Fix FallbackModel to respect each model's model settings (#2540)

Author: jerry-reevo

pydantic_ai_slim/pydantic_ai/models/fallback.py

@@ -11,6 +11,7 @@
 from pydantic_ai.models.instrumented import InstrumentedModel
 
 from ..exceptions import FallbackExceptionGroup, ModelHTTPError
+from ..settings import merge_model_settings
 from . import KnownModelName, Model, ModelRequestParameters, StreamedResponse, infer_model
 
 if TYPE_CHECKING:
@@ -65,8 +66,9 @@ async def request(
 
         for model in self.models:
             customized_model_request_parameters = model.customize_request_parameters(model_request_parameters)
+            merged_settings = merge_model_settings(model.settings, model_settings)
             try:
-                response = await model.request(messages, model_settings, customized_model_request_parameters)
+                response = await model.request(messages, merged_settings, customized_model_request_parameters)
             except Exception as exc:
                 if self._fallback_on(exc):
                     exceptions.append(exc)
@@ -91,10 +93,13 @@ async def request_stream(
 
         for model in self.models:
             customized_model_request_parameters = model.customize_request_parameters(model_request_parameters)
+            merged_settings = merge_model_settings(model.settings, model_settings)
             async with AsyncExitStack() as stack:
                 try:
                     response = await stack.enter_async_context(
-                        model.request_stream(messages, model_settings, customized_model_request_parameters, run_context)
+                        model.request_stream(
+                            messages, merged_settings, customized_model_request_parameters, run_context
+                        )
                     )
                 except Exception as exc:
                     if self._fallback_on(exc):

tests/models/test_fallback.py

@@ -1,17 +1,21 @@
 from __future__ import annotations
 
+import json
 import sys
 from collections.abc import AsyncIterator
 from datetime import timezone
 from typing import Any
 
 import pytest
+from dirty_equals import IsJson
 from inline_snapshot import snapshot
+from pydantic_core import to_json
 
 from pydantic_ai import Agent, ModelHTTPError
 from pydantic_ai.messages import ModelMessage, ModelRequest, ModelResponse, TextPart, UserPromptPart
 from pydantic_ai.models.fallback import FallbackModel
 from pydantic_ai.models.function import AgentInfo, FunctionModel
+from pydantic_ai.settings import ModelSettings
 from pydantic_ai.usage import Usage
 
 from ..conftest import IsNow, try_import
@@ -445,3 +449,67 @@ async def test_fallback_condition_tuple() -> None:
 
     response = await agent.run('hello')
     assert response.output == 'success'
+
+
+async def test_fallback_model_settings_merge():
+    """Test that FallbackModel properly merges model settings from wrapped model and runtime settings."""
+
+    def return_settings(_: list[ModelMessage], info: AgentInfo) -> ModelResponse:
+        return ModelResponse(parts=[TextPart(to_json(info.model_settings).decode())])
+
+    base_model = FunctionModel(return_settings, settings=ModelSettings(temperature=0.1, max_tokens=1024))
+    fallback_model = FallbackModel(base_model)
+
+    # Test that base model settings are preserved when no additional settings are provided
+    agent = Agent(fallback_model)
+    result = await agent.run('Hello')
+    assert result.output == IsJson({'max_tokens': 1024, 'temperature': 0.1})
+
+    # Test that runtime model_settings are merged with base settings
+    agent_with_settings = Agent(fallback_model, model_settings=ModelSettings(temperature=0.5, parallel_tool_calls=True))
+    result = await agent_with_settings.run('Hello')
+    expected = {'max_tokens': 1024, 'temperature': 0.5, 'parallel_tool_calls': True}
+    assert result.output == IsJson(expected)
+
+    # Test that run-time model_settings override both base and agent settings
+    result = await agent_with_settings.run(
+        'Hello', model_settings=ModelSettings(temperature=0.9, extra_headers={'runtime_setting': 'runtime_value'})
+    )
+    expected = {
+        'max_tokens': 1024,
+        'temperature': 0.9,
+        'parallel_tool_calls': True,
+        'extra_headers': {
+            'runtime_setting': 'runtime_value',
+        },
+    }
+    assert result.output == IsJson(expected)
+
+
+async def test_fallback_model_settings_merge_streaming():
+    """Test that FallbackModel properly merges model settings in streaming mode."""
+
+    async def return_settings_stream(_: list[ModelMessage], info: AgentInfo):
+        # Yield the merged settings as JSON to verify they were properly combined
+        yield to_json(info.model_settings).decode()
+
+    base_model = FunctionModel(
+        stream_function=return_settings_stream,
+        settings=ModelSettings(temperature=0.1, extra_headers={'anthropic-beta': 'context-1m-2025-08-07'}),
+    )
+    fallback_model = FallbackModel(base_model)
+
+    # Test that base model settings are preserved in streaming mode
+    agent = Agent(fallback_model)
+    async with agent.run_stream('Hello') as result:
+        output = await result.get_output()
+
+    assert json.loads(output) == {'extra_headers': {'anthropic-beta': 'context-1m-2025-08-07'}, 'temperature': 0.1}
+
+    # Test that runtime model_settings are merged with base settings in streaming mode
+    agent_with_settings = Agent(fallback_model, model_settings=ModelSettings(temperature=0.5))
+    async with agent_with_settings.run_stream('Hello') as result:
+        output = await result.get_output()
+
+    expected = {'extra_headers': {'anthropic-beta': 'context-1m-2025-08-07'}, 'temperature': 0.5}
+    assert json.loads(output) == expected