Every language has a good validator. The bugs are not inside a language, they are at the seam between two, where each disagrees about what the data means. JavaScript loses integers past 2^53. Python cannot tell absent from null. Seam makes the contract a file. Python, Node and the browser validate against the same compiled Rust engine, not the same spec written three times. 95 conformance cases run from all four in CI, so agreement is tested. pip install seam-schema npm install seam-schema