Interface Contract Verifier Agent Skill
Verify that interface and class contracts (preconditions, postconditions, invariants) are preserved across program versions. Use when validating refactorings, checking API compatibility, verifying design-by-contract implementations, or ensuring behavioral contracts remain intact after code changes. Automatically detects contract violations, identifies affected methods and classes, and provides actionable guidance for resolving violations while maintaining program correctness.
Install
one command, takes just this skill from the repositorynpx skills add https://github.com/ArabelaTso/Skills-4-SE --skill interface-contract-verifierThe instruction itself
20 sections, as written by the authorInterface Contract Verifier
Overview
Verify that formal or structured contracts (preconditions, postconditions, invariants) defined in interfaces and classes are preserved when updating to a new program version.
Core Workflow
1. Extract Contracts
Extract contracts from both versions:
python scripts/contract_extractor.py --program old_version --output old_contracts.json
python scripts/contract_extractor.py --program new_version --output new_contracts.json
2. Verify Contracts
Compare and verify contract preservation:
python scripts/contract_verifier.py --old old_contracts.json --new new_contracts.json --output report.json
3. Review Violations
Examine violations: weakened preconditions, strengthened postconditions, broken invariants.
Contract Types
Preconditions
Requirements before method execution:
def withdraw(amount):
"""Precondition: amount > 0 and amount <= balance"""
Postconditions
Guarantees after method execution:
def deposit(amount):
"""Postcondition: balance == old(balance) + amount"""
Invariants
Properties always true for a class:
class BankAccount:
"""Invariant: balance >= 0"""
Verification Rules
Liskov Substitution Principle:
- Preconditions can be weakened (accept more)
- Postconditions can be strengthened (guarantee more)
- Invariants must be maintained
Violation Types
Strengthened Precondition (Error)
New version rejects previously valid inputs.
Weakened Postcondition (Error)
New version guarantees less.
Broken Invariant (Critical)
Class invariant no longer holds.
Resolution Guidance
Fix Strengthened Precondition
Relax precondition or update callers.
Fix Weakened Postcondition
Strengthen implementation to meet original guarantee.
Fix Broken Invariant
Add checks to maintain invariant in all methods.
Resources
- references/design_by_contract.md: Design by Contract principles
- scripts/contract_extractor.py: Extract contracts from code
- scripts/contract_verifier.py: Verify contract preservation
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How to use it
Take arabelatso/interface-contract-verifier from the repository into ~/.claude/skills for personal
use, or into .claude/skills inside a project.
The agent identifies a skill by the name field in its header. Two skills with the
same name cannot sit side by side — one of them will be ignored.