchiotis/library-first-engineering/lfe-tdd"><img src="https://agentmods.dev/badge/skills/stchiotis/library-first-engineering/lfe-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 | $0.00032 | $0.01162 |
| Opus 5 | $0.00016 | $0.00581 |
| Sonnet 5 | $0.00006 | $0.00232 |
| Haiku 4.5 | $0.00003 | $0.00116 |
Grade A, and why
lfe-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 — 143 lines — stays where its author put it; the contents beside it link to each section on GitHub.
LFE TDD
Position in Pipeline
- Phase: 2 (Builder sub-pipeline, Step 2)
- Persona: Builder
- Input:
active_plan.md+.plans/builder_done.md(resume marker — confirms implementation phase completed) + code written by lfe-builder - Output:
.plans/tdd_report.md
Philosophy
Core principle: Tests verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.
Good tests are integration-style: they exercise real code paths through public APIs. They describe what the system does, not how. A good test reads like a specification.
Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means. Warning sign: your test breaks when you refactor, but behavior hasn't changed.
See tests.md, mocking.md for examples.
Anti-Pattern: Horizontal Slices
Write incrementally — one test, then its implementation — rather than all tests up front. Writing all tests first is "horizontal slicing."
Correct approach: Vertical slices via tracer bullets. One test → one implementation → repeat.
WRONG (horizontal):
RED: test1, test2, test3, test4, test5
GREEN: impl1, impl2, impl3, impl4, impl5
RIGHT (vertical):
RED→GREEN: test1→impl1
RED→GREEN: test2→impl2
RED→GREEN: test3→impl3
Workflow
1. Review
Read active_plan.md and review what the Builder implemented. Identify behaviors that need test coverage. Use the project's domain glossary for test names.
Confirm with the human which behaviors to prioritize for testing. Testing everything is impossible — prioritize.
2. Tracer Bullet
Write ONE test that confirms ONE thing:
RED: Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes
3. Incremental Loop
For each remaining behavior:
RED: Write next test → fails
GREEN: Minimal code to pass → passes
Rules:
- One test at a time
- Only enough code to pass current test
- Write only for the current test, not anticipated future ones
- Keep tests focused on observable 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.
- 5d ago First seen · 143 lines · 32 tokens per session scan A 49205e883f71
lfe-tdd is a skill published in the GitHub repository StChiotis/Library-First-Engineering (43 stars, last pushed 1mo ago), licensed MIT. It adds 32 tokens to every session and 1,162 once invoked, about $0.0002 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 2026-08-30.
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