diff --git a/tests/test_oidc_storage.py b/tests/test_oidc_storage.py
index 6f92e3ae..f15b9d82 100644
--- a/tests/test_oidc_storage.py
+++ b/tests/test_oidc_storage.py
@@ -550,3 +550,50 @@ class TestReplaceOIDCRoles:
         assert removed == {"role-oidc-old"}
         roles = {r["role_id"]: r["assigned_by"] for r in db.list_user_roles("u1")}
         assert roles == {"role-manual": "admin-ui", "role-default": "oidc-default"}
+
+    def test_replace_oidc_roles_no_op_steady_state(self, db):
+        """Steady-state re-login: claims unchanged, function must short-circuit.
+
+        This pins the contract that drives the SQLite optimistic-read fast
+        path — the common case (token refresh with identical role claims)
+        must not acquire a write lock.
+        """
+        db.create_user("u1", "alice", "Alice", "h")
+        self._seed_role(db, "role-a")
+        self._seed_role(db, "role-b")
+        db.assign_role("u1", "role-a", "oidc")
+        db.assign_role("u1", "role-b", "oidc")
+
+        added, removed = db.replace_oidc_roles("u1", {"role-a", "role-b"})
+
+        assert added == set()
+        assert removed == set()
+        # All rows still oidc-assigned with identical membership.
+        roles = {r["role_id"]: r["assigned_by"] for r in db.list_user_roles("u1")}
+        assert roles == {"role-a": "oidc", "role-b": "oidc"}
+
+    def test_replace_oidc_roles_returns_post_lock_diff(self, db):
+        """Returned (added, removed) reflects the post-lock state, not the optimistic read.
+
+        The SQLite implementation re-reads under the write lock to defend
+        against races; the values returned must come from that re-read so
+        callers (apply_role_mapping audit logs) see the actual transition
+        that hit the table. Steady-state input must collapse to empty
+        sets and leave row timestamps unchanged.
+        """
+        db.create_user("u1", "alice", "Alice", "h")
+        self._seed_role(db, "role-a")
+        db.assign_role("u1", "role-a", "oidc")
+
+        before = db.list_user_roles("u1")
+        assert len(before) == 1
+        original_created = before[0]["assignment_created"]
+
+        added, removed = db.replace_oidc_roles("u1", {"role-a"})
+
+        assert added == set()
+        assert removed == set()
+        # No write occurred — the assignment row's timestamp is untouched.
+        after = db.list_user_roles("u1")
+        assert len(after) == 1
+        assert after[0]["assignment_created"] == original_created
diff --git a/turnstone/core/storage/_postgresql.py b/turnstone/core/storage/_postgresql.py
index 8ae4f559..68fb9a96 100644
--- a/turnstone/core/storage/_postgresql.py
+++ b/turnstone/core/storage/_postgresql.py
@@ -2105,10 +2105,25 @@ class PostgreSQLBackend:
 
         now = datetime.now(UTC).strftime("%Y-%m-%dT%H:%M:%S")
         with self._conn() as conn:
+            # `with_for_update()` takes per-row locks on this user's
+            # `user_roles` rows for the duration of the transaction.
+            # Two concurrent OIDC callbacks for the same `user_id` (e.g.
+            # racing token refreshes with differing claim sets) would
+            # otherwise both read the same baseline under READ COMMITTED
+            # and produce a final role state matching neither caller's
+            # intent. The lock is per-`user_id`, so unrelated user writes
+            # are unaffected.
+            #
+            # Note: FOR UPDATE on an empty result set acquires no locks,
+            # so on a brand-new user with no rows yet, two concurrent
+            # callers can proceed in parallel; their inserts merge via
+            # ON CONFLICT DO NOTHING (final state is the union of the
+            # two desired sets). The next single-caller reconciliation
+            # cycle self-heals.
             existing_rows = conn.execute(
-                sa.select(user_roles.c.role_id, user_roles.c.assigned_by).where(
-                    user_roles.c.user_id == user_id
-                )
+                sa.select(user_roles.c.role_id, user_roles.c.assigned_by)
+                .where(user_roles.c.user_id == user_id)
+                .with_for_update()
             ).fetchall()
             current_oidc: set[str] = {r[0] for r in existing_rows if r[1] == "oidc"}
             # Roles assigned by any other source (admin-ui, oidc-default, etc.)
diff --git a/turnstone/core/storage/_sqlite.py b/turnstone/core/storage/_sqlite.py
index e95dc93a..ce266372 100644
--- a/turnstone/core/storage/_sqlite.py
+++ b/turnstone/core/storage/_sqlite.py
@@ -2238,9 +2238,24 @@ class SQLiteBackend:
     def replace_oidc_roles(
         self, user_id: str, desired_role_ids: set[str]
     ) -> tuple[set[str], set[str]]:
+        # Double-check pattern: the steady-state OIDC re-login (claims
+        # unchanged from the last login) is overwhelmingly the common
+        # case, and SQLite's `BEGIN IMMEDIATE` takes the database-wide
+        # write lock — serialising every unrelated writer in the
+        # process. Acquiring it for a no-op diff is pure waste.
+        #
+        # Phase 1 reads under the default deferred transaction (no
+        # write lock) and bails out cheaply when the diff is empty.
+        # Phase 2 only fires when work is needed: it commits the read
+        # txn, escalates to `BEGIN IMMEDIATE`, and re-reads + re-diffs
+        # under the lock. The re-read is required — between the two
+        # reads any concurrent writer (admin-ui assignment, another
+        # racing OIDC callback) could have changed the row set, and
+        # acting on the optimistic snapshot would clobber that change.
+        # The post-lock diff is what we return so callers see the
+        # actual transition that hit the table.
         now = datetime.now(UTC).strftime("%Y-%m-%dT%H:%M:%S")
         with self._conn() as conn:
-            conn.execute(sa.text("BEGIN IMMEDIATE"))
             existing_rows = conn.execute(
                 sa.select(user_roles.c.role_id, user_roles.c.assigned_by).where(
                     user_roles.c.user_id == user_id
@@ -2251,6 +2266,27 @@ class SQLiteBackend:
             # are off-limits to OIDC reconciliation per apply_role_mapping's contract.
             blocked: set[str] = {r[0] for r in existing_rows if r[1] != "oidc"}
 
+            effective_desired = desired_role_ids - blocked
+            added = effective_desired - current_oidc
+            removed = current_oidc - effective_desired
+            if not added and not removed:
+                # Steady state: claims unchanged from prior login. Skip
+                # the write lock entirely — this is the perf win.
+                return set(), set()
+
+            # Mutation needed. Release the implicit read txn, acquire
+            # the SQLite write lock, and re-read so the diff reflects
+            # any state change that landed between the two reads.
+            conn.commit()
+            conn.execute(sa.text("BEGIN IMMEDIATE"))
+            existing_rows = conn.execute(
+                sa.select(user_roles.c.role_id, user_roles.c.assigned_by).where(
+                    user_roles.c.user_id == user_id
+                )
+            ).fetchall()
+            current_oidc = {r[0] for r in existing_rows if r[1] == "oidc"}
+            blocked = {r[0] for r in existing_rows if r[1] != "oidc"}
+
             effective_desired = desired_role_ids - blocked
             added = effective_desired - current_oidc
             removed = current_oidc - effective_desired