diff --git a/src/agents/agent_tool_input.py b/src/agents/agent_tool_input.py
index 19a81e62e6..c744be5476 100644
--- a/src/agents/agent_tool_input.py
+++ b/src/agents/agent_tool_input.py
@@ -212,10 +212,21 @@ def _describe_json_schema_field(
     if not isinstance(field_schema, dict):
         return None
 
-    if any(key in field_schema for key in ("properties", "items", "oneOf", "anyOf", "allOf")):
+    if any(key in field_schema for key in ("properties", "items", "oneOf", "allOf")):
         return None
 
     description = _read_schema_description(field_schema)
+
+    # Pydantic renders `T | None` as `anyOf: [{type: T}, {type: "null"}]`.
+    # Treat that exact shape as the same as `type: ["T", "null"]` so optional
+    # simple fields still appear in the schema summary.
+    any_of = field_schema.get("anyOf")
+    if any_of is not None:
+        nullable_label = _describe_nullable_anyof(any_of)
+        if nullable_label is None:
+            return None
+        return _SchemaFieldDescription(type=nullable_label, description=description)
+
     raw_type = field_schema.get("type")
 
     if isinstance(raw_type, list):
@@ -245,6 +256,42 @@ def _describe_json_schema_field(
     return None
 
 
+def _describe_nullable_anyof(any_of: Any) -> str | None:
+    """Render a 2-branch `anyOf` of a simple type and `null` as `T | null`.
+
+    Also handles `Optional[Literal[...]]`, which Pydantic emits as an `enum`/`const`
+    branch (possibly with `type: "string"`) plus a `null` branch.
+    """
+    if not isinstance(any_of, list) or len(any_of) != 2:
+        return None
+    base_label: str | None = None
+    has_null = False
+    for entry in any_of:
+        if not isinstance(entry, dict):
+            return None
+        entry_type = entry.get("type")
+        if entry_type == "null":
+            has_null = True
+            continue
+        if base_label is not None:
+            return None
+        # Prefer `enum`/`const` over a bare `type` so `Optional[Literal[...]]`
+        # surfaces the allowed values rather than just e.g. `string | null`.
+        if isinstance(entry.get("enum"), list):
+            base_label = _format_enum_label(entry.get("enum"))
+            continue
+        if "const" in entry:
+            base_label = _format_literal_label(entry)
+            continue
+        if entry_type in _SIMPLE_JSON_SCHEMA_TYPES:
+            base_label = cast(str, entry_type)
+            continue
+        return None
+    if base_label is None or not has_null:
+        return None
+    return f"{base_label} | null"
+
+
 def _read_schema_description(value: Any) -> str | None:
     if not isinstance(value, dict):
         return None
diff --git a/tests/test_agent_tool_input.py b/tests/test_agent_tool_input.py
index 93f72efc7b..fed9719d8b 100644
--- a/tests/test_agent_tool_input.py
+++ b/tests/test_agent_tool_input.py
@@ -125,3 +125,68 @@ def test_private_schema_helper_edge_cases() -> None:
     assert _format_enum_label([]) == "enum"
     assert "..." in _format_enum_label([1, 2, 3, 4, 5, 6])
     assert _format_literal_label({}) == "literal"
+
+
+def test_schema_summary_handles_pydantic_optional_anyof() -> None:
+    # Pydantic emits `T | None` as anyOf:[{type:T},{type:"null"}]; without
+    # support for that shape, a single Optional field nukes the whole summary.
+    schema = {
+        "type": "object",
+        "properties": {
+            "count": {
+                "anyOf": [{"type": "integer"}, {"type": "null"}],
+                "description": "Optional count.",
+            },
+            "name": {"type": "string", "description": "A name."},
+        },
+        "required": ["name"],
+    }
+    summary = _build_schema_summary(schema)
+    assert summary is not None
+    assert "- count (integer | null, optional) - Optional count." in summary
+    assert "- name (string, required) - A name." in summary
+
+    # Non-nullable anyOf shapes (e.g. union of two simple types, or anyOf with
+    # nested objects) should still be rejected.
+    assert (
+        _build_schema_summary(
+            {
+                "type": "object",
+                "description": "x",
+                "properties": {
+                    "u": {"anyOf": [{"type": "integer"}, {"type": "string"}]},
+                },
+            }
+        )
+        is None
+    )
+
+
+def test_schema_summary_preserves_enum_const_in_nullable_anyof() -> None:
+    # Pydantic renders `Optional[Literal["a", "b"]]` as an anyOf branch carrying
+    # both an `enum` (with `type: "string"`) and a `null` branch. Surface the
+    # allowed values rather than collapsing to `string | null`.
+    schema = {
+        "type": "object",
+        "properties": {
+            "color": {
+                "anyOf": [
+                    {"enum": ["red", "blue"], "type": "string"},
+                    {"type": "null"},
+                ],
+                "description": "Pick a color.",
+            },
+            "tag": {
+                "anyOf": [
+                    {"const": "x", "type": "string"},
+                    {"type": "null"},
+                ],
+                "description": "A tag.",
+            },
+        },
+        "required": ["color", "tag"],
+    }
+    summary = _build_schema_summary(schema)
+    assert summary is not None
+    assert '- color (enum("red" | "blue") | null, required) - Pick a color.' in summary
+    assert '- tag (literal("x") | null, required) - A tag.' in summary