diff --git a/core/wren/src/wren/genbi/composer.py b/core/wren/src/wren/genbi/composer.py
index 726b8d5860..3fdfbaabab 100644
--- a/core/wren/src/wren/genbi/composer.py
+++ b/core/wren/src/wren/genbi/composer.py
@@ -57,7 +57,18 @@ def _format_model_inventory(models: list[dict]) -> str:
         return "- (no models found — run `wren context build` first)"
     lines = []
     for model in models:
-        cols = ", ".join(c.get("name", "?") for c in model.get("columns", []))
+        # `columns` is YAML-sourced; only the list container/type is untrusted here.
+        # Models themselves come from load_models (already dict-filtered).
+        raw_cols = model.get("columns", [])
+        if not isinstance(raw_cols, list):
+            raw_cols = []
+        names = []
+        for c in raw_cols:
+            if isinstance(c, dict):
+                names.append(str(c.get("name", "?")))
+            elif isinstance(c, str) and c:
+                names.append(c)
+        cols = ", ".join(names)
         lines.append(f"- **{model.get('name', '?')}**: {cols}")
     return "\n".join(lines)
 
diff --git a/core/wren/tests/unit/test_genbi_build.py b/core/wren/tests/unit/test_genbi_build.py
index 6713cca315..c8b1e34bea 100644
--- a/core/wren/tests/unit/test_genbi_build.py
+++ b/core/wren/tests/unit/test_genbi_build.py
@@ -94,6 +94,29 @@ def test_build_includes_model_inventory(tmp_path: Path) -> None:
     assert "duckdb" in result.output
 
 
+def test_build_inventory_tolerates_non_list_columns(tmp_path: Path) -> None:
+    """YAML may set columns: to a string; inventory formatting must not crash."""
+    from wren.genbi.composer import compose_build_instruction
+
+    models = [
+        {"name": "orders", "columns": "id, customer_id"},
+        {"name": "customers", "columns": [{"name": "id"}, "bare", 3]},
+    ]
+    text = compose_build_instruction(
+        app_name="demo",
+        data_mode="snapshot",
+        user_prompt="hi",
+        mdl_path=tmp_path / "mdl.json",
+        app_dir=tmp_path / "app",
+        models=models,
+        data_source="duckdb",
+    )
+    assert "**orders**" in text
+    assert "**customers**" in text
+    assert "id" in text
+
+
+
 def test_build_includes_wasm_wiring_and_final_steps(tmp_path: Path) -> None:
     project = _make_project(tmp_path)