API Drift: A Hidden Crisis in MCP Servers

Abstract: We recently tested popular MCP servers (Postman, Hugging Face and GitHub) using Specmatic’s MCP Auto-Test (open-source). What we discovered is not just about a few bugs – it’s about a deeper issue of trust in API contracts.

Here are three patterns that stood out:

Schema–Implementation Drift
Servers declared fields as optional in schemas, yet rejected requests when those fields were missing. This silent misalignment creates brittle integrations and surprises downstream.

Erosion of Standards
Key constraints like min, max, and enum weren’t defined in standard JSON Schema keywords. Relying on natural language descriptions leaves consumers guessing—and guessing is not engineering.

The AI Angle
In a world where APIs feed into LLMs and autonomous agents, vague schemas don’t just mean failed tests. They risk teaching AI the wrong rules, leading to hallucinations and unpredictable system behaviour.

The takeaway is clear: without precise, tested, and enforceable API contracts, we are building on shaky ground.Drift isn’t a theoretical risk – it’s already showing up in widely used platforms.

Join this session to understand what you can do while developing your MCP servers to ensure such issues don’t leak to production.