Make cross-boundary behaviour explicit.

Minimum contract questions

• Who provides and who consumes the behaviour?
• What inputs and outputs exist?
• What are the types, nullability, ranges, and units?
• Which errors are observable?
• What are retry and timeout rules?
• What ordering and concurrency guarantees exist?
• How is data serialized?
• Who owns persistent state?
• How is compatibility versioned?
• What is explicitly not guaranteed?

Contract artifact

name:
version:
provider:
consumers:
inputs:
outputs:
errors:
timeouts:
retries:
ordering:
concurrency:
ownership:
compatibility:
migration:
verification:

Contract evolution

Changing a contract should be a controlled engineering event. Identify the owner, boundary, contract impact, observable acceptance evidence, and migration or recovery path.