diff --git a/src/config.py b/src/config.py
index 1703b5cd..88c04673 100644
--- a/src/config.py
+++ b/src/config.py
@@ -926,6 +926,10 @@ class DialecticSettings(HonchoSettings):
                                     del base_mc[k]
                         level_override[mc_key] = {**base_mc, **override_mc}
                 levels_raw[level_name] = {**base, **level_override}
+        # Backfill any reasoning levels the operator didn't explicitly set with the default values.
+        for default_level_name, default_level in defaults.items():
+            if default_level_name not in levels_raw:
+                levels_raw[default_level_name] = default_level.model_dump(by_alias=True)
         return data  # pyright: ignore[reportUnknownVariableType]
 
     @model_validator(mode="after")
diff --git a/tests/llm/test_model_config.py b/tests/llm/test_model_config.py
index 0f0664ff..4ed1ca06 100644
--- a/tests/llm/test_model_config.py
+++ b/tests/llm/test_model_config.py
@@ -515,3 +515,50 @@ 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(
+    monkeypatch: pytest.MonkeyPatch,
+) -> 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,
+        _default_dialectic_levels,  # pyright: ignore[reportPrivateUsage]
+    )
+
+    for key in list(os.environ):
+        if key.startswith("DIALECTIC_LEVELS"):
+            monkeypatch.delenv(key)
+
+    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()
+    defaults = _default_dialectic_levels()
+    assert (
+        settings.LEVELS["minimal"].MAX_TOOL_ITERATIONS
+        == defaults["minimal"].MAX_TOOL_ITERATIONS
+    )
+    assert (
+        settings.LEVELS["max"].MAX_TOOL_ITERATIONS
+        == defaults["max"].MAX_TOOL_ITERATIONS
+    )