Every chat request hands each builtin tool a fresh copy of its cached spec, because callers mutate what they get. That copy was a full deepcopy of a nested dict, repeated per tool per message.
The builder now caches the spec already serialized, so a request only parses it back. Parsing is what produces the independent tree callers mutate, and the cached value becomes an immutable string, so a request can no longer reach the cached object at all.
Measured on CPython 3.12 with a 1.1 KB spec and 20 builtin tools per request:
| | before | after |
|---|---|---|
| stdlib json, the default | 276.2 us | 66.4 us |
| orjson | 279.5 us | 37.5 us |
Builtin specs are plain JSON by construction: pydantic normalizes every default before it reaches the schema, so a tuple, set, enum or datetime cannot appear in one, and an unserializable default is dropped rather than embedded.
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