Spec-First Engineering with Specmatic: Contract-Testing a Multi-Agent AI Assistant

Short summary

Spec-first engineering inverts traditional API design: write the OpenAPI contract first, then build code that conforms to it. Using Specmatic, the author tested TRIO, a multi-agent AI assistant with local LLM integration, discovering how contract-based testing and schema mutation prevent API drift and handle non-determinism in AI outputs. Key lessons: mock real service contracts faithfully, separate contract tests from resiliency tests, and document negative examples as first-class test cases.

• •Spec-first approach makes OpenAPI contracts executable, turning them into the source of truth for API behavior
• •Contract testing + generative mutation testing catches API drift and validates graceful degradation under invalid input
• •Practical patterns for testing non-deterministic AI systems: mock fidelity, CI environment constraints, and negative example documentation

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