[Kayvan-Zahiri]

A recursive Pydantic object model (for example Node with children: list[Node]) emits a root $ref into $defs. The SDK's structured-output check only inspected the root type, so it rejected that valid object model with TypeError: structured output schema must describe a top-level object before a turn could start.

Resolve local JSON Pointer references for the existing top-level-object check, including escaped pointer tokens and cycle detection. The original schema is preserved on the wire and Pydantic still validates the returned object. External, missing, cyclic, and non-object references remain rejected.

Validation:

• Reproduced the recursive Pydantic model failure on the original code; the regression now passes and adapts a nested object successfully.
• Added seven cases: recursive model, escaped local pointer and schema preservation, plus five invalid/non-object reference cases.
• Full offline suite: 1,539 passed, 15 skipped. Live Droid/model tests were skipped; no paid inference or hosted product run was performed.
• Ruff lint/format, mypy (45 configured files), and Pyright (explicit test-environment Python) pass. Mypy prints the existing unrecognized warn_untyped_fields configuration-option notice.

AI assistance: Codex independently reproduced, implemented, and tested this change with the account owner's authorization. Draft for maintainer review; no human implementation review yet.