diff --git a/tests/llm/test_model_config.py b/tests/llm/test_model_config.py
index 0f0664ff..6eb1b5c9 100644
--- a/tests/llm/test_model_config.py
+++ b/tests/llm/test_model_config.py
@@ -515,3 +515,34 @@ def test_dialectic_level_transport_override_drops_default_thinking_params(
     assert minimal_mc["model"] == "gpt-4.1-mini"
     assert "thinking_budget_tokens" not in minimal_mc
     assert "thinking_effort" not in minimal_mc
+
+
+def test_dialectic_settings_backfills_missing_levels() -> None:
+    """Operators only need to override the levels they care about.
+
+    Env-var overrides replace the LEVELS dict wholesale (bypassing the
+    default_factory), so without a backfill the unmentioned levels would be
+    dropped and _validate_all_levels_present would fail.
+    """
+    from src.config import DialecticSettings
+
+    settings = DialecticSettings(
+        LEVELS={  # pyright: ignore[reportArgumentType]
+            "low": {
+                "MODEL_CONFIG": {
+                    "transport": "anthropic",
+                    "model": "claude-haiku-4-5-20251001",
+                    "thinking_budget_tokens": 1024,
+                },
+                "MAX_OUTPUT_TOKENS": 2500,
+            }
+        }
+    )
+
+    assert set(settings.LEVELS.keys()) == {"minimal", "low", "medium", "high", "max"}
+    assert settings.LEVELS["low"].MODEL_CONFIG.transport == "anthropic"
+    assert settings.LEVELS["low"].MODEL_CONFIG.model == "claude-haiku-4-5-20251001"
+    assert settings.LEVELS["low"].MAX_OUTPUT_TOKENS == 2500
+    # Backfilled levels come from _default_dialectic_levels()
+    assert settings.LEVELS["minimal"].MAX_TOOL_ITERATIONS == 1
+    assert settings.LEVELS["max"].MAX_TOOL_ITERATIONS == 10