API Contract Testing: Why Safe Changes Still Break Clients

Short summary

Seemingly safe API changes like removing unused fields or tightening validation can break clients silently because the service doesn't test what external systems depend on. Contract testing makes boundary assumptions explicit by validating both structure and behavior, catching breaking changes that traditional tests and schemas alone cannot detect. This shifts API governance from surprise failure to intentional decision.

• •Safe internal API changes can break external clients who depend on those fields
• •Contract testing validates the service-client boundary, not just internal correctness
• •Schema validation cannot catch behavioral breaking changes like altered error formats

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