diff --git a/heart/checks/test_run.py b/heart/checks/test_run.py
index b12bce6..c9ec0be 100644
--- a/heart/checks/test_run.py
+++ b/heart/checks/test_run.py
@@ -225,10 +225,6 @@ def run(results_dir: Path | None = None, fetch_cloud: bool | None = None) -> dic
         summary["cloud_url"] = cloud["url"]
         summary["source"] = "cloud"
 
-    sys.path.insert(0, str(HEART_HOME))
-    from heart import state
-
-    state.atomic_write_json(HEART_STATE_DIR / "test_run.json", summary)
     return summary
 
 
@@ -237,6 +233,12 @@ def main(argv: list[str]) -> int:
     summary = run(results_dir)
 
     sys.path.insert(0, str(HEART_HOME))
+    from heart import state
+
+    # Persist only here, at the tick/CLI entrypoint — run() is side-effect-free
+    # so library callers (and the test suite) can never clobber live state.
+    state.atomic_write_json(HEART_STATE_DIR / "test_run.json", summary)
+
     from heart.heart_color import c_ok, c_warn, c_fail, c_info, c_meta, glyph_ok, glyph_warn, glyph_fail
 
     if not summary:
diff --git a/heart/checks/version_skew.py b/heart/checks/version_skew.py
index 0f330eb..7e36086 100644
--- a/heart/checks/version_skew.py
+++ b/heart/checks/version_skew.py
@@ -157,16 +157,18 @@ def run(root: Path = PYAUTO_ROOT) -> dict[str, Any]:
             }
         )
     result = {"workspaces": workspaces}
-    sys.path.insert(0, str(HEART_HOME))
-    from heart import state
-
-    state.atomic_write_json(HEART_STATE_DIR / "version_skew.json", result)
     return result
 
 
 def main(argv: list[str]) -> int:
     result = run()
     sys.path.insert(0, str(HEART_HOME))
+    from heart import state
+
+    # Persist only here, at the tick/CLI entrypoint — run() is side-effect-free
+    # so library callers (and the test suite) can never clobber live state.
+    state.atomic_write_json(HEART_STATE_DIR / "version_skew.json", result)
+
     from heart.heart_color import c_ok, c_warn, c_fail, c_info, c_meta, glyph_ok, glyph_warn, glyph_fail
 
     workspaces = result["workspaces"]
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..7fc695e
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,16 @@
+"""tests/conftest.py — sandbox HEART_STATE_DIR for the whole suite.
+
+heart/checks/* resolve HEART_STATE_DIR at import time, so this assignment must
+happen before any test module imports them; conftest import is pytest's
+earliest hook, which is why this is a module-level statement and not a fixture.
+Belt-and-braces on top of the checks' run()/main() split (state writes live
+only in main()): no test, present or future, can reach the developer's live
+~/.pyauto-heart — that state is the input to the release gate.
+"""
+
+from __future__ import annotations
+
+import os
+import tempfile
+
+os.environ["HEART_STATE_DIR"] = tempfile.mkdtemp(prefix="pyauto-heart-test-state-")
diff --git a/tests/test_test_run.py b/tests/test_test_run.py
index 984c2fb..eb45884 100644
--- a/tests/test_test_run.py
+++ b/tests/test_test_run.py
@@ -150,3 +150,30 @@ def test_server_verdict_prefers_agent_file_over_gh(monkeypatch, tmp_path):
 def test_agent_supplied_verdict_absent_is_none(monkeypatch, tmp_path):
     monkeypatch.setattr(tr, "VALIDATION_FILE", tmp_path / "nope.json")
     assert tr._agent_supplied_verdict() is None
+
+
+# --- state-dir isolation (the 2026-07-15 clobber incident) ---------------------
+
+def test_run_writes_nothing_to_state_dir(tmp_path):
+    """run() must be side-effect-free: the write lives in main() only, so tests
+    (and any library caller) can never clobber live Heart state."""
+    import os
+    from pathlib import Path
+    state_dir = Path(os.environ["HEART_STATE_DIR"])
+    before = set(state_dir.glob("**/*")) if state_dir.exists() else set()
+    (tmp_path / "report.json").write_text(json.dumps({
+        "ready": True, "run_label": "R1", "summary": {"passed": 1}}))
+    tr.run(results_dir=tmp_path)
+    after = set(state_dir.glob("**/*")) if state_dir.exists() else set()
+    assert after == before
+
+
+def test_main_persists_summary_to_state_dir(tmp_path):
+    """The tick path (python -m heart.checks.test_run) must still persist."""
+    import os
+    from pathlib import Path
+    (tmp_path / "report.json").write_text(json.dumps({
+        "ready": True, "run_label": "R1", "summary": {"passed": 1}}))
+    assert tr.main(["test_run", str(tmp_path)]) == 0
+    written = json.loads((Path(os.environ["HEART_STATE_DIR"]) / "test_run.json").read_text())
+    assert written["ready"] is True and written["passed"] == 1
diff --git a/tests/test_version_skew.py b/tests/test_version_skew.py
index 28424f6..0fc255e 100644
--- a/tests/test_version_skew.py
+++ b/tests/test_version_skew.py
@@ -119,3 +119,28 @@ def test_autolens_assistant_is_a_pinned_workspace():
     mapping = vs.workspace_library()
     assert "autolens_assistant" in mapping
     assert mapping["autolens_assistant"] == ("PyAutoLens", "autolens")
+
+
+# --- state-dir isolation (the 2026-07-15 clobber incident's sibling) -----------
+
+def test_run_writes_nothing_to_state_dir(tmp_path):
+    """run() must be side-effect-free: the write lives in main() only, so tests
+    (and any library caller) can never clobber live Heart state."""
+    import os
+    from pathlib import Path
+    state_dir = Path(os.environ["HEART_STATE_DIR"])
+    before = set(state_dir.glob("**/*")) if state_dir.exists() else set()
+    vs.run(root=tmp_path)
+    after = set(state_dir.glob("**/*")) if state_dir.exists() else set()
+    assert after == before
+
+
+def test_main_persists_result_to_state_dir(monkeypatch):
+    """The tick path (python -m heart.checks.version_skew) must still persist."""
+    import json
+    import os
+    from pathlib import Path
+    monkeypatch.setattr(vs, "run", lambda root=vs.PYAUTO_ROOT: {"workspaces": []})
+    assert vs.main(["version_skew"]) == 0
+    written = json.loads((Path(os.environ["HEART_STATE_DIR"]) / "version_skew.json").read_text())
+    assert written == {"workspaces": []}