Isabelle

Free plan LinuxMacSelf-hostedWindows

What Isabelle is

Isabelle is a free proof assistant for expressing mathematical formulas in a formal language and proving them through logical rules. It is used to formalize mathematical proofs and verify properties of hardware, software, programming languages, and protocols. Isabelle/HOL provides a higher-order logic environment for large applications, with tools including a classical reasoner, simplifier, linear arithmetic automation, algebraic decision procedures, and Sledgehammer access to external first-order provers. The structured Isar proof language is designed to be readable by people as well as computers. Isabelle/HOL can generate SML, OCaml, Haskell, and Scala from executable specifications. Isabelle/jEdit is the default interface, with ongoing proof checking and real-time feedback; the Isabelle2025-2 release also includes Isabelle/VSCode panels for documentation, symbols, and Sledgehammer. The distribution contains a library of formally verified mathematics, and the Archive of Formal Proofs has additional applications. Isabelle runs on Linux, macOS, and Windows, and can be self-hosted. Its main code base is subject to BSD-style regulations. On Windows and macOS, missing developer signatures and certificates mean the operating systems reject the app by default when first opened.

Who it suits

Isabelle suits people working on formal mathematics or verification of hardware, software, programming languages, and protocols. It is available for Linux, macOS, Windows, and self-hosted use.

What it does well

Free distribution with open-source licenses
Includes a theory library of verified mathematics
Generates code in four listed languages
Provides real-time proof checking in Isabelle/jEdit

What to know first

Windows rejects the app by default at first run
macOS requires users to allow the unsigned app

Plans

Plan	Billed and limits	Price
Isabelle	Distributed for free · open-source licenses, with the main code-base subject to BSD-style regulations	Free

As isabelle.in.tum.de lists them, read 30 Sept 2026. The maker's own page has the final word.

Getting started

Visit the Isabelle website.
Choose the distribution for Linux, macOS, or Windows, or use a self-hosted deployment.
Install Isabelle and open Isabelle/jEdit, the default interface.
On Windows, address the system rejection of the unsigned application when it first runs.
On macOS, allow the unsigned application to open.
Use the documentation, project mailing lists, Zulip, or listed Q&A sites for support.

Verdict

Isabelle combines formal proof development with tools for verification and code generation. Its proof environments are available across several platforms, though first launch on Windows and macOS requires addressing their rejection of unsigned applications.

Questions

Is Isabelle free?

Yes. The Isabelle plan is free, and the distribution is available under open-source licenses. The main code base is subject to BSD-style regulations.

Which platforms does Isabelle support?

It supports Linux, macOS, Windows, and self-hosted deployment.

What can Isabelle be used to prove?

Its main uses include formalizing mathematical proofs and verifying properties of hardware, software, programming languages, and protocols.

What interfaces are available?

Isabelle/jEdit is the default interface and Prover IDE. Isabelle2025-2 also includes Isabelle/VSCode with Documentation, Symbols, and Sledgehammer panels.

Can Isabelle generate code?

Yes. Isabelle/HOL can generate SML, OCaml, Haskell, and Scala from executable specifications.

Who makes Isabelle?

The website is isabelle.in.tum.de. Support is available through official documentation, the isabelle-users and isabelle-dev mailing lists, Zulip, and Q&A sites listed by the project.

Facts from Isabelle's own pages

Additional editor: The Isabelle2025-2 release includes Isabelle/VSCode, with GUI panels for Documentation, Symbols, and Sledgehammer. · isabelle.in.tum.de, 30 Sept 2026
Code generation: Isabelle/HOL can turn executable specifications into code in SML, OCaml, Haskell, and Scala. · isabelle.in.tum.de, 30 Sept 2026
IDE: Isabelle/jEdit is the default user interface and Prover IDE, providing continuous proof checking with real-time feedback and semantic markup. · isabelle.in.tum.de, 30 Sept 2026
Isabelle/HOL: Isabelle/HOL provides a higher-order logic theorem proving environment intended for large applications. · isabelle.in.tum.de, 30 Sept 2026
Libraries and examples: The distribution includes a large theory library of formally verified mathematics, and the Archive of Formal Proofs provides additional applications from mathematics and software engineering. · isabelle.in.tum.de, 30 Sept 2026
macOS limitation: The macOS application lacks developer signatures and certificates, so Apple rejects it by default and requires the user to allow it to open. · isabelle.in.tum.de, 30 Sept 2026
Main uses: Its main applications are formalizing mathematical proofs and formal verification, including proving properties of hardware, software, programming languages, and protocols. · isabelle.in.tum.de, 30 Sept 2026
Platforms: The application bundles support Linux, Windows, and macOS; the site also provides a self-contained Docker image without GUI support. · isabelle.in.tum.de, 30 Sept 2026
Proof language: Isar is a structured proof language whose proof text is intended to be understandable to both people and computers. · isabelle.in.tum.de, 30 Sept 2026
Proof tools: Proof productivity tools include a classical reasoner, a simplifier, linear arithmetic automation, algebraic decision procedures, and access to external first-order provers through Sledgehammer. · isabelle.in.tum.de, 30 Sept 2026
Security design: The overview says Isabelle follows the LCF system approach so users can write proof procedures and theory extension packages in ML without breaking system soundness. · isabelle.in.tum.de, 30 Sept 2026
Support: Support is available through official documentation, the isabelle-users and isabelle-dev mailing lists, Zulip, and Q&A sites listed by the project. · isabelle.in.tum.de, 30 Sept 2026
What it does: Isabelle is a generic proof assistant for expressing mathematical formulas in a formal language and proving them in a logical calculus. · isabelle.in.tum.de, 30 Sept 2026
Windows limitation: The Windows application lacks developer signatures and certificates, so Microsoft rejects it by default when first run. · isabelle.in.tum.de, 30 Sept 2026

Where Isabelle stands on Freedom251

Formal Verification Tools#10 of 34