diff --git a/tests/tools/test_mcp_oauth_manager.py b/tests/tools/test_mcp_oauth_manager.py
index 87e454a970..d3e3fcc87e 100644
--- a/tests/tools/test_mcp_oauth_manager.py
+++ b/tests/tools/test_mcp_oauth_manager.py
@@ -136,6 +136,8 @@ async def test_first_observation_401_still_allows_in_place_refresh(tmp_path, mon
     (token_dir / "srv.json").write_text(json.dumps({"access_token": "OLD"}), encoding="utf-8")
 
     class _Ctx:
+        current_tokens = object()  # tokens already in memory (in-process sign-in)
+
         def can_refresh_token(self):
             return True
 
@@ -154,6 +156,33 @@ async def test_first_observation_401_still_allows_in_place_refresh(tmp_path, mon
     assert mgr._entries[mgr._key("srv")].last_mtime_ns != 0
 
 
+@pytest.mark.asyncio
+async def test_external_login_after_absent_tokens_file_forces_reload(tmp_path, monkeypatch):
+    """Provider built before login (no file, no in-memory tokens) must reload
+    once another process writes the tokens file (GH#39551 review)."""
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+    from tools.mcp_oauth_manager import MCPOAuthManager, _ProviderEntry
+
+    class _Ctx:
+        current_tokens = None
+
+    class _Provider:
+        context = _Ctx()
+        _initialized = True
+
+    mgr = MCPOAuthManager()
+    provider = _Provider()
+    mgr._entries[mgr._key("srv")] = _ProviderEntry(
+        server_url="https://example.com/mcp", oauth_config=None, provider=provider,
+    )
+    assert await mgr.invalidate_if_disk_changed("srv") is False  # file absent
+    token_dir = tmp_path / "mcp-tokens"
+    token_dir.mkdir(parents=True)
+    (token_dir / "srv.json").write_text(json.dumps({"access_token": "NEW"}), encoding="utf-8")
+    assert await mgr.invalidate_if_disk_changed("srv") is True
+    assert provider._initialized is False
+
+
 @pytest.mark.asyncio
 async def test_handle_401_dedup_survives_even_if_task_reference_dropped(tmp_path, monkeypatch):
     """Concurrent 401s share one handler task and all callers resolve.
diff --git a/tools/mcp_oauth_manager.py b/tools/mcp_oauth_manager.py
index e40ff4663d..9f3a2a8339 100644
--- a/tools/mcp_oauth_manager.py
+++ b/tools/mcp_oauth_manager.py
@@ -355,10 +355,12 @@ class MCPOAuthManager:
             if mtime_ns == entry.last_mtime_ns:
                 return False
             old, entry.last_mtime_ns = entry.last_mtime_ns, mtime_ns
-            if old == 0:
-                # First observation only seeds the baseline. Forcing a
-                # reload here can reset the provider in the middle of its
-                # first auth handshake and tear down HTTP MCP discovery.
+            if old == 0 and getattr(getattr(entry.provider, "context", None), "current_tokens", None) is not None:
+                # First observation with tokens already in memory only seeds the
+                # baseline: the file was written by this process's own first
+                # sign-in, and reloading on the next request would tear down the
+                # live HTTP MCP session. With no tokens in memory (started before
+                # an external `hermes mcp login`), fall through and reload.
                 return False
             # `_initialized` is private SDK API but stable across the pinned versions (>=1.26.0).
             if hasattr(entry.provider, "_initialized"):