Vexil

Vexil is a schema language and toolchain for exact binary protocols. The schema defines the data model and its representation: bit widths, field ordinals, integer encodings, collection bounds, and evolution metadata are reviewable in one contract.

Pre-1.0: Vexil's components version independently. Read the support matrix and compatibility limits before adoption.

The defining choice

In many schema systems, a type describes a value while the codec decides how it is represented. Vexil makes representation part of the type contract:

message Reading {
    channel  @0 : u4
    value    @1 : u16
    sequence @2 : u32 @varint
    offset   @3 : i32 @zigzag
}

channel is four bits. sequence uses unsigned LEB128. offset uses ZigZag followed by LEB128. Those are language rules rather than conventions hidden in application code.

What the toolchain provides

• a compiler with source-spanned diagnostics;
• Rust, TypeScript, Go, and Python code generation;
• deterministic canonical schema hashes using BLAKE3;
• compatibility classification for schema evolution;
• Rust, TypeScript, Go, and Python runtimes with different documented evidence levels;
• a conformance corpus and golden byte vectors.

The wire is not self-describing, and Vexil does not define transport, authentication, discovery, or compression. Applications own those layers.

A practical path

1. Install the compiler.
2. Write and check a schema.
3. Generate code for your target.
4. Run the curated examples.
5. Review the support matrix for the exact target combination you intend to ship.