diff --git a/.gitignore b/.gitignore
index d935b35..4007403 100644
--- a/.gitignore
+++ b/.gitignore
@@ -211,9 +211,7 @@ __marimo__/
 
 # Output artifacts
 decoded/
-tests/ACMS80217/
-tests/ACMS80221/
-tests/ACMS80227/
+tests/ACMS*/
 .qc_cache/
 8-00014-OIS_100-Software/
 8-00015-Other-HS32-Software/
diff --git a/pyproject.toml b/pyproject.toml
index 035a997..4b4f86a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -44,7 +44,6 @@ combine-as-imports = true
 known-first-party = ["hardsector_tool"]
 
 [tool.pytest.ini_options]
-pythonpath = ["src"]
 testpaths = ["tests"]
 addopts = "-ra"
 markers = [
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..7face3a
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+import importlib.util
+import sys
+from pathlib import Path
+
+import pytest
+
+
+def repo_src_path() -> Path:
+    """Return the repository's ``src`` directory."""
+
+    return Path(__file__).resolve().parents[1] / "src"
+
+
+def _ensure_repo_on_path() -> None:
+    if importlib.util.find_spec("hardsector_tool") is None:
+        sys.path.insert(0, str(repo_src_path()))
+
+
+_ensure_repo_on_path()
+
+
+def require_fixture(path: Path) -> Path:
+    if not path.exists():
+        pytest.skip(f"fixture not available: {path}", allow_module_level=True)
+    return path
diff --git a/tests/fixtures/README.md b/tests/fixtures/README.md
new file mode 100644
index 0000000..6d6db9c
--- /dev/null
+++ b/tests/fixtures/README.md
@@ -0,0 +1,17 @@
+# SCP fixtures
+
+Large SCP captures used by the slow tests are not checked into git. To exercise
+those tests, place the files in the following locations:
+
+- `tests/ACMS80217/ACMS80217-HS32.scp`
+- `tests/ACMS80221/ACMS80221-HS32.scp`
+- `tests/ACMS80227/ACMS80227-HS32.scp`
+
+Additional ACMS fixture directories follow the same pattern under `tests/ACMS*/`.
+
+By default the tests will skip scenarios that require these captures when the
+files are absent. To run them explicitly, supply the slow marker:
+
+```bash
+pytest -m slow
+```
diff --git a/tests/test_dominant_prefix_rescue.py b/tests/test_dominant_prefix_rescue.py
index cb7057d..ba95060 100644
--- a/tests/test_dominant_prefix_rescue.py
+++ b/tests/test_dominant_prefix_rescue.py
@@ -2,15 +2,18 @@
 
 import pytest
 
+from conftest import require_fixture
 from hardsector_tool.scp import SCPImage
 from hardsector_tool.wang import checksum_prefixes, dominant_prefix, reconstruct_track
 
-FIXTURE = Path("tests/ACMS80221/ACMS80221-HS32.scp")
+pytestmark = pytest.mark.slow
 
+FIXTURE = Path("tests/ACMS80221/ACMS80221-HS32.scp")
 
-@pytest.mark.skipif(not FIXTURE.exists(), reason="ACMS80221 fixture missing")
 def test_dominant_prefix_rescue() -> None:
-    image = SCPImage.from_file(FIXTURE)
+    fixture = require_fixture(FIXTURE)
+
+    image = SCPImage.from_file(fixture)
     sector_map, _, _ = reconstruct_track(
         image,
         track_number=0,
diff --git a/tests/test_fm_decode.py b/tests/test_fm_decode.py
index b78e5f0..574ac11 100644
--- a/tests/test_fm_decode.py
+++ b/tests/test_fm_decode.py
@@ -1,5 +1,8 @@
 from pathlib import Path
 
+import pytest
+
+from conftest import require_fixture
 from hardsector_tool.fm import (
     best_aligned_bytes,
     brute_force_mark_payloads,
@@ -14,7 +17,9 @@
 )
 from hardsector_tool.scp import SCPImage
 
-FIXTURE = Path("tests/ACMS80217/ACMS80217-HS32.scp")
+pytestmark = pytest.mark.slow
+
+FIXTURE = require_fixture(Path("tests/ACMS80217/ACMS80217-HS32.scp"))
 
 
 def test_fm_decode_produces_bytes() -> None:
diff --git a/tests/test_gw_hardsector_clockfactor.py b/tests/test_gw_hardsector_clockfactor.py
index 6531f8d..8fffd3f 100644
--- a/tests/test_gw_hardsector_clockfactor.py
+++ b/tests/test_gw_hardsector_clockfactor.py
@@ -1,12 +1,18 @@
 from pathlib import Path
 
+from pathlib import Path
+
 import pytest
 
+from conftest import require_fixture
 from hardsector_tool.fm import pll_decode_fm_bytes
 from hardsector_tool.hardsector import payload_metrics
 from hardsector_tool.scp import SCPImage
 
 
+pytestmark = pytest.mark.slow
+
+
 FIXTURES = [
     Path("tests/ACMS80217/ACMS80217-HS32.scp"),
     Path("tests/ACMS80221/ACMS80221-HS32.scp"),
@@ -19,8 +25,7 @@
     ids=[path.parent.name for path in FIXTURES],
 )
 def test_clock_factor_one_increases_entropy(scp_path: Path) -> None:
-    if not scp_path.exists():
-        pytest.skip(f"Fixture not present: {scp_path}")
+    scp_path = require_fixture(scp_path)
 
     image = SCPImage.from_file(scp_path)
     track = image.read_track(0)
diff --git a/tests/test_hardsector_grouping.py b/tests/test_hardsector_grouping.py
index c208d28..3aff26d 100644
--- a/tests/test_hardsector_grouping.py
+++ b/tests/test_hardsector_grouping.py
@@ -1,5 +1,8 @@
 from pathlib import Path
 
+import pytest
+
+from conftest import require_fixture
 from hardsector_tool.hardsector import (
     FORMAT_PRESETS,
     best_sector_map,
@@ -9,12 +12,16 @@
 )
 from hardsector_tool.scp import RevolutionEntry, SCPImage, TrackData
 
+pytestmark = pytest.mark.slow
+
 FIXTURE = Path("tests/ACMS80217/ACMS80217-HS32.scp")
 FIXTURE_221 = Path("tests/ACMS80221/ACMS80221-HS32.scp")
 
 
 def test_grouping_matches_expected_rotations() -> None:
-    image = SCPImage.from_file(FIXTURE)
+    fixture = require_fixture(FIXTURE)
+
+    image = SCPImage.from_file(fixture)
     track = image.read_track(0)
     assert track is not None
 
@@ -28,7 +35,9 @@ def test_grouping_matches_expected_rotations() -> None:
 
 
 def test_grouping_pairs_holes_into_logical_sectors() -> None:
-    image = SCPImage.from_file(FIXTURE)
+    fixture = require_fixture(FIXTURE)
+
+    image = SCPImage.from_file(fixture)
     track = image.read_track(0)
     assert track is not None
 
@@ -40,7 +49,9 @@ def test_grouping_pairs_holes_into_logical_sectors() -> None:
 
 
 def test_detects_short_pair_position() -> None:
-    image = SCPImage.from_file(FIXTURE)
+    fixture = require_fixture(FIXTURE)
+
+    image = SCPImage.from_file(fixture)
     track = image.read_track(0)
     assert track is not None
 
@@ -53,7 +64,9 @@ def test_detects_short_pair_position() -> None:
 
 
 def test_short_pair_position_other_fixture() -> None:
-    image = SCPImage.from_file(FIXTURE_221)
+    fixture = require_fixture(FIXTURE_221)
+
+    image = SCPImage.from_file(fixture)
     track = image.read_track(0)
     assert track is not None
 
@@ -154,7 +167,9 @@ def test_merges_shortest_adjacent_intervals() -> None:
 
 
 def test_normalization_yields_single_index_pair_per_rotation() -> None:
-    image = SCPImage.from_file(FIXTURE)
+    fixture = require_fixture(FIXTURE)
+
+    image = SCPImage.from_file(fixture)
     track = image.read_track(0)
     assert track is not None
 
diff --git a/tests/test_reconstruct_disk_cli.py b/tests/test_reconstruct_disk_cli.py
index 4611277..b58e2da 100644
--- a/tests/test_reconstruct_disk_cli.py
+++ b/tests/test_reconstruct_disk_cli.py
@@ -8,6 +8,11 @@
 
 import pytest
 
+from conftest import repo_src_path, require_fixture
+
+
+pytestmark = pytest.mark.slow
+
 
 FIXTURES = [
     Path("tests/ACMS80217/ACMS80217-HS32.scp"),
@@ -17,9 +22,12 @@
 
 @pytest.mark.parametrize("fixture_path", FIXTURES)
 def test_reconstruct_disk_track0(tmp_path: Path, fixture_path: Path) -> None:
+    fixture_path = require_fixture(fixture_path)
     out_dir = tmp_path / "reconstruct"
     env = os.environ.copy()
-    env["PYTHONPATH"] = str(Path(__file__).resolve().parents[1] / "src")
+    env["PYTHONPATH"] = os.pathsep.join(
+        filter(None, [str(repo_src_path()), env.get("PYTHONPATH")])
+    )
 
     cmd = [
         sys.executable,
@@ -51,9 +59,12 @@ def test_reconstruct_disk_track0(tmp_path: Path, fixture_path: Path) -> None:
 
 @pytest.mark.parametrize("fixture_path", FIXTURES)
 def test_reconstruct_disk_track_mapping(tmp_path: Path, fixture_path: Path) -> None:
+    fixture_path = require_fixture(fixture_path)
     out_dir = tmp_path / "reconstruct_mapping"
     env = os.environ.copy()
-    env["PYTHONPATH"] = str(Path(__file__).resolve().parents[1] / "src")
+    env["PYTHONPATH"] = os.pathsep.join(
+        filter(None, [str(repo_src_path()), env.get("PYTHONPATH")])
+    )
 
     cmd = [
         sys.executable,
diff --git a/tests/test_scp_parser.py b/tests/test_scp_parser.py
index 4bd1515..d920443 100644
--- a/tests/test_scp_parser.py
+++ b/tests/test_scp_parser.py
@@ -2,9 +2,12 @@
 
 import pytest
 
+from conftest import require_fixture
 from hardsector_tool.scp import SCPImage
 
-FIXTURE = Path("tests/ACMS80217/ACMS80217-HS32.scp")
+pytestmark = pytest.mark.slow
+
+FIXTURE = require_fixture(Path("tests/ACMS80217/ACMS80217-HS32.scp"))
 
 
 @pytest.fixture(scope="session")
diff --git a/tests/test_wang.py b/tests/test_wang.py
index a4faf57..a7d37f0 100644
--- a/tests/test_wang.py
+++ b/tests/test_wang.py
@@ -2,16 +2,21 @@
 
 import pytest
 
+from conftest import require_fixture
 from hardsector_tool.hardsector import group_hard_sectors, pair_holes
 from hardsector_tool.scp import SCPImage
 from hardsector_tool.wang import reconstruct_track, scan_wang_frames
 
+pytestmark = pytest.mark.slow
+
 FIXTURE = Path("tests/ACMS80217/ACMS80217-HS32.scp")
 FIXTURE_221 = Path("tests/ACMS80221/ACMS80221-HS32.scp")
 
 
 def test_wang_scan_frame_detects_headers() -> None:
-    image = SCPImage.from_file(FIXTURE)
+    fixture = require_fixture(FIXTURE)
+
+    image = SCPImage.from_file(fixture)
     track = image.read_track(0)
     assert track is not None
 
@@ -44,7 +49,9 @@ def test_scan_wang_frames_accepts_plausible_offsets() -> None:
 
 
 def test_reconstruct_track_sets_window_metadata() -> None:
-    image = SCPImage.from_file(FIXTURE)
+    fixture = require_fixture(FIXTURE)
+
+    image = SCPImage.from_file(fixture)
     reconstructed, recon, _ = reconstruct_track(
         image,
         0,
@@ -61,7 +68,9 @@ def test_reconstruct_track_sets_window_metadata() -> None:
 
 
 def test_reconstruct_track_supports_unpaired_sectors() -> None:
-    image = SCPImage.from_file(FIXTURE)
+    fixture = require_fixture(FIXTURE)
+
+    image = SCPImage.from_file(fixture)
     reconstructed, _, _ = reconstruct_track(
         image,
         0,
@@ -78,10 +87,9 @@ def test_reconstruct_track_supports_unpaired_sectors() -> None:
 
 
 def test_reconstruct_track_promotes_dominant_transform_family() -> None:
-    if not FIXTURE_221.exists():
-        pytest.skip("ACMS80221 fixture missing")
+    fixture = require_fixture(FIXTURE_221)
 
-    image = SCPImage.from_file(FIXTURE_221)
+    image = SCPImage.from_file(fixture)
     reconstructed, _, _ = reconstruct_track(
         image,
         0,