Intermittent AttributeError in beta.messages.stream() with effort parameter: 'dict' object has no attribute 'to_dict'

[SergioHC95]

Summary

Using client.beta.messages.stream() with Claude Opus 4.5 and the effort-2025-11-24 beta intermittently raises AttributeError: 'dict' object has no attribute 'to_dict' when calling stream.get_final_message().

Environment

• SDK: anthropic>=0.75.0
• Python: 3.11+
• Model: claude-opus-4-5-20251101
• Betas: code-execution-2025-08-25, effort-2025-11-24

Minimal Reproduction

with client.beta.messages.stream(
    model="claude-opus-4-5-20251101",
    max_tokens=64000,
    messages=[{"role": "user", "content": [{"type": "text", "text": "..."}]}],
    betas=["code-execution-2025-08-25", "effort-2025-11-24"],
    output_config={"effort": "medium"},
) as stream:
    for event in stream:
        pass
    resp = stream.get_final_message()  # Intermittent error here

Observed Behavior

• Intermittent: Same request fails on attempts 1-3, then succeeds on retry
• Non-deterministic: Identical code works in one run, fails in another
• Beta-specific: Only affects Opus 4.5 via client.beta.messages; non-beta streaming works reliably

Error

AttributeError: 'dict' object has no attribute 'to_dict'

Hypothesis

The beta streaming accumulator occasionally returns a raw dict instead of a Pydantic model, causing internal .to_dict() calls to fail. Possibly a race condition in response accumulation.