diff --git a/pr_split/config.py b/pr_split/config.py
index 9c4abca..b803fa8 100644
--- a/pr_split/config.py
+++ b/pr_split/config.py
@@ -59,7 +59,9 @@ def set_default_model(self):
     @model_validator(mode="after")
     def auto_derive_min_loc(self):
         if self.min_loc is None and self.max_refinement_iterations > 0:
-            self.min_loc = self.max_loc // DEFAULT_MIN_LOC_RATIO
+            # Floor at 1 so a tiny max_loc cannot derive a min_loc that the
+            # explicit field constraint (ge=1) would have rejected.
+            self.min_loc = max(1, self.max_loc // DEFAULT_MIN_LOC_RATIO)
         return self
 
     @model_validator(mode="after")
diff --git a/tests/test_config.py b/tests/test_config.py
index 06d3ff9..1ea04f0 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -182,3 +182,24 @@ def test_empty_string_key_raises_openai(self, monkeypatch: pytest.MonkeyPatch) -
         monkeypatch.setenv("OPENAI_API_KEY", "")
         with pytest.raises(ValidationError, match="OPENAI_API_KEY"):
             Settings(provider=Provider.OPENAI)
+
+
+class TestDerivedMinLocFloor:
+    @pytest.mark.parametrize("max_loc", [2, 3])
+    def test_tiny_max_loc_derives_min_loc_of_one(
+        self, monkeypatch: pytest.MonkeyPatch, max_loc: int
+    ) -> None:
+        monkeypatch.setenv("ANTHROPIC_API_KEY", "sk-test")
+        settings = Settings(max_loc=max_loc, max_refinement_iterations=1)
+        assert settings.min_loc == 1
+
+    def test_max_loc_of_one_with_refinement_is_rejected(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.setenv("ANTHROPIC_API_KEY", "sk-test")
+        with pytest.raises(ValueError, match="min_loc 1 must be less than max_loc 1"):
+            Settings(max_loc=1, max_refinement_iterations=1)
+
+    def test_normal_max_loc_uses_ratio(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        monkeypatch.setenv("ANTHROPIC_API_KEY", "sk-test")
+        assert Settings(max_loc=400, max_refinement_iterations=1).min_loc == 100