fix: Copy extra_headers to allow model_settings reuse (#3628)

Author: Wh1isper

pydantic_ai_slim/pydantic_ai/models/anthropic.py

@@ -424,7 +424,7 @@ def _get_betas_and_extra_headers(
         Handles merging custom `anthropic-beta` header from `extra_headers` into betas set
         and ensuring `User-Agent` is set.
         """
-        extra_headers = model_settings.get('extra_headers', {})
+        extra_headers = dict(model_settings.get('extra_headers', {}))
         extra_headers.setdefault('User-Agent', get_user_agent())
 
         betas: set[str] = set()

tests/models/test_anthropic.py

@@ -786,6 +786,49 @@ def memory(**command: Any) -> Any:  # pragma: no cover
     )
 
 
+async def test_model_settings_reusable_with_beta_headers(allow_model_requests: None):
+    """Verify that model_settings with extra_headers can be reused across multiple runs.
+
+    This test ensures that the beta header extraction doesn't mutate the original model_settings,
+    allowing the same settings to be used for multiple agent runs.
+    """
+    c = completion_message(
+        [BetaTextBlock(text='Hello!', type='text')],
+        BetaUsage(input_tokens=5, output_tokens=10),
+    )
+    mock_client = MockAnthropic.create_mock(c)
+
+    model_settings = AnthropicModelSettings(extra_headers={'anthropic-beta': 'custom-feature-1, custom-feature-2'})
+
+    model = AnthropicModel(
+        'claude-sonnet-4-5',
+        provider=AnthropicProvider(anthropic_client=mock_client),
+        settings=model_settings,
+    )
+
+    agent = Agent(model)
+
+    # First run
+    await agent.run('Hello')
+
+    # Verify the original model_settings is not mutated
+    assert model_settings.get('extra_headers') == {'anthropic-beta': 'custom-feature-1, custom-feature-2'}
+
+    # Second run should work with the same beta headers
+    await agent.run('Hello again')
+
+    # Verify again after second run
+    assert model_settings.get('extra_headers') == {'anthropic-beta': 'custom-feature-1, custom-feature-2'}
+
+    # Verify both runs had the correct betas
+    all_kwargs = get_mock_chat_completion_kwargs(mock_client)
+    assert len(all_kwargs) == 2
+    for completion_kwargs in all_kwargs:
+        betas = completion_kwargs['betas']
+        assert 'custom-feature-1' in betas
+        assert 'custom-feature-2' in betas
+
+
 async def test_anthropic_mixed_strict_tool_run(allow_model_requests: None, anthropic_api_key: str):
     """Exercise both strict=True and strict=False tool definitions against the live API."""
     m = AnthropicModel('claude-sonnet-4-5', provider=AnthropicProvider(api_key=anthropic_api_key))