F*
F*: A broad, self-hosted verifier for effectful programs with precise specifications. Ranked #11 of 34 in Formal Verification Tools by our editors (6.2/10); pricing: Open source; best for effectful programs with precise specifications.
At a glance
- Editor score6.2 / 10
- PricingOpen source
- Best forEffectful programs with precise specifications
- Paid fromNone
- Verification methodHybrid
- Facts checked20 Sep 2026
Where it wins
- Combines dependent types, refinements, effects, and SMT automation
- Supports tactics, symbolic proofs, termination checking, and verification conditions
- Extracts verified code to OCaml, F#, C, Wasm, and assembly
Where it doesn't
- Self-hosted deployment requires local installation and environment management
- Its workflows are centered on the F* language and theorem proving
- Interactive proofs can require specialized formal-methods expertise
Our verdict on F*
F* is an open-source, dependently typed programming language, proof assistant, and program verification engine for functional and effectful software. It is aimed at developers working on high-assurance cryptographic and systems projects who need specifications close to implementation. Programs can express behavior through dependent types, refinement types, lemmas, and effects, while verification combines SMT-based automation, symbolic computation, normalization, and interactive tactics. The deployment model is self-hosted, with local binaries or source-based installations for Windows, Linux, and macOS.
Its strongest differentiator is the depth of its effect-aware verification model. F* can generate and check verification conditions, model state, exceptions, concurrency, and other effects, and check termination for recursive total functions. Z3-based automation handles parts of the proof process, while tactics and metaprogramming support more interactive workflows. Once code is verified, extraction and compilation targets include OCaml, F#, C, Wasm, and assembly. That combination gives teams a path from precise specifications to deployable outputs without limiting verification to purely functional examples.
F* is a strong fit when a project needs detailed specifications, explicit effect modeling, and control over proof automation. Its breadth also means the learning and proof-engineering burden can be significant, particularly for teams without experience with dependent types or interactive theorem proving. Teams seeking a hosted SaaS verifier, a general-purpose interface independent of F*, or a lighter scheduling-style workflow should choose another category of tool. F* suits engineering groups prepared to manage a local toolchain and invest in formal proofs for effectful software.
F* pricing
F* fact sheet
| Free plan | Not verified |
|---|---|
| Paid from | None |
| Verification method | Hybrid |
| Supported formalisms | Theorem-proving |
| Counterexamples | Not verified |
| Proof artifacts | Not verified |
| Input languages | F* |
| Deployment | Self-hosted |
| Deployment | Self-hosted |
| Platforms | Windows, Linux, macOS |
| Support | Email, Community, Docs |
| Built for | Small business, Mid-market, Enterprise (editorial estimate) |
| Pricing | Open source |
| Website | fstar-lang.org |
| Facts checked | 20 Sep 2026 |
Alternatives to F*
- IsabelleA free, open-source proof assistant for broad mathematical and systems verification.9.0
- RocqA free, kernel-checked theorem prover with dependent types, automation, and certified extraction.8.2
- PVSA broad, self-hosted environment for hybrid formal verification workflows.7.7
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Reviewed by iTechGuides Editors · Editorial team · Updated Sep 2026
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