fix(middleware): fix DanglingToolCallMiddleware inserting patches at wrong position (#904)

Previously used before_model which returned {"messages": patches}, causing
LangGraph's add_messages reducer to append patches at the end of the message
list. This resulted in invalid ordering (ToolMessage after a HumanMessage)
that LLMs reject with tool call ID mismatch errors.

Switch to wrap_model_call/awrap_model_call to insert synthetic ToolMessages
immediately after each dangling AIMessage before the request reaches the LLM,
without persisting the patches to state.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/agents/middlewares/dangling_tool_call_middleware.py

@@ -4,17 +4,23 @@ A dangling tool call occurs when an AIMessage contains tool_calls but there are
 no corresponding ToolMessages in the history (e.g., due to user interruption or
 request cancellation). This causes LLM errors due to incomplete message format.
 
-This middleware runs before the model call to detect and patch such gaps by
+This middleware intercepts the model call to detect and patch such gaps by
-inserting synthetic ToolMessages with an error indicator.
+inserting synthetic ToolMessages with an error indicator immediately after the
+AIMessage that made the tool calls, ensuring correct message ordering.
+
+Note: Uses wrap_model_call instead of before_model to ensure patches are inserted
+at the correct positions (immediately after each dangling AIMessage), not appended
+to the end of the message list as before_model + add_messages reducer would do.
 """
 
 import logging
+from collections.abc import Awaitable, Callable
 from typing import override
 
 from langchain.agents import AgentState
 from langchain.agents.middleware import AgentMiddleware
+from langchain.agents.middleware.types import ModelCallResult, ModelRequest, ModelResponse
 from langchain_core.messages import ToolMessage
-from langgraph.runtime import Runtime
 
 logger = logging.getLogger(__name__)
 
@@ -23,33 +29,51 @@ class DanglingToolCallMiddleware(AgentMiddleware[AgentState]):
     """Inserts placeholder ToolMessages for dangling tool calls before model invocation.
 
     Scans the message history for AIMessages whose tool_calls lack corresponding
-    ToolMessages, and injects synthetic error responses so the LLM receives a
+    ToolMessages, and injects synthetic error responses immediately after the
-    well-formed conversation.
+    offending AIMessage so the LLM receives a well-formed conversation.
     """
 
-    def _fix_dangling_tool_calls(self, state: AgentState) -> dict | None:
+    def _build_patched_messages(self, messages: list) -> list | None:
-        messages = state.get("messages", [])
+        """Return a new message list with patches inserted at the correct positions.
-        if not messages:
-            return None
 
+        For each AIMessage with dangling tool_calls (no corresponding ToolMessage),
+        a synthetic ToolMessage is inserted immediately after that AIMessage.
+        Returns None if no patches are needed.
+        """
         # Collect IDs of all existing ToolMessages
         existing_tool_msg_ids: set[str] = set()
         for msg in messages:
             if isinstance(msg, ToolMessage):
                 existing_tool_msg_ids.add(msg.tool_call_id)
 
-        # Find dangling tool calls and build patch messages
+        # Check if any patching is needed
-        patches: list[ToolMessage] = []
+        needs_patch = False
         for msg in messages:
             if getattr(msg, "type", None) != "ai":
                 continue
-            tool_calls = getattr(msg, "tool_calls", None)
+            for tc in getattr(msg, "tool_calls", None) or []:
-            if not tool_calls:
-                continue
-            for tc in tool_calls:
                 tc_id = tc.get("id")
                 if tc_id and tc_id not in existing_tool_msg_ids:
-                    patches.append(
+                    needs_patch = True
+                    break
+            if needs_patch:
+                break
+
+        if not needs_patch:
+            return None
+
+        # Build new list with patches inserted right after each dangling AIMessage
+        patched: list = []
+        patched_ids: set[str] = set()
+        patch_count = 0
+        for msg in messages:
+            patched.append(msg)
+            if getattr(msg, "type", None) != "ai":
+                continue
+            for tc in getattr(msg, "tool_calls", None) or []:
+                tc_id = tc.get("id")
+                if tc_id and tc_id not in existing_tool_msg_ids and tc_id not in patched_ids:
+                    patched.append(
                         ToolMessage(
                             content="[Tool call was interrupted and did not return a result.]",
                             tool_call_id=tc_id,
@@ -57,18 +81,30 @@ class DanglingToolCallMiddleware(AgentMiddleware[AgentState]):
                             status="error",
                         )
                     )
-                    existing_tool_msg_ids.add(tc_id)
+                    patched_ids.add(tc_id)
+                    patch_count += 1
 
-        if not patches:
+        logger.warning(f"Injecting {patch_count} placeholder ToolMessage(s) for dangling tool calls")
-            return None
+        return patched
-
-        logger.warning(f"Injecting {len(patches)} placeholder ToolMessage(s) for dangling tool calls")
-        return {"messages": patches}
 
     @override
-    def before_model(self, state: AgentState, runtime: Runtime) -> dict | None:
+    def wrap_model_call(
-        return self._fix_dangling_tool_calls(state)
+        self,
+        request: ModelRequest,
+        handler: Callable[[ModelRequest], ModelResponse],
+    ) -> ModelCallResult:
+        patched = self._build_patched_messages(request.messages)
+        if patched is not None:
+            request = request.override(messages=patched)
+        return handler(request)
 
     @override
-    async def abefore_model(self, state: AgentState, runtime: Runtime) -> dict | None:
+    async def awrap_model_call(
-        return self._fix_dangling_tool_calls(state)
+        self,
+        request: ModelRequest,
+        handler: Callable[[ModelRequest], Awaitable[ModelResponse]],
+    ) -> ModelCallResult:
+        patched = self._build_patched_messages(request.messages)
+        if patched is not None:
+            request = request.override(messages=patched)
+        return await handler(request)