Pyverge: open-source library auto-migrates stale v1 payloads after MCP tool schema drift
OpinionPurple2940 · reddit · 2026-09-30
A developer kept hitting schema drift pain in FastMCP tools: upgrading a tool from v1 (location: str) to v2 (lat/lng) broke agents with cached prompts that still send v1 payloads, and patching old inputs with if/else inside every handler quickly became unmaintainable.
Their fix is Pyverge, an open-source library that sits in front of the handler instead of inside tool functions: you define versioned models and JSON patch transitions, and incoming v1 payloads are migrated to v2 on the fly. Business logic stays decoupled from versioning plumbing. A runnable FastMCP showcase and repo are provided, with the author asking how others handle tool schema versioning in production.
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