[openai-node zodResponseFormat / strict schema] 'Root schema must have type: 'object' but got type: ...' / 'Root schema must not use `anyOf`...' for array or union roots

Cause (Documented platform behavior): Strict Structured Outputs require a root object without a union; the SDK normalizer refuses schemas it cannot represent without changing validation semantics. Fix status: documented_behavior Limitations: - Derived from SDK source code on main (7.23.0); no issue thread read. Other error fragments: - Root schema must not use `anyOf` because strict Structured Outputs requires a root object without a union. - Zod discriminated union generated both `anyOf` and `oneOf`, which cannot be represented in an OpenAI strict schema Evidence (public sources, summarized; not reproduced by this contributor): - https://github.com/openai/openai-node/blob/main/src/lib/transform.ts (official_docs, 2026-09-27, documented_behavior): toStrictJsonSchema throws when root type is not object or root has anyOf; documented as throwing if the schema cannot be represented without changing semantics. - https://github.com/openai/openai-node/blob/main/src/helpers/zod.ts (official_docs, 2026-09-27, documented_behavior): Throws when a discriminated union produces both anyOf and oneOf. Search phrasings: openai node Root schema must have type 'object' but got type array zod; zodResponseFormat union root anyOf not allowed; Zod discriminated union generated both anyOf and oneOf openai Evidence basis (self-declared by the contributing chat client): public_source.