diff --git a/tests/conftest.py b/tests/conftest.py
index f7c9e8c..1873223 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,4 +1,5 @@
 """Shared pytest fixtures for the Gitea-Tools test suite."""
+import os
 import sys
 
 import pytest
@@ -16,13 +17,41 @@ def _reset_mutation_authority(monkeypatch):
     the next test. It must never replace verify_mutation_authority with a
     no-op: individual tests that need a specific authority state set it up
     explicitly.
+
+    Session-state isolation (#559 / #590 / #594): many tests use
+    ``patch.dict(os.environ, ..., clear=True)``, which drops
+    ``GITEA_MCP_SESSION_STATE_DIR``. Pin ``default_state_dir`` /
+    ``DEFAULT_STATE_DIR`` to a per-test temp dir so durable load/save never
+    touches host state even after env clears.
     """
     monkeypatch.delenv("GITEA_SESSION_PROFILE_LOCK", raising=False)
     # Isolate durable session-state files so tests never share host cache (#559).
     import tempfile
 
     _state_tmp = tempfile.TemporaryDirectory(prefix="gitea-session-state-")
-    monkeypatch.setenv("GITEA_MCP_SESSION_STATE_DIR", _state_tmp.name)
+    state_dir = _state_tmp.name
+    monkeypatch.setenv("GITEA_MCP_SESSION_STATE_DIR", state_dir)
+    try:
+        import mcp_session_state
+    except Exception:
+        mcp_session_state = None
+    if mcp_session_state is not None:
+        monkeypatch.setattr(
+            mcp_session_state, "DEFAULT_STATE_DIR", state_dir, raising=False
+        )
+
+        def _isolated_default_state_dir(
+            _fallback: str = state_dir,
+            _env_key: str = mcp_session_state.STATE_DIR_ENV,
+        ) -> str:
+            raw = (os.environ.get(_env_key) or "").strip()
+            return raw or _fallback
+
+        monkeypatch.setattr(
+            mcp_session_state,
+            "default_state_dir",
+            _isolated_default_state_dir,
+        )
     try:
         import mcp_server
     except Exception:
@@ -40,6 +69,11 @@ def _reset_mutation_authority(monkeypatch):
     monkeypatch.setattr(mcp_server, "_preflight_capability_baseline_porcelain", None)
     monkeypatch.setattr(mcp_server, "_preflight_resolved_task", None)
     monkeypatch.setattr(mcp_server, "_preflight_reviewer_violation_files", [])
+    monkeypatch.setattr(
+        mcp_server,
+        "_check_mcp_runtimes_diagnostics",
+        lambda *args, **kwargs: [],
+    )
     try:
         import review_workflow_load
         review_workflow_load._REVIEW_WORKFLOW_LOAD = None
diff --git a/tests/test_mcp_server.py b/tests/test_mcp_server.py
index d52263b..a793e15 100644
--- a/tests/test_mcp_server.py
+++ b/tests/test_mcp_server.py
@@ -774,6 +774,9 @@ class TestMergePR(unittest.TestCase):
     def setUp(self):
         import reviewer_pr_lease
 
+        # Clean ledger each merge test so residual/host durable #332 state
+        # cannot short-circuit eligibility gates (#590 / #594).
+        mcp_server._save_review_decision_lock(None)
         mcp_server.gitea_load_review_workflow()
         self._lease_patch = _install_owned_reviewer_lease(8)
         self._lease_patch.start()