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 michael-denyer/pstack-claude --skill principle-test-behavior-not-implementationgit clone --depth 1 https://github.com/michael-denyer/pstack-claudeWrote 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/michael-denyer/pstack-claude/principle-test-behavior-not-implementation)<a href="https://agentmods.dev/skills/michael-denyer/pstack-claude/principle-test-behavior-not-implementation"><img src="https://agentmods.dev/badge/skills/michael-denyer/pstack-claude/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/michael-denyer/pstack-claude/principle-test-behavior-not-implementation"><img src="https://agentmods.dev/badge/skills/michael-denyer/pstack-claude/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 | $0.00063 | $0.00604 |
| Opus 5 | $0.00032 | $0.00302 |
| Sonnet 5 | $0.00013 | $0.00121 |
| Haiku 4.5 | $0.00006 | $0.00060 |
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 yesterday.
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.
What it actually says
Test Behavior, Not Implementation
A test calls the code the way its users do and asserts the result they observe against a literal expected value. A test that asserts which calls the code made, or restates a constant the code contains, does neither.
The check: before you keep a test, ask whether it would still pass if every function it imports returned undefined. If yes, it observes no behavior and cannot fail for a defect. Rewrite the assertion or delete the test.
Why: A test that cannot fail for a defect costs CI time and review attention and catches nothing. A constant pin also fails when someone edits the constant or the prompt it restates, so it prevents that edit.
Five shapes that still pass when every imported function returns undefined:
- Weak or no assertion. No
expect, or onlytoBeDefined,toBeTruthy,not.toThrow,toBeInstanceOf,toBeGreaterThan(0). - Mock or absence only. Only
toHaveBeenCalled,not.toHaveBeenCalled,toBeUndefined,toEqual([]),toHaveLength(0),not.toBe(wrongValue). - Self-referential. The expected value comes from the code under test:
expect(f(a)).toBe(f(a)),expect(parsed.url).toBe(buildUrl(...)). - Constant pin. The assertion restates a hand-maintained constant, config default, table row, or prompt string:
expect(LIMITS.maxTools).toBe(8),expect(PROMPT).toContain("You are"). - Fixture asserts fixture. The assertion reads data the test built or a value computed in
beforeEach, and the subject never runs inside the body.
The fix: call the subject inside the test body with one concrete input and assert the literal output or the observable effect, expect(slugify("Hello, World!")).toBe("hello-world"). For an absence, assert the presence on the other input in the same test. For a constant, test the mechanism that reads it with one input instead of restating the value. For a mock, assert the payload it received or the state after the call, not that it was called. When no such assertion exists, delete the test.
Keep a test of a relation across a table's rows (a key present in two tables, a parent that exists), and a compile-time check in a *.test-d.ts file.
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.
- yesterday First seen · 26 lines · 63 tokens per session scan A 08f6e5f38643
principle-test-behavior-not-implementation is a skill published in the GitHub repository michael-denyer/pstack-claude (281 stars, last pushed yesterday), licensed MIT. It adds 63 tokens to every session and 604 once invoked, about $0.0003 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 2026-09-08.
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