Data contracts are executable agreements between data producers and consumers that formalize expectations around schema, semantics, quality, and delivery of data products. Rooted in the API-first principles of software engineering, they shift data quality left—enforcing validation at the point of production rather than downstream, reducing pipeline failures by up to 80% in production environments. Unlike passive documentation or schema registries, data contracts are enforced in code through automated validation, version control, and CI/CD integration, making them a critical defense against schema drift, breaking changes, and trust erosion in modern data architectures. One non-obvious insight: contracts are most effective when they embrace bounded flexibility—strict on critical invariants (schema, nullability, uniqueness) but lenient on non-breaking additions, allowing systems to evolve without constant renegotiation.