fix: use override not direct attr access for wrap style hooks (#1525)

Author: sydney-runkle

src/oss/langchain/agents.mdx

@@ -163,8 +163,7 @@ def dynamic_model_selection(request: ModelRequest, handler) -> ModelResponse:
     else:
         model = basic_model
 
-    request.model = model
-    return handler(request)
+    return handler(request.override(model=model))
 
 agent = create_agent(
     model=basic_model,  # Default model

src/oss/python/releases/langchain-v1.mdx

@@ -173,9 +173,7 @@ class ExpertiseBasedToolMiddleware(AgentMiddleware):
             model = ChatOpenAI(model="gpt-5-nano")
             tools = [simple_search, basic_calculator]
 
-        request.model = model
-        request.tools = tools
-        return handler(request)
+        return handler(request.override(model=model, tools=tools))
 
 agent = create_agent(
     model="claude-sonnet-4-5-20250929",