Interesting approach to constraining what agents can express syntactically. I've been thinking about a related but different problem: it's not just about how agents write code, it's about what they do at runtime.
I've been building agents that call external APIs (ticket systems, chat tools, cloud infra) and the bigger risk I keep hitting isn't bad code, it's bad data flowing through correct code. An agent can write perfectly valid, type-safe code that reads a customer record and posts it somewhere, and the code is fine but the payload has an SSN in it.
Curious whether Sigil's contract system could extend to runtime data constraints on API call payloads, not just function inputs. Like requires payload contains no PII as a contract on an external call. That would be genuinely novel.