diff --git a/core/wren/src/wren/memory/schema_indexer.py b/core/wren/src/wren/memory/schema_indexer.py
index 90728b6bdb..126be84a80 100644
--- a/core/wren/src/wren/memory/schema_indexer.py
+++ b/core/wren/src/wren/memory/schema_indexer.py
@@ -162,9 +162,26 @@ def _describe_column(col: dict, lines: list[str]) -> None:
     lines.append("".join(parts))
 
 
+def _relationship_models(rel: dict, name: str) -> list:
+    """Return relationship endpoint models; raise on wrong-typed ``models``.
+
+    Missing/null ``models`` is empty. A present non-list is structural error
+    (same policy as :func:`_iter_section` for top-level sections).
+    """
+    models = rel.get("models")
+    if models is None:
+        return []
+    if not isinstance(models, list):
+        raise ValueError(
+            f"relationship {name!r}: 'models' must be a list, "
+            f"got {type(models).__name__}"
+        )
+    return models
+
+
 def _describe_relationship(rel: dict, lines: list[str]) -> None:
     name = rel["name"]
-    models = rel.get("models") or []
+    models = _relationship_models(rel, name)
     left = models[0] if len(models) > 0 else "?"
     right = models[1] if len(models) > 1 else "?"
     join_type = rel.get("joinType", "")
@@ -374,7 +391,7 @@ def _column_record(col: dict, model_name: str, mdl_h: str, now: datetime) -> dic
 
 def _relationship_record(rel: dict, mdl_h: str, now: datetime) -> dict:
     name = rel["name"]
-    models = rel.get("models") or []
+    models = _relationship_models(rel, name)
     join_type = rel.get("joinType", "")
     condition = rel.get("condition", "")
 
diff --git a/core/wren/tests/unit/test_schema_indexer_rel_models_non_list.py b/core/wren/tests/unit/test_schema_indexer_rel_models_non_list.py
new file mode 100644
index 0000000000..bafd2d2b2c
--- /dev/null
+++ b/core/wren/tests/unit/test_schema_indexer_rel_models_non_list.py
@@ -0,0 +1,75 @@
+"""relationship models must be a list — wrong types raise ValueError (#2590)."""
+
+from __future__ import annotations
+
+import pytest
+
+from wren.memory.schema_indexer import describe_schema, extract_schema_items
+
+
+@pytest.mark.parametrize(
+    ("bad", "got"), [({"a": 1}, "dict"), ("orders", "str"), (5, "int")]
+)
+def test_describe_schema_non_list_relationship_models_raises(bad, got):
+    with pytest.raises(
+        ValueError, match=rf"relationship 'r': 'models' must be a list, got {got}"
+    ):
+        describe_schema({"relationships": [{"name": "r", "models": bad}]})
+
+
+@pytest.mark.parametrize(
+    ("bad", "got"), [({"a": 1}, "dict"), ("orders", "str"), (5, "int")]
+)
+def test_extract_schema_items_non_list_relationship_models_raises(bad, got):
+    with pytest.raises(
+        ValueError, match=rf"relationship 'r': 'models' must be a list, got {got}"
+    ):
+        extract_schema_items({"relationships": [{"name": "r", "models": bad}]})
+
+
+def test_string_models_do_not_emit_truncated_endpoints():
+    """Regression: str models used to index first two characters as endpoints."""
+    m = {
+        "relationships": [
+            {
+                "name": "orders_customers",
+                "models": "orders",
+                "joinType": "MANY_TO_ONE",
+                "condition": "o.cid = c.id",
+            }
+        ]
+    }
+    with pytest.raises(ValueError):
+        describe_schema(m)
+    with pytest.raises(ValueError):
+        extract_schema_items(m)
+
+
+def test_missing_models_still_ok():
+    text = describe_schema({"relationships": [{"name": "r1"}]})
+    assert "r1" in text
+    items = extract_schema_items({"relationships": [{"name": "r1"}]})
+    assert any(i.get("item_name") == "r1" for i in items)
+
+
+def test_explicit_none_models_still_ok():
+    """`models:` with no value parses to None, the shape users hit by accident."""
+    text = describe_schema({"relationships": [{"name": "r1", "models": None}]})
+    assert "r1" in text
+    items = extract_schema_items({"relationships": [{"name": "r1", "models": None}]})
+    assert any(i.get("item_name") == "r1" for i in items)
+
+
+def test_valid_list_models_unchanged():
+    text = describe_schema(
+        {
+            "relationships": [
+                {
+                    "name": "r1",
+                    "models": ["orders", "customers"],
+                    "joinType": "MANY_TO_ONE",
+                }
+            ]
+        }
+    )
+    assert "orders → customers" in text