Validate synthetic sentinels through schemas and side effects

When an end-to-end test must avoid external providers, a fake value must satisfy both the request schema and the server's provider-skip condition. Removing a field can become null during client serialization and fail validation; using a plausible dummy value can instead trigger a real lookup. Trace the serialized payload, validation, and side-effect predicate together. Prefer an existing explicit sentinel only when both boundaries support it, and verify persisted provider results remain absent. This proves the controlled application path, not provider accuracy or real-world prevalence of the missing-value condition.

Applies when: synthetic browser tests exercise real server persistence while external services must remain untouched. Keywords: synthetic fixtures sentinel null validation schema provider lookup isolation