`build_strict_openai_schema` emits `oneOf` for discriminated unions, which OpenAI strict mode rejects

[inickt]

Bug Description

When a tool input model contains a Pydantic discriminated union, build_strict_openai_schema(...) emits oneOf in the generated schema.

That appears to be incompatible with OpenAI strict tool schemas. The resulting tool registration fails with:

Invalid schema for function '<function>': In context=(), 'oneOf' is not permitted.

The same union without an explicit discriminator produces anyOf instead and works.

Expected Behavior

to_strict_json_schema(...) should normalize discriminated unions into a strict-mode-compatible schema shape:
https://github.com/livekit/agents/blob/main/livekit-agents/livekit/agents/llm/_strict.py

Reproduction Steps

from typing import Annotated, Literal

from pydantic import BaseModel, Field

class Car(BaseModel):
    vehicle: Literal["Car"]
    brand: str
    color: str


class Bike(BaseModel):
    vehicle: Literal["Bike"]
    brand: str
    color: str


type CarOrBike = Annotated[Car | Bike, Field(discriminator="vehicle")]
# type CarOrBike = Annotated[Car | Bike, Field()]  # this version works

and then use `CarOrBike` in a `@function_tool` parameter.

Relevant schema difference:

With discriminator:


{
  "$defs": {
    "CarOrBike": {
      "oneOf": [
        { "$ref": "#/$defs/Car" },
        { "$ref": "#/$defs/Bike" }
      ]
    }
  }
}


Without discriminator:


{
  "$defs": {
    "CarOrBike": {
      "anyOf": [
        { "$ref": "#/$defs/Car" },
        { "$ref": "#/$defs/Bike" }
      ]
    }
  }
}

Operating System

macOS/Linux

Models Used

OpenAI GPT 4.1, should occur with any OpenAI model with strict mode enabled

Package Versions

livekit-agents==1.4.6

Session/Room/Call IDs

N/A

Proposed Solution

Normalize `oneOf` back to `anyOf` when building the strict OpenAI schema.

Additional Context

Discriminated unions are the recommended Pydantic shape for tagged unions:
https://docs.pydantic.dev/latest/concepts/unions/#discriminated-unions

Screenshots and Recordings

No response