fix(langchain): ensure HITL middleware edits persist correctly

Fix issues #33787 and #33784 where Human-in-the-Loop middleware edits
were not persisting correctly in the agent's message history.

The problem occurred because the middleware was directly mutating the
AIMessage.tool_calls attribute, but LangGraph's state management doesn't
properly persist direct object mutations. This caused the agent to see
the original (unedited) tool calls in subsequent model invocations,
leading to duplicate or incorrect tool executions.

Changes:
- Create new AIMessage instance instead of mutating the original
- Ensure message has an ID (generate UUID if needed) so add_messages
  reducer properly replaces instead of appending
- Add comprehensive test case that reproduces and verifies the fix

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: zhangzhefang-github

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

@@ -345,7 +345,25 @@ def after_model(self, state: AgentState, runtime: Runtime) -> dict[str, Any] | N
             if tool_message:
                 artificial_tool_messages.append(tool_message)
 
-        # Update the AI message to only include approved tool calls
-        last_ai_msg.tool_calls = revised_tool_calls
+        # Create a new AIMessage with updated tool calls instead of mutating the original
+        # This ensures LangGraph's state management properly persists the changes
+        # (fixes Issues #33787 and #33784 where edits weren't persisted correctly)
+        #
+        # CRITICAL: We must ensure last_ai_msg has an ID for the add_messages reducer
+        # to properly replace it (not append a duplicate). If no ID exists, generate one.
+        if last_ai_msg.id is None:
+            import uuid
+
+            last_ai_msg.id = str(uuid.uuid4())
+
+        updated_ai_msg = AIMessage(
+            content=last_ai_msg.content,
+            tool_calls=revised_tool_calls,
+            id=last_ai_msg.id,  # Same ID ensures replacement, not appending
+            name=last_ai_msg.name,
+            additional_kwargs=last_ai_msg.additional_kwargs,
+            response_metadata=last_ai_msg.response_metadata,
+            usage_metadata=last_ai_msg.usage_metadata,
+        )
 
-        return {"messages": [last_ai_msg, *artificial_tool_messages]}
+        return {"messages": [updated_ai_msg, *artificial_tool_messages]}

libs/langchain_v1/tests/unit_tests/agents/test_middleware_agent.py

@@ -1024,6 +1024,161 @@ def mock_capture_requests(request):
         assert captured_request["action_requests"][1]["description"] == "Static description"
 
 
+def test_human_in_the_loop_middleware_edit_actually_executes_with_edited_args(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Test that HITL edit decision properly replaces original tool call (Issues #33787, #33784).
+
+    This test reproduces the bug where after editing a tool call:
+    1. The edited tool executes correctly
+    2. BUT the agent's next model call sees the ORIGINAL (unedited) tool_calls in the AIMessage
+    3. This causes the agent to re-attempt the original tool call
+
+    The bug happens because HumanInTheLoopMiddleware directly modifies the AIMessage
+    object's tool_calls attribute, but LangGraph's state management may not properly
+    persist this mutation, causing subsequent reads to see the original values.
+    """
+    # Track what arguments tools were actually called with
+    send_email_calls = []
+
+    @tool
+    def send_email_tool(to: str, subject: str, body: str) -> str:
+        """Send an email to a recipient.
+
+        Args:
+            to: Email address of the recipient
+            subject: Subject line of the email
+            body: Body content of the email
+        """
+        send_email_calls.append({"to": to, "subject": subject, "body": body})
+        return f"Email sent successfully to {to} with subject: {subject}"
+
+    # Create agent with HITL middleware
+    # Simulate the exact scenario from issue #33787:
+    # 1. First model call: agent wants to send email to [email protected]
+    # 2. After edit (to [email protected]), the model should see the edited params
+    #    and not re-attempt the original call
+    agent = create_agent(
+        model=FakeToolCallingModel(
+            tool_calls=[
+                # First call: agent proposes sending to [email protected]
+                [
+                    {
+                        "args": {"to": "[email protected]", "subject": "Test", "body": "Hello"},
+                        "id": "call_001",
+                        "name": "send_email_tool",
+                    }
+                ],
+                # Second call (after edited tool execution): Agent should see the EDITED
+                # tool call in message history. If model still had original params in its
+                # context, it might try again. But with the fix, it sees edited params.
+                # For testing, we configure model to not make additional tool calls
+                # (empty list) to verify the agent loop completes successfully.
+                [],  # No more tools - task completed
+            ]
+        ),
+        tools=[send_email_tool],
+        middleware=[
+            HumanInTheLoopMiddleware(
+                interrupt_on={
+                    "send_email_tool": {"allowed_decisions": ["approve", "edit", "reject"]}
+                }
+            )
+        ],
+        checkpointer=sync_checkpointer,
+    )
+
+    thread = {"configurable": {"thread_id": "test-hitl-bug-33787"}}
+
+    # === STEP 1: First invocation - should interrupt before sending email ===
+    result = agent.invoke(
+        {
+            "messages": [
+                HumanMessage(
+                    "Send an email to [email protected] with subject 'Test' and body 'Hello'"
+                )
+            ]
+        },
+        thread,
+    )
+
+    # Verify we got an interrupt (email not sent yet)
+    assert len(send_email_calls) == 0, "Email should not have been sent yet"
+    last_ai_msg = result["messages"][-1]
+    assert isinstance(last_ai_msg, AIMessage)
+    assert len(last_ai_msg.tool_calls) == 1
+    assert last_ai_msg.tool_calls[0]["args"]["to"] == "[email protected]"
+
+    # === STEP 2: Resume with edit decision - change recipient to [email protected] ===
+    from langgraph.types import Command
+
+    result = agent.invoke(
+        Command(
+            resume={
+                "decisions": [
+                    {
+                        "type": "edit",
+                        "message": "Changing recipient to test address",
+                        "edited_action": Action(
+                            name="send_email_tool",
+                            args={
+                                "to": "[email protected]",
+                                "subject": "this is a test",
+                                "body": "don't reply",
+                            },
+                        ),
+                    }
+                ]
+            }
+        ),
+        thread,
+    )
+
+    # CRITICAL ASSERTION 1: Email should be sent to EDITED address ([email protected])
+    assert len(send_email_calls) == 1, "Exactly one email should have been sent"
+    assert send_email_calls[0]["to"] == "[email protected]", (
+        f"Email should be sent to edited address '[email protected]', got '{send_email_calls[0]['to']}'"
+    )
+    assert send_email_calls[0]["subject"] == "this is a test"
+
+    # Verify the tool message reflects the edited execution
+    tool_messages = [m for m in result["messages"] if isinstance(m, ToolMessage)]
+    assert len(tool_messages) >= 1
+    assert "[email protected]" in tool_messages[-1].content
+
+    # CRITICAL ASSERTION 2: Verify the AIMessage in history was properly updated
+    # This is the core fix - the AIMessage with original params should be replaced
+    # with the edited version so that subsequent model calls see the correct context
+    ai_msg_with_original_id = None
+    for msg in result["messages"]:
+        if isinstance(msg, AIMessage) and getattr(msg, "id", None) == "0":
+            ai_msg_with_original_id = msg
+            break
+
+    assert ai_msg_with_original_id is not None, "Should find the AIMessage that was edited"
+    assert len(ai_msg_with_original_id.tool_calls) == 1
+
+    # THE KEY ASSERTION: The AIMessage stored in state should have EDITED params
+    # Without the fix, this would still have [email protected] due to mutation issues
+    assert ai_msg_with_original_id.tool_calls[0]["args"]["to"] == "[email protected]", (
+        f"BUG DETECTED (Issue #33787): AIMessage in state still has original params "
+        f"'{ai_msg_with_original_id.tool_calls[0]['args']['to']}' instead of edited '[email protected]'. "
+        f"The middleware should have created a NEW AIMessage with edited params to replace the original."
+    )
+
+    # CRITICAL ASSERTION 3: Agent should not have re-executed original tool call
+    # Only the EDITED call should have been executed
+    assert len(send_email_calls) == 1, (
+        f"Expected exactly 1 email (the edited one), but {len(send_email_calls)} were sent. "
+        f"This suggests the agent re-attempted the original tool call."
+    )
+
+    # Final verification: the execution log confirms only edited params were used
+    assert send_email_calls[0]["to"] == "[email protected]"
+    assert send_email_calls[0]["subject"] == "this is a test"
+    assert send_email_calls[0]["body"] == "don't reply"
+
+
 # Tests for SummarizationMiddleware
 def test_summarization_middleware_initialization() -> None:
     """Test SummarizationMiddleware initialization."""