API Compatibility
Did externally observable contracts change? A sensor of boundary stability — can old and new versions coexist?
API compatibility checking verifies that a change doesn’t break existing consumers. This is the contract test applied to change: not “does the contract hold now” but “did the contract change between versions.”
Sensor properties
| Property | Value |
|---|---|
| Oracle strength | High — breaking changes are definitive |
| Independence | High — schema/contract is external |
| Scope | Service-level |
| Feedback latency | Seconds |
| Actionability | Guiding — shows which API surface changed |
| Type | Predictive |
What it cannot detect
API compatibility checks the shape of the interface, not the behavioral meaning. A response with the same fields and types but different semantics will pass compatibility checks.