feat(client): support custom middleware injection (#1520)

* feat(client): support custom middleware injection

Add support for custom middleware, allowing custom middleware list to be passed when initializing DeerFlowClient. These middleware will be injected after the default middleware when creating the agent, extending the agent's functionality.

* feat: inject custom middlewares before ClarificationMiddleware to preserve ordering

- Add `custom_middlewares` param to `_build_middlewares`
- Inject custom middlewares right before `ClarificationMiddleware` to keep it as the last in the chain
- Remove unsafe `.extend()` in `client.py`
- Update tests in `test_client.py` and `test_lead_agent_model_resolution.py` to assert correct injection ordering

backend/packages/harness/deerflow/client.py

@@ -22,12 +22,13 @@ import mimetypes
 import shutil
 import tempfile
 import uuid
-from collections.abc import Generator
+from collections.abc import Generator, Sequence
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any
 
 from langchain.agents import create_agent
+from langchain.agents.middleware import AgentMiddleware
 from langchain_core.messages import AIMessage, HumanMessage, SystemMessage, ToolMessage
 from langchain_core.runnables import RunnableConfig
 
@@ -116,6 +117,7 @@ class DeerFlowClient:
         subagent_enabled: bool = False,
         plan_mode: bool = False,
         agent_name: str | None = None,
+        middlewares: Sequence[AgentMiddleware] | None = None,
     ):
         """Initialize the client.
 
@@ -131,6 +133,7 @@ class DeerFlowClient:
             subagent_enabled: Enable subagent delegation.
             plan_mode: Enable TodoList middleware for plan mode.
             agent_name: Name of the agent to use.
+            middlewares: Optional list of custom middlewares to inject into the agent.
         """
         if config_path is not None:
             reload_app_config(config_path)
@@ -145,6 +148,7 @@ class DeerFlowClient:
         self._subagent_enabled = subagent_enabled
         self._plan_mode = plan_mode
         self._agent_name = agent_name
+        self._middlewares = list(middlewares) if middlewares else []
 
         # Lazy agent — created on first call, recreated when config changes.
         self._agent = None
@@ -217,7 +221,7 @@ class DeerFlowClient:
         kwargs: dict[str, Any] = {
             "model": create_chat_model(name=model_name, thinking_enabled=thinking_enabled),
             "tools": self._get_tools(model_name=model_name, subagent_enabled=subagent_enabled),
-            "middleware": _build_middlewares(config, model_name=model_name, agent_name=self._agent_name),
+            "middleware": _build_middlewares(config, model_name=model_name, agent_name=self._agent_name, custom_middlewares=self._middlewares),
             "system_prompt": apply_prompt_template(
                 subagent_enabled=subagent_enabled,
                 max_concurrent_subagents=max_concurrent_subagents,