fix: deprecate setattr on ModelCallRequest (#34022)

* one alternative considered was setting `frozen=True` on the dataclass,
but this is breaking, so a deprecation is a nicer approach

libs/partners/anthropic/langchain_anthropic/middleware/prompt_caching.py

@@ -94,14 +94,6 @@ class AnthropicPromptCachingMiddleware(AgentMiddleware):
         )
         return messages_count >= self.min_messages_to_cache
 
-    def _apply_cache_control(self, request: ModelRequest) -> None:
-        """Apply cache control settings to the request.
-
-        Args:
-            request: The model request to modify.
-        """
-        request.model_settings["cache_control"] = {"type": self.type, "ttl": self.ttl}
-
     def wrap_model_call(
         self,
         request: ModelRequest,
@@ -119,8 +111,12 @@ class AnthropicPromptCachingMiddleware(AgentMiddleware):
         if not self._should_apply_caching(request):
             return handler(request)
 
-        self._apply_cache_control(request)
-        return handler(request)
+        model_settings = request.model_settings
+        new_model_settings = {
+            **model_settings,
+            "cache_control": {"type": self.type, "ttl": self.ttl},
+        }
+        return handler(request.override(model_settings=new_model_settings))
 
     async def awrap_model_call(
         self,
@@ -139,5 +135,9 @@ class AnthropicPromptCachingMiddleware(AgentMiddleware):
         if not self._should_apply_caching(request):
             return await handler(request)
 
-        self._apply_cache_control(request)
-        return await handler(request)
+        model_settings = request.model_settings
+        new_model_settings = {
+            **model_settings,
+            "cache_control": {"type": self.type, "ttl": self.ttl},
+        }
+        return await handler(request.override(model_settings=new_model_settings))

libs/partners/anthropic/tests/unit_tests/middleware/test_prompt_caching.py

@@ -82,12 +82,17 @@ def test_anthropic_prompt_caching_middleware_initialization() -> None:
         model_settings={},
     )
 
+    modified_request: ModelRequest | None = None
+
     def mock_handler(req: ModelRequest) -> ModelResponse:
+        nonlocal modified_request
+        modified_request = req
         return ModelResponse(result=[AIMessage(content="mock response")])
 
     middleware.wrap_model_call(fake_request, mock_handler)
     # Check that model_settings were passed through via the request
-    assert fake_request.model_settings == {
+    assert modified_request is not None
+    assert modified_request.model_settings == {
         "cache_control": {"type": "ephemeral", "ttl": "5m"}
     }
 
@@ -162,13 +167,18 @@ async def test_anthropic_prompt_caching_middleware_async() -> None:
         model_settings={},
     )
 
+    modified_request: ModelRequest | None = None
+
     async def mock_handler(req: ModelRequest) -> ModelResponse:
+        nonlocal modified_request
+        modified_request = req
         return ModelResponse(result=[AIMessage(content="mock response")])
 
     result = await middleware.awrap_model_call(fake_request, mock_handler)
     assert isinstance(result, ModelResponse)
     # Check that model_settings were passed through via the request
-    assert fake_request.model_settings == {
+    assert modified_request is not None
+    assert modified_request.model_settings == {
         "cache_control": {"type": "ephemeral", "ttl": "1h"}
     }
 
@@ -237,13 +247,18 @@ async def test_anthropic_prompt_caching_middleware_async_min_messages() -> None:
         model_settings={},
     )
 
+    modified_request: ModelRequest | None = None
+
     async def mock_handler(req: ModelRequest) -> ModelResponse:
+        nonlocal modified_request
+        modified_request = req
         return ModelResponse(result=[AIMessage(content="mock response")])
 
     result = await middleware.awrap_model_call(fake_request, mock_handler)
     assert isinstance(result, ModelResponse)
     # Cache control should NOT be added when message count is below minimum
-    assert fake_request.model_settings == {}
+    assert modified_request is not None
+    assert modified_request.model_settings == {}
 
 
 async def test_anthropic_prompt_caching_middleware_async_with_system_prompt() -> None:
@@ -268,13 +283,18 @@ async def test_anthropic_prompt_caching_middleware_async_with_system_prompt() ->
         model_settings={},
     )
 
+    modified_request: ModelRequest | None = None
+
     async def mock_handler(req: ModelRequest) -> ModelResponse:
+        nonlocal modified_request
+        modified_request = req
         return ModelResponse(result=[AIMessage(content="mock response")])
 
     result = await middleware.awrap_model_call(fake_request, mock_handler)
     assert isinstance(result, ModelResponse)
     # Cache control should be added when system prompt pushes count to minimum
-    assert fake_request.model_settings == {
+    assert modified_request is not None
+    assert modified_request.model_settings == {
         "cache_control": {"type": "ephemeral", "ttl": "1h"}
     }
 
@@ -300,12 +320,17 @@ async def test_anthropic_prompt_caching_middleware_async_default_values() -> Non
         model_settings={},
     )
 
+    modified_request: ModelRequest | None = None
+
     async def mock_handler(req: ModelRequest) -> ModelResponse:
+        nonlocal modified_request
+        modified_request = req
         return ModelResponse(result=[AIMessage(content="mock response")])
 
     result = await middleware.awrap_model_call(fake_request, mock_handler)
     assert isinstance(result, ModelResponse)
     # Check that model_settings were added with default values
-    assert fake_request.model_settings == {
+    assert modified_request is not None
+    assert modified_request.model_settings == {
         "cache_control": {"type": "ephemeral", "ttl": "5m"}
     }