fix(langchain): frame custom HITL rejection reasons (#39773)

Human-provided HITL rejection reasons now retain enough context for the
model to understand that the tool was rejected rather than executed.

Previously, a custom `RejectDecision.message` replaced all rejection
framing, while the bare-rejection fallback instructed the model not to
retry. `HumanInTheLoopMiddleware` now uses minimal user-rejection
context in both cases without prescribing future model behavior.

## Release note
Human-in-the-loop rejection results now identify the user rejection
without instructing the model whether to retry.

_Developed with AI-agent assistance._

Made by [Open
SWE](https://openswe.vercel.app/agents/3df8f29d-d7d4-5296-9369-26755096058d)

---------

Co-authored-by: Harrison Chase <11986836+hwchase17@users.noreply.github.com>
Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

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

@@ -101,10 +101,11 @@ class RejectDecision(TypedDict):
     """The type of response when a human rejects the action."""
 
     message: NotRequired[str]
-    """The message sent to the model explaining why the action was rejected.
+    """The human-provided reason for rejecting the action.
 
-    If omitted, the model is told that the tool was not executed and should not
-    retry the same tool call unless the user asks for it.
+    The reason is framed as a user rejection when sent to the model. If omitted,
+    the model is told that the tool was not executed and should not retry the same
+    tool call unless the user asks for it.
     """
 
 
@@ -334,10 +335,15 @@ class HumanInTheLoopMiddleware(AgentMiddleware[StateT, ContextT, ResponseT]):
                 None,
             )
         if decision["type"] == "reject" and "reject" in allowed_decisions:
-            content = decision.get("message") or (
-                f"User rejected the tool call for `{tool_call['name']}` with id {tool_call['id']}. "
-                "The tool was not executed. Do not retry this tool call unless the user "
-                "explicitly requests it."
+            reason = decision.get("message")
+            content = (
+                f"User rejected the tool call for `{tool_call['name']}` with reason: {reason}"
+                if reason
+                else (
+                    f"User rejected the tool call for `{tool_call['name']}` with id "
+                    f"{tool_call['id']}. The tool was not executed. Do not retry this tool "
+                    "call unless the user explicitly requests it."
+                )
             )
             tool_message = ToolMessage(
                 content=content,

libs/langchain_v1/tests/unit_tests/agents/middleware/implementations/test_human_in_the_loop.py

@@ -149,8 +149,8 @@ def test_human_in_the_loop_middleware_single_tool_edit() -> None:
         assert result["messages"][0].tool_calls[0]["id"] == "1"  # ID should be preserved
 
 
-def test_human_in_the_loop_middleware_single_tool_response() -> None:
-    """Test HumanInTheLoopMiddleware with single tool response with custom message."""
+def test_human_in_the_loop_middleware_single_tool_rejection_reason() -> None:
+    """Test a custom rejection reason retains its human-provided context."""
     middleware = HumanInTheLoopMiddleware(
         interrupt_on={"test_tool": {"allowed_decisions": ["approve", "edit", "reject"]}}
     )
@@ -173,7 +173,9 @@ def test_human_in_the_loop_middleware_single_tool_response() -> None:
         assert len(result["messages"]) == 2
         assert isinstance(result["messages"][0], AIMessage)
         assert isinstance(result["messages"][1], ToolMessage)
-        assert result["messages"][1].content == "Custom response message"
+        assert result["messages"][1].content == (
+            "User rejected the tool call for `test_tool` with reason: Custom response message"
+        )
         assert result["messages"][1].name == "test_tool"
         assert result["messages"][1].tool_call_id == "1"
 
@@ -458,7 +460,10 @@ def test_human_in_the_loop_middleware_multiple_tools_mixed_responses() -> None:
         # Second message should be the tool message for the rejected tool call
         tool_message = result["messages"][1]
         assert isinstance(tool_message, ToolMessage)
-        assert tool_message.content == "User rejected this tool call"
+        assert tool_message.content == (
+            "User rejected the tool call for `get_temperature` with reason: "
+            "User rejected this tool call"
+        )
         assert tool_message.name == "get_temperature"
 
 
@@ -1014,7 +1019,9 @@ def test_human_in_the_loop_middleware_preserves_order_with_rejections() -> None:
         # Check rejection tool message
         tool_message = result["messages"][1]
         assert isinstance(tool_message, ToolMessage)
-        assert tool_message.content == "Rejected tool B"
+        assert tool_message.content == (
+            "User rejected the tool call for `tool_b` with reason: Rejected tool B"
+        )
         assert tool_message.tool_call_id == "id_b"