The narrow JsonSchema surface here didn't model JSON Schema's two
sharing primitives — \$ref (reference into a definitions map) and
\$defs (the definitions map itself). Consumers that want to DRY a
large repeated subschema (e.g. rabit's set_plan tool, where the
extraTraits.from enum of ~300 std-behavior paths is currently inlined
per orbital branch) have no typed way to construct one.

Additive only; no field renames, no breaking changes. OpenAI strict
tool calling + DeepSeek's OpenAI-compatible API both resolve \$ref
against \$defs at the parameters root.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>