diff --git a/conftest.py b/conftest.py
new file mode 100644
index 000000000..434f8cf2f
--- /dev/null
+++ b/conftest.py
@@ -0,0 +1,19 @@
+"""Give each test an empty TimeNet home and no other ``TIMENET_*`` setting."""
+
+import os
+
+import pytest
+
+from timenet.config import TimeNetSettings
+
+
+@pytest.fixture(autouse=True)
+def _timenet_state(tmp_path_factory, monkeypatch):
+    """Set an empty temporary TimeNet home and clear other ``TIMENET_*`` variables.
+
+    Ignore settings from ``.env`` files.
+    """
+    for name in [name for name in os.environ if name.startswith("TIMENET_")]:
+        monkeypatch.delenv(name)
+    monkeypatch.setitem(TimeNetSettings.model_config, "env_file", None)
+    monkeypatch.setenv("TIMENET_HOME", str(tmp_path_factory.mktemp("timenet-home")))
diff --git a/packages/timenet-connectors/tests/test_build_backend.py b/packages/timenet-connectors/tests/test_build_backend.py
index ab179a74c..e0e6dd95f 100644
--- a/packages/timenet-connectors/tests/test_build_backend.py
+++ b/packages/timenet-connectors/tests/test_build_backend.py
@@ -129,7 +129,6 @@ def test_build_in_process_round_trips_the_connector_default_zarr_backend(tmp_pat
 # Only BuildOptions.quiet silences the child. An SDK build without a progress callback still prints.
 @pytest.mark.parametrize("quiet", [False, True])
 def test_build_runs_an_isolated_child_with_the_release_and_the_pins(tmp_path, monkeypatch, quiet):
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     monkeypatch.setattr(backend_module, "run_pipeline", _unexpected_pipeline)
     calls = []
     monkeypatch.setattr(
@@ -148,7 +147,6 @@ def test_build_runs_an_isolated_child_with_the_release_and_the_pins(tmp_path, mo
 
 
 def test_build_rejects_another_version(tmp_path, monkeypatch):
-    monkeypatch.setenv("TIMENET_ISOLATION", "on")
     monkeypatch.setattr(backend_module, "run_isolated_build", _unexpected_isolation)
     monkeypatch.setattr(backend_module, "run_pipeline", _unexpected_pipeline)
     release = DatasetRef.parse("timenet/hello-world@9.9.9")
diff --git a/packages/timenet-connectors/tests/test_build_cli.py b/packages/timenet-connectors/tests/test_build_cli.py
index 308f05928..0b81d818e 100644
--- a/packages/timenet-connectors/tests/test_build_cli.py
+++ b/packages/timenet-connectors/tests/test_build_cli.py
@@ -51,7 +51,6 @@ def _unexpected(*args, **kwargs):
 
 
 def test_build_runs_isolated_by_default(monkeypatch, tmp_path):
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     calls = []
     monkeypatch.setattr(backend_module, "run_isolated_build", _isolated_child(calls))
 
@@ -67,9 +66,7 @@ def test_build_runs_in_process_without_isolation(monkeypatch, tmp_path, how):
     calls = []
     monkeypatch.setattr(backend_module, "run_isolated_build", _unexpected)
     monkeypatch.setattr(backend_module, "run_pipeline", _in_process(calls))
-    if how == "flag":
-        monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
-    else:
+    if how == "setting":
         monkeypatch.setenv("TIMENET_ISOLATION", "off")
 
     result = runner.invoke(
@@ -104,7 +101,6 @@ def test_build_without_isolation_shows_each_build_line_once(monkeypatch, tmp_pat
 
 
 def test_a_build_that_reuses_the_release_says_so_on_stderr(monkeypatch, tmp_path):
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     # The line names a long path, so the console must not wrap it.
     monkeypatch.setattr(cli_module.console, "rich", Console(stderr=True, width=1000))
     calls = []
@@ -122,7 +118,6 @@ def test_a_build_that_reuses_the_release_says_so_on_stderr(monkeypatch, tmp_path
 
 
 def test_build_takes_an_exact_release_and_refuses_another_version(monkeypatch, tmp_path):
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     calls = []
     monkeypatch.setattr(backend_module, "run_isolated_build", _isolated_child(calls))
     result = runner.invoke(app, ["build", "timenet/hello-world@1.0.0", "--out", str(tmp_path)])
@@ -162,7 +157,6 @@ def test_build_rejects_an_id_that_no_card_declares_and_names_the_known_ids(tmp_p
 
 @pytest.mark.parametrize("quiet", [False, True])
 def test_build_forwards_quiet_to_the_isolated_child(monkeypatch, tmp_path, quiet):
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     # The console is a process-wide singleton; monkeypatch restores it for the tests that follow.
     monkeypatch.setattr(cli_module.console, "quiet", False)
     calls = []
diff --git a/packages/timenet-connectors/tests/test_build_plan.py b/packages/timenet-connectors/tests/test_build_plan.py
index 89111b6a4..57fef1214 100644
--- a/packages/timenet-connectors/tests/test_build_plan.py
+++ b/packages/timenet-connectors/tests/test_build_plan.py
@@ -299,7 +299,6 @@ def child(release, store_root, *, pinned_by, **_):
         else:
             _store_from_card(release.dataset_id, store_root, force=False)
 
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     monkeypatch.setattr(backend_module, "run_isolated_build", child)
 
     if backend == ValuesBackend.PARQUET:
@@ -510,7 +509,6 @@ def child_builds(monkeypatch) -> list:
     Returns:
         The list that receives one ``(dataset_id, force)`` entry for each build.
     """
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     calls = []
     monkeypatch.setattr(backend_module, "run_isolated_build", _prepared_builds(calls))
     return calls
@@ -519,7 +517,6 @@ def child_builds(monkeypatch) -> list:
 def test_build_prepares_parents_before_the_isolated_child(tmp_path, monkeypatch):
     cards = _install_cards(monkeypatch, child={"parent": PARENT, "grand": GRAND}, parent={})
     monkeypatch.setattr(cli_module, "known_ids", lambda: list(cards))
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     origin = LocalRegistry(tmp_path / "origin")
     origin.put_dataset(make_dataset())
     out = tmp_path / "out"
@@ -541,7 +538,6 @@ def test_build_prepares_parents_before_the_isolated_child(tmp_path, monkeypatch)
 @pytest.mark.parametrize("obtain", ["fetch_from", "build_parents"])
 def test_load_obtains_a_missing_parent_before_it_builds_and_reads(tmp_path, monkeypatch, obtain):
     cards = _install_cards(monkeypatch, child={"parent": PARENT}, parent={})
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     monkeypatch.setenv("TIMENET_HOME", str(tmp_path / "home"))
     monkeypatch.setenv("TIMENET_REGISTRY", str(tmp_path / "out"))
     calls = []
@@ -601,7 +597,6 @@ def child(command, env):
         result = runner.invoke(app, argv)
         return "", result.exit_code
 
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     monkeypatch.setattr(env_module, "_run_build", child)
     monkeypatch.setattr(cli_module, "known_ids", lambda: list(cards))
     monkeypatch.setattr(backend_module, "run_pipeline", run_pipeline)
@@ -666,7 +661,6 @@ def _remote_out(tmp_path, monkeypatch, cards) -> tuple[_Hosted, LocalRegistry, l
     monkeypatch.setattr(cli_module, "_resolve_target", lambda target: remote)
     monkeypatch.setattr(cli_module, "known_ids", lambda: list(cards))
     monkeypatch.setenv("TIMENET_HOME", str(tmp_path / "home"))
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     calls = []
     monkeypatch.setattr(backend_module, "run_isolated_build", _prepared_builds(calls))
     return remote, LocalRegistry(tmp_path / "home/registry"), calls
diff --git a/packages/timenet-connectors/tests/test_package_api.py b/packages/timenet-connectors/tests/test_package_api.py
index c9f53d388..74166be0e 100644
--- a/packages/timenet-connectors/tests/test_package_api.py
+++ b/packages/timenet-connectors/tests/test_package_api.py
@@ -40,7 +40,6 @@ def test_build_then_load_round_trip(monkeypatch, tmp_path):
 def test_build_runs_in_an_isolated_environment_by_default(monkeypatch, tmp_path):
     # The parent reads only the card. It does not import the connector, which needs dependencies
     # that only the environment of the child has.
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     monkeypatch.setattr(backend_module, "resolve", _unexpected_resolve)
     calls = []
 
@@ -58,7 +57,6 @@ def fake_isolated(release, store_root, *, force=False, keep_cache=False, quiet=F
 
 
 def test_build_rejects_a_version_the_connector_does_not_declare(monkeypatch, tmp_path):
-    monkeypatch.delenv("TIMENET_ISOLATION", raising=False)
     monkeypatch.setattr(backend_module, "resolve", _unexpected_resolve)
     monkeypatch.setattr(backend_module, "run_isolated_build", _unexpected_isolation)
 
diff --git a/packages/timenet/tests/conftest.py b/packages/timenet/tests/conftest.py
index cc6be6f89..b7f785f6f 100644
--- a/packages/timenet/tests/conftest.py
+++ b/packages/timenet/tests/conftest.py
@@ -9,6 +9,9 @@
 import sys
 
 from hypothesis import HealthCheck, settings
+import pytest
+
+from timenet.testing import install_backends
 
 
 # Make the test helpers of this directory (e.g. ``_demo_child``) importable by name under pytest's
@@ -24,3 +27,9 @@
     suppress_health_check=[HealthCheck.too_slow],
 )
 settings.load_profile(os.environ.get("HYPOTHESIS_PROFILE", "ci"))
+
+
+@pytest.fixture(autouse=True)
+def _no_installed_connectors(monkeypatch):
+    """Install no builder backends, so that no test reads the connector packages of the environment."""
+    install_backends(monkeypatch)
diff --git a/packages/timenet/tests/test_cli.py b/packages/timenet/tests/test_cli.py
index 991fcbede..e03bf46c6 100644
--- a/packages/timenet/tests/test_cli.py
+++ b/packages/timenet/tests/test_cli.py
@@ -8,14 +8,13 @@
 import typer
 from typer.testing import CliRunner
 
-from timenet import builders
 from timenet.cache import cached_datasets, raw_cache_size
 from timenet.cli.app import _enum_list, app
 from timenet.dataset import TimeFDataset
 from timenet.errors import TimeNetDatasetNotFoundError
 from timenet.reader import TimeFReader
 from timenet.registry import open_local_version
-from timenet.testing import install_backends, make_dataset
+from timenet.testing import make_dataset
 from timenet.types import Access, Domain
 from timenet.types.reference_keys import ParentAlias
 from timenet.types.references import DatasetRef
@@ -154,8 +153,6 @@ def test_info_lists_the_access_page_of_each_release_that_sets_one(tmp_path):
 
 
 def test_download_command_copies_into_the_store_and_then_has_nothing_to_do(registry_root, tmp_path, monkeypatch):
-    # The default sources end with the installed connectors. Keep the real ones out of the resolution.
-    monkeypatch.setattr(builders, "entry_points", lambda group: [])
     # The last line names a long path, so the console must not wrap it.
     monkeypatch.setattr(_cli_module.ui, "rich", Console(stderr=True, width=1000))
     monkeypatch.setenv("TIMENET_HOME", str(tmp_path / "home"))
@@ -180,7 +177,6 @@ def test_download_command_copies_into_the_store_and_then_has_nothing_to_do(regis
 
 
 def test_download_command_with_force_copies_a_held_release_again(registry_root, tmp_path, monkeypatch):
-    install_backends(monkeypatch)
     monkeypatch.setenv("TIMENET_HOME", str(tmp_path / "home"))
     monkeypatch.setenv("TIMENET_REGISTRY", str(tmp_path / "store"))
     command = ["download", "timenet/hello-world", "--registry", str(registry_root)]
diff --git a/packages/timenet/tests/test_client.py b/packages/timenet/tests/test_client.py
index 7f696b85d..5fa13c006 100644
--- a/packages/timenet/tests/test_client.py
+++ b/packages/timenet/tests/test_client.py
@@ -8,7 +8,6 @@
 from pydantic import ValidationError
 import pytest
 
-from timenet import builders as builders_module
 from timenet.client import TimeNet, default_sources
 from timenet.connectors import BaseConnector
 from timenet.dataset import Record, RegularAxis, Signal, Source, TimeFDataset
@@ -65,8 +64,7 @@ def build(self, release, store, *, force, pinned_by, options):
         store.put_dataset(dataset, force=force, values_backend=self.BACKEND, pinned_by=pinned_by or None)
 
 
-def test_a_child_reads_its_missing_payload_source_from_a_later_registry(tmp_path, monkeypatch):
-    monkeypatch.setattr(builders_module, "entry_points", lambda group: [])
+def test_a_child_reads_its_missing_payload_source_from_a_later_registry(tmp_path):
     origin, reference = _child_of_the_demo(tmp_path / "origin")
     cache = tmp_path / "cache"
     shutil.copytree(origin.root / "test/child/1.0.0", cache / "test/child/1.0.0")
@@ -221,7 +219,6 @@ def test_the_default_list_follows_timenet_registry(tmp_path, monkeypatch, regist
 
 
 def test_timenet_registry_env_selects_a_local_registry(tmp_path, monkeypatch):
-    monkeypatch.setattr(builders_module, "entry_points", lambda group: [])
     monkeypatch.setenv("TIMENET_HOME", str(tmp_path / "home"))
     monkeypatch.setenv("TIMENET_REGISTRY", str(tmp_path / "home" / "registry"))
     dataset = make_dataset()
@@ -363,8 +360,7 @@ def test_download_of_latest_fails_when_a_connector_declares_a_newer_version(regi
     assert client.download("timenet/hello-world") == store.root / "timenet/hello-world/1.0.0"
 
 
-def test_download_of_a_release_that_no_source_provides_names_each_source(tmp_path, monkeypatch):
-    monkeypatch.setattr(builders_module, "entry_points", lambda group: [])
+def test_download_of_a_release_that_no_source_provides_names_each_source(tmp_path):
     sources = [LocalRegistry(tmp_path / "store"), LocalRegistry(tmp_path / "empty"), InstalledConnectors()]
 
     with pytest.raises(TimeNetDatasetNotFoundError) as caught:
@@ -438,7 +434,6 @@ def test_download_names_the_store_when_no_other_registry_provides_its_copy(tmp_p
 
 @pytest.mark.parametrize("case", ["force", "damaged"])
 def test_download_counts_a_registry_over_the_directory_of_the_store_as_the_store(tmp_path, monkeypatch, case):
-    monkeypatch.setattr(builders_module, "entry_points", lambda group: [])
     root = tmp_path / "reg"
     LocalRegistry(root).put_dataset(make_dataset())
     directory = root / "timenet/hello-world/1.0.0"
@@ -773,8 +768,7 @@ def test_load_names_the_buildable_version_when_a_pin_cannot_be_built(registry_ro
 
 
 @pytest.mark.parametrize("connectors", [True, False])
-def test_a_release_that_no_source_provides_names_how_to_get_it(tmp_path, monkeypatch, connectors):
-    monkeypatch.setattr(builders_module, "entry_points", lambda group: [])
+def test_a_release_that_no_source_provides_names_how_to_get_it(tmp_path, connectors):
     sources = [
         LocalRegistry(tmp_path / "reg"),
         LocalRegistry(tmp_path / "empty"),
@@ -858,8 +852,7 @@ def test_load_rejects_a_credentialed_dataset_from_a_hosted_registry(tmp_path):
     assert not store.root.exists()
 
 
-def test_the_access_hint_says_that_no_installed_connector_builds_the_release(tmp_path, monkeypatch):
-    monkeypatch.setattr(builders_module, "entry_points", lambda group: [])
+def test_the_access_hint_says_that_no_installed_connector_builds_the_release(tmp_path):
     card = _card("org/gated", Access.CREDENTIALED, "https://physionet.example/dua")
     client = TimeNet(sources=[LocalRegistry(tmp_path / "store"), _hosted(tmp_path, card), InstalledConnectors()])
 
diff --git a/packages/timenet/tests/test_conftest.py b/packages/timenet/tests/test_conftest.py
new file mode 100644
index 000000000..a7aa29752
--- /dev/null
+++ b/packages/timenet/tests/test_conftest.py
@@ -0,0 +1,55 @@
+"""Tests of the root ``conftest.py``: each test gets its own state."""
+
+import os
+from pathlib import Path
+import subprocess
+import sys
+
+from timenet.config import settings
+
+
+pytest_plugins = ["pytester"]
+
+ROOT_CONFTEST = Path(__file__).resolve().parents[3] / "conftest.py"
+
+PATHS = ("registry_path", "storage_dir", "cache_dir", "control_cache_dir")
+"""The settings properties that point at local state."""
+
+
+def test_each_test_starts_without_the_timenet_variables_of_the_shell(pytester, monkeypatch):
+    for name, value in {
+        "TIMENET_REGISTRY": "timenet://",
+        "TIMENET_STORAGE": "/shell/storage",
+        "TIMENET_CACHE": "/shell/cache",
+        "TIMENET_TOKEN": "secret",
+        "TIMENET_ISOLATION": "off",
+    }.items():
+        monkeypatch.setenv(name, value)
+    pytester.makeconftest(ROOT_CONFTEST.read_text())
+    pytester.makepyfile(
+        """
+        import os
+
+        def test_only_the_home_is_set():
+            names = sorted(name for name in os.environ if name.startswith("TIMENET_"))
+            assert names == ["TIMENET_HOME"]
+        """
+    )
+    pytester.runpytest().assert_outcomes(passed=1)
+
+
+def test_every_settings_path_lies_in_the_home_of_the_test_also_in_a_child_process(tmp_path):
+    home = Path(os.environ["TIMENET_HOME"])
+    in_process = [getattr(settings(), name) for name in PATHS]
+    code = f"from timenet.config import settings; print(*(getattr(settings(), name) for name in {PATHS}), sep='\\n')"
+    child = subprocess.run([sys.executable, "-c", code], cwd=tmp_path, capture_output=True, text=True, check=True)
+    in_child = [Path(line) for line in child.stdout.splitlines()]
+    assert len(in_child) == len(in_process) == 4
+    assert all(path.is_relative_to(home) for path in [*in_process, *in_child])
+
+
+def test_a_dotenv_file_in_the_working_directory_does_not_change_the_settings(monkeypatch, tmp_path):
+    (tmp_path / ".env").write_text("TIMENET_REGISTRY=timenet://\nTIMENET_TOKEN=secret\nTIMENET_ISOLATION=off\n")
+    monkeypatch.chdir(tmp_path)
+    cfg = settings()
+    assert (cfg.registry, cfg.token, cfg.isolation) == (None, None, "on")
diff --git a/packages/timenet/tests/test_settings_remote.py b/packages/timenet/tests/test_settings_remote.py
index 6624e3daa..8724b6046 100644
--- a/packages/timenet/tests/test_settings_remote.py
+++ b/packages/timenet/tests/test_settings_remote.py
@@ -1,15 +1,6 @@
 from timenet.config import settings
 
 
-def test_token_defaults_to_none(monkeypatch, tmp_path):
-    # A .env in the working directory supplies a token of its own, and pydantic-settings reads it
-    # when the environment variable is absent. Run from an empty directory so the assertion tests
-    # the default rather than whatever the checkout happens to sit next to.
-    monkeypatch.chdir(tmp_path)
-    monkeypatch.delenv("TIMENET_TOKEN", raising=False)
-    assert settings().token is None
-
-
 def test_token_read_from_env(monkeypatch):
     monkeypatch.setenv("TIMENET_TOKEN", "tok_abc")
     assert settings().token == "tok_abc"
diff --git a/packages/timenet/tests/test_sources.py b/packages/timenet/tests/test_sources.py
index 904d3863a..8a0c7e518 100644
--- a/packages/timenet/tests/test_sources.py
+++ b/packages/timenet/tests/test_sources.py
@@ -2,7 +2,6 @@
 
 import pytest
 
-from timenet import builders
 from timenet.errors import (
     TimeFValidationError,
     TimeNetRegistryError,
@@ -132,8 +131,7 @@ def test_installed_connectors_answer_not_found_for_another_version(monkeypatch):
     assert InstalledConnectors().resolve("x/y", None) == NotFound()
 
 
-def test_installed_connectors_without_a_package_answer_not_found(monkeypatch):
-    monkeypatch.setattr(builders, "entry_points", lambda group: [])
+def test_installed_connectors_without_a_package_answer_not_found():
     assert InstalledConnectors().resolve("timenet/hello-world", None) == NotFound()
     assert "no connector package is installed" in str(InstalledConnectors())