diff --git a/tests/cli/conftest.py b/tests/cli/conftest.py
index a0079bdd6..4c0822296 100644
--- a/tests/cli/conftest.py
+++ b/tests/cli/conftest.py
@@ -152,17 +152,24 @@ def plan_file(temp_dir):
 
 # Environment configuration for tests
 @pytest.fixture(autouse=True)
-def test_environment():
-    """Set up test environment variables"""
+def test_environment(monkeypatch):
+    """Set up test environment variables.
+
+    Both go through ``monkeypatch``. Autouse fixtures in the root conftest and in
+    this one create ``monkeypatch`` before this fixture, so its undo runs after the
+    restore below. Set straight into ``os.environ``, these values were what a test's
+    own ``monkeypatch.delenv`` or ``setenv`` recorded as the original, and its undo put
+    them back for the rest of the session (see ``test_conftest_env_isolation.py``).
+    """
     original_env = os.environ.copy()
 
     # Set test environment variables
-    os.environ["FLUID_ENV"] = "test"
-    os.environ["FLUID_LOG_LEVEL"] = "DEBUG"
+    monkeypatch.setenv("FLUID_ENV", "test")
+    monkeypatch.setenv("FLUID_LOG_LEVEL", "DEBUG")
 
     yield
 
-    # Restore original environment
+    # Restore anything a test changed in os.environ directly
     os.environ.clear()
     os.environ.update(original_env)
 
diff --git a/tests/cli/test_conftest_env_isolation.py b/tests/cli/test_conftest_env_isolation.py
new file mode 100644
index 000000000..215644ac0
--- /dev/null
+++ b/tests/cli/test_conftest_env_isolation.py
@@ -0,0 +1,55 @@
+# Copyright 2024-2026 Agentics Transformation Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""``tests/cli/conftest.py::test_environment`` must not leak what it sets.
+
+It wrote ``FLUID_ENV`` and ``FLUID_LOG_LEVEL`` straight into ``os.environ`` and
+restored a snapshot afterwards. Autouse fixtures in the root conftest and in this
+directory's conftest create ``monkeypatch`` before it, so ``monkeypatch.undo()`` ran
+after that restore. A test that called ``monkeypatch.delenv`` or ``monkeypatch.setenv``
+on either variable recorded the fixture's value as the original, and the undo put it
+back for the rest of the session. ``test_cli_output_keeps_brackets.py`` delenvs every
+``FLUID_*`` key. Later subprocess tests inherited ``FLUID_LOG_LEVEL=DEBUG``, and
+``TestValidateStrict`` in ``tests/test_overlay_keyed_merge.py`` saw the loader log its
+warning a second time.
+"""
+
+from __future__ import annotations
+
+import os
+
+import pytest
+
+# Everything test_environment sets.
+_FIXTURE_VARS = ("FLUID_ENV", "FLUID_LOG_LEVEL")
+
+
+@pytest.fixture(scope="module", autouse=True)
+def _nothing_outlives_the_module():
+    # Module scope: torn down after every function-scoped fixture of the last test,
+    # monkeypatch's undo included, so this sees what the next module would inherit.
+    saved = {name: os.environ.pop(name, None) for name in _FIXTURE_VARS}
+    yield
+    leaked = {name: os.environ.pop(name) for name in _FIXTURE_VARS if name in os.environ}
+    for name, value in saved.items():
+        if value is not None:
+            os.environ[name] = value
+    assert not leaked, f"{leaked} outlived this module's tests"
+
+
+def test_a_test_may_delenv_what_the_fixture_set(monkeypatch):
+    assert (os.environ["FLUID_ENV"], os.environ["FLUID_LOG_LEVEL"]) == ("test", "DEBUG")
+    for name in _FIXTURE_VARS:
+        monkeypatch.delenv(name)
+    assert not any(name in os.environ for name in _FIXTURE_VARS)
diff --git a/tests/test_overlay_keyed_merge.py b/tests/test_overlay_keyed_merge.py
index c8d823b92..ddb9d7919 100644
--- a/tests/test_overlay_keyed_merge.py
+++ b/tests/test_overlay_keyed_merge.py
@@ -428,9 +428,11 @@ def _write_product(tmp_path: Path, version: str, overlay: Dict[str, Any]) -> Non
 PROD_BY_POSITION = {"builds": [{}, {"id": "transform", "description": "8 executors"}]}
 
 #: Environment variables a CLI subprocess here must not inherit: API keys, and
-#: the debug level ``tests/cli/conftest.py`` sets, which some xdist workers keep.
-#: At DEBUG the loader also logs a warning the command reports, and the tests
-#: below count that the terminal shows it once.
+#: debug switches, such as one a developer exported. (``tests/cli/conftest.py``
+#: used to leak its ``FLUID_LOG_LEVEL=DEBUG`` into later tests; see
+#: ``tests/cli/test_conftest_env_isolation.py``.) At DEBUG the loader also logs a
+#: warning the command reports, and the tests below count that the terminal shows
+#: it once.
 _NOT_INHERITED = ("FLUID_LOG_LEVEL", "FLUID_DEBUG", "FLUID_SHOW_REGISTRATION")