diff --git a/dandi/cli/tests/test_cmd_ls.py b/dandi/cli/tests/test_cmd_ls.py
index 13dbde12d..eec7e4c7c 100644
--- a/dandi/cli/tests/test_cmd_ls.py
+++ b/dandi/cli/tests/test_cmd_ls.py
@@ -1,5 +1,8 @@
+from __future__ import annotations
+
 import json
-import os
+from pathlib import Path
+from typing import Any
 from unittest.mock import ANY
 
 from click.testing import CliRunner
@@ -15,25 +18,30 @@
 @pytest.mark.parametrize(
     "format", ("auto", "json", "json_pp", "json_lines", "yaml", "pyout")
 )
-def test_smoke(simple1_nwb_metadata, simple1_nwb, format):
+def test_smoke(
+    simple1_nwb_metadata: dict[str, Any], simple1_nwb: Path, format: str
+) -> None:
     runner = CliRunner()
-    r = runner.invoke(ls, ["-f", format, simple1_nwb])
+    r = runner.invoke(ls, ["-f", format, str(simple1_nwb)])
     assert r.exit_code == 0, f"Exited abnormally. out={r.stdout}"
     # we would need to redirect pyout for its analysis
     out = r.stdout
 
     if format == "json_lines":
-        load = json.loads
+
+        def load(s: str) -> Any:
+            return json.loads(s)
+
     elif format.startswith("json"):
 
-        def load(s):
+        def load(s: str) -> Any:
             obj = json.loads(s)
             assert len(obj) == 1  # will be a list with a single elem
             return obj[0]
 
     elif format == "yaml":
 
-        def load(s):
+        def load(s: str) -> Any:
             obj = yaml_load(s, typ="base")
             assert len(obj) == 1  # will be a list with a single elem
             return obj[0]
@@ -49,20 +57,20 @@ def load(s):
         assert metadata[f] == simple1_nwb_metadata[f]
 
 
-def test_ls_nwb_file(simple2_nwb):
-    bids_file_path = "simple2.nwb"
-    bids_file_path = os.path.join(simple2_nwb, bids_file_path)
-    r = CliRunner().invoke(ls, ["-f", "yaml", bids_file_path])
+def test_ls_nwb_file(simple2_nwb: Path) -> None:
+    bids_file_path = simple2_nwb / "simple2.nwb"
+    r = CliRunner().invoke(ls, ["-f", "yaml", str(bids_file_path)])
     assert r.exit_code == 0, r.output
     data = yaml_load(r.stdout, "safe")
     assert len(data) == 1
 
 
 @mark.skipif_no_network
-def test_ls_bids_file(bids_examples):
-    bids_file_path = "asl003/sub-Sub1/anat/sub-Sub1_T1w.nii.gz"
-    bids_file_path = os.path.join(bids_examples, bids_file_path)
-    r = CliRunner().invoke(ls, ["-f", "yaml", bids_file_path])
+def test_ls_bids_file(bids_examples: Path) -> None:
+    bids_file_path = (
+        bids_examples / "asl003" / "sub-Sub1" / "anat" / "sub-Sub1_T1w.nii.gz"
+    )
+    r = CliRunner().invoke(ls, ["-f", "yaml", str(bids_file_path)])
     assert r.exit_code == 0, r.output
     data = yaml_load(r.stdout, "safe")
     assert len(data) == 1
@@ -70,12 +78,16 @@ def test_ls_bids_file(bids_examples):
 
 
 @mark.skipif_no_network
-def test_ls_zarrbids_file(bids_examples):
+def test_ls_zarrbids_file(bids_examples: Path) -> None:
     bids_file_path = (
-        "micr_SEMzarr/sub-01/ses-01/micr/sub-01_ses-01_sample-A_SPIM.ome.zarr"
+        bids_examples
+        / "micr_SEMzarr"
+        / "sub-01"
+        / "ses-01"
+        / "micr"
+        / "sub-01_ses-01_sample-A_SPIM.ome.zarr"
     )
-    bids_file_path = os.path.join(bids_examples, bids_file_path)
-    r = CliRunner().invoke(ls, ["-f", "yaml", bids_file_path])
+    r = CliRunner().invoke(ls, ["-f", "yaml", str(bids_file_path)])
     assert r.exit_code == 0, r.output
     data = yaml_load(r.stdout, "safe")
     assert len(data) == 1
@@ -83,7 +95,7 @@ def test_ls_zarrbids_file(bids_examples):
 
 
 @mark.skipif_no_network
-def test_ls_dandiset_url():
+def test_ls_dandiset_url() -> None:
     r = CliRunner().invoke(
         ls, ["-f", "yaml", "https://api.dandiarchive.org/api/dandisets/000027"]
     )
@@ -94,7 +106,7 @@ def test_ls_dandiset_url():
 
 
 @mark.skipif_no_network
-def test_ls_dandiset_url_recursive():
+def test_ls_dandiset_url_recursive() -> None:
     r = CliRunner().invoke(
         ls, ["-f", "yaml", "-r", "https://api.dandiarchive.org/api/dandisets/000027"]
     )
@@ -106,7 +118,7 @@ def test_ls_dandiset_url_recursive():
 
 
 @mark.skipif_no_network
-def test_ls_path_url():
+def test_ls_path_url() -> None:
     r = CliRunner().invoke(
         ls,
         [
@@ -124,7 +136,7 @@ def test_ls_path_url():
     assert data[0]["path"] == "sub-RAT123/sub-RAT123.nwb"
 
 
-def test_smoke_local_schema(simple1_nwb):
+def test_smoke_local_schema(simple1_nwb: Path) -> None:
     runner = CliRunner()
     r = runner.invoke(
         ls,
@@ -133,7 +145,7 @@ def test_smoke_local_schema(simple1_nwb):
             "json",
             "--schema",
             DANDI_SCHEMA_VERSION,
-            simple1_nwb,
+            str(simple1_nwb),
         ],
     )
     assert r.exit_code == 0, f"Exited abnormally. out={r.stdout}"
diff --git a/dandi/cli/tests/test_cmd_validate.py b/dandi/cli/tests/test_cmd_validate.py
index 234cbed23..afa430a00 100644
--- a/dandi/cli/tests/test_cmd_validate.py
+++ b/dandi/cli/tests/test_cmd_validate.py
@@ -1,5 +1,4 @@
 import json
-import os
 from pathlib import Path
 
 from click.testing import CliRunner
@@ -10,44 +9,40 @@
 
 
 @pytest.mark.parametrize("dataset", BIDS_ERROR_TESTDATA_SELECTION)
-def test_validate_bids_error(bids_error_examples, dataset):
-
-    broken_dataset = os.path.join(bids_error_examples, dataset)
-    with open(os.path.join(broken_dataset, ".ERRORS.json")) as f:
+def test_validate_bids_error(bids_error_examples: Path, dataset: str) -> None:
+    broken_dataset = bids_error_examples / dataset
+    with (broken_dataset / ".ERRORS.json").open() as f:
         expected_errors = json.load(f)
-    r = CliRunner().invoke(validate, [broken_dataset])
+    r = CliRunner().invoke(validate, [str(broken_dataset)])
     # Does it break?
     assert r.exit_code == 1
-
     # Does it detect all errors?
     for key in expected_errors:
         assert key in r.output
 
 
-def test_validate_nwb_error(simple3_nwb):
-    """
-    Do we fail on critical NWB validation errors?
-    """
-
-    r = CliRunner().invoke(validate, [simple3_nwb])
+def test_validate_nwb_error(simple3_nwb: Path) -> None:
+    """Do we fail on critical NWB validation errors?"""
+    r = CliRunner().invoke(validate, [str(simple3_nwb)])
     # does it fail? as per:
     # https://github.com/dandi/dandi-cli/pull/1157#issuecomment-1312546812
     assert r.exit_code != 0
 
 
-def test_validate_bids_grouping_error(bids_error_examples, dataset="invalid_asl003"):
+def test_validate_bids_grouping_error(
+    bids_error_examples: Path, dataset: str = "invalid_asl003"
+) -> None:
     """
-    This is currently a placeholder test, and should be updated once we have paths with
-    multiple errors for which grouping functionality can actually be tested.
+    This is currently a placeholder test, and should be updated once we have
+    paths with multiple errors for which grouping functionality can actually be
+    tested.
     """
-
-    dataset = os.path.join(bids_error_examples, dataset)
-    r = CliRunner().invoke(validate, ["--grouping=path", dataset])
+    bids_dataset = bids_error_examples / dataset
+    r = CliRunner().invoke(validate, ["--grouping=path", str(bids_dataset)])
     # Does it break?
     assert r.exit_code == 1
-
     # Does it detect all errors?
-    assert dataset in r.output
+    assert str(bids_dataset) in r.output
 
 
 def test_validate_nwb_path_grouping(organized_nwb_dir3: Path) -> None:
@@ -64,15 +59,13 @@ def test_validate_nwb_path_grouping(organized_nwb_dir3: Path) -> None:
 
 
 def test_validate_bids_error_grouping_notification(
-    bids_error_examples, dataset="invalid_asl003"
-):
+    bids_error_examples: Path, dataset: str = "invalid_asl003"
+) -> None:
     """Test user notification for unimplemented parameter value."""
-
-    broken_dataset = os.path.join(bids_error_examples, dataset)
-    r = CliRunner().invoke(validate, ["--grouping=error", broken_dataset])
+    broken_dataset = bids_error_examples / dataset
+    r = CliRunner().invoke(validate, ["--grouping=error", str(broken_dataset)])
     # Does it break?
     assert r.exit_code == 2
-
     # Does it notify the user correctly?
     notification_substring = "Invalid value for '--grouping'"
     assert notification_substring in r.output
diff --git a/dandi/tests/fixtures.py b/dandi/tests/fixtures.py
index e0b473d4c..1184b159f 100644
--- a/dandi/tests/fixtures.py
+++ b/dandi/tests/fixtures.py
@@ -55,9 +55,10 @@
 BIDS_ERROR_TESTDATA_SELECTION = ["invalid_asl003", "invalid_pet001"]
 
 
-def copytree(src, dst, symlinks=False, ignore=None):
-    """Function mimicking `shutil.copytree()` behaviour but supporting existing target
-    directories.
+def copytree(src: str | Path, dst: str | Path) -> None:
+    """
+    Function mimicking `shutil.copytree()` behaviour but supporting existing
+    target directories.
 
     Notes
     -----
@@ -75,7 +76,7 @@ def copytree(src, dst, symlinks=False, ignore=None):
         s = os.path.join(src, item)
         d = os.path.join(dst, item)
         if os.path.isdir(s):
-            copytree(s, d, symlinks, ignore)
+            copytree(s, d)
         else:
             if not os.path.exists(d) or os.stat(s).st_mtime - os.stat(d).st_mtime > 1:
                 shutil.copy2(s, d)
@@ -112,9 +113,9 @@ def simple1_nwb_metadata() -> Dict[str, Any]:
 @pytest.fixture(scope="session")
 def simple1_nwb(
     simple1_nwb_metadata: Dict[str, Any], tmp_path_factory: pytest.TempPathFactory
-) -> str:
+) -> Path:
     return make_nwb_file(
-        str(tmp_path_factory.mktemp("simple1") / "simple1.nwb"),
+        tmp_path_factory.mktemp("simple1") / "simple1.nwb",
         **simple1_nwb_metadata,
     )
 
@@ -122,10 +123,10 @@ def simple1_nwb(
 @pytest.fixture(scope="session")
 def simple2_nwb(
     simple1_nwb_metadata: Dict[str, Any], tmp_path_factory: pytest.TempPathFactory
-) -> str:
+) -> Path:
     """With a subject"""
     return make_nwb_file(
-        str(tmp_path_factory.mktemp("simple2") / "simple2.nwb"),
+        tmp_path_factory.mktemp("simple2") / "simple2.nwb",
         subject=pynwb.file.Subject(
             subject_id="mouse001",
             date_of_birth=datetime(2016, 12, 1, tzinfo=tzutc()),
@@ -139,10 +140,10 @@ def simple2_nwb(
 @pytest.fixture(scope="session")
 def simple3_nwb(
     simple1_nwb_metadata: Dict[str, Any], tmp_path_factory: pytest.TempPathFactory
-) -> str:
+) -> Path:
     """With a subject, but no subject_id."""
     return make_nwb_file(
-        str(tmp_path_factory.mktemp("simple3") / "simple3.nwb"),
+        tmp_path_factory.mktemp("simple3") / "simple3.nwb",
         subject=pynwb.file.Subject(
             age="P1D/",
             sex="O",
@@ -153,7 +154,7 @@ def simple3_nwb(
 
 
 @pytest.fixture(scope="session")
-def simple4_nwb(tmp_path_factory: pytest.TempPathFactory) -> str:
+def simple4_nwb(tmp_path_factory: pytest.TempPathFactory) -> Path:
     """
     With subject, subject_id, species, but including data orientation ambiguity,
     the only currently non-critical issue in the dandi schema for nwbinspector validation:
@@ -183,7 +184,7 @@ def simple4_nwb(tmp_path_factory: pytest.TempPathFactory) -> str:
         ),
     )
     nwbfile.add_acquisition(time_series)
-    filename = str(tmp_path_factory.mktemp("simple4") / "simple4.nwb")
+    filename = tmp_path_factory.mktemp("simple4") / "simple4.nwb"
     with pynwb.NWBHDF5IO(filename, "w") as io:
         io.write(nwbfile, cache_spec=False)
     return filename
@@ -191,7 +192,7 @@ def simple4_nwb(tmp_path_factory: pytest.TempPathFactory) -> str:
 
 @pytest.fixture(scope="session")
 def organized_nwb_dir(
-    simple2_nwb: str, tmp_path_factory: pytest.TempPathFactory
+    simple2_nwb: Path, tmp_path_factory: pytest.TempPathFactory
 ) -> Path:
     tmp_path = tmp_path_factory.mktemp("organized_nwb_dir")
     (tmp_path / dandiset_metadata_file).write_text("{}\n")
@@ -205,7 +206,7 @@ def organized_nwb_dir(
 @pytest.fixture(scope="session")
 def organized_nwb_dir2(
     simple1_nwb_metadata: Dict[str, Any],
-    simple2_nwb: str,
+    simple2_nwb: Path,
     tmp_path_factory: pytest.TempPathFactory,
 ) -> Path:
     tmp_path = tmp_path_factory.mktemp("organized_nwb_dir2")
@@ -214,7 +215,7 @@ def organized_nwb_dir2(
     # file to be "organized"
     shutil.copy(simple2_nwb, tmp_path)
     make_nwb_file(
-        str(tmp_path / "simple3.nwb"),
+        tmp_path / "simple3.nwb",
         subject=pynwb.file.Subject(
             subject_id="lizard001",
             date_of_birth=datetime(2016, 12, 1, tzinfo=tzutc()),
@@ -232,7 +233,7 @@ def organized_nwb_dir2(
 
 @pytest.fixture(scope="session")
 def organized_nwb_dir3(
-    simple4_nwb: str, tmp_path_factory: pytest.TempPathFactory
+    simple4_nwb: Path, tmp_path_factory: pytest.TempPathFactory
 ) -> Path:
     tmp_path = tmp_path_factory.mktemp("organized_nwb_dir")
     (tmp_path / dandiset_metadata_file).write_text("{}\n")
@@ -260,18 +261,18 @@ def get_gitrepo_fixture(
     committish: Optional[str] = None,
     scope: Scope = "session",
     make_subdirs_dandisets: bool = False,
-) -> Callable[[pytest.TempPathFactory], str]:
+) -> Callable[[pytest.TempPathFactory], Path]:
 
     if committish:
         raise NotImplementedError()
 
     @pytest.fixture(scope=scope)
-    def fixture(tmp_path_factory: pytest.TempPathFactory) -> str:
+    def fixture(tmp_path_factory: pytest.TempPathFactory) -> Path:
         skipif.no_network()
         skipif.no_git()
-        path = str(tmp_path_factory.mktemp("gitrepo"))
+        path = tmp_path_factory.mktemp("gitrepo")
         lgr.debug("Cloning %r into %r", url, path)
-        run(["git", "clone", "--depth=1", url, path], check=True)
+        run(["git", "clone", "--depth=1", url, str(path)], check=True)
         if make_subdirs_dandisets:
             _make_subdirs_dandisets(path)
         return path
@@ -283,14 +284,14 @@ def get_filtered_gitrepo_fixture(
     url: str,
     whitelist: List[str],
     make_subdirs_dandisets: Optional[bool] = False,
-) -> Callable[[pytest.TempPathFactory], Iterator[str]]:
+) -> Callable[[pytest.TempPathFactory], Iterator[Path]]:
     @pytest.fixture(scope="session")
     def fixture(
         tmp_path_factory: pytest.TempPathFactory,
-    ) -> Iterator[str]:
+    ) -> Iterator[Path]:
         skipif.no_network()
         skipif.no_git()
-        path = str(tmp_path_factory.mktemp("gitrepo"))
+        path = tmp_path_factory.mktemp("gitrepo")
         lgr.debug("Cloning %r into %r", url, path)
         run(
             [
@@ -300,15 +301,15 @@ def fixture(
                 "--filter=blob:none",
                 "--sparse",
                 url,
-                path,
+                str(path),
             ],
             check=True,
         )
         # cwd specification is VERY important, not only to achieve the correct
         # effects, but also to avoid dropping files from your repository if you
         # were to run `git sparse-checkout` inside the software repo.
-        run(["git", "sparse-checkout", "init", "--cone"], cwd=path, check=True)
-        run(["git", "sparse-checkout", "set"] + whitelist, cwd=path, check=True)
+        run(["git", "sparse-checkout", "init", "--cone"], cwd=str(path), check=True)
+        run(["git", "sparse-checkout", "set"] + whitelist, cwd=str(path), check=True)
         if make_subdirs_dandisets:
             _make_subdirs_dandisets(path)
         yield path
@@ -316,9 +317,8 @@ def fixture(
     return fixture
 
 
-def _make_subdirs_dandisets(path: str) -> None:
-    for i in os.listdir(path):
-        bids_dataset_path = Path(path) / i
+def _make_subdirs_dandisets(path: Path) -> None:
+    for bids_dataset_path in path.iterdir():
         if bids_dataset_path.is_dir():
             (bids_dataset_path / dandiset_metadata_file).write_text(" \n")
 
@@ -547,45 +547,36 @@ def zarr_dandiset(new_dandiset: SampleDandiset) -> SampleDandiset:
 
 
 @pytest.fixture()
-def bids_dandiset(new_dandiset: SampleDandiset, bids_examples: str) -> SampleDandiset:
-    copytree(
-        os.path.join(bids_examples, "asl003"),
-        str(new_dandiset.dspath) + "/",
-    )
+def bids_dandiset(new_dandiset: SampleDandiset, bids_examples: Path) -> SampleDandiset:
+    copytree(bids_examples / "asl003", new_dandiset.dspath)
     (new_dandiset.dspath / "CHANGES").write_text("0.1.0 2014-11-03\n")
     return new_dandiset
 
 
 @pytest.fixture()
 def bids_nwb_dandiset(
-    new_dandiset: SampleDandiset, bids_examples: str
+    new_dandiset: SampleDandiset, bids_examples: Path
 ) -> SampleDandiset:
-    copytree(
-        os.path.join(bids_examples, "ieeg_epilepsyNWB"),
-        str(new_dandiset.dspath) + "/",
-    )
+    copytree(bids_examples / "ieeg_epilepsyNWB", new_dandiset.dspath)
     (new_dandiset.dspath / "CHANGES").write_text("0.1.0 2014-11-03\n")
     return new_dandiset
 
 
 @pytest.fixture()
 def bids_dandiset_invalid(
-    new_dandiset: SampleDandiset, bids_error_examples: str
+    new_dandiset: SampleDandiset, bids_error_examples: Path
 ) -> SampleDandiset:
     dataset_path = new_dandiset.dspath
-    copytree(
-        os.path.join(bids_error_examples, "invalid_pet001"),
-        str(dataset_path) + "/",
-    )
-    os.remove(os.path.join(dataset_path, "README"))
+    copytree(bids_error_examples / "invalid_pet001", dataset_path)
+    (dataset_path / "README").unlink()
     return new_dandiset
 
 
 @pytest.fixture()
-def video_files(tmp_path):
-    video_paths = []
+def video_files(tmp_path: Path) -> list[tuple[Path, Path]]:
     import cv2
 
+    video_paths = []
     video_path = tmp_path / "video_files"
     video_path.mkdir()
     for no in range(2):
diff --git a/dandi/tests/test_dandiapi.py b/dandi/tests/test_dandiapi.py
index 8e2580063..09717d7b9 100644
--- a/dandi/tests/test_dandiapi.py
+++ b/dandi/tests/test_dandiapi.py
@@ -1,7 +1,6 @@
 import builtins
 from datetime import datetime, timezone
 import logging
-import os.path
 from pathlib import Path
 import random
 import re
@@ -32,7 +31,9 @@
 from ..utils import list_paths
 
 
-def test_upload(new_dandiset: SampleDandiset, simple1_nwb: str, tmp_path: Path) -> None:
+def test_upload(
+    new_dandiset: SampleDandiset, simple1_nwb: Path, tmp_path: Path
+) -> None:
     d = new_dandiset.dandiset
     assert d.version_id == DRAFT
     d.upload_raw_asset(simple1_nwb, {"path": "testing/simple1.nwb"})
@@ -41,7 +42,7 @@ def test_upload(new_dandiset: SampleDandiset, simple1_nwb: str, tmp_path: Path)
     d.download_directory("", tmp_path)
     paths = list_paths(tmp_path)
     assert paths == [tmp_path / "testing" / "simple1.nwb"]
-    assert paths[0].stat().st_size == os.path.getsize(simple1_nwb)
+    assert paths[0].stat().st_size == simple1_nwb.stat().st_size
 
 
 def test_publish_and_manipulate(new_dandiset: SampleDandiset, tmp_path: Path) -> None:
@@ -92,7 +93,7 @@ def test_publish_and_manipulate(new_dandiset: SampleDandiset, tmp_path: Path) ->
     )
 
 
-def test_get_asset_metadata(new_dandiset: SampleDandiset, simple1_nwb: str) -> None:
+def test_get_asset_metadata(new_dandiset: SampleDandiset, simple1_nwb: Path) -> None:
     d = new_dandiset.dandiset
     d.upload_raw_asset(simple1_nwb, {"path": "testing/simple1.nwb", "foo": "bar"})
     (asset,) = d.get_assets()
diff --git a/dandi/tests/test_download.py b/dandi/tests/test_download.py
index 7570b4ecd..21d62a5bc 100644
--- a/dandi/tests/test_download.py
+++ b/dandi/tests/test_download.py
@@ -1,6 +1,5 @@
 import json
 import os
-import os.path as op
 from pathlib import Path
 import re
 from shutil import rmtree
@@ -123,12 +122,10 @@ def test_download_000027_resume(
     with (dldir / "checksum").open("w") as fp:
         json.dump(digests, fp)
     download(url, tmp_path, get_metadata=False)
-    contents = [
-        op.relpath(op.join(dirpath, entry), dsdir)
-        for (dirpath, dirnames, filenames) in os.walk(dsdir)
-        for entry in dirnames + filenames
+    assert list_paths(dsdir, dirs=True) == [
+        dsdir / "sub-RAT123",
+        dsdir / "sub-RAT123" / "sub-RAT123.nwb",
     ]
-    assert sorted(contents) == ["sub-RAT123", op.join("sub-RAT123", "sub-RAT123.nwb")]
     assert nwb.stat().st_size == size
     assert digester(str(nwb)) == digests
 
diff --git a/dandi/tests/test_files.py b/dandi/tests/test_files.py
index 99b50ea03..7e0ad2aed 100644
--- a/dandi/tests/test_files.py
+++ b/dandi/tests/test_files.py
@@ -310,7 +310,7 @@ def test_dandi_file_zarr_with_excluded_dotfiles(tmp_path: Path) -> None:
     assert isinstance(zf, ZarrAsset)
 
 
-def test_validate_simple1(simple1_nwb):
+def test_validate_simple1(simple1_nwb: Path) -> None:
     # this file should be ok as long as schema_version is specified
     errors = dandi_file(simple1_nwb).get_validation_errors(
         schema_version=get_schema_version()
@@ -318,9 +318,14 @@ def test_validate_simple1(simple1_nwb):
     assert [e.message for e in errors] == ["File is not inside a Dandiset"]
 
 
-def test_validate_simple1_no_subject(simple1_nwb):
+def test_validate_simple1_no_subject(simple1_nwb: Path) -> None:
     errors = dandi_file(simple1_nwb).get_validation_errors()
-    assert sorted(e.message for e in errors) == [
+    errmsgs = []
+    for e in errors:
+        assert e.message is not None
+        errmsgs.append(e.message)
+    errmsgs.sort()
+    assert errmsgs == [
         "File is not inside a Dandiset",
         "Subject is missing.",
     ]
@@ -344,9 +349,14 @@ def test_validate_simple2_new(organized_nwb_dir: Path) -> None:
     assert not errors
 
 
-def test_validate_simple3_no_subject_id(simple3_nwb):
+def test_validate_simple3_no_subject_id(simple3_nwb: Path) -> None:
     errors = dandi_file(simple3_nwb).get_validation_errors()
-    assert sorted(e.message for e in errors) == [
+    errmsgs = []
+    for e in errors:
+        assert e.message is not None
+        errmsgs.append(e.message)
+    errmsgs.sort()
+    assert errmsgs == [
         "File is not inside a Dandiset",
         "subject_id is missing.",
     ]
diff --git a/dandi/tests/test_metadata.py b/dandi/tests/test_metadata.py
index f02e2666a..2bd14c4bb 100644
--- a/dandi/tests/test_metadata.py
+++ b/dandi/tests/test_metadata.py
@@ -1,6 +1,5 @@
 from datetime import datetime, timedelta
 import json
-import os
 from pathlib import Path
 import shutil
 from shutil import copyfile
@@ -53,7 +52,7 @@
 METADATA_DIR = Path(__file__).with_name("data") / "metadata"
 
 
-def test_get_metadata(simple1_nwb: str, simple1_nwb_metadata: Dict[str, Any]) -> None:
+def test_get_metadata(simple1_nwb: Path, simple1_nwb_metadata: Dict[str, Any]) -> None:
     target_metadata = simple1_nwb_metadata.copy()
     # we will also get some counts
     target_metadata["number_of_electrodes"] = 0
@@ -65,7 +64,7 @@ def test_get_metadata(simple1_nwb: str, simple1_nwb_metadata: Dict[str, Any]) ->
     # we do not populate any subject fields in our simple1_nwb
     for f in metadata_nwb_subject_fields:
         target_metadata[f] = None
-    metadata = get_metadata(str(simple1_nwb))
+    metadata = get_metadata(simple1_nwb)
     # we also load nwb_version field, so it must not be degenerate and ATM
     # it is 2.X.Y. And since I don't know how to query pynwb on what
     # version it currently "supports", we will just pop it
@@ -73,7 +72,7 @@ def test_get_metadata(simple1_nwb: str, simple1_nwb_metadata: Dict[str, Any]) ->
     assert target_metadata == metadata
 
 
-def test_bids_nwb_metadata_integration(bids_examples, tmp_path):
+def test_bids_nwb_metadata_integration(bids_examples: Path, tmp_path: Path) -> None:
     """
     Notes
     -----
@@ -82,15 +81,16 @@ def test_bids_nwb_metadata_integration(bids_examples, tmp_path):
         https://github.com/dandi/dandi-cli/pull/1183#discussion_r1061622910
     """
 
-    source_dpath = os.path.join(bids_examples, "ieeg_epilepsyNWB")
-    dpath = os.path.join(tmp_path, "ieeg_epilepsyNWB")
+    source_dpath = bids_examples / "ieeg_epilepsyNWB"
+    dpath = tmp_path / "ieeg_epilepsyNWB"
     shutil.copytree(source_dpath, dpath)
 
-    file_path = os.path.join(
-        dpath,
-        *"sub-01/ses-postimp/ieeg/sub-01_ses-postimp_task-seizure_run-01_ieeg.nwb".split(
-            "/"
-        )
+    file_path = (
+        dpath
+        / "sub-01"
+        / "ses-postimp"
+        / "ieeg"
+        / "sub-01_ses-postimp_task-seizure_run-01_ieeg.nwb"
     )
     metadata = get_metadata(file_path)
     # This is a key sourced from both NWB and BIDS:
@@ -726,7 +726,7 @@ def test_ndtypes(ndtypes, asset_dict):
 
 
 @mark.skipif_no_network
-def test_nwb2asset(simple2_nwb: str) -> None:
+def test_nwb2asset(simple2_nwb: Path) -> None:
     assert nwb2asset(simple2_nwb, digest=DUMMY_DANDI_ETAG) == BareAsset.unvalidated(
         schemaKey="Asset",
         schemaVersion=DANDI_SCHEMA_VERSION,
@@ -767,7 +767,7 @@ def test_nwb2asset(simple2_nwb: str) -> None:
         contentSize=19664,
         encodingFormat="application/x-nwb",
         digest={DigestType.dandi_etag: "dddddddddddddddddddddddddddddddd-1"},
-        path=simple2_nwb,
+        path=str(simple2_nwb),
         dateModified=ANY_AWARE_DATETIME,
         blobDateModified=ANY_AWARE_DATETIME,
         wasAttributedTo=[
diff --git a/dandi/tests/test_organize.py b/dandi/tests/test_organize.py
index 7eb203470..990157e99 100644
--- a/dandi/tests/test_organize.py
+++ b/dandi/tests/test_organize.py
@@ -1,6 +1,5 @@
 from __future__ import annotations
 
-from glob import glob
 import os
 import os.path as op
 from pathlib import Path
@@ -111,8 +110,8 @@ def c() -> Any:  # shortcut
 
 @pytest.mark.integration
 @pytest.mark.parametrize("mode", no_move_modes)
-def test_organize_nwb_test_data(nwb_test_data: str, tmp_path: Path, mode: str) -> None:
-    outdir = str(tmp_path / "organized")
+def test_organize_nwb_test_data(nwb_test_data: Path, tmp_path: Path, mode: str) -> None:
+    outdir = tmp_path / "organized"
 
     relative = False
     if mode == "symlink-relative":
@@ -123,8 +122,8 @@ def test_organize_nwb_test_data(nwb_test_data: str, tmp_path: Path, mode: str) -
         # organize also organize using relative paths in case of 'symlink'
         # mode
         cwd = os.getcwd()
-        nwb_test_data = op.relpath(nwb_test_data, cwd)
-        outdir = op.relpath(outdir, cwd)
+        nwb_test_data = Path(op.relpath(nwb_test_data, cwd))
+        outdir = Path(op.relpath(outdir, cwd))
 
     src = tmp_path / "src"
     src.touch()
@@ -151,16 +150,16 @@ def test_organize_nwb_test_data(nwb_test_data: str, tmp_path: Path, mode: str) -
     elif mode == "hardlink" and not hardlinks_work:
         pytest.skip("Hard links not supported")
 
-    input_files = op.join(nwb_test_data, "v2.0.1")
+    input_files = nwb_test_data / "v2.0.1"
 
-    cmd = ["-d", outdir, "--files-mode", mode, input_files]
+    cmd = ["-d", str(outdir), "--files-mode", mode, str(input_files)]
     r = CliRunner().invoke(organize, cmd)
 
     # with @map_to_click_exceptions we loose original str of message somehow
     # although it is shown to the user - checked. TODO - figure it out
     # assert "not containing all" in str(r.exc_info[1])
     assert r.exit_code != 0, "Must have aborted since many files lack subject_id"
-    assert not glob(op.join(outdir, "*")), "no files should have been populated"
+    assert not any(outdir.glob("*")), "no files should have been populated"
 
     r = CliRunner().invoke(organize, cmd + ["--invalid", "warn"])
     assert r.exit_code == 0
@@ -187,19 +186,19 @@ def test_organize_nwb_test_data(nwb_test_data: str, tmp_path: Path, mode: str) -
         assert not any(op.islink(p) for p in produced_paths)
 
 
-def test_ambiguous(simple2_nwb: str, tmp_path: Path) -> None:
+def test_ambiguous(simple2_nwb: Path, tmp_path: Path) -> None:
     copy2 = copy_nwb_file(simple2_nwb, tmp_path)
-    outdir = str(tmp_path / "organized")
-    args = ["--files-mode", "copy", "-d", outdir, simple2_nwb, copy2]
+    outdir = tmp_path / "organized"
+    args = ["--files-mode", "copy", "-d", str(outdir), str(simple2_nwb), copy2]
     r = CliRunner().invoke(organize, args)
     assert r.exit_code == 0
-    produced_paths = sorted(find_files(".*", paths=outdir))
-    produced_paths_rel = [op.relpath(p, outdir) for p in produced_paths]
-    assert produced_paths_rel == sorted(
-        op.join(
-            "sub-mouse001", "sub-mouse001_obj-%s.nwb" % get_obj_id(get_object_id(f))
-        )
-        for f in [simple2_nwb, copy2]
+    assert list_paths(outdir) == sorted(
+        [
+            outdir
+            / "sub-mouse001"
+            / f"sub-mouse001_obj-{get_obj_id(get_object_id(f))}.nwb"
+            for f in [simple2_nwb, Path(copy2)]
+        ]
     )
 
 
@@ -350,7 +349,7 @@ def test_validate_organized_path(path: str, error_ids: list[str]) -> None:
     assert [e.id for e in errors] == error_ids
 
 
-def test_organize_required_field(simple2_nwb: str, tmp_path: Path) -> None:
+def test_organize_required_field(simple2_nwb: Path, tmp_path: Path) -> None:
     (tmp_path / dandiset_metadata_file).write_text("{}\n")
     r = CliRunner().invoke(
         organize,
@@ -360,7 +359,7 @@ def test_organize_required_field(simple2_nwb: str, tmp_path: Path) -> None:
             "--dandiset-path",
             str(tmp_path),
             "--required-field=session_id",
-            simple2_nwb,
+            str(simple2_nwb),
         ],
     )
     assert r.exit_code == 0
diff --git a/dandi/tests/test_pynwb_utils.py b/dandi/tests/test_pynwb_utils.py
index 298d973ff..9f19998bd 100644
--- a/dandi/tests/test_pynwb_utils.py
+++ b/dandi/tests/test_pynwb_utils.py
@@ -1,4 +1,5 @@
 from datetime import datetime, timezone
+from pathlib import Path
 import re
 from typing import Any, Callable, NoReturn
 
@@ -8,10 +9,10 @@
 from ..pynwb_utils import _sanitize_nwb_version, nwb_has_external_links
 
 
-def test_pynwb_io(simple1_nwb: str) -> None:
+def test_pynwb_io(simple1_nwb: Path) -> None:
     # To verify that our dependencies spec is sufficient to avoid
     # stepping into known pynwb/hdmf issues
-    with NWBHDF5IO(str(simple1_nwb), "r", load_namespaces=True) as reader:
+    with NWBHDF5IO(simple1_nwb, "r", load_namespaces=True) as reader:
         nwbfile = reader.read()
     assert repr(nwbfile)
     assert str(nwbfile)
diff --git a/dandi/tests/test_validate.py b/dandi/tests/test_validate.py
index e80552e2b..90e51cd2a 100644
--- a/dandi/tests/test_validate.py
+++ b/dandi/tests/test_validate.py
@@ -1,6 +1,5 @@
 import json
-import os
-import pathlib
+from pathlib import Path
 
 import pytest
 
@@ -8,25 +7,21 @@
 from ..validate import validate
 
 
-def test_validate_nwb_error(simple3_nwb):
-    """
-    Do we fail on critical NWB validation errors?
-    """
-
+def test_validate_nwb_error(simple3_nwb: Path) -> None:
+    """Do we fail on critical NWB validation errors?"""
     validation_result = validate(simple3_nwb)
     assert len([i for i in validation_result if i.severity]) > 0
 
 
 @pytest.mark.parametrize("dataset", BIDS_TESTDATA_SELECTION)
-def test_validate_bids(bids_examples, tmp_path, dataset):
-
-    selected_dataset = os.path.join(bids_examples, dataset)
+def test_validate_bids(bids_examples: Path, tmp_path: Path, dataset: str) -> None:
+    selected_dataset = bids_examples / dataset
     validation_result = validate(selected_dataset)
     for i in validation_result:
         assert i.severity is None
 
 
-def test_validate_bids_onefile(bids_error_examples, tmp_path):
+def test_validate_bids_onefile(bids_error_examples: Path, tmp_path: Path) -> None:
     """
     Dedicated test using single-file validation.
 
@@ -39,37 +34,30 @@ def test_validate_bids_onefile(bids_error_examples, tmp_path):
     a whole anyway.
     """
 
-    selected_dataset, error_file = (
-        "invalid_asl003",
-        "sub-Sub1/perf/sub-Sub1_headshape.jpg",
-    )
+    selected_dataset = "invalid_asl003"
+    error_file = Path("sub-Sub1/perf/sub-Sub1_headshape.jpg")
 
-    bids_file_path = os.path.join(bids_error_examples, selected_dataset, error_file)
-    bids_dataset_path = pathlib.Path(
-        bids_error_examples, selected_dataset
-    )
-    error_reference = os.path.join(bids_dataset_path, ".ERRORS.json")
-    with open(error_reference) as f:
+    bids_file_path = bids_error_examples / selected_dataset / error_file
+    error_reference = bids_error_examples / selected_dataset / ".ERRORS.json"
+    with error_reference.open() as f:
         expected_errors = json.load(f)
     validation_result = validate(bids_file_path)
     for i in validation_result:
         error_id = i.id
-        error_path = i.path
-        relative_error_path = os.path.relpath(error_path, i.dataset_path)
-        relative_error_path = pathlib.Path(relative_error_path).as_posix()
+        assert i.path is not None
+        assert i.dataset_path is not None
+        relative_error_path = i.path.relative_to(i.dataset_path).as_posix()
         assert relative_error_path in expected_errors[error_id.lstrip("BIDS.")]["scope"]
 
 
 @pytest.mark.parametrize("dataset", BIDS_ERROR_TESTDATA_SELECTION)
-def test_validate_bids_errors(bids_error_examples, dataset):
-    # This only checks that the error we found is correct, not that we found all errors.
-    # ideally make a list and erode etc.
-
-    selected_dataset = os.path.join(bids_error_examples, dataset)
-    validation_result = validate(selected_dataset)
-    with open(os.path.join(selected_dataset, ".ERRORS.json")) as f:
+def test_validate_bids_errors(bids_error_examples: Path, dataset: str) -> None:
+    # This only checks that the error we found is correct, not that we found
+    # all errors.  ideally make a list and erode etc.
+    selected_dataset = bids_error_examples / dataset
+    validation_result = list(validate(selected_dataset))
+    with (selected_dataset / ".ERRORS.json").open() as f:
         expected_errors = json.load(f)
-    validation_result = [i for i in validation_result]
 
     # We know that these datasets contain errors.
     assert len(validation_result) > 0
@@ -79,10 +67,9 @@ def test_validate_bids_errors(bids_error_examples, dataset):
         if i.id == "BIDS.MATCH":
             continue
         error_id = i.id
-        if i.path:
-            error_path = i.path
-            relative_error_path = os.path.relpath(error_path, i.dataset_path)
-            relative_error_path = pathlib.Path(relative_error_path).as_posix()
+        if i.path is not None:
+            assert i.dataset_path is not None
+            relative_error_path = i.path.relative_to(i.dataset_path).as_posix()
             assert (
                 relative_error_path
                 in expected_errors[error_id.lstrip("BIDS.")]["scope"]
diff --git a/dandi/validate.py b/dandi/validate.py
index 0bdfec666..509dd0bc3 100644
--- a/dandi/validate.py
+++ b/dandi/validate.py
@@ -130,7 +130,7 @@ def validate_bids(
 
 
 def validate(
-    *paths: str,
+    *paths: str | Path,
     schema_version: Optional[str] = None,
     devel_debug: bool = False,
     allow_any_path: bool = False,