Summary

transform_schema() drops a top-level $defs when the schema root is a $ref, leaving a dangling reference that the API rejects.

The early return at the top of transform_schema copies $ref and discards every sibling key — including $defs — before the $defs handling below it can run:

ref = json_schema.pop("$ref", None)
if ref is not None:
    strict_schema["$ref"] = ref
    return strict_schema

That's fine for the recursive case (a nested property that is just {"$ref": ...}), but wrong at the root: $ref with a sibling $defs is valid JSON Schema (draft 2019-09+), and it's exactly what Pydantic v2 emits for RootModel types:

TypeAdapter(RootModel[Tier]).json_schema()
# {"$ref": "#/$defs/Tier", "$defs": {"Tier": {...}}}

Any RootModel output type fed through the strict-mode bindings hits this. Reported by a user; verified present from at least v0.84.0 through v0.105.2.

Repro

from anthropic.lib._parse._transform import transform_schema

schema = {
    "$ref": "#/$defs/Tier",
    "$defs": {"Tier": {"type": "string", "enum": ["free", "pro", "enterprise"], "title": "Tier"}},
}
transform_schema(schema)
# before: {"$ref": "#/$defs/Tier"}            <- $defs dropped, dangling ref
# after:  {"$defs": {"Tier": {...}}, "$ref": "#/$defs/Tier"}

Fix

Process $defs before the $ref early return. Bare-$ref schemas (no siblings) are unchanged — $defs is only emitted when present in the input.

Not addressed here: other root-$ref siblings (e.g. title) are still dropped by the early return. $defs is the one that makes the schema invalid; the rest are cosmetic.

Tests

• Added test_ref_schema_with_defs covering the RootModel shape (root $ref + sibling $defs); the existing test_ref_schema covers the bare-$ref case.
• tests/lib/_parse/test_transform.py: 14/14 pass.