diff --git a/hermes_cli/auth.py b/hermes_cli/auth.py
index 3547bb87323..91911e77e17 100644
--- a/hermes_cli/auth.py
+++ b/hermes_cli/auth.py
@@ -43,7 +43,12 @@ from urllib.parse import parse_qs, urlencode, urlparse
 
 import httpx
 
-from hermes_cli.config import get_hermes_home, get_config_path, read_raw_config
+from hermes_cli.config import (
+    get_hermes_home,
+    get_config_path,
+    read_raw_config,
+    require_readable_config_before_write,
+)
 from hermes_constants import OPENROUTER_BASE_URL, secure_parent_dir
 from agent.credential_persistence import sanitize_borrowed_credential_payload
 from utils import atomic_replace, atomic_yaml_write, env_float, is_truthy_value
@@ -6335,6 +6340,7 @@ def _update_config_for_provider(
     # Update config.yaml model section
     config_path = get_config_path()
     config_path.parent.mkdir(parents=True, exist_ok=True)
+    require_readable_config_before_write(config_path)
 
     config = read_raw_config()
 
@@ -6427,6 +6433,7 @@ def _reset_config_provider() -> Path:
     config_path = get_config_path()
     if not config_path.exists():
         return config_path
+    require_readable_config_before_write(config_path)
 
     config = read_raw_config()
     if not config:
diff --git a/hermes_cli/config.py b/hermes_cli/config.py
index c9f267996c7..08ca6cadc27 100644
--- a/hermes_cli/config.py
+++ b/hermes_cli/config.py
@@ -6499,6 +6499,30 @@ def read_raw_config() -> Dict[str, Any]:
         return data
 
 
+def require_readable_config_before_write(config_path: Optional[Path] = None) -> None:
+    """Refuse to replace an existing config.yaml that cannot be read."""
+    if config_path is None:
+        config_path = get_config_path()
+    try:
+        config_path.stat()
+    except FileNotFoundError:
+        return
+    except OSError as exc:
+        raise RuntimeError(
+            f"Refusing to overwrite {config_path}: existing config.yaml cannot be accessed "
+            f"({exc}). Fix the file permissions or move it aside first."
+        ) from exc
+
+    try:
+        with open(config_path, "rb") as f:
+            f.read(1)
+    except OSError as exc:
+        raise RuntimeError(
+            f"Refusing to overwrite {config_path}: existing config.yaml cannot be read "
+            f"({exc}). Fix the file permissions or move it aside first."
+        ) from exc
+
+
 def load_config() -> Dict[str, Any]:
     """Load configuration from ~/.hermes/config.yaml.
 
@@ -6864,6 +6888,7 @@ def save_config(
 
         ensure_hermes_home()
         config_path = get_config_path()
+        require_readable_config_before_write(config_path)
         # Compute explicit user paths BEFORE any normalisation --------
         # _normalize_max_turns_config may inject agent.max_turns from
         # DEFAULT_CONFIG; using the raw dict preserves which paths the
@@ -7843,6 +7868,7 @@ def set_config_value(key: str, value: str):
     # Read the raw user config (not merged with defaults) to avoid
     # dumping all default values back to the file
     config_path = get_config_path()
+    require_readable_config_before_write(config_path)
     user_config = {}
     if config_path.exists():
         try:
diff --git a/tests/hermes_cli/test_config.py b/tests/hermes_cli/test_config.py
index 37740c2ea75..60d23e2a80a 100644
--- a/tests/hermes_cli/test_config.py
+++ b/tests/hermes_cli/test_config.py
@@ -24,6 +24,7 @@ from hermes_cli.config import (
     save_env_value,
     save_env_value_secure,
     sanitize_env_file,
+    set_config_value,
     write_platform_config_field,
     _sanitize_env_lines,
 )
@@ -267,6 +268,17 @@ class TestLoadConfigParseFailure:
 
 
 class TestSaveAndLoadRoundtrip:
+    @staticmethod
+    def _deny_config_reads(config_path):
+        real_open = open
+
+        def fake_open(file, mode="r", *args, **kwargs):
+            if Path(file) == config_path and "r" in mode:
+                raise PermissionError("denied")
+            return real_open(file, mode, *args, **kwargs)
+
+        return fake_open
+
     def test_roundtrip(self, tmp_path):
         with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
             config = load_config()
@@ -282,6 +294,30 @@ class TestSaveAndLoadRoundtrip:
             assert saved["agent"]["max_turns"] == 42
             assert "max_turns" not in saved
 
+    def test_save_config_refuses_to_overwrite_unreadable_existing_config(self, tmp_path):
+        config_path = tmp_path / "config.yaml"
+        original = "model: test/original\n"
+        config_path.write_text(original, encoding="utf-8")
+
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            with patch("builtins.open", side_effect=self._deny_config_reads(config_path)):
+                with pytest.raises(RuntimeError, match="Refusing to overwrite"):
+                    save_config({"model": "test/replacement"})
+
+        assert config_path.read_text(encoding="utf-8") == original
+
+    def test_config_set_refuses_to_overwrite_unreadable_existing_config(self, tmp_path):
+        config_path = tmp_path / "config.yaml"
+        original = "model:\n  provider: openrouter\n"
+        config_path.write_text(original, encoding="utf-8")
+
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            with patch("builtins.open", side_effect=self._deny_config_reads(config_path)):
+                with pytest.raises(RuntimeError, match="Refusing to overwrite"):
+                    set_config_value("model.provider", "openai")
+
+        assert config_path.read_text(encoding="utf-8") == original
+
     def test_save_config_normalizes_legacy_root_level_max_turns(self, tmp_path):
         with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
             save_config({"model": "test/custom-model", "max_turns": 37})