Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add ArabelaTso/Skills-4-SE --skill interface-contract-verifiergit clone --depth 1 https://github.com/ArabelaTso/Skills-4-SEWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/arabelatso/skills-4-se/interface-contract-verifier)<a href="https://agentmods.dev/skills/arabelatso/skills-4-se/interface-contract-verifier"><img src="https://agentmods.dev/badge/skills/arabelatso/skills-4-se/interface-contract-verifier/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/arabelatso/skills-4-se/interface-contract-verifier"><img src="https://agentmods.dev/badge/skills/arabelatso/skills-4-se/interface-contract-verifier.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00077 | $0.00533 |
| Opus 5 | $0.00039 | $0.00267 |
| Sonnet 5 | $0.00015 | $0.00107 |
| Haiku 4.5 | $0.00008 | $0.00053 |
Grade A, and why
interface-contract-verifier scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 9d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 92 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Interface 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
What ships with it
3 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 9d ago First seen · 92 lines · 77 tokens per session scan A 75bac2fd14e0
interface-contract-verifier is a skill published in the GitHub repository ArabelaTso/Skills-4-SE (252 stars, last pushed 22d ago), licensed Apache-2.0. It adds 77 tokens to every session and 533 once invoked, about $0.0004 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.
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