Talos – Open-source WASM interpreter for Lean
At Cajal (YC W26) we’re excited to share Talos (https://github.com/cajal-technologies/talos), an open source framework for formal verification of WebAssembly modules in Lean.AI is now writing tons of the code that gets pushed to production. As code generation gets cheaper, verification becomes the bottleneck. We believe in a future where every piece of software comes with a mathematical proof that it does what its author intended - in doing so, eliminating many classes of exploits. Talos is part of the foundation for that.Talos provides a Wasm interpreter optimized for reasoning at the binary level, together with a weakest-precondition calculus layer for proving properties about programs. Because we reason directly about WebAssembly, any language with a Wasm backend is in scope: Rust, C++, Go, C, Swift, Kotlin, Zig, C#, and many more.To make this possible, we use Lean: a programming language and theorem prover that lets you both write software and mathematically prove that it's correct - all in one system. That's what lets Talos double as both an executable interpreter and the formal object Lean reasons about. Lean also integrates with modern AI proving tools, discharging goals automatically via both proof search and direct evaluation.To see Talos in action check out a proof for Stein's GCD algorithm, implemented in the popular Rust crate num-integer: https://github.com/cajal-technologies/talos/blob/main/progra....Our roadmap:- Full Wasm coverage by first passing the official W3C testsuite, then later verifying against SpecTec (formal Wasm spec) - Arbitrary crate verification - any Rust crate that compiles to Wasm should be in scope - Building our proof library codelib, to make verifying increasingly complex programs tractableWe would love to hear the community’s feedback on Talos and comments on the state of formal verification right now. Contributions are also welcome!
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A formal verification tool that proves WebAssembly code is correct by reasoning about compiled binaries in Lean, targeting the growing problem of AI-generated code reaching production without guarantees.
The pain
The gap
Build angle
Strengths
- Addresses a real bottleneck: as code generation gets cheaper, verification becomes the constraint
- Works at binary level, making it language-agnostic across Rust, C++, Go, etc.
- Lean integration with AI proving tools means proofs can be partially automated
- YC backing and open-source approach builds trust in a domain where correctness claims matter
- Clear technical differentiation: executable interpreter + formal reasoning in one system
- Timing is good: AI code generation is accelerating, creating urgency for verification
Risks
- Adoption barrier is high: formal verification requires expertise most teams don't have; even with automation, users need to understand what they're proving
- Market size unclear: how many teams actually need mathematical proofs vs. better testing? Enterprise adoption of formal methods has been slow for decades
- Competitive pressure from simpler alternatives: static analysis, fuzzing, and property-based testing solve 80% of problems for 5% of the cost
- Lean ecosystem is small; building a proof library (codelib) from scratch is a multi-year effort with uncertain ROI
- Full WASM coverage is a long slog; W3C testsuite compliance doesn't guarantee practical usefulness
- Chicken-and-egg problem: tool is only valuable if proofs exist for the code people actually write; building that library requires either community contribution or significant internal effort
Fly Labs Method
Is the pain real, is there a gap, is it the right time, can one person build it.
Value Equation
Dream outcome and how likely it feels, against the time and effort it costs.
One-Person Business
Curiosity pull, identity fit, and a path from free value to paid for a solo creator.
Viral Frameworks
Hook strength, shareability, and how cheaply it can be tested.
Builder Lens
Evidence the problem exists, timing, defensibility, and a model that fits on a napkin.
Five lenses, one composite. How scoring works
The angle
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