Most production JSON bugs are not parse errors but semantic drift: dates that…

Most production JSON bugs are not parse errors but semantic drift: dates that become strings and change timezone, BigInts that round to nearby floats, enums that gain members, and undefined turning into omitted keys or nulls. Test the contract by round-tripping schemas across your client and server runtimes — TypeScript, .NET, Java — and include any LLM/tools that generate or transform payloads, not just hand-checked examples. Do this before agent tooling talks to business logic: a failing round-trip test is the cheapest alarm you can buy for silent corruption.