diff --git a/strix/auth/codex.py b/strix/auth/codex.py
index 6b5551e9..3843428b 100644
--- a/strix/auth/codex.py
+++ b/strix/auth/codex.py
@@ -32,6 +32,8 @@ from typing import TYPE_CHECKING, Any
 
 
 if TYPE_CHECKING:
+    from collections.abc import Iterator
+
     from openai import AsyncOpenAI
 
 
@@ -77,6 +79,36 @@ _EXPIRY_SKEW_S = 300
 _refresh_lock = threading.Lock()
 
 
+@contextlib.contextmanager
+def _refresh_guard() -> Iterator[None]:
+    """Serialize a token refresh within and across Strix processes.
+
+    The in-process lock covers concurrent agents in one process; a best-effort
+    file lock (``flock``) covers concurrent Strix processes sharing one login, so
+    two of them can't both spend the single-use refresh token and leave one with
+    ``invalid_grant``. Degrades to the in-process lock alone where ``flock`` is
+    unavailable (e.g. Windows).
+    """
+    with _refresh_lock:
+        try:
+            import fcntl
+
+            lock_path = store.AUTH_PATH.with_suffix(".lock")
+            lock_path.parent.mkdir(parents=True, exist_ok=True)
+            handle = lock_path.open("w")
+        except (ImportError, OSError):
+            yield
+            return
+        try:
+            with contextlib.suppress(OSError):
+                fcntl.flock(handle.fileno(), fcntl.LOCK_EX)
+            yield
+        finally:
+            with contextlib.suppress(OSError):
+                fcntl.flock(handle.fileno(), fcntl.LOCK_UN)
+            handle.close()
+
+
 class CodexAuthError(Exception):
     """A Codex auth step failed. ``code`` is a stable, machine-readable reason."""
 
@@ -295,16 +327,19 @@ def _near_expiry(record: dict[str, Any]) -> bool:
 def get_valid_token() -> tuple[str, str]:
     """Return ``(access_token, account_id)``, refreshing if near expiry.
 
-    Refreshes under a lock and re-reads the store after acquiring it, so that
-    when many agents fire at once only one refresh happens — OpenAI invalidates a
-    refresh token as soon as it is used, so a concurrent stampede would fail.
+    Refreshes under a within- and cross-process lock and re-reads the store after
+    acquiring it, so that when many agents (or parallel Strix processes sharing
+    one login) fire at once only one refresh happens — OpenAI invalidates a
+    refresh token as soon as it is used, so a concurrent stampede would fail. The
+    re-read means a caller that loses the race picks up the token the winner just
+    rotated instead of exchanging the now-dead one.
     """
     record = read_record()
     if record is None:
         raise CodexAuthError("not_authenticated", "not signed in; run: strix auth login")
     if not _near_expiry(record):
         return record["access"], record["account_id"]
-    with _refresh_lock:
+    with _refresh_guard():
         record = read_record()
         if record is None:
             raise CodexAuthError("not_authenticated", "not signed in; run: strix auth login")
diff --git a/tests/test_codex_auth.py b/tests/test_codex_auth.py
index 28682db2..5feacbd0 100644
--- a/tests/test_codex_auth.py
+++ b/tests/test_codex_auth.py
@@ -183,6 +183,49 @@ def test_get_valid_token_refreshes_and_persists_rotation(monkeypatch: pytest.Mon
     assert codex.read_record()["refresh"] == "r2"
 
 
+def test_get_valid_token_uses_token_rotated_by_another_process(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    # Simulate a parallel Strix process rotating the token while we wait for the
+    # refresh guard: the pre-guard read sees the stale token, the in-guard read
+    # sees the winner's fresh one, so we must NOT exchange the now-dead refresh.
+    records = [
+        {
+            "type": "oauth",
+            "provider": "codex",
+            "access": "stale",
+            "refresh": "r1",
+            "account_id": "acct",
+            "expires_at": time.time() - 10,
+        },
+        {
+            "type": "oauth",
+            "provider": "codex",
+            "access": "fresh-from-other-process",
+            "refresh": "r2",
+            "account_id": "acct",
+            "expires_at": time.time() + 3600,
+        },
+    ]
+    calls = {"n": 0}
+
+    def _fake_read() -> dict[str, Any]:
+        record = records[min(calls["n"], len(records) - 1)]
+        calls["n"] += 1
+        return record
+
+    def _boom(_payload: dict[str, str]) -> dict[str, Any]:
+        msg = "must not refresh a token another process already rotated"
+        raise AssertionError(msg)
+
+    monkeypatch.setattr(codex, "read_record", _fake_read)
+    monkeypatch.setattr(codex, "_post_form", _boom)
+
+    access, account_id = codex.get_valid_token()
+    assert access == "fresh-from-other-process"
+    assert account_id == "acct"
+
+
 def test_get_valid_token_raises_when_not_signed_in() -> None:
     with pytest.raises(codex.CodexAuthError) as exc:
         codex.get_valid_token()