ADR 0266 — A discovered schema’s unions are judged by its server

Invariants: I2, I13. Follows ADR 0205.

Context

A connection restores each discovered tool’s JSON Schema into Zod (zodObjectFromJsonSchema) and parses the call’s arguments against it before sending them, so a CLI coerces argv to the right types and refuses a missing field without a round trip. That schema is the remote’s presentation, not its contract. ADR 0205 joins a discriminated union into one object for models: the discriminator becomes an enum and a field every variant shares becomes an unlabelled anyOf of the variants’ shapes. Every accepted value still passes; which shape goes with which discriminator value is gone by design.

Parsed locally, such an anyOf is a plain union, and a wrong value inside it can only be refused with every branch listed. A consuming project’s lesson tool with a 12-variant block union answered a missing artifactId with No union branch matched — branch 1 … and 6 more branches, while its server, holding the original schema, answered data.artifactId.

Putting the correlation back into the schema — oneOf with a const discriminator, or a discriminator map — would undo the reason the join exists (weaker models drop oneOf), and a map would be a vendor keyword outside the one schema profile every provider is served (ADR 0047).

Decision

A schema restored from a foreign document is marked at the boundary as one whose unions the remote judges. When the local parse of a call fails, the invalid_union refusals of such a schema are set aside: if nothing else was refused, the coerced arguments are sent unchanged and the remote’s refusal is what the caller sees; if something else was, only that is reported. A schema authored in this process keeps its full verdict, union summary included.

Consequences

A wrong value inside a union of a discovered tool costs one round trip and is named by the server exactly as on its own transports. A missing field, a wrong scalar or an unknown key outside a union is still refused before anything is sent, and no longer carries union lines beside it. A foreign server that does not validate its input sees the values its own schema advertised as acceptable; that is its contract to keep. Proven end to end against a real stitchkit MCP server with flattenUnionInput in tests/connections-remote-union-verdict.test.ts.