fix(agent): reserve ellipsis room so the local title respects max_chars (#4052)

`_fallback_title` sliced the user message to `min(max_chars, 50)` and then
appended a three-character ellipsis, so the returned title could be three
characters longer than the configured cap. `_parse_title`, six lines above,
slices the model's answer to `max_chars` exactly -- both read the same
`TitleConfig.max_chars`, only one honoured it.

This is the default path, not an error branch: `config.example.yaml` ships
`title.model_name: null` ("null = fast local fallback"), so every title is
produced here unless the operator opts into a title model. `max_chars` is a
documented key with a pydantic range of 10..200; any value in 10..52 makes a
long first message overshoot its cap.

Reserve room for the ellipsis before slicing. At the shipped `max_chars: 60`
the body is still 50 characters, so default output is unchanged.

The existing `test_sync_generate_title_respects_fallback_truncation` asserted
the shape of the truncation but never its length -- at its own `max_chars=50`
it was passing on a 53-character title. It now asserts the bound it is named
after.

backend/packages/harness/deerflow/agents/middlewares/title_middleware.py

@@ -162,7 +162,11 @@ class TitleMiddleware(AgentMiddleware[TitleMiddlewareState]):
         config = self._get_title_config()
         fallback_chars = min(config.max_chars, 50)
         if len(user_msg) > fallback_chars:
-            return user_msg[:fallback_chars].rstrip() + "..."
+            # Reserve room for the ellipsis so this path honours ``max_chars``
+            # exactly as ``_parse_title`` does on the model path.
+            ellipsis = "..."
+            body = min(fallback_chars, config.max_chars - len(ellipsis))
+            return user_msg[:body].rstrip() + ellipsis
         return user_msg if user_msg else "New Conversation"
 
     def _get_runnable_config(self) -> dict[str, Any]:

backend/tests/test_title_middleware_core_logic.py

@@ -4,6 +4,7 @@ import asyncio
 from types import SimpleNamespace
 from unittest.mock import AsyncMock, MagicMock
 
+import pytest
 from langchain_core.messages import AIMessage, HumanMessage
 from langgraph.constants import TAG_NOSTREAM
 
@@ -319,6 +320,51 @@ class TestTitleMiddlewareCoreLogic:
         result = middleware._generate_title_result(state)
         assert result["title"].endswith("...")
         assert result["title"].startswith("这是一个非常长的问题描述")
+        assert len(result["title"]) <= 50
+
+    @pytest.mark.parametrize("max_chars", [10, 20, 40, 49, 50, 52, 53, 60, 200])
+    def test_fallback_title_never_exceeds_max_chars(self, max_chars):
+        """``max_chars`` bounds the fallback title, not just its body.
+
+        The ellipsis is part of the returned title, so a body of exactly
+        ``min(max_chars, 50)`` characters overshot the configured cap by three.
+        ``model_name: null`` is the shipped default, so this is the path every
+        title takes out of the box -- not an error branch.
+        """
+        _set_test_title_config(max_chars=max_chars, model_name=None)
+        middleware = TitleMiddleware()
+
+        title = middleware._fallback_title("x" * 200)
+
+        assert len(title) <= max_chars
+        assert title.endswith("...")
+
+    @pytest.mark.parametrize("max_chars", [10, 20, 50])
+    def test_fallback_title_honours_the_same_cap_as_the_model_path(self, max_chars):
+        """Both title paths read ``config.max_chars``; both must respect it.
+
+        ``_parse_title`` slices the model's answer to ``max_chars`` exactly. The
+        local path is the other half of the same contract.
+        """
+        _set_test_title_config(max_chars=max_chars, model_name=None)
+        middleware = TitleMiddleware()
+        long_text = "x" * 200
+
+        assert len(middleware._parse_title(long_text)) <= max_chars
+        assert len(middleware._fallback_title(long_text)) <= max_chars
+
+    def test_fallback_title_keeps_default_config_output_unchanged(self):
+        """The default ``max_chars=60`` leaves room for the ellipsis already.
+
+        Reserving that room must not shorten titles that were never over the
+        cap, so the shipped configuration keeps emitting a 50-character body.
+        """
+        _set_test_title_config(max_chars=60, model_name=None)
+        middleware = TitleMiddleware()
+
+        title = middleware._fallback_title("x" * 200)
+
+        assert title == "x" * 50 + "..."
 
     def test_parse_title_strips_think_tags(self):
         """Title model responses with <think>...</think> blocks are stripped before use."""