From da577e082162af7e9d2f45f83eb6804d141b9213 Mon Sep 17 00:00:00 2001
From: Tony Meyer <tony.meyer@canonical.com>
Date: Fri, 19 Dec 2025 11:18:41 +1300
Subject: [PATCH 1/2] fix: correct the value of additional_parameters in Juju
 4.

---
 ops/charm.py       |  6 +++++-
 test/test_charm.py | 12 ++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/ops/charm.py b/ops/charm.py
index a5b155905..8225ea7c2 100644
--- a/ops/charm.py
+++ b/ops/charm.py
@@ -19,6 +19,7 @@
 import dataclasses
 import enum
 import logging
+import os
 import pathlib
 import warnings
 from collections.abc import Mapping
@@ -45,6 +46,7 @@
     Object,
     ObjectEvents,
 )
+from .jujuversion import JujuVersion
 
 if TYPE_CHECKING:
     from typing_extensions import Required
@@ -2209,7 +2211,9 @@ def __init__(self, name: str, raw: dict[str, Any] | None = None):
         self.description = raw.get('description', '')
         self.parameters = raw.get('params', {})  # {<parameter name>: <JSON Schema definition>}
         self.required = raw.get('required', [])  # [<parameter name>, ...]
-        self.additional_properties = raw.get('additionalProperties', True)
+        # The default in Juju 4 is False. The default in earlier Juju versions is True.
+        juju_version = JujuVersion(os.environ.get('JUJU_VERSION', '0.0.0'))
+        self.additional_properties = raw.get('additionalProperties', juju_version < '4.0.0')
 
 
 @dataclasses.dataclass(frozen=True)
diff --git a/test/test_charm.py b/test/test_charm.py
index 0bb24141c..b6e52f6bd 100644
--- a/test/test_charm.py
+++ b/test/test_charm.py
@@ -644,6 +644,18 @@ def test_actions_from_charm_root():
         assert meta.actions['foo'].description == 'foos the bar'
 
 
+@pytest.mark.parametrize(
+    'juju_version,additional_properties',
+    [('2.9', True), ('3.6.12', True), ('4.0.0', False), ('4.1', False)],
+)
+def test_actions_additional_properties(
+    monkeypatch: pytest.MonkeyPatch, juju_version: str, additional_properties: bool
+):
+    monkeypatch.setenv('JUJU_VERSION', juju_version)
+    action = ops.ActionMeta('foo', {})
+    assert action.additional_properties == additional_properties
+
+
 def _setup_test_action(fake_script: FakeScript):
     fake_script.write('action-get', """echo '{"foo-name": "name", "silent": true}'""")
     fake_script.write('action-set', '')

From 19a6902942940c08254cfbea90f8fce28255080f Mon Sep 17 00:00:00 2001
From: Tony Meyer <tony.meyer@canonical.com>
Date: Fri, 19 Dec 2025 18:43:00 +1300
Subject: [PATCH 2/2] Add tests for missing envvar and both explicit values.

---
 test/test_charm.py | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/test/test_charm.py b/test/test_charm.py
index b6e52f6bd..464abce4d 100644
--- a/test/test_charm.py
+++ b/test/test_charm.py
@@ -627,11 +627,17 @@ def test_config_from_raw():
     assert meta.config['sssh'].description == 'a user secret'
 
 
-def test_actions_from_charm_root():
+@pytest.mark.parametrize('additional_properties', [False, True])
+def test_actions_from_charm_root(additional_properties: bool):
     with tempfile.TemporaryDirectory() as d:
         td = pathlib.Path(d)
         (td / 'actions.yaml').write_text(
-            yaml.safe_dump({'foo': {'description': 'foos the bar', 'additionalProperties': False}})
+            yaml.safe_dump({
+                'foo': {
+                    'description': 'foos the bar',
+                    'additionalProperties': additional_properties,
+                }
+            })
         )
         (td / 'metadata.yaml').write_text(
             yaml.safe_dump({'name': 'bob', 'requires': {'foo': {'interface': 'bar'}}})
@@ -640,18 +646,21 @@ def test_actions_from_charm_root():
         meta = ops.CharmMeta.from_charm_root(td)
         assert meta.name == 'bob'
         assert meta.requires['foo'].interface_name == 'bar'
-        assert not meta.actions['foo'].additional_properties
+        assert meta.actions['foo'].additional_properties == additional_properties
         assert meta.actions['foo'].description == 'foos the bar'
 
 
 @pytest.mark.parametrize(
     'juju_version,additional_properties',
-    [('2.9', True), ('3.6.12', True), ('4.0.0', False), ('4.1', False)],
+    [('2.9', True), ('3.6.12', True), ('4.0.0', False), ('4.1', False), (None, True)],
 )
 def test_actions_additional_properties(
-    monkeypatch: pytest.MonkeyPatch, juju_version: str, additional_properties: bool
+    monkeypatch: pytest.MonkeyPatch, juju_version: str | None, additional_properties: bool
 ):
-    monkeypatch.setenv('JUJU_VERSION', juju_version)
+    if juju_version is None:
+        monkeypatch.delenv('JUJU_VERSION', raising=False)
+    else:
+        monkeypatch.setenv('JUJU_VERSION', juju_version)
     action = ops.ActionMeta('foo', {})
     assert action.additional_properties == additional_properties