Theorem (OSS Verify): developer tools for open-source maintainers
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Theorem (OSS Verify) helps you build software by this route: machine-checked proofs for open-source code, plus a Claude skill to get your project started. It is best for open-source maintainers who want proof, not just tests, that their code cannot crash. Anthropic notes the proofs cover single-threaded, static RISC-V binaries and that formally verified does not mean bug-free.
What it does
Anthropic reports that, with Theorem, Claude proved there are no crashes on any input up to 64 MB for 239 fuzz-test entry points across 123 OSS-Fuzz libraries, in about 180 million lines of Rocq proofs. Anthropic notes the proofs cover single-threaded, static RISC-V binaries and that formally verified does not mean bug-free. Theorem’s oss-verify repository includes an add-project Claude skill that walks a maintainer through enrolling a project.
Official site:theorem.dev.
Editorial note
Theorem is a formal-verification company that worked with Anthropic on Claude-written proofs for open-source libraries. Anthropic says Theorem is offering to run the same pipeline for other open-source projects and publishing a Claude skill to help developers start.
Who it’s for
- For: Open-source maintainers who want proof, not just tests, that their code cannot crash.
- Not ideal for: closed-source apps looking for a free plan; commercial work is by contact.
Key use cases
- Build apps & code
What you can do with it
- Free access: Theorem’s GitHub describes OSS Verify as free machine-checked proofs for open-source fuzz targets. This matters when you want to test before you pay.
- Commercial plan: Tell Theorem the properties to verify and it delivers a CI-checkable proof, per its site. This matters when the free tier is not enough.
- Help documentation: Official pricing and product pages at theorem.dev. This matters when you set it up yourself.
How to use Theorem (OSS Verify): step-by-step tutorial
Five to eight steps from the account to a first useful result. Where the official guide names a control, the step uses that name. Where it does not, the step stays general.
- Open the theorem-labs/oss-verify repository on GitHub and read its README.
- Point Claude at the add-project skill in .claude/skills.
- Answer its questions about what “correct” means for your library.
- Let it write the project files and fuzz targets in projects/.
- Open the pull request yourself; Theorem then proves theorems about the built binaries.
These steps follow the official guide: Anthropic: formal verification at scale, Oct 22, 2026.
Frequently asked questions
What is Theorem?
A formal-verification company that, per Anthropic, collaborated on Claude-written proofs for 123 open-source libraries.
Is OSS Verify free?
Theorem’s GitHub describes it as free machine-checked proofs for open-source fuzz targets, checked October 25, 2026.
Does verified mean bug-free?
No. Anthropic says formally verified does not mean bug-free; proofs only cover what the spec models.
Is there a Claude skill?
Yes. Theorem’s oss-verify repository includes an add-project skill for Claude.
Pricing
Prices checked on the official pricing page.
- OSS Verify
-
Free for open-source projects
Enroll a project; Theorem proves theorems about the binaries its fuzz targets build
- Commercial
-
Contact Theorem
Tell Theorem the properties to verify and it delivers a CI-checkable proof, per its site
- Limits: Proofs currently model single-threaded static RISC-V binaries on Linux, per Anthropic.
- Plans change: vendors update limits and prices; confirm on the official page before you buy.
Facts
- Access
- Ongoing free plan
- Paid plans from
- Free (OSS)
- Checked
- Category
- Category: Development tools (31 in the library).
- On the record
- Theorem’s GitHub describes OSS Verify as free machine-checked proofs for open-source fuzz targets.