Create a formal verification skill

On Codex, read the platform mapping, including its per-skill notes, before following this skill.

On GitHub Copilot, read the platform mapping, including its per-skill notes, before following this skill.

A test checks the cases that someone thought to write. Formal verification checks every case in a stated scope: every order of steps in a model, every input up to a bound, or every input. This skill finds the part of the repo where that matters most, verifies one property of it, and leaves a project-local skill that lets the next agent check a change against that property. Write that skill for an agent who reads it cold, mid-change, and has never seen your work. The user must be able to trust the result and act on it without learning the verification language.

Choose the target from the code

Answer from the codebase, and ask the user only what you cannot observe. If the user named an area or a bug, start there.

A good target is a promise the code makes, where a break harms users and tests rarely reach the case that breaks it. Choose the strongest such target in the repo, not the one that is easiest to verify.

Let the question choose the tool

Pick the tool that answers the question at the lowest cost: a model checker, a proof assistant, an SMT-backed verifier, or another tool. Whether events can happen in a bad order usually suits a model checker over a small model of the design, and TLA+ is the default for that in any language. Whether the code itself behaves for every input, as with memory safety, overflow, or a data-structure invariant, usually suits a verifier that runs on the code. If no verifier runs on this language, say so, and test the real code with generated inputs, not a hand model of the function. A property that must hold at every size, where a bound is not enough, can need a proof. Prefer a tool that the repo's developers can install and run, and tell the user why you chose it.

What makes it good

• True to the code. Every part of the model or proof points to code that a reviewer can open. What the verification treats as one indivisible step is one indivisible step in the code. Names come from this repo.
• Able to fail. A check that you have never seen fail proves nothing. Show each property failing on a broken variant of the code it guards, such as a removed check or a reordered step.
• Findings stay findings. When the current code breaks the property, report it with the counterexample in the code's terms. Do not weaken the property or the model until it passes, and do not change product code unless the user asks.
• Honest about scope. Say what is verified, under which bounds and assumptions, and what is left out. A pass covers the model or the bound, not the whole system.
• Readable without the tool. State the property and the harm in one plain sentence each. Tell a counterexample as a short sequence of steps in the code's words.
• Cheap to rerun. One command, a reproducible tool version, and a run short enough that the next agent will use it on every relevant change.
• Kept in step with the code. Record which files and functions each check covers, and the commit at which you last confirmed that the verification describes them. When the covered code changes after that commit, the next agent can see that the verification needs another look.

Generate the formal verification skill

Write a project-local skill with frontmatter that names the area and says when to use it. Name the skill for the area, never verify or a verify- prefix, which belong to the driver skill /create-verification-skill writes. On Claude Code it lives at .claude/skills/<area>/SKILL.md. Without frontmatter, the skill never registers. From the skill alone, the next agent must be able to tell whether a change touches the verified code, run the check, tell a real bug from a modeling error, and update the verification when a change is intended. Put the coverage record and the sync commit in the skill, and tell the next agent to move the sync commit forward only after it confirms that the verification describes the code at the new commit. Use this repo's real commands, paths, and nouns, and leave no placeholders.

Prove it before you hand off

Follow the generated skill once, as the next agent would, on a scratch change that breaks the property in the real code. It must catch the break. Then drop the scratch change and confirm that the check is back where it started: passing, or failing only on the findings you will report. A formal verification skill that was never run is a draft. If the skill misses the break or a step does not run, fix the skill or the verification, not the scratch change.

Hand off

Tell the user what you verified, which tool you used and why, the bounds, what the check found in plain words, how to run it, and what it does not cover. Suggest a second target only if you found a close one. Point the user at /maintain-formal-verification-skill for bringing the verification back in line after changes that did not recheck it.