inherit-legacy-style

name	inherit-legacy-style
description	Legacy-project style inheritance skill. Use when the user types /inherit-legacy-style, or when onboarding an AI coding agent onto a hand-written legacy project and you need to prevent "style drift" (the model imposing its pretrained mainstream idioms onto the project).
origin	community
allowed-tools	Read, Glob, Grep, Bash, Edit, Write, AskUserQuestion

Inherit Legacy Style

Prevents AI code style drift in legacy projects by scanning the codebase for implicit conventions across 4 meta-architecture dimensions, resolving conflicts with the user one at a time, and crystallizing the consensus into an enforceable .ai-style-rules.md. Fully language- and framework-agnostic.

When to Activate

• User types /inherit-legacy-style
• User mentions onboarding AI onto a hand-written legacy project
• User is worried about AI-generated code "drifting" from existing project conventions
• User wants to extract and codify their project's implicit coding rules

When to Use

Use this skill when you need to preserve legacy project style and prevent AI-generated style drift. See When to Activate above for trigger conditions.

Prerequisites

• Git (recommended; non-Git projects fall back to file timestamps for incremental mode)
• Read/Write access to the project root (generates .ai-style-rules.md and optionally CLAUDE.md)

Workflow

Step 0 — Auto-Detect Mode

Silently check for .ai-style-rules.md at the project root:

File exists?	Mode
No	Branch A — First-time Full-Scan
Yes	Branch B — Incremental Sniff

Announce the mode in one line and proceed — never ask the user to pick.

Branch A — First-time Full-Scan

1. Measure scale, pick a scanning tier

git ls-files | grep -cE '\.(js|ts|jsx|tsx|vue|py|go|rs|java|kt|rb|php|cs|swift|c|cpp|h)$'

Tier	Source files	Strategy
Small	≲ 50	Full close-read every source
Medium	50–500	Infra layer = full read; business layer = sample 2–3 per dimension
Large	≳ 500	Strict sampling + budget cap; --stat summary first, then targeted reads

2. Scan along 4 dimensions

1. File Anatomy — in-file declaration order (imports → types → main logic → helpers → export)
2. State & Control Flow — naming conventions for async state, pagination, flags
3. Infrastructure — where cross-cutting utils live (interceptors, formatters, middleware)
4. Error Handling — try/catch vs global interceptor vs Result return; null-check habits

3. Apply signal-threshold noise reduction

Before interrupting the user, evaluate signal strength:

• Weak signal → auto-suppress: minority <5% AND count <10 → majority wins, minority goes to DONTs
• Strong signal → grill: near-even split, or semantic fork on a core dimension
• Small-project exception: sources ≲50, "3 vs 2" is NOT a majority → grill it

4. Resolve conflicts one at a time (Grilling Protocol)

For each strong-signal conflict, present exactly ONE question with 4 options:

Evidence: pathA uses style X, pathB uses style Y WARNING: Risk: mixing both fractures the project style Choose: 1 follow X 2 follow Y 3 this is evolution, update rules 4 I have a new rule

Suspend until the user answers, then proceed to the next conflict. Never stack questions.

5. Generate .ai-style-rules.md with three mandatory sections:

• [Golden Files] — real exemplar paths annotated with what they demonstrate
• [Naming & State-Control Rules] — concrete, checkable conventions
• [DONTs] — anti-patterns that must not propagate

6. Install the persistent hook

Ask the user for enforcement strength (use AskUserQuestion):

Option	Mechanism
1 Soft hook (recommended)	Write @.ai-style-rules.md reference into project CLAUDE.md
2 Hard hook	Soft hook + PreToolUse[Write|Edit|MultiEdit] Hook in settings.json
3 No hook	Keep the rules file; user references manually

Branch B — Incremental Sniff

1. Read existing .ai-style-rules.md; if it has a commit fingerprint, git diff <last_hash> HEAD --stat to pinpoint delta
2. Read recent Git changes (git log -3 --stat → inspect suspect files on demand)
3. For oversized diffs (>hundreds of files): --stat summary only + sample the largest changes
4. Compare new code against recorded rules → conflicts go through Grilling Protocol
5. Append evolution log at the end of .ai-style-rules.md (never overwrite old rules)

Per-Turn Enforcement

When .ai-style-rules.md is in context (loaded via CLAUDE.md), every code-writing task must open with a compliance declaration in the reasoning chain, naming the exemplar being followed and the DONTs being avoided.

How It Works

This skill auto-detects whether it's a first-time or incremental run via .ai-style-rules.md presence:

• First-time (Branch A) — Measures project scale, scans codebase across 4 meta-architecture dimensions (File Anatomy, State & Control Flow, Infrastructure, Error Handling), applies signal-threshold noise reduction to suppress weak conflicts, resolves strong-signal conflicts one-at-a-time with the user, generates .ai-style-rules.md with Golden Files / Naming Rules / DONTs, and offers optional enforcement hooks.
• Incremental (Branch B) — Reads existing rules, checks recent Git diffs for new or conflicting patterns, runs the same one-at-a-time grilling protocol for any conflicts found, and appends evolution logs without overwriting existing rules.
• Per-Turn Enforcement — When hooked via CLAUDE.md, every code-writing task opens with a compliance declaration naming the exemplar followed and the DONTs avoided.

Output Specification

• .ai-style-rules.md at project root (with commit fingerprint + scale tier in header)
• Optionally CLAUDE.md with @.ai-style-rules.md reference
• Evolution logs appended as ### [YYYY-MM-DD] Style Evolution Log entries

Anti-Patterns

• FAIL: Do NOT skip the scale measurement step — sampling a 30-file project "starves" it; full-scanning a 5,000-file repo blows up
• FAIL: Do NOT stack multiple conflict questions at once — grilling is strictly one-at-a-time
• FAIL: Do NOT overwrite old rules in incremental mode — always append evolution logs
• FAIL: Do NOT default to "hard hook" without asking — enforcement strength is the user's call
• FAIL: Do NOT judge syntax or tech-stack quality — this skill aligns meta-architecture only
• FAIL: Do NOT copy bugs from exemplar files — reuse structure, flag defects

Best Practices

• Announce the detected mode (first-time vs incremental) and scale tier in one line before scanning
• For large projects, read --stat summaries first, then targeted Read on suspect files
• Let the signal threshold handle noise — a 843-vs-8 naming split should auto-resolve without user interruption
• When in doubt about signal strength, lean toward asking
• The CLAUDE.md soft hook (@.ai-style-rules.md) is usually sufficient; hard hook only if the user wants mechanical enforcement

Related Skills

• init — initialize a new CLAUDE.md with codebase documentation
• code-review — review diffs for correctness and style issues
• simplify — review code for reuse and simplification opportunities

Examples

1. First-time onboarding

   • User: "Help me onboard AI to this older codebase without changing its style."
   • Action: Run Branch A full-scan → measure scale → scan 4 dimensions → grill conflicts → generate .ai-style-rules.md → offer hook strength (soft/hard/none).

2. Incremental update after team changes

   • User: "We added a new module; keep existing style rules intact."
   • Action: Run Branch B incremental sniff → compare Git deltas to recorded rules → grill any new conflicts → append evolution log without overwriting.

3. Enforcing DONTs via CLAUDE.md

   • User: "Make sure all new code stays consistent with the project's rules."
   • Action: Soft hook installed → .ai-style-rules.md auto-loaded every session → every code-writing task opens with compliance declaration, reusing exemplar patterns and avoiding DONTs.

Activation Triggers

Language- and framework-agnostic — it aligns meta-architecture only, not syntax. Once run, it becomes a behavioral constraint on all subsequent coding tasks. Do NOT use for pure research or one-off questions unrelated to code-style alignment.