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 minghinmatthewlam/agent-guards --skill principle-test-behavior-not-implementationgit clone --depth 1 https://github.com/minghinmatthewlam/agent-guardsWrote 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/minghinmatthewlam/agent-guards/principle-test-behavior-not-implementation)<a href="https://agentmods.dev/skills/minghinmatthewlam/agent-guards/principle-test-behavior-not-implementation"><img src="https://agentmods.dev/badge/skills/minghinmatthewlam/agent-guards/principle-test-behavior-not-implementation/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/minghinmatthewlam/agent-guards/principle-test-behavior-not-implementation"><img src="https://agentmods.dev/badge/skills/minghinmatthewlam/agent-guards/principle-test-behavior-not-implementation.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 | $0.00052 | $0.00319 |
| Opus 5.5 | $0.00021 | $0.00128 |
| Sonnet 5.5 | $0.00010 | $0.00064 |
| Haiku 4.5 | $0.00005 | $0.00032 |
Grade A, and why
principle-test-behavior-not-implementation 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 2d 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.
This is a copy
88% identical to principle-test-behavior-not-implementation — 2 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
What it actually says
Test Behavior, Not Implementation
A test calls the code as its users do and asserts the exact result they see. Rewrite or delete any test that would still pass if every imported function returned undefined or a wrong value.
These shapes fail that check:
- Loose or no assertion:
toBeDefined,toBeTruthy,not.toThrow,toBeGreaterThan(0). - Call or absence only:
toHaveBeenCalled,not.toHaveBeenCalled,toBeUndefined,toEqual([]). - Expected value computed by the code under test:
expect(f(a)).toBe(f(a)). - Restated constant, config default, table row, or prompt string.
- Assertions on data the test built, with no call to the subject.
Fix: call the subject in the test body with one concrete input and assert the literal output or effect. For a mock, assert the payload it received. For an absence, also assert the presence on another input. For a constant, test the mechanism that reads it with one input instead of restating the value.
Keep relation checks across table rows (a key in two tables, a parent that exists) and *.test-d.ts type checks.
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.
- 2d ago First seen · 22 lines · 52 tokens per session scan A f9a7cc57395f
principle-test-behavior-not-implementation is a skill published in the GitHub repository minghinmatthewlam/agent-guards (47 stars, last pushed 2d ago), licensed MIT. It adds 52 tokens to every session and 319 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. It is 88% identical to principle-test-behavior-not-implementation, differing in 2 lines, and is treated as a copy.
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Apply when you write, change, or keep a test. Assert what the code's users can observe, so that the test fails when that behavior breaks and keeps passing when only the internals change.
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Rebuild a package's unit test suite around a behavior specification (SPEC.md) derived from the implementation, with every test citing a numbered rule ID. Use when asked to do a spec-driven test rebuild, write a SPEC.md for a package, repeat the state or store spec process (PRs.