principle-test-behavior-not-implementation SKILL.md skillA
This file is byte-identical to the first copy the registry indexed, one of 3 indexed copies. Same content hash, so the same checks give both the same grade.
principle-test-behavior-not-implementation from cursor/plugins is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
mdr add cursor/plugins/principle-test-behavior-not-implementation@git:20261016.182f2cePins git:20261016.182f2ce in mdr.lock. Pin the fingerprint instead.
- Audit grade
- A, 17 of 17 checks passed
- Kind
- SKILL.md skill
- Repository
- cursor/plugins
- Path
harness/pstack/claude-code/skills/principle-test-behavior-not-implementation/SKILL.md- Versions recorded
- 1
- Latest version
- git:20261016.182f2ce, committed Oct 16, 2026
- Size
- 2,614 bytes
- Repository stars
- 13,494
- License
- none declared
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.
Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file the same grade.
What the file says
# Test Behavior, Not Implementation A test is a claim about what the code does for its users: a caller, an end user, or another service. Behavior is what those users can observe through the interface they use. That includes return values, errors, state they can read back, and effects on systems outside the code, such as a request sent, a row written, or a message emitted. Implementation is what they cannot observe: private helpers, internal call order, intermediate data, and which internal collaborator did the work. Judge each test against two imagined edits: - **Break the behavior.** Make the code return a wrong value, skip an effect, or do the effect twice. If the test still passes, it proves nothing. The usual causes are an assertion looser than the contract, an expected value computed by the code under test, a restated constant or prompt string that never runs the code that reads it, and an assertion on data that the test built itself. - **Change the internals.** Keep every observable result the same and restructure the code underneath. If the test now fails, it is pinned to the implementation. It will need a rewrite on every refactor and still catches no bug. …
Read the whole file at its exact version.
How to install
mdr add cursor/plugins/principle-test-behavior-not-implementation@sha256:f7223698623b6a26The line at the top of this page pins the label git:20261016.182f2ce, which is what most people want. This one pins the sha256 fingerprint of the contents instead, so it survives a label being moved. Either way the exact version is written to mdr.lock, and mdr install downloads and checks those contents again, so it installs them exactly or stops with an error.
[](https://markdownregistry.com/a/art_4kvw3bkk2sgyremi)
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Versions
The registry has recorded 1 version of this file, committed on Oct 16, 2026.
Audit of the latest version
- pass: Frontmatter block present
- pass: Frontmatter declares a name
- pass: Frontmatter declares a description
- pass: Size between 200 bytes and 200 KB (2614 bytes)
- pass: No zero-width or bidi control characters
- pass: No instruction hidden inside an HTML comment
- pass: No link to an exfiltration or paste host
- pass: No credential-shaped string
- pass: No instruction to send local credentials anywhere
- pass: No text hidden with inline styles
- pass: No prompt-injection phrasing
- pass: No curl or wget piped into a shell
- pass: No recursive delete of root, home or parent
- pass: No instruction to read or print local credentials
- pass: No base64 blob over 200 characters
- pass: No link to a raw IP address
- pass: No script tag
Source
cursor/plugins · 13,494 stars · license none declared · pushed 2026-10-22 · branch main
API
GET https://markdownregistry.com/api/v1/artifacts/art_4kvw3bkk2sgyremi GET https://markdownregistry.com/api/v1/resolve?ref=cursor/plugins/principle-test-behavior-not-implementation GET https://markdownregistry.com/api/v1/blob/f7223698623b6a26fea49c751e466b38e6691b1f32ce841d3983e7ffbe2b74a6
Your AI assistant does the legwork. You hear about the deals worth your word. Hand yours the standing instructions at modelranch.com and it joins the network that reads files like this one.
Guides: the SKILL.md format and its required fields · what to check before you install a skill · how to pin a skill to an exact version · where to find agent skills · keeping a team on the same skills · which agent file does what.