Sort extra keys when formatting ForbiddenExtraKeysError

HISTORY.md

@@ -11,6 +11,11 @@ The third number is for emergencies when we need to start branches for older rel
 
 Our backwards-compatibility policy can be found [here](https://github.com/python-attrs/cattrs/blob/main/.github/SECURITY.md).
 
+## NEXT (UNRELEASED)
+
+- Fix {func}`transform_error <cattrs.transform_error>` listing the extra keys of a `ForbiddenExtraKeysError` in set iteration order, which made the message differ between runs; the keys are now sorted, like the error's own `__str__` already sorts them.
+  ([#776](https://github.com/python-attrs/cattrs/pull/776))
+
 ## 26.2.0 (2026-09-08)
 
 - Fix the `msgpack` and `cbor2` converters unstructuring naive datetimes as local time, which made the serialized value depend on the timezone of the machine doing the unstructuring; naive datetimes are now assumed to be UTC, matching what the structure hooks already read back.

src/cattrs/v.py

@@ -41,7 +41,7 @@ def format_exception(exc: BaseException, type: Union[type, None]) -> str:
             tn = type.__name__ if hasattr(type, "__name__") else repr(type)
             res = f"invalid value for type, expected {tn}"
     elif isinstance(exc, ForbiddenExtraKeysError):
-        res = f"extra fields found ({', '.join(exc.extra_fields)})"
+        res = f"extra fields found ({', '.join(sorted(exc.extra_fields))})"
     elif isinstance(exc, AttributeError) and exc.args[0].endswith(
         "object has no attribute 'items'"
     ):

tests/test_v.py

@@ -106,6 +106,27 @@ class C:
         ]
 
 
+def test_extra_keys_are_sorted(c: Converter) -> None:
+    """Extra keys are reported in a stable order.
+
+    `ForbiddenExtraKeysError.extra_fields` is a set, so the message used to
+    depend on the iteration order of that set, which differs between processes.
+    """
+
+    @define
+    class C:
+        a: int
+
+    c.register_structure_hook(
+        C, make_dict_structure_fn(C, c, _cattrs_forbid_extra_keys=True)
+    )
+
+    with raises(Exception) as exc_info:
+        c.structure({"a": 1, "e": 2, "c": 3, "b": 4, "d": 5}, C)
+
+    assert transform_error(exc_info.value) == ["extra fields found (b, c, d, e) @ $"]
+
+
 def test_untyped_class_errors(c: Converter) -> None:
     """Errors on untyped attrs classes transform correctly."""