diff --git a/backend/packages/harness/deerflow/config/title_config.py b/backend/packages/harness/deerflow/config/title_config.py
index 5aa750bc78..679183e6ce 100644
--- a/backend/packages/harness/deerflow/config/title_config.py
+++ b/backend/packages/harness/deerflow/config/title_config.py
@@ -1,6 +1,6 @@
 """Configuration for automatic thread title generation."""
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, ValidationInfo, field_validator
 
 
 class TitleConfig(BaseModel):
@@ -31,6 +31,15 @@ class TitleConfig(BaseModel):
         description="Prompt template for LLM title generation when model_name is set",
     )
 
+    @field_validator("max_words", "max_chars", mode="before")
+    @classmethod
+    def _reject_boolean_limits(cls, value: object, info: ValidationInfo) -> object:
+        # YAML `true`/`false` next to neighbouring boolean knobs must not coerce
+        # into integers (Pydantic would turn `true` into 1 before ge/le runs).
+        if isinstance(value, bool):
+            raise ValueError(f"{info.field_name} must be an integer, not a boolean")
+        return value
+
 
 # Global configuration instance
 _title_config: TitleConfig = TitleConfig()
diff --git a/backend/tests/test_title_generation.py b/backend/tests/test_title_generation.py
index 53b0a50102..61ce71eab6 100644
--- a/backend/tests/test_title_generation.py
+++ b/backend/tests/test_title_generation.py
@@ -44,6 +44,19 @@ class TestTitleConfig:
         with pytest.raises(ValueError):
             TitleConfig(max_chars=201)
 
+    @pytest.mark.parametrize("field_name", ["max_words", "max_chars"])
+    @pytest.mark.parametrize("bad_value", [True, False])
+    def test_config_rejects_boolean_limits(self, field_name: str, bad_value: bool):
+        """YAML booleans must fail loudly instead of coercing into integers."""
+        with pytest.raises(ValueError, match="must be an integer, not a boolean"):
+            TitleConfig(**{field_name: bad_value})
+
+    def test_config_still_accepts_numeric_strings(self):
+        """Numeric strings remain valid after the boolean guard."""
+        config = TitleConfig(max_words="8", max_chars="80")
+        assert config.max_words == 8
+        assert config.max_chars == 80
+
     def test_get_set_config(self):
         """Test global config getter and setter."""
         original_config = get_title_config()