diff --git a/pyproject.toml b/pyproject.toml
index 01711eed5..bb3803cd3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -86,11 +86,10 @@ dev = [
 test = [
   "inline-snapshot>=0.31.1",
   "msgspec>=0.18",
-  "pytest>=6.1",
   "pytest>=8.3.4",
-  "pytest-cov>=2.12.1",
   "pytest-cov>=5",
   "pytest-mock>=3.14",
+  "pytest-timeout>=2.4",
   "pytest-xdist>=3.3.1",
   "time-machine>=3.1",
   "watchfiles>=1.1",
diff --git a/src/datamodel_code_generator/model/base.py b/src/datamodel_code_generator/model/base.py
index 7110f58bc..75bb109a7 100644
--- a/src/datamodel_code_generator/model/base.py
+++ b/src/datamodel_code_generator/model/base.py
@@ -223,7 +223,7 @@ def _build_union_type_hint(self) -> str | None:
         """Build Union[] type hint from data_type.data_types if forward reference requires it."""
         if not (self._use_union_operator != self.data_type.use_union_operator and self.data_type.is_union):
             return None
-        parts = [dt.type_hint for dt in self.data_type.data_types if dt.type_hint]
+        parts = dict.fromkeys(dt.type_hint for dt in self.data_type.data_types if dt.type_hint).keys()
         if len(parts) > 1:
             return f"Union[{', '.join(parts)}]"
         return None  # pragma: no cover
@@ -232,7 +232,7 @@ def _build_base_union_type_hint(self) -> str | None:  # pragma: no cover
         """Build Union[] base type hint from data_type.data_types if forward reference requires it."""
         if not (self._use_union_operator != self.data_type.use_union_operator and self.data_type.is_union):
             return None
-        parts = [dt.base_type_hint for dt in self.data_type.data_types if dt.base_type_hint]
+        parts = dict.fromkeys(dt.base_type_hint for dt in self.data_type.data_types if dt.base_type_hint).keys()
         if len(parts) > 1:
             return f"Union[{', '.join(parts)}]"
         return None
diff --git a/src/datamodel_code_generator/parser/base.py b/src/datamodel_code_generator/parser/base.py
index ecbad8e72..363f9e060 100644
--- a/src/datamodel_code_generator/parser/base.py
+++ b/src/datamodel_code_generator/parser/base.py
@@ -493,7 +493,7 @@ def add_model_path_to_list(
     return paths
 
 
-def sort_data_models(  # noqa: PLR0912, PLR0915
+def sort_data_models(  # noqa: PLR0912, PLR0914, PLR0915
     unsorted_data_models: list[DataModel],
     sorted_data_models: SortedDataModels | None = None,
     require_update_action_models: list[str] | None = None,
@@ -502,8 +502,10 @@ def sort_data_models(  # noqa: PLR0912, PLR0915
     """Sort data models by dependency order for correct forward references."""
     if sorted_data_models is None:
         sorted_data_models = OrderedDict()
+
     if require_update_action_models is None:
         require_update_action_models = []
+
     sorted_model_count: int = len(sorted_data_models)
 
     unresolved_references: list[DataModel] = []
@@ -521,6 +523,7 @@ def sort_data_models(  # noqa: PLR0912, PLR0915
                 add_model_path_to_list(require_update_action_models, model)
         else:
             unresolved_references.append(model)
+
     if unresolved_references:
         if sorted_model_count != len(sorted_data_models) and recursion_count:
             try:
@@ -534,6 +537,7 @@ def sort_data_models(  # noqa: PLR0912, PLR0915
                 pass
 
         # sort on base_class dependency
+        seen_orderings: set[tuple[str, ...]] = set()
         while True:
             ordered_models: list[tuple[int, DataModel]] = []
             # Build lookup dict for O(1) index access instead of O(n) list.index()
@@ -552,6 +556,7 @@ def sort_data_models(  # noqa: PLR0912, PLR0915
                         for b in model.base_classes
                         if b.reference and b.reference.path in path_to_index
                     ]
+
                 if indexes:
                     ordered_models.append((
                         max(indexes),
@@ -562,9 +567,19 @@ def sort_data_models(  # noqa: PLR0912, PLR0915
                         -1,
                         model,
                     ))
+
             sorted_unresolved_models = [m[1] for m in sorted(ordered_models, key=operator.itemgetter(0))]
             if sorted_unresolved_models == unresolved_references:
                 break
+
+            sig = tuple(m.path for m in sorted_unresolved_models)
+            if sig in seen_orderings:
+                # Base-class dependency order has no fixed point (e.g. cyclic inheritance with
+                # discriminators). Further iterations only permute the list; use stable order.
+                unresolved_references.sort(key=lambda m: m.path)
+                break
+
+            seen_orderings.add(sig)
             unresolved_references = sorted_unresolved_models
 
         # circular reference
@@ -578,16 +593,19 @@ def sort_data_models(  # noqa: PLR0912, PLR0915
                 if update_action_parent:
                     add_model_path_to_list(require_update_action_models, model)
                 continue
+
             if not unresolved_model - unsorted_data_model_names:
                 sorted_data_models[model.path] = model
                 add_model_path_to_list(require_update_action_models, model)
                 continue
+
             # unresolved
             unresolved_classes = ", ".join(
                 f"[class: {item.path} references: {item.reference_classes}]" for item in unresolved_references
             )
             msg = f"A Parser can not resolve classes: {unresolved_classes}."
             raise Exception(msg)  # noqa: TRY002
+
     return unresolved_references, sorted_data_models, require_update_action_models
 
 
@@ -1576,11 +1594,14 @@ def __apply_discriminator_type(  # noqa: PLR0912, PLR0914, PLR0915
                     discriminator_values: list[DiscriminatorValue] = []
 
                     def check_paths(
-                        model: pydantic_model_v2.BaseModel | Reference,
+                        model: pydantic_model_v2.BaseModel | Reference | None,
                         mapping: dict[str, str],
                         discriminator_values: list[DiscriminatorValue] = discriminator_values,
                     ) -> None:
                         """Validate discriminator mapping paths for a model."""
+                        if model is None:
+                            return
+
                         for name, path in mapping.items():
                             if (model.path.split("#/")[-1] != path.split("#/")[-1]) and (
                                 path.startswith("#/") or model.path[:-1] != path.split("/")[-1]
@@ -1624,6 +1645,9 @@ def get_discriminator_field_value(
 
                         if len(discriminator_values) == 0:
                             for base_class in discriminator_model.base_classes:
+                                if not base_class.reference:
+                                    continue
+
                                 check_paths(base_class.reference, mapping)  # ty: ignore
 
                         if not discriminator_values:
diff --git a/tests/data/expected/main/openapi/openapi_discriminated_oneof_allof_cycle.py b/tests/data/expected/main/openapi/openapi_discriminated_oneof_allof_cycle.py
new file mode 100644
index 000000000..f4a2870df
--- /dev/null
+++ b/tests/data/expected/main/openapi/openapi_discriminated_oneof_allof_cycle.py
@@ -0,0 +1,53 @@
+# generated by datamodel-codegen:
+#   filename:  openapi_discriminated_oneof_allof_cycle.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Literal, Union
+
+from pydantic import BaseModel, Field, RootModel
+
+
+class ASchema(BaseModel):
+    pass
+
+
+class BSchema(BaseModel):
+    pass
+
+
+class A1(BaseModel):
+    kind: Literal['a']
+
+
+class B1(BaseModel):
+    kind: Literal['b']
+
+
+class A(RootModel[Union["A2", "A3"]]):
+    root: Union["A2", "A3"]
+
+
+class A2(A1):
+    pass
+
+
+class A3(A1):
+    pass
+
+
+class B(RootModel["B2"]):
+    root: "B2"
+
+
+class B2(B1):
+    pass
+
+
+class X(RootModel[A | B]):
+    root: A | B = Field(..., discriminator='kind')
+
+
+A.model_rebuild()
+B.model_rebuild()
diff --git a/tests/data/openapi/openapi_discriminated_oneof_allof_cycle.json b/tests/data/openapi/openapi_discriminated_oneof_allof_cycle.json
new file mode 100644
index 000000000..aa84a4f15
--- /dev/null
+++ b/tests/data/openapi/openapi_discriminated_oneof_allof_cycle.json
@@ -0,0 +1,74 @@
+{
+  "openapi": "3.1.0",
+  "info": {
+    "title": "",
+    "version": ""
+  },
+  "paths": {},
+  "components": {
+    "schemas": {
+      "ASchema": {
+        "type": "object",
+        "description": "Schema referenced by discriminator mapping."
+      },
+      "BSchema": {
+        "type": "object",
+        "description": "Schema referenced by discriminator mapping."
+      },
+      "X": {
+        "type": "object",
+        "discriminator": {
+          "propertyName": "kind",
+          "mapping": {
+            "a": "#/components/schemas/ASchema",
+            "b": "#/components/schemas/BSchema"
+          }
+        },
+        "oneOf": [
+          {
+            "$ref": "#/components/schemas/A"
+          },
+          {
+            "$ref": "#/components/schemas/B"
+          }
+        ]
+      },
+      "A": {
+        "allOf": [
+          {
+            "$ref": "#/components/schemas/X"
+          },
+          {
+            "type": "object",
+            "required": [
+              "kind"
+            ],
+            "properties": {
+              "kind": {
+                "const": "a"
+              }
+            }
+          }
+        ]
+      },
+      "B": {
+        "allOf": [
+          {
+            "$ref": "#/components/schemas/X"
+          },
+          {
+            "type": "object",
+            "required": [
+              "kind"
+            ],
+            "properties": {
+              "kind": {
+                "const": "b"
+              }
+            }
+          }
+        ]
+      }
+    }
+  }
+}
diff --git a/tests/main/openapi/test_main_openapi.py b/tests/main/openapi/test_main_openapi.py
index 65121180d..dd4a5a757 100644
--- a/tests/main/openapi/test_main_openapi.py
+++ b/tests/main/openapi/test_main_openapi.py
@@ -5316,3 +5316,18 @@ def test_main_reuse_model_with_type_alias(output_file: Path) -> None:
             "--use-type-alias",
         ],
     )
+
+
+@pytest.mark.timeout(30)
+def test_main_openapi_discriminated_oneof_allof_cycle(output_file: Path) -> None:
+    """Discriminated oneOf with variants that allOf the parent (circular graph).
+
+    Covers sort_data_models ordering for cyclic base dependencies and discriminator
+    handling (mapping + RootModel) on a minimal OpenAPI spec.
+    """
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "openapi_discriminated_oneof_allof_cycle.json",
+        output_path=output_file,
+        input_file_type="openapi",
+        assert_func=assert_file_content,
+    )
diff --git a/uv.lock b/uv.lock
index 55a81c343..a12766c67 100644
--- a/uv.lock
+++ b/uv.lock
@@ -857,6 +857,7 @@ dev = [
     { name = "pytest" },
     { name = "pytest-cov" },
     { name = "pytest-mock" },
+    { name = "pytest-timeout" },
     { name = "pytest-xdist" },
     { name = "time-machine" },
     { name = "twine" },
@@ -901,6 +902,7 @@ test = [
     { name = "pytest" },
     { name = "pytest-cov" },
     { name = "pytest-mock" },
+    { name = "pytest-timeout" },
     { name = "pytest-xdist" },
     { name = "time-machine" },
     { name = "watchfiles" },
@@ -914,6 +916,7 @@ type = [
     { name = "pytest" },
     { name = "pytest-cov" },
     { name = "pytest-mock" },
+    { name = "pytest-timeout" },
     { name = "pytest-xdist" },
     { name = "time-machine" },
     { name = "ty" },
@@ -975,11 +978,10 @@ dev = [
     { name = "inline-snapshot", specifier = ">=0.31.1" },
     { name = "msgspec", specifier = ">=0.18" },
     { name = "prek", specifier = ">=0.2.22" },
-    { name = "pytest", specifier = ">=6.1" },
     { name = "pytest", specifier = ">=8.3.4" },
-    { name = "pytest-cov", specifier = ">=2.12.1" },
     { name = "pytest-cov", specifier = ">=5" },
     { name = "pytest-mock", specifier = ">=3.14" },
+    { name = "pytest-timeout", specifier = ">=2.4" },
     { name = "pytest-xdist", specifier = ">=3.3.1" },
     { name = "time-machine", specifier = ">=3.1" },
     { name = "twine", specifier = ">=6.1" },
@@ -1009,11 +1011,10 @@ test = [
     { name = "diff-cover", specifier = ">=9.7.2" },
     { name = "inline-snapshot", specifier = ">=0.31.1" },
     { name = "msgspec", specifier = ">=0.18" },
-    { name = "pytest", specifier = ">=6.1" },
     { name = "pytest", specifier = ">=8.3.4" },
-    { name = "pytest-cov", specifier = ">=2.12.1" },
     { name = "pytest-cov", specifier = ">=5" },
     { name = "pytest-mock", specifier = ">=3.14" },
+    { name = "pytest-timeout", specifier = ">=2.4" },
     { name = "pytest-xdist", specifier = ">=3.3.1" },
     { name = "time-machine", specifier = ">=3.1" },
     { name = "watchfiles", specifier = ">=1.1" },
@@ -1024,11 +1025,10 @@ type = [
     { name = "diff-cover", specifier = ">=9.7.2" },
     { name = "inline-snapshot", specifier = ">=0.31.1" },
     { name = "msgspec", specifier = ">=0.18" },
-    { name = "pytest", specifier = ">=6.1" },
     { name = "pytest", specifier = ">=8.3.4" },
-    { name = "pytest-cov", specifier = ">=2.12.1" },
     { name = "pytest-cov", specifier = ">=5" },
     { name = "pytest-mock", specifier = ">=3.14" },
+    { name = "pytest-timeout", specifier = ">=2.4" },
     { name = "pytest-xdist", specifier = ">=3.3.1" },
     { name = "time-machine", specifier = ">=3.1" },
     { name = "ty", specifier = ">=0.0.8" },
@@ -2417,6 +2417,18 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/5a/cc/06253936f4a7fa2e0f48dfe6d851d9c56df896a9ab09ac019d70b760619c/pytest_mock-3.15.1-py3-none-any.whl", hash = "sha256:0a25e2eb88fe5168d535041d09a4529a188176ae608a6d249ee65abc0949630d", size = 10095, upload-time = "2025-09-16T16:37:25.734Z" },
 ]
 
+[[package]]
+name = "pytest-timeout"
+version = "2.4.0"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "pytest" },
+]
+sdist = { url = "https://files.pythonhosted.org/packages/ac/82/4c9ecabab13363e72d880f2fb504c5f750433b2b6f16e99f4ec21ada284c/pytest_timeout-2.4.0.tar.gz", hash = "sha256:7e68e90b01f9eff71332b25001f85c75495fc4e3a836701876183c4bcfd0540a", size = 17973, upload-time = "2025-05-05T19:44:34.99Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/fa/b6/3127540ecdf1464a00e5a01ee60a1b09175f6913f0644ac748494d9c4b21/pytest_timeout-2.4.0-py3-none-any.whl", hash = "sha256:c42667e5cdadb151aeb5b26d114aff6bdf5a907f176a007a30b940d3d865b5c2", size = 14382, upload-time = "2025-05-05T19:44:33.502Z" },
+]
+
 [[package]]
 name = "pytest-xdist"
 version = "3.8.0"