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

Author: sydney-runkle

.gitignore

@@ -163,3 +163,6 @@ node_modules
 
 prof
 virtualenv/
+scratch/
+
+.langgraph_api/

libs/langchain_v1/langchain/agents/middleware/context_editing.py

@@ -10,6 +10,7 @@
 from __future__ import annotations
 
 from collections.abc import Awaitable, Callable, Iterable, Sequence
+from copy import deepcopy
 from dataclasses import dataclass
 from typing import Literal
 
@@ -238,10 +239,11 @@ def count_tokens(messages: Sequence[BaseMessage]) -> int:
                     system_msg + list(messages), request.tools
                 )
 
+        edited_messages = deepcopy(list(request.messages))
         for edit in self.edits:
-            edit.apply(request.messages, count_tokens=count_tokens)
+            edit.apply(edited_messages, count_tokens=count_tokens)
 
-        return handler(request)
+        return handler(request.override(messages=edited_messages))
 
     async def awrap_model_call(
         self,
@@ -266,10 +268,11 @@ def count_tokens(messages: Sequence[BaseMessage]) -> int:
                     system_msg + list(messages), request.tools
                 )
 
+        edited_messages = deepcopy(list(request.messages))
         for edit in self.edits:
-            edit.apply(request.messages, count_tokens=count_tokens)
+            edit.apply(edited_messages, count_tokens=count_tokens)
 
-        return await handler(request)
+        return await handler(request.override(messages=edited_messages))
 
 
 __all__ = [

libs/langchain_v1/langchain/agents/middleware/model_fallback.py

@@ -92,9 +92,8 @@ def wrap_model_call(
 
         # Try fallback models
         for fallback_model in self.models:
-            request.model = fallback_model
             try:
-                return handler(request)
+                return handler(request.override(model=fallback_model))
             except Exception as e:  # noqa: BLE001
                 last_exception = e
                 continue
@@ -127,9 +126,8 @@ async def awrap_model_call(
 
         # Try fallback models
         for fallback_model in self.models:
-            request.model = fallback_model
             try:
-                return await handler(request)
+                return await handler(request.override(model=fallback_model))
             except Exception as e:  # noqa: BLE001
                 last_exception = e
                 continue

libs/langchain_v1/langchain/agents/middleware/todo.py

@@ -194,22 +194,22 @@ def wrap_model_call(
         handler: Callable[[ModelRequest], ModelResponse],
     ) -> ModelCallResult:
         """Update the system prompt to include the todo system prompt."""
-        request.system_prompt = (
+        new_system_prompt = (
             request.system_prompt + "\n\n" + self.system_prompt
             if request.system_prompt
             else self.system_prompt
         )
-        return handler(request)
+        return handler(request.override(system_prompt=new_system_prompt))
 
     async def awrap_model_call(
         self,
         request: ModelRequest,
         handler: Callable[[ModelRequest], Awaitable[ModelResponse]],
     ) -> ModelCallResult:
         """Update the system prompt to include the todo system prompt (async version)."""
-        request.system_prompt = (
+        new_system_prompt = (
             request.system_prompt + "\n\n" + self.system_prompt
             if request.system_prompt
             else self.system_prompt
         )
-        return await handler(request)
+        return await handler(request.override(system_prompt=new_system_prompt))

libs/langchain_v1/langchain/agents/middleware/tool_selection.py

@@ -255,8 +255,7 @@ def _process_selection_response(
         # Also preserve any provider-specific tool dicts from the original request
         provider_tools = [tool for tool in request.tools if isinstance(tool, dict)]
 
-        request.tools = [*selected_tools, *provider_tools]
-        return request
+        return request.override(tools=[*selected_tools, *provider_tools])
 
     def wrap_model_call(
         self,

libs/langchain_v1/langchain/agents/middleware/types.py

@@ -94,6 +94,31 @@ class ModelRequest:
     runtime: Runtime[ContextT]  # type: ignore[valid-type]
     model_settings: dict[str, Any] = field(default_factory=dict)
 
+    def __setattr__(self, name: str, value: Any) -> None:
+        """Set an attribute with a deprecation warning.
+
+        Direct attribute assignment on `ModelRequest` is deprecated. Use the
+        `override()` method instead to create a new request with modified attributes.
+
+        Args:
+            name: Attribute name.
+            value: Attribute value.
+        """
+        import warnings
+
+        # Allow setting attributes during __init__ (when object is being constructed)
+        if not hasattr(self, "__dataclass_fields__") or not hasattr(self, name):
+            object.__setattr__(self, name, value)
+        else:
+            warnings.warn(
+                f"Direct attribute assignment to ModelRequest.{name} is deprecated. "
+                f"Use request.override({name}=...) instead to create a new request "
+                f"with the modified attribute.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+            object.__setattr__(self, name, value)
+
     def override(self, **overrides: Unpack[_ModelRequestOverrides]) -> ModelRequest:
         """Replace the request with a new request with the given overrides.
 
@@ -446,7 +471,14 @@ def wrap_tool_call(
 
                 ```python
                 def wrap_tool_call(self, request, handler):
-                    request.tool_call["args"]["value"] *= 2
+                    modified_call = {
+                        **request.tool_call,
+                        "args": {
+                            **request.tool_call["args"],
+                            "value": request.tool_call["args"]["value"] * 2,
+                        },
+                    }
+                    request = request.override(tool_call=modified_call)
                     return handler(request)
                 ```
 
@@ -1337,7 +1369,7 @@ async def async_wrapped(
                 handler: Callable[[ModelRequest], Awaitable[ModelResponse]],
             ) -> ModelCallResult:
                 prompt = await func(request)  # type: ignore[misc]
-                request.system_prompt = prompt
+                request = request.override(system_prompt=prompt)
                 return await handler(request)
 
             middleware_name = cast("str", getattr(func, "__name__", "DynamicPromptMiddleware"))
@@ -1358,7 +1390,7 @@ def wrapped(
             handler: Callable[[ModelRequest], ModelResponse],
         ) -> ModelCallResult:
             prompt = cast("str", func(request))
-            request.system_prompt = prompt
+            request = request.override(system_prompt=prompt)
             return handler(request)
 
         async def async_wrapped_from_sync(
@@ -1368,7 +1400,7 @@ async def async_wrapped_from_sync(
         ) -> ModelCallResult:
             # Delegate to sync function
             prompt = cast("str", func(request))
-            request.system_prompt = prompt
+            request = request.override(system_prompt=prompt)
             return await handler(request)
 
         middleware_name = cast("str", getattr(func, "__name__", "DynamicPromptMiddleware"))
@@ -1469,7 +1501,7 @@ def fallback_model(request, handler):
                     pass
 
                 # Try fallback model
-                request.model = fallback_model_instance
+                request = request.override(model=fallback_model_instance)
                 return handler(request)
             ```
 
@@ -1632,7 +1664,14 @@ async def async_retry(request, handler):
             ```python
             @wrap_tool_call
             def modify_args(request, handler):
-                request.tool_call["args"]["value"] *= 2
+                modified_call = {
+                    **request.tool_call,
+                    "args": {
+                        **request.tool_call["args"],
+                        "value": request.tool_call["args"]["value"] * 2,
+                    },
+                }
+                request = request.override(tool_call=modified_call)
                 return handler(request)
             ```
 

libs/langchain_v1/tests/unit_tests/agents/middleware/core/test_composition.py

@@ -230,9 +230,7 @@ def inner(request, handler):
         test_runtime = {"test": "runtime"}
 
         # Create request with state and runtime
-        test_request = create_test_request()
-        test_request.state = test_state
-        test_request.runtime = test_runtime
+        test_request = create_test_request(state=test_state, runtime=test_runtime)
         result = composed(test_request, create_mock_base_handler())
 
         # Both handlers should see same state and runtime

libs/langchain_v1/tests/unit_tests/agents/middleware/core/test_decorators.py

@@ -90,8 +90,7 @@ def test_on_model_call_decorator() -> None:
 
     @wrap_model_call(state_schema=CustomState, tools=[test_tool], name="CustomOnModelCall")
     def custom_on_model_call(request, handler):
-        request.system_prompt = "Modified"
-        return handler(request)
+        return handler(request.override(system_prompt="Modified"))
 
     # Verify all options were applied
     assert isinstance(custom_on_model_call, AgentMiddleware)
@@ -277,8 +276,7 @@ def test_async_on_model_call_decorator() -> None:
 
     @wrap_model_call(state_schema=CustomState, tools=[test_tool], name="AsyncOnModelCall")
     async def async_on_model_call(request, handler):
-        request.system_prompt = "Modified async"
-        return await handler(request)
+        return await handler(request.override(system_prompt="Modified async"))
 
     assert isinstance(async_on_model_call, AgentMiddleware)
     assert async_on_model_call.state_schema == CustomState

libs/langchain_v1/tests/unit_tests/agents/middleware/core/test_tools.py

@@ -79,8 +79,8 @@ def wrap_model_call(
             handler: Callable[[ModelRequest], AIMessage],
         ) -> AIMessage:
             # Only allow tool_a and tool_b
-            request.tools = [t for t in request.tools if t.name in ["tool_a", "tool_b"]]
-            return handler(request)
+            filtered_tools = [t for t in request.tools if t.name in ["tool_a", "tool_b"]]
+            return handler(request.override(tools=filtered_tools))
 
     # Model will try to call tool_a
     model = FakeToolCallingModel(
@@ -123,8 +123,7 @@ def wrap_model_call(
             handler: Callable[[ModelRequest], AIMessage],
         ) -> AIMessage:
             # Add an unknown tool
-            request.tools = request.tools + [unknown_tool]
-            return handler(request)
+            return handler(request.override(tools=request.tools + [unknown_tool]))
 
     agent = create_agent(
         model=FakeToolCallingModel(),
@@ -163,7 +162,8 @@ def wrap_model_call(
         ) -> AIMessage:
             # Remove admin_tool if not admin
             if not request.state.get("is_admin", False):
-                request.tools = [t for t in request.tools if t.name != "admin_tool"]
+                filtered_tools = [t for t in request.tools if t.name != "admin_tool"]
+                request = request.override(tools=filtered_tools)
             return handler(request)
 
     model = FakeToolCallingModel()
@@ -200,7 +200,7 @@ def wrap_model_call(
             handler: Callable[[ModelRequest], AIMessage],
         ) -> AIMessage:
             # Remove all tools
-            request.tools = []
+            request = request.override(tools=[])
             return handler(request)
 
     model = FakeToolCallingModel()
@@ -244,7 +244,8 @@ def wrap_model_call(
         ) -> AIMessage:
             modification_order.append([t.name for t in request.tools])
             # Remove tool_c
-            request.tools = [t for t in request.tools if t.name != "tool_c"]
+            filtered_tools = [t for t in request.tools if t.name != "tool_c"]
+            request = request.override(tools=filtered_tools)
             return handler(request)
 
     class SecondMiddleware(AgentMiddleware):
@@ -257,7 +258,8 @@ def wrap_model_call(
             # Should not see tool_c here
             assert all(t.name != "tool_c" for t in request.tools)
             # Remove tool_b
-            request.tools = [t for t in request.tools if t.name != "tool_b"]
+            filtered_tools = [t for t in request.tools if t.name != "tool_b"]
+            request = request.override(tools=filtered_tools)
             return handler(request)
 
     agent = create_agent(