I lost an afternoon to an order ID that was correct in the database, correct in the Python logs, and wrong by 83 by the time it reached the Node consumer. Nothing threw. JSON.parse cannot hold an integer past 2^53, and no validator running afterwards brings the bits back. That is not a bug in either service. It is the seam between them, and every language has its own version of it: Python cannot tell a missing key from a null one, JavaScript has three kinds of empty, and none of the three agree on what a date without a timezone means. So I stopped trying to keep three validators in sync and made the contract a file. One .seam, one compiled Rust engine, loaded into Python, Node and the browser. The part I am proudest of is not the validation. It is the conformance suite: 95 plain text cases that run identically from all four runtimes in CI. If Rust and the browser ever disagree, the build fails. That is what turns "no drift" into something that can break rather than something I claim. What changed most while building it was how much I had to decline. Custom validators and regex patterns both need an expression language, and a schema that cannot execute anything turned out to be worth more than either. It is 0.1.3, so the contract can still move. I would rather hear where it is wrong than where it is nice. It is on PyPI, npm and crates.io right now, not a waitlist: pip install seam-schema Python 3.9+ npm install seam-schema Node 20+ npm install seam-schema-wasm browsers cargo add seam-core the engine A demo you can clone and run in a minute, with the ID arriving wrong and then right: github.com/angeldevmobile/seam-demo

Seam

One schema file. Python, Node and the browser agree.

One schema file. Python, Node and the browser agree.