Borrowing it
Nothing to install: this file belongs to tiago-peixoto/claude-shapeup. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/tiago-peixoto/claude-shapeup/main/.claude/skills/behavior-tdd/SKILL.mdgit clone --depth 1 https://github.com/tiago-peixoto/claude-shapeupWrote 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/tiago-peixoto/claude-shapeup/behavior-tdd)<a href="https://agentmods.dev/skills/tiago-peixoto/claude-shapeup/behavior-tdd"><img src="https://agentmods.dev/badge/skills/tiago-peixoto/claude-shapeup/behavior-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/tiago-peixoto/claude-shapeup/behavior-tdd"><img src="https://agentmods.dev/badge/skills/tiago-peixoto/claude-shapeup/behavior-tdd.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 | $0.00092 | $0.01559 |
| Opus 5 | $0.00046 | $0.00779 |
| Sonnet 5 | $0.00018 | $0.00312 |
| Haiku 4.5 | $0.00009 | $0.00156 |
Grade A, and why
behavior-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 9d 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 — 117 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Behavior-change TDD (for this plugin)
This repo's agent behavior is defined by prompt text (skills/*/SKILL.md, references/*), not
code. A prompt edit can silently change outcomes while every deterministic test still passes.
This skill is the mandatory procedure for changing behavior here. The rule:
Deterministic checks for scaffolding. LLM-as-judge for outcomes. An ablation proves the prompt is what caused the change — not the model's defaults.
Read CLAUDE.md and CONTRIBUTING.md first; this skill operationalizes them.
When to use
- Editing any
skills/<phase>/SKILL.md, areferences/*.md, or a shared script the skills call. - Adding or changing a behavioral invariant (a new commit rule, a renamed status, a reshaped artifact, a new "the agent must/never…").
Do NOT use for pure scaffolding (hook/script logic with exact-text assertions) — that's the unit layer only. And never edit speculatively: start from a concrete signal (a failing test, a user complaint, an observed bad run). No signal → no edit.
The loop (RED → edit → GREEN → regression → commit)
1. Establish the behavioral RED — and prove the test bites
- Write or locate a criterion (
tests/behavioral/criteria/<name>.md) and a scenario (tests/behavioral/scenarios/<name>.md). Criteria are binary pass/fail propositions that name the exact observable behavior; the judge must cite evidence (seejudge-rubric.md). - Confirm it FAILs against the current prompt:
bash tests/behavioral/run-behavioral.sh <scenario> - Ablation (negative control) — non-negotiable. Confirm the criterion needs the prompt:
If ablation printsbash tests/behavioral/run-behavioral.sh --ablate <scenario> # must print "OK: fails without the skill"CONTAMINATED, the criterion passes with a neutral prompt — it's measuring the model (or the scenario leaks the answer), not your SKILL. Tighten the criterion until it hinges on prompt-specific mechanism, then re-ablate. (This is the "confirm-the-red" / scaffold-removal discipline; a passing ablation is a failed test.)
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.
- 9d ago First seen · 117 lines · 92 tokens per session scan A fea6c45647fc
behavior-tdd is a skill published in the GitHub repository tiago-peixoto/claude-shapeup (22 stars, last pushed 2mo ago), licensed MIT. It adds 92 tokens to every session and 1,559 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 2026-08-30.
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