Vary

Catch the tests that LLMs lie about

You ask an LLM to write tests for your code. It generates a full suite. Coverage hits 90%. Everything passes. You merge it.

A week later, a bug ships. You look at the tests and realize they were checking almost nothing. They asserted that functions returned without errors, but never verified the actual results. The LLM wrote tests that looked right but lied about what they proved.

This is the problem Vary solves. By building mutation testing directly into the language, Vary can flip your bytecode and check whether your tests notice. If a mutation survives, your test was never really protecting you. Run `vary mutate` and find out which tests are real and which ones are theater.

Vary is a statically-typed language that compiles to JVM bytecode. Clean syntax, type inference, null safety at compile time, no semicolons. The compiler has mutation testing built in: it flips bytecode instructions and checks whether your tests notice. If they don't, those tests were never protecting you. One CLI binary handles running, checking, testing, formatting, and mutating. Nothing extra to install.

varylang.com Documentation

Features

Built-in mutation testing via `vary mutate` - not a plugin, part of the compiler
Bytecode-level mutation that flips JVM instructions to verify test effectiveness
Clean, readable syntax with braces, type inference, and no semicolons
Null safety checked at compile time
Unified CLI: run, check, test, format, and mutate from a single binary
Compiles to standard JVM bytecode
VS Code extension support

About Curtis

Curtis is a technologist who works to make systems safer and easier to use. He adopts a socio-technical approach to complex challenges via human-centred solutions. Throughout his career, which has spanned work with university libraries, start-ups and enterprises, he has been a trusted advisor to customers and stakeholders. He is constantly learning and growing, and through organisations like TAICO, he helps others do the same.