Stainless

Free#33 of 35 in Formal Verification Tools

Stainless: A focused, self-hosted verifier for Scala 3 contracts, safety properties, and termination. Ranked #33 of 35 in Formal Verification Tools by our editors (6.2/10); pricing: Free plan; best for contract verification for Scala.

6.2/10★★★☆☆Editor score

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At a glance

• Editor score
  6.2 / 10
• Pricing
  Free plan
• Best for
  Contract verification for Scala
• Free plan
  Yes
• Paid from
  Free
• Verification method
  Deductive
• Facts checked
  20 Sep 2026

• Where it wins

  • Concrete counterexamples help diagnose invalid contracts.
  • Uses Z3, cvc5, and Princess for automated reasoning.
  • Supports Scala-to-C99 generation, Coq export, and SBT integration.

• Where it doesn't

  • Focused on a supported subset of Scala 3.
  • Self-hosted deployment requires local environment management.
  • Verification results can be valid, invalid, or unknown.

Our verdict on Stainless

Stainless is an open-source formal verification framework for a supported subset of Scala 3. It targets developers and teams that need deductive verification of higher-order Scala programs, including function contracts, runtime-safety properties, termination, and algebraic data type invariants. The framework runs locally through standalone releases, package managers, source builds, SBT integration, or browser-based GitHub Codespaces. Its free, open-source model suits projects that want to inspect and manage verification tooling within their own development environment.

The main depth is its contract and proof system. Stainless checks preconditions, postconditions, loop invariants, and ADT invariants, and it can verify quantified properties and natural-deduction proofs. For termination, it supports inferred or user-specified measures. When a contract fails, the tool provides concrete counterexamples, which can make invalid specifications easier to investigate. Automated reasoning uses Z3, cvc5, and Princess, while verification reports distinguish valid, invalid, and unknown results. This combination gives Scala developers a broad set of specification constructs without narrowing the workflow to simple assertions.

Its ecosystem fit is strongest for Scala projects already using SBT or requiring adjacent artifacts. Stainless offers SBT plugin integration, Coq program export, and Scala-to-C99 code generation, alongside support for Windows, macOS, Linux, and an API. The trade-off is focus: Stainless is designed for Scala 3 and a supported language subset rather than broad multi-language verification. Teams working primarily in another language, or those seeking a general-purpose formal verification platform, should choose an alternative. Scala developers who want local, contract-focused verification, counterexamples, termination checking, and proof-oriented specifications should consider Stainless.

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Alternatives to Stainless

• IIsabelleA 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
• PPVSA broad, self-hosted environment for hybrid formal verification workflows.9.2

See all Stainless alternatives →

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Featured on iTechGuides

Reviewed by iTechGuides Editors · Editorial team · Updated Oct 2026

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Last updated 7 Oct 2026 · How we research and update