No. 30 of 33 ·Formal Verification Tools

Why3

Web · Windows · Linux · Self-hosted · API

Freedom report

Two barsScore 5.3

Free tierA free tier is on its own pricing page
Open codeNo open-source code on record
Runs widely3 of 6 device platforms
DocumentedThinly documented

Why3 is ranked #30 of 33 in formal verification tools on Freedom251. It runs on API, Linux, Self-hosted, Web, Windows. There is a free plan.

Why3 plans and pricing

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Why3 Free GNU Lesser General Public License version 2.1 with a special linking exception why3.org · 10 Oct 2026

Compared on formal verification tools

Verification method: deductive
Supported formalisms: contracts
Counterexamples: Yes
Input languages: WhyML, micro-C, micro-Python, MLCFG, Coma
Deployment: both

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The record

Purpose: Why3 is a platform for deductive program verification that uses WhyML and external automated or interactive theorem provers to discharge verification conditions.
WhyML: WhyML supports specifications and programs with preconditions, postconditions, assertions, and loop invariants.
Standard library: Why3 includes logical theories and programming data structures such as arrays, queues, and hash tables.
Prover support: Why3 supports a range of external theorem provers, which must be installed separately and configured for use.
Extensibility: Why3 can be extended with support for new external provers and used as a software library through its API.
Code extraction: WhyML programs can be automatically extracted into correct-by-construction OCaml programs.
Language verification: WhyML is used as an intermediate language for verification of C, Java, Rust, and Ada programs.
Browser access: TryWhy3 is a JavaScript version of the Why3 Verification Platform that runs in a browser.
Installation: Why3 can be installed via Opam, run from a Docker image, or compiled from source.
Editor integrations: Why3 distributions include configuration files for Emacs and Vim, plus shell completion files for bash and zsh.
Proof sessions: The graphical interface saves proof attempts and transformations in an XML file and can replay obsolete proof attempts.
License: Why3 is open source and freely available under the GNU LGPL 2.1.
Support: Users can discuss Why3 on its Zulip forum or public mailing list, and report bugs through its bug tracking system or Zulip.
Notable limitation: Loop invariant inference is described as work in progress, with many features still very limited.
Price from: Free

Where it ranks on Freedom251

Best Formal Verification Tools in 2026#30 of 33

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Sources

why3.org/doc/foreword.html· checked 9 Oct 2026
why3.org/doc/whyml.html· checked 9 Oct 2026
why3.org· checked 9 Oct 2026
why3.org/doc/install.html· checked 9 Oct 2026
why3.org/try/· checked 9 Oct 2026
why3.org/doc/starting.html· checked 9 Oct 2026