Equiv, check that an AI refactor did not change what your code does

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An LLM should not be the only thing reviewing LLM-written code.

equiv runs a changed function against its previous version on the same deterministically generated inputs and reports whether the behaviour changed. If it did, you get the exact input where they differ. Either way you get a reproducible, signed receipt: re-run the check on any machine and you get the same answer, byte for byte, without trusting any model's opinion.

Most code is now written by AI and reviewed by AI. A model saying "this looks fine" is not verification. A deterministic check you can re-run yourself is.

Quickstart: the PR gate

List the functions whose behaviour must be preserved across a PR in a manifest at the repository root. The format of each line is : : , where arg types are int, str, or list[int], comma separated:

src/math.py : total : int

Add the workflow at .github/workflows/equiv-review.yml:

on: pull_request permissions: { contents: read, pull-requests: write, id-token: write } jobs: review: runs-on: ubuntu-latest steps:

• uses: actions/checkout@v4

with: { fetch-depth: 0 }

• uses: Neelagiri65/[email protected]

with: { keyless: "true" }

Pin to a released tag (@v0.1.0) rather than @main so runs are reproducible and do not change under you.

Each PR receives a comment. Every changed function is tested against its version on the base branch. A change that preserves behaviour passes. A change that does not is reported with the input that distinguishes the two versions. That fails the check. Receipts are signed with Sigstore keyless signing, which stores no key. They can be verified with cosign.

CLI

curl --proto '=https' --tlsv1.2 -LsSf \ https://github.com/Neelagiri65/equiv/releases/latest/download/equiv-cli-installer.sh | sh

equiv review candidate.py reference.py equiv verify-receipt

Exit codes: 0 equivalent, 1 diverges with a printed counterexample, 2 could not check.

Scope

equiv checks behavioural equivalence of a function against a reference, on deterministically generated inputs. This is bounded random testing, not exhaustive verification: a pass means no divergence was found on the generated inputs. It can still miss an edge case that only shows up for an input that was not generated. It does not check intent, architecture, security. It cannot judge new functionality that has no reference to compare against. A passing result means behaviour was preserved on the tested inputs. It does not mean the change is correct. Supported input types in this version are int, str and list[int].

How it works

Input generation and the verdict are computed in Rust from a fixed seed. The language runtime is used only as an evaluator and never decides anything that reaches the receipt. Receipts are identical across hosts. Receipts can be signed with a local ed25519 key or with keyless Sigstore (OIDC). The keyless path binds the signature to a verifiable CI identity rather than a stored secret. The tool is a single static binary with no runtime dependencies, prebuilt for macOS, Linux and Windows.

Documentation

docs/signing-model.md: receipt signing with ed25519 and keyless Sigstore.

docs/RELEASING.md: building prebuilt binaries with cargo-dist.

crates/: the Rust workspace (equiv-core, equiv-engine, equiv-review, equiv-cli).

License: Apache-2.0.

About

Deterministic checker for behaviour-preserving code changes. Signed, re-runnable receipts; PR gate; single static binary.

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License

Apache-2.0 license

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Releases 2

v0.1.0

Latest

Jun 13, 2026

+ 1 release

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Rust 98.7%

Shell 1.3%