---

name: apply-key-principles description: Apply and validate the 7 Key Principles to code - TDD, Fail Fast, Modular, Reuse, Open Source, No Debt, Excellence. Checks code compliance and suggests improvements. Use during code review or refactoring. allowed-tools: [Read, Grep, Glob, Bash]

apply-key-principles

Skill Type: Validator/Sensor Purpose: Validate code compliance with 7 Key Principles Prerequisites: Code exists to validate

---

Agent Instructions

You are validating code against the 7 Key Principles.

The 7 Key Principles:

1. Test Driven Development - "No code without tests"
2. Fail Fast & Root Cause - "Break loudly, fix completely"
3. Modular & Maintainable - "Single responsibility, loose coupling"
4. Reuse Before Build - "Check first, create second"
5. Open Source First - "Suggest alternatives, human decides"
6. No Legacy Baggage - "Clean slate, no debt"
7. Perfectionist Excellence - "Best of breed only"

Your role: Check code compliance and report violations.

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Validation Checks

Principle #1: Test Driven Development

Check: Does code have tests?

Validation:

# For each file in src/, check if tests exist
for file in src/**/*.py; do
  test_file="tests/$(basename $file | sed 's/\.py$//')"
  if [ ! -f "${test_file}_test.py" ]; then
    echo "VIOLATION: $file has no tests"
  fi
done

✅ Pass:

• Every production file has corresponding test file
• Tests were written first (git history shows RED before GREEN commits)
• Coverage >= 80%

❌ Fail:

• Code without tests
• Tests written after code (git history shows)
• Coverage < 80%

---

Principle #2: Fail Fast & Root Cause

Check: Does code fail loudly?

Validation:

# Look for silent failures
grep -rn "except.*pass" src/          # Empty except blocks (silent failures)
grep -rn "return None" src/ | grep -v "Optional"  # Silent None returns
grep -rn "# TODO: error handling" src/  # Deferred error handling

✅ Pass:

• Exceptions raised for invalid states
• Assertions check preconditions
• Specific error messages
• Logging for debugging

❌ Fail:

• Silent failures (empty except blocks)
• Generic error messages
• Swallowing exceptions
• No logging

---

Principle #3: Modular & Maintainable

Check: Is code modular?

Validation:

# Check file/function sizes
find src -name "*.py" -exec wc -l {} \; | awk '$1 > 300'  # Files > 300 lines

# Check cyclomatic complexity
radon cc src/ -a | grep "F"  # Functions with F rating (too complex)

✅ Pass:

• Files < 300 lines
• Functions < 50 lines
• Cyclomatic complexity <= 10
• Single responsibility per module
• Low coupling

❌ Fail:

• Large files (> 300 lines)
• Large functions (> 50 lines)
• High complexity (> 10)
• Mixed responsibilities

---

Principle #4: Reuse Before Build

Check: Is this functionality already available?

Validation:

# Search for similar code in project
grep -rn "function_name" src/

# Check for duplicate code
jscpd src/  # Copy-paste detector

✅ Pass:

• No duplicate code
• Reusing existing functions/classes
• Using standard libraries where appropriate

❌ Fail:

• Duplicate code blocks
• Reimplementing existing functionality
• Not using available libraries

---

Principle #5: Open Source First

Check: Could we use an open source library?

Validation:

• For custom implementations, check if library exists
• Document decision if building custom

✅ Pass:

• Using well-maintained libraries
• Documented decision to build custom (ADR)
• Libraries chosen after research

❌ Fail:

• Building custom without research
• Reinventing wheel (custom date parser, custom validation, etc.)

---

Principle #6: No Legacy Baggage

Check: Is code clean of technical debt?

Validation:

# Unused imports
grep -rn "^import\|^from" src/ | check_usage

# Dead code
find_functions_with_zero_callers src/

# Commented code
grep -rn "# " src/ | grep -v "^#" | check_if_code

# Complexity
radon cc src/ -a | grep -E "C|D|E|F"

✅ Pass:

• No unused imports
• No dead code
• No commented-out code
• Complexity <= 10
• No TODOs without tickets

❌ Fail:

• Any technical debt present

If FAIL: Invoke prune-unused-code, simplify-complex-code

---

Principle #7: Perfectionist Excellence

Check: Is this excellent code?

Validation:

# Naming quality
grep -rn " x " src/  # Single-letter variables
grep -rn " temp" src/  # Temp variables

# Documentation
find src -name "*.py" -exec grep -L '"""' {} \;  # Files without docstrings

# Type hints (Python)
grep -rn "def " src/ | grep -v " -> "  # Functions without return types

# Style compliance
pylint src/ --errors-only
black src/ --check

✅ Pass:

• Clear, descriptive names
• Comprehensive documentation
• Type hints/annotations
• Follows style guide (PEP 8, etc.)
• Code review ready

❌ Fail:

• Vague naming
• Missing documentation
• No type hints
• Style violations

---

Output Format

When all principles satisfied:

[APPLY KEY PRINCIPLES]

Validating code against 7 Key Principles...

✅ Principle #1: Test Driven Development
   - All files have tests
   - Tests written first (git history verified)
   - Coverage: 95.2%

✅ Principle #2: Fail Fast & Root Cause
   - No silent failures
   - Specific error messages
   - Comprehensive logging

✅ Principle #3: Modular & Maintainable
   - Max file size: 187 lines (< 300)
   - Max function size: 23 lines (< 50)
   - Max complexity: 6 (< 10)

✅ Principle #4: Reuse Before Build
   - No duplicate code
   - Using standard libraries (bcrypt, datetime)
   - Searched codebase first

✅ Principle #5: Open Source First
   - Using bcrypt library (not custom hashing)
   - Documented in ADR-001

✅ Principle #6: No Legacy Baggage
   - Tech debt: 0 violations
   - No unused imports
   - No dead code
   - No commented code
   - Max complexity: 6

✅ Principle #7: Perfectionist Excellence
   - Clear naming (validate_email, normalize_email)
   - Comprehensive docstrings
   - Type hints on all functions
   - Style: PEP 8 compliant

Result: 7/7 Principles Satisfied ✅

Quality: EXCELLENT 🔥
Ready for commit/deployment

When violations found:

[APPLY KEY PRINCIPLES - VIOLATIONS FOUND]

❌ Principle #6: No Legacy Baggage

Violations (5):
  1. Unused import: import hashlib (src/auth.py:3)
  2. Dead function: legacy_hash_password() (src/auth.py:67-74)
  3. Commented code: Lines 120-135 (src/auth.py)
  4. High complexity: login() complexity 14 (src/auth.py:89)
  5. TODO without ticket: # TODO: Add rate limiting (src/auth.py:145)

❌ Principle #7: Perfectionist Excellence

Violations (3):
  1. Missing docstring: _check_password() (src/auth.py:156)
  2. No type hint: def validate(email) (src/auth.py:178)
  3. Vague naming: variable 'x' (src/auth.py:192)

Result: 5/7 Principles Satisfied ⚠️

Violations: 8 total
Quality: NEEDS IMPROVEMENT

Actions Required:
  1. Invoke 'prune-unused-code' to fix Principle #6
  2. Fix naming, docs, types for Principle #7
  3. Re-run validation after fixes

Blocked: Fix violations before commit

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Prerequisites Check

Before invoking:

1. Code exists to validate
2. Testing tools available (for Principle #1)

---

Configuration

plugins:
  - name: "@aisdlc/principles-key"
    config:
      principles:
        enforce_tdd: true
        enforce_fail_fast: true
        enforce_modular: true
        enforce_reuse_first: true
        enforce_open_source_first: true
        enforce_no_legacy: true
        enforce_excellence: true
        block_on_violation: true

      thresholds:
        max_file_lines: 300
        max_function_lines: 50
        max_complexity: 10
        min_coverage: 80

---

Notes

Why apply principles?

• Operational enforcement (not just aspirational)
• Measurable (can check compliance automatically)
• Quality gate (blocks bad code)
• Continuous validation (run on every commit)

Principles manifest in code:

1. TDD → Tests exist, coverage high
2. Fail Fast → Exceptions, assertions, logging
3. Modular → Small files/functions, low complexity
4. Reuse → No duplication, using libraries
5. Open Source → Libraries documented
6. No Debt → Zero unused code, low complexity
7. Excellence → Clear naming, docs, types

Homeostasis Goal:

desired_state:
  all_seven_principles_satisfied: true
  violations: 0
  code_quality: excellent

"Excellence or nothing" 🔥