diff --git a/cli_auth.py b/cli_auth.py
index 61c9f25..8e913f8 100644
--- a/cli_auth.py
+++ b/cli_auth.py
@@ -2,6 +2,11 @@
 
 Called by pat_rotator._persist_token() every 10 minutes. Lightweight —
 just swaps token values in existing files, no installs or script runs.
+
+All writes are atomic (write to `.tmp`, then `os.replace`) so a Hermes / OpenCode
+/ Codex invocation that reads the file mid-update sees the old token whole or
+the new token whole — never a half-written file. Errors other than "file does
+not exist" surface as warnings rather than being silently swallowed.
 """
 
 import json
@@ -16,6 +21,20 @@
     _HOME = "/app/python/source_code"
 
 
+def _atomic_write_text(path, content):
+    """Write `content` to `path` atomically via tmp file + rename.
+
+    Prevents the read-while-rewriting race that bit Hermes specifically:
+    Hermes reads `~/.hermes/config.yaml` on every invocation, so a bare
+    open(path, 'w') by the rotator could leave the file in a partial state
+    visible to a concurrent Hermes call → 403 Invalid access token.
+    """
+    tmp = f"{path}.tmp"
+    with open(tmp, "w") as f:
+        f.write(content)
+    os.replace(tmp, path)
+
+
 def update_cli_tokens(token):
     """Update the literal token in all CLI config files."""
     _update_claude(token)
@@ -28,15 +47,16 @@ def update_cli_tokens(token):
 def _update_claude(token):
     """Update ANTHROPIC_AUTH_TOKEN in ~/.claude/settings.json."""
     path = os.path.join(_HOME, ".claude", "settings.json")
+    if not os.path.exists(path):
+        return  # setup_claude.py hasn't run yet
     try:
         with open(path) as f:
             settings = json.load(f)
         if "env" in settings and "ANTHROPIC_AUTH_TOKEN" in settings["env"]:
             settings["env"]["ANTHROPIC_AUTH_TOKEN"] = token
-            with open(path, "w") as f:
-                json.dump(settings, f, indent=2)
-    except (OSError, json.JSONDecodeError):
-        pass  # file doesn't exist yet — initial setup hasn't run
+            _atomic_write_text(path, json.dumps(settings, indent=2))
+    except (OSError, json.JSONDecodeError) as e:
+        logger.warning("Failed to update Claude token in %s: %s", path, e)
 
 
 def _update_codex(token):
@@ -48,6 +68,8 @@ def _update_codex(token):
 def _update_opencode(token):
     """Update api_key values in ~/.local/share/opencode/auth.json."""
     path = os.path.join(_HOME, ".local", "share", "opencode", "auth.json")
+    if not os.path.exists(path):
+        return  # setup_opencode.py hasn't run yet
     try:
         with open(path) as f:
             auth = json.load(f)
@@ -57,10 +79,9 @@ def _update_opencode(token):
                 provider["api_key"] = token
                 changed = True
         if changed:
-            with open(path, "w") as f:
-                json.dump(auth, f, indent=2)
-    except (OSError, json.JSONDecodeError):
-        pass
+            _atomic_write_text(path, json.dumps(auth, indent=2))
+    except (OSError, json.JSONDecodeError) as e:
+        logger.warning("Failed to update OpenCode token in %s: %s", path, e)
 
 
 def _update_gemini(token):
@@ -72,6 +93,8 @@ def _update_gemini(token):
 def _update_hermes(token):
     """Update api_key lines in ~/.hermes/config.yaml."""
     path = os.path.join(_HOME, ".hermes", "config.yaml")
+    if not os.path.exists(path):
+        return  # setup_hermes.py hasn't run yet
     try:
         with open(path) as f:
             content = f.read()
@@ -82,14 +105,15 @@ def _update_hermes(token):
             flags=re.MULTILINE
         )
         if new_content != content:
-            with open(path, "w") as f:
-                f.write(new_content)
-    except OSError:
-        pass
+            _atomic_write_text(path, new_content)
+    except OSError as e:
+        logger.warning("Failed to update Hermes token in %s: %s", path, e)
 
 
 def _replace_dotenv_key(path, key, value):
     """Replace a KEY=value line in a dotenv file."""
+    if not os.path.exists(path):
+        return  # caller's setup script hasn't run yet
     try:
         with open(path) as f:
             content = f.read()
@@ -100,7 +124,6 @@ def _replace_dotenv_key(path, key, value):
             flags=re.MULTILINE
         )
         if new_content != content:
-            with open(path, "w") as f:
-                f.write(new_content)
-    except OSError:
-        pass
+            _atomic_write_text(path, new_content)
+    except OSError as e:
+        logger.warning("Failed to update %s in %s: %s", key, path, e)