MultiProvider not exported in public API but used as default in RunConfig

[kobol]

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Describe the bug

The MultiProvider class is used as the default factory for RunConfig.model_provider but is not exported in the public API (__init__.py). This creates an inconsistency where users cannot directly access the class that's used internally as the default.

Current Behavior

# In run.py
model_provider: ModelProvider = field(default_factory=MultiProvider)

# But in __init__.py, MultiProvider is not exported
# Users cannot do:
from agents import MultiProvider  # This fails

Expected Behavior

Users should be able to access MultiProvider directly since it's used as the default model provider.

Impact

1. API Inconsistency: The default behavior uses a class that's not publicly accessible
2. User Confusion: Users might want to customize the default provider but can't access the class
3. Limited Flexibility: Users cannot create custom MultiProvider instances with specific configurations

Proposed Solutions

Export MultiProvider directly

Add MultiProvider to the __all__ list in __init__.py:

from .models.multi_provider import MultiProvider

__all__ = [
    # ... existing exports
    "MultiProvider",
    # ...
]

Debug information

• Agents SDK version: (e.g. v0.2.2)
• Python version (e.g. Python 3.12)