diff --git a/py2http/decorators.py b/py2http/decorators.py
index dbfad1f..5bf3200 100644
--- a/py2http/decorators.py
+++ b/py2http/decorators.py
@@ -357,10 +357,9 @@ def __init__(
         self._dflt_default = _dflt_default
         self._kind = _kind
 
-        if hasattr(self, "__annotations__"):
-            self.__annotations__.update(_annotations)
-        else:
-            self.__annotations__ = _annotations
+        # A new dict on the instance: updating ``self.__annotations__`` in place would
+        # mutate the class's dict, leaking these annotations into every later instance.
+        self.__annotations__ = {**getattr(self, "__annotations__", {}), **_annotations}
 
         reserved = {
             "_annotations",
diff --git a/py2http/tests/test_not_set_defaults.py b/py2http/tests/test_not_set_defaults.py
index 5faab49..6c17a28 100644
--- a/py2http/tests/test_not_set_defaults.py
+++ b/py2http/tests/test_not_set_defaults.py
@@ -110,3 +110,15 @@ class K:
 
     specifier = ParamsSpecifier.from_func(method, _dflt_default="dflt")
     assert specifier._name_and_dflts == {"y": "dflt", "z": "dflt", "d": 1.5}
+
+
+def test_params_specifier_annotations_do_not_leak_between_instances():
+    """An instance's annotations used to be written into the class's own dict."""
+
+    def g(y: int, z: float = 0.0): ...
+
+    def f(a, b: int): ...
+
+    ParamsSpecifier.__annotations__  # as on 3.10, where the class dict then exists
+    ParamsSpecifier.from_func(g)
+    assert [p["name"] for p in ParamsSpecifier.from_func(f)()] == ["a", "b"]