Code Review
imbenrabi/Financial-Modeling-Prep-MCP-Server
Review a PR or working diff against this repo's intent layer (the AGENTS.md hierarchy), toolception pitfalls, and core invariants.
Write architecture fitness functions — deterministic tests that enforce a project's hard rules (module boundaries, offline guarantees, content contracts) from inside its own test target.
Install Fitness Functions
$ npx skills add rshankras/claude-code-apple-skills --skill fitness-functions -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install rshankras/claude-code-apple-skills fitness-functions --agent claude-codeProject scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).
$ git clone --depth 1 https://github.com/rshankras/claude-code-apple-skills.git skills-src && mkdir -p .claude/skills && cp -r skills-src/skills/testing/fitness-functions .claude/skills/fitness-functions && rm -rf skills-srcUse ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.
Claude Code skills documentation · loads skills from .claude/skills/
Install the "fitness-functions" agent skill from https://github.com/rshankras/claude-code-apple-skills/tree/main/skills/testing/fitness-functions into .claude/skills/fitness-functions/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "fitness-functions", then confirm the skill loads.Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$skill-installer install https://github.com/rshankras/claude-code-apple-skills/tree/main/skills/testing/fitness-functionsType this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.
$ npx skills add rshankras/claude-code-apple-skills --skill fitness-functions -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install rshankras/claude-code-apple-skills fitness-functions --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/rshankras/claude-code-apple-skills.git skills-src && mkdir -p .agents/skills && cp -r skills-src/skills/testing/fitness-functions .agents/skills/fitness-functions && rm -rf skills-srcUse ~/.agents/skills/ instead of .agents/skills for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "fitness-functions" agent skill from https://github.com/rshankras/claude-code-apple-skills/tree/main/skills/testing/fitness-functions into .agents/skills/fitness-functions/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "fitness-functions", then confirm the skill loads.Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add rshankras/claude-code-apple-skills --skill fitness-functions -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install rshankras/claude-code-apple-skills fitness-functions --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/rshankras/claude-code-apple-skills.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/skills/testing/fitness-functions .cursor/skills/fitness-functions && rm -rf skills-srcUse ~/.cursor/skills/ instead of .cursor/skills for a personal install.
Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/
Install the "fitness-functions" agent skill from https://github.com/rshankras/claude-code-apple-skills/tree/main/skills/testing/fitness-functions into .cursor/skills/fitness-functions/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "fitness-functions", then confirm the skill loads.Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gemini skills install https://github.com/rshankras/claude-code-apple-skills.git --path skills/testing/fitness-functions--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.
$ npx skills add rshankras/claude-code-apple-skills --skill fitness-functions -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install rshankras/claude-code-apple-skills fitness-functions --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/rshankras/claude-code-apple-skills.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/skills/testing/fitness-functions .gemini/skills/fitness-functions && rm -rf skills-srcUse ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.
Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/
Install the "fitness-functions" agent skill from https://github.com/rshankras/claude-code-apple-skills/tree/main/skills/testing/fitness-functions into .gemini/skills/fitness-functions/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "fitness-functions", then confirm the skill loads.Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gh skill install rshankras/claude-code-apple-skills fitness-functionsInstalls for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).
$ npx skills add rshankras/claude-code-apple-skills --skill fitness-functions -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/rshankras/claude-code-apple-skills.git skills-src && mkdir -p .github/skills && cp -r skills-src/skills/testing/fitness-functions .github/skills/fitness-functions && rm -rf skills-srcUse ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.
GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/
Install the "fitness-functions" agent skill from https://github.com/rshankras/claude-code-apple-skills/tree/main/skills/testing/fitness-functions into .github/skills/fitness-functions/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "fitness-functions", then confirm the skill loads.GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add rshankras/claude-code-apple-skills --skill fitness-functions -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install rshankras/claude-code-apple-skills fitness-functions --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/rshankras/claude-code-apple-skills.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/skills/testing/fitness-functions .opencode/skills/fitness-functions && rm -rf skills-srcUse ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.
OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/
Install the "fitness-functions" agent skill from https://github.com/rshankras/claude-code-apple-skills/tree/main/skills/testing/fitness-functions into .opencode/skills/fitness-functions/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "fitness-functions", then confirm the skill loads.OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
Facts
- Skill name
fitness-functions- GitHub stars
- 787
- Token cost
- ~2k tokens
- SKILL.md length
- 971 words
- Files
- 4
- Skills in repo
- 41
- Repo updated
- First seen
- Licence
- MIT
At a glance
Write architecture fitness functions — deterministic tests that enforce a project's hard rules (module boundaries, offline guarantees, content contracts) from inside its own test target.
- Works in 5 steps: Identify the Invariant → Pick the Pattern → Write the Suite → …
- A constraint lives only in prose (CLAUDE.md
- SKILL.md covers When This Skill Activates, Why a Test, Not a Lint Rule, The Three Patterns and Process, plus 3 more sections
- Runs Swift scripts from its folder
What it does
Fitness Functions is an agent skill from rshankras/claude-code-apple-skills. Write architecture fitness functions — deterministic tests that enforce a project's hard rules (module boundaries, offline guarantees, content contracts) from inside its own test target. Use when a constraint lives only in prose (CLAUDE.md, code review) and should become un-arguable.
Its SKILL.md is about 2k tokens, which your agent loads only when the skill is triggered. The skill folder holds 4 other files.
It sits in Development, covering Agent instruction files and Code review. The repository describes itself as: Claude Code skills for Apple platform development (iOS, macOS, iPadOS) — product validation, code generation, App Store optimization, and more. The licence is MIT.
When your agent uses it
- A constraint lives only in prose (CLAUDE.md
- Code review) and should become un-arguable
Example prompts
- “/fitness-functions”
Requirements
- Pre-approved tools (allowed-tools): Read, Write, Edit, Glob, Grep, Bash
Workflow steps
5 steps, taken from the step headings in SKILL.md.
What it can do on your machine
Read from SKILL.md and the folder at commit 9ffb831. It shows what the files ask for, not the result of running them.
Tool permissions
Pre-approves these tools, so the agent can use them without asking each time:
ReadWriteEditGlobGrepBash
From allowed-tools in the SKILL.md frontmatter.
Runs code
Ships script files (Swift), which the agent can run.
From the folder's file list and the shell code blocks in SKILL.md.
Network
No URLs in SKILL.md.
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.
Context cost
Fitness Functions loads about 2k tokens when it runs. Until then it costs about 76 tokens; SKILL.md has 971 words of instructions outside code blocks.
Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.
Safety
Auto-check: notes
The automated check noted patterns worth knowing about, such as sudo or a known installer.
- NotePre-approves every shell command (allowed-tools: Bash)SKILL.md
allowed-tools: Read, Write, Edit, Glob, Grep, Bash
Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.
SKILL.md
The full file from rshankras/claude-code-apple-skills at commit 9ffb831, republished under its MIT licence (© rshankras). 971 words, ~2,024 tokens.
Download SKILL.mdSave it as
.claude/skills/fitness-functions/SKILL.md (or your agent's skills folder). This skill also uses 3 other files; get the full folder from GitHub.- name
- fitness-functions
- description
- Write architecture fitness functions — deterministic tests that enforce a project's hard rules (module boundaries, offline guarantees, content contracts) from inside its own test target. Use when a constraint lives only in prose (CLAUDE.md, code review) and should become un-arguable.
- allowed-tools
- Read, Write, Edit, Glob, Grep, Bash
- last_verified
- 2026-07-24
- review_by
- 2027-07-01
Architecture Fitness Functions
A fitness function is an ordinary test that enforces an architectural rule instead of a behavior. It lives in the app's own test target, runs under every existing test gate forever, and turns "the agent said it's fine" into an exit code. This is the gauntlet's answer for invariants no unit test of behavior can see: a file quietly importing a framework it must not touch, a registry drifting from its documented size, user-facing copy breaking its format contract.
When This Skill Activates
Use this skill when:
- A project hard rule exists only in prose — CLAUDE.md constraints, file-header comments, review checklists ("deck games must run fully offline", "only the sync layer may import CloudKit")
- A code review keeps re-checking the same structural rule by hand
- An
/apple:reviewstructural finding is being graduated into permanent enforcement - A count, boundary, or content format was broken silently once already
Why a Test, Not a Lint Rule
A per-line regex (SwiftLint custom_rules) can flag a line. A fitness function can assert a set: "the files importing MusicKit are exactly these four", "the mode registry has exactly 21 entries", "every case's copy fits the format". Sets, counts, and cross-file facts need code, and putting that code in the test target means no new tooling, no CI wiring — it rides the test gate that already exists.
The Three Patterns
Pattern 1: Import-Boundary Allowlist Scan
Enforces "only these files may import X." The test walks the shipped source tree and compares the actual importer set against an allowlist — so a fifth importer fails a test, not a code review.
Key rules (learned in production):
- Resolve the source root from
#filePath, never a hardcoded path — the test stays correct wherever the repo is checked out. - Match exact trimmed lines (
line == "import X"), notcontains— comments documenting the contract itself must not count as hits. - Compare full sets, not counts — the failure message then names the drifted file.
- Exclude non-shipping directories deliberately (DEBUG-only spikes, generated code) and say why in a comment: the guarantee is about what ships.
- The allowlist is a fence, not a headcount. Widening it is a legitimate, documented decision: when a new file genuinely belongs inside the boundary, the test failing is the system working — record why in the allowlist comment, then add it.
- Add a companion API-surface scan for bypass routes the import check can't see. An import-only check misses a file calling the underlying API directly (every file already imports Foundation, which is enough to reach
URLSessionwithout importing the fenced framework). Scan for the raw API strings (URLSession(,URLSession.shared) outside the boundary too.
Template: templates/ImportBoundaryTests.swift
Pattern 2: Runtime Sentinel
Enforces "this code path never attempts X" — stronger than "happens to succeed without X." The canonical case is an offline guarantee: register a URLProtocol subclass that intercepts, counts, and fails every network request, drive the real production path, and assert the counter stayed at zero.
Key rules:
- Register the sentinel per-test and unregister in
defer. - The sentinel fails requests loudly (never lets them reach the network) — a regression fails in CI instead of silently succeeding against a live server.
- Assert
interceptedRequestCount == 0: the contract is zero attempts, not zero successes. - Drive the real path (real service, real composition), with only storage in-memory.
Template: templates/NetworkSentinelTests.swift
Show full SKILL.md (427 more words)Show less
Pattern 3: Contract Pins
Pins facts that must change deliberately, never by drift.
Count pins — registry and configuration sizes:
// Phase 3.5 added 3 modes — 9 modes, 5 streaming.
// Phase 3.8 added the four reunion music games — 15 modes, 9 streaming.
// Phase 3.10 added the DJ-host quartet — 18 modes, 10 streaming.
#expect(PartyOccasion.bigParty.modes.count == 18)
#expect(PartyOccasion.bigParty.modes.filter(\.usesDJEngine).count == 10)The comment trail is mandatory: each pinned number carries its history, so updating the pin is a reviewed decision with a written reason, not a chore silenced with an edit.
Copy-contract pins — structural rules on user-facing content, iterated over CaseIterable so new cases are covered automatically:
@Test func everyModeHasExactlyThreeBeats() {
for mode in GameMode.allCases {
#expect(mode.howItPlaysBeats.count == 3, "\(mode.rawValue) has \(mode.howItPlaysBeats.count) beats, expected 3")
}
}Plus a few verbatim exemplars pinned with exact equality — a future content pass must change those deliberately, not by accident.
Template: templates/ContractPinTests.swift
Process
Phase 1: Identify the Invariant
Read CLAUDE.md / APP.md / file-header comments for hard rules currently enforced by nothing. Good candidates state a set or bound: "only", "never", "exactly", "at most", "every".
Phase 2: Pick the Pattern
| The rule is about… | Pattern |
|---|---|
| Which files may use a framework/API | 1 — import-boundary scan (+ API-surface companion) |
| What a code path may do at runtime | 2 — runtime sentinel |
| A size, format, or exact content | 3 — contract pins |
Phase 3: Write the Suite
Copy the matching template into the existing unit-test target and adapt. One suite per invariant; name it after the guarantee (OfflineGuaranteeTests), not the mechanism. Every failure message must tell the fixer which side to change — the code or the contract — and where the contract is documented.
Phase 4: Prove It Red (Deliberate-Break Drill)
A fitness function that has never failed is unverified. Before committing:
- Introduce the violation for real (add the forbidden import to one file; change a pinned count).
- Run the suite — it MUST fail, with a message that names the offender.
- Revert. Run again — green.
Record the red/green pair in the commit message or PR. Repeat the drill whenever the suite's scanning logic (not its data) changes.
Phase 5: Commit
The suite ships in the normal test target. No special CI wiring — every existing test gate now enforces the rule.
Output Format
## Fitness Function: [invariant]
- **Rule source**: CLAUDE.md hard rule N / [file header]
- **Pattern**: import-boundary | runtime sentinel | contract pin
- **Suite**: Tests/[Name]Tests.swift
- **Deliberate-break drill**: [what was broken] → failed with "[message]" → reverted → greenCommon Pitfalls
| Pitfall | Problem | Solution |
|---|---|---|
contains("import X") matching | Comments about the rule count as violations | Compare exact trimmed lines |
| Hardcoded source paths | Breaks on other checkouts/CI | Derive root from #filePath |
| Pinning counts without history | Updating the pin becomes a mindless chore | Comment trail explaining every number |
| Import check only | Direct API calls bypass the fence | Add the API-surface companion scan |
| Never proven red | Scan bug silently passes everything | Deliberate-break drill before commit |
| Allowlist grows without comment | Fence decays into a headcount | Each entry documents why it's inside the boundary |
References
testing/tdd-bug-fix/— prove-red discipline for behavioral bugstesting/test-contract/— runtime behavioral invariants across implementationsswift/code-size/— the complexity/size half of the deterministic gauntlettesting/coverage-ratchet/— coverage floor that only rises
© rshankras, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file
Files
SKILL.md and 3 other files in skills/testing/fitness-functions of rshankras/claude-code-apple-skills.
- SKILL.md
- templates/ContractPinTests.swift
- templates/ImportBoundaryTests.swift
- templates/NetworkSentinelTests.swift
Open the folder on GitHubat commit 9ffb831
Compare with similar skills
Fitness Functions next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.
| Skill | Stars | Used in | Tokens | Auto-check | Licence | Repo updated |
|---|---|---|---|---|---|---|
| Fitness Functions this skillrshankras/claude-code-apple-skills | 787 | — | ~2k | Automated safety check: Notes | MIT | |
| Code Reviewimbenrabi/Financial-Modeling-Prep-MCP-Server | 150 | — | ~2.6k | Automated safety check: Pass | Apache-2.0 | |
| Shipmillionco/react-doctor | 15k | — | ~716 | Automated safety check: Pass | Custom licence | |
| Promptkitmicrosoft/PromptKit | 111 | — | ~420 | Automated safety check: Pass | MIT | |
| Megingiard Code Reviewstormpanda/megingiard | 155 | — | ~2.9k | Automated safety check: Pass | Custom licence | |
| Ad ReviewCorridorTech/PoseCap | 224 | — | ~2.4k | Automated safety check: Notes | Apache-2.0 |
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Questions about Fitness Functions
What does Fitness Functions do?
Write architecture fitness functions — deterministic tests that enforce a project's hard rules (module boundaries, offline guarantees, content contracts) from inside its own test target. Fitness Functions is an agent skill from rshankras/claude-code-apple-skills. Write architecture fitness functions — deterministic tests that enforce a project's hard rules (module boundaries, offline guarantees, content contracts) from inside its own test target.
When should I use Fitness Functions?
Fitness Functions fits situations like: A constraint lives only in prose (CLAUDE.md; code review) and should become un-arguable.
How do I install Fitness Functions in Claude Code?
Run `npx skills add rshankras/claude-code-apple-skills --skill fitness-functions -a claude-code`. Or copy the skill folder (skills/testing/fitness-functions in rshankras/claude-code-apple-skills) into .claude/skills/fitness-functions in your project. Claude Code loads it when a task matches its description.
How do I install Fitness Functions in Codex?
Run `npx skills add rshankras/claude-code-apple-skills --skill fitness-functions -a codex`. Or copy the skill folder (skills/testing/fitness-functions in rshankras/claude-code-apple-skills) into .agents/skills/fitness-functions in your project. Codex loads it when a task matches its description.
Can I use Fitness Functions in Cursor, Gemini CLI or GitHub Copilot?
Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add rshankras/claude-code-apple-skills --skill fitness-functions -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/fitness-functions, .gemini/skills/fitness-functions, .github/skills/fitness-functions and .opencode/skills/fitness-functions in your project.
What does Fitness Functions need to run?
Going by SKILL.md and its folder, Fitness Functions needs Swift for the scripts in its folder. Its frontmatter pre-approves these tools: Read, Write, Edit, Glob, Grep, Bash.
Does Fitness Functions access the network?
SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.
Is Fitness Functions safe to install?
Our automated static check of SKILL.md found notes only (pre-approves every shell command (allowed-tools: bash)), nothing it rates as a warning. It is not a guarantee. Review the folder before installing.
What licence does Fitness Functions use?
Fitness Functions is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.
How many tokens does Fitness Functions use?
About 2k tokens (SKILL.md is roughly 8.1k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.
What are the alternatives to Fitness Functions?
Skills that share tags, products or a category with Fitness Functions: Code Review (imbenrabi/Financial-Modeling-Prep-MCP-Server, 150 stars), Ship (millionco/react-doctor, 15k stars), Promptkit (microsoft/PromptKit, 111 stars) and Megingiard Code Review (stormpanda/megingiard, 155 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Fitness Functions?
rshankras (a GitHub user) maintains it in rshankras/claude-code-apple-skills, which has 787 GitHub stars. The repository holds 41 skills in this directory. The repository was last updated on July 24, 2026.
Source: rshankras/claude-code-apple-skills on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.