diff --git a/src/dolphin/workflows/config/_yaml_model.py b/src/dolphin/workflows/config/_yaml_model.py
index 2bd44527..55658632 100644
--- a/src/dolphin/workflows/config/_yaml_model.py
+++ b/src/dolphin/workflows/config/_yaml_model.py
@@ -171,15 +171,30 @@ def _add_comments(
                 subsequent_indent=" " * indent_per_level,
             )
         )
-        if "anyOf" in val:
+        if "anyOf" in val or "oneOf" in val:
             #   'anyOf': [{'type': 'string'}, {'type': 'null'}],
-            # Join the options with a pipe, like Python types
-            type_str = " | ".join(d["type"] for d in val["anyOf"])
+            # or for a Union of submodels (plain or discriminated):
+            #   'anyOf': [{'$ref': '#/$defs/A'}, {'$ref': '#/$defs/B'}]
+            #   'oneOf': [{'$ref': '#/$defs/A'}, {'$ref': '#/$defs/B'}]
+            # `oneOf` shows up when the field uses
+            # `Annotated[Union[...], Field(discriminator=...)]`. Join the
+            # options with a pipe, like Python types; fall back to the
+            # sub-model name for `$ref` entries that have no primitive
+            # `type` key.
+            def _union_label(d: dict) -> str:
+                if "type" in d:
+                    return d["type"]
+                if "$ref" in d:
+                    return d["$ref"].rsplit("/", 1)[-1]
+                return "object"
+
+            entries = val.get("anyOf") or val.get("oneOf") or []
+            type_str = " | ".join(_union_label(d) for d in entries)
             type_str.replace("null", "None")
         elif "const" in val:
             type_str = val["const"]
         else:
-            type_str = val["type"]
+            type_str = val.get("type", "object")
         type_line = f"\n  Type: {type_str}."
         choices = f"\n  Options: {val['enum']}." if "enum" in val else ""