Summary

Build a Python proof-of-concept for the custom filter interface. Validates the design shape recommended in SPIKE #646 before locking the full implementation in both SDKs.

Issued separately from the TypeScript PoC to avoid SDK dependencies between them.

Acceptance criteria

• PoC implements the custom filter design recommended in SPIKE: Custom filter design and SDK integration strategy #646
• PoC demonstrates a custom filter applied to a sample CommonGrants search request (customFilters property, standard Filter schema per ADR-0012)
• Findings and design questions captured to inform full SDK implementation