Why3
Why3: An open-source, multi-prover environment for contract-based program verification. Ranked #14 of 35 in Formal Verification Tools by our editors (7.9/10); pricing: Open source; best for multi-prover program verification.
At a glance
- Editor score7.9 / 10
- PricingOpen source
- Best forMulti-prover program verification
- Paid fromFree
- Verification methodDeductive
- Facts checked20 Sep 2026
Where it wins
- Connects automated and interactive external theorem provers
- Combines graphical IDE, CLI tools, proof replay, and reporting
- Supports counterexamples, multiple input formats, and OCaml extraction
Where it doesn't
- External theorem provers generally require separate installation
- Requires familiarity with deductive verification and contracts
- Does not include AI features
Our verdict on Why3
Why3 is an open-source platform for deductive program verification, developed by the Toccata team at Inria with CNRS and Université Paris-Saclay. It is designed for teams and researchers who need to specify programs with contracts, generate verification conditions, and delegate proofs to automated or interactive external theorem provers. Why3 includes the WhyML specification and programming language, supports WhyML as well as micro-C, micro-Python, MLCFG, and Coma inputs, and can extract OCaml programs. Its web, Linux, and Windows availability gives it a broad deployment footprint.
Its strongest differentiator is the prover ecosystem. Why3 integrates with Alt-Ergo, CVC4, CVC5, Z3, Coq, Isabelle/HOL, and PVS, allowing verification workflows to combine automated solvers with interactive theorem proving. Verification-condition generation and proof strategies help organize proof work, while proof-session storage, replay, and reporting support repeatable review. Potential and validated counterexample generation adds diagnostic value for supported provers. The OCaml API also allows Why3 to be used as a software library, extending it beyond its graphical IDE and command-line tools.
The main trade-off is setup complexity: external theorem provers generally need separate installation, so teams should plan for a multi-component environment rather than a single self-contained application. Why3 fits users who value formal contracts, multiple prover back ends, counterexamples, and detailed proof-session management. It is less suitable for teams seeking a narrow, single-prover workflow or AI-assisted verification, since it has no AI features. Choose Why3 when interoperability and deductive depth matter more than minimizing the number of tools involved.
Questions about Why3
What does Why3 run on? Web, Linux and Windows. It runs in the cloud, self-hosted and as a desktop app.
What does Why3 integrate with? 7 integrations, including Alt-Ergo, CVC4, CVC5, Z3 and Coq.
Who is Why3 built for? Solo users, small businesses, mid-market companies and enterprises (our editorial estimate).
Your product? Our editors can test Why3 in depth and publish a review, labelled as paid · from $149/mo →
Why3 pricing
Why3 fact sheet
| Free plan | Not verified |
|---|---|
| Paid from | Free |
| Verification method | Deductive |
| Supported formalisms | Contracts |
| Counterexamples | Yes |
| Proof artifacts | Not verified |
| Input languages | WhyML, micro-C, micro-Python, MLCFG, Coma |
| Deployment | Both |
| Deployment | Cloud, Self-hosted, Desktop |
| Platforms | Web, Linux, Windows |
| Support | Live chat, Community, Docs |
| Built for | Solo, Small business, Mid-market, Enterprise (editorial estimate) |
| Integrations | 7 integrations: Alt-Ergo, CVC4, CVC5, Z3, Coq, Isabelle/HOL … |
| Pricing | Open source |
| Website | why3.org |
| Facts checked | 20 Sep 2026 |
Why3 integrations
Why3 lists 7 integrations on its own site.
- Alt-Ergo
- CVC4
- CVC5
- Z3
- Coq
- Isabelle/HOL
- PVS
Alternatives to Why3
- IsabelleA free, open-source proof assistant for broad mathematical and systems verification.9.4
- RocqA free, kernel-checked theorem prover with dependent types, automation, and certified extraction.9.3
- PVSA broad, self-hosted environment for hybrid formal verification workflows.9.2
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Reviewed by iTechGuides Editors · Editorial team · Updated Nov 2026
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