diff --git a/tests/hermes_cli/test_custom_provider_normalize_no_mutate.py b/tests/hermes_cli/test_custom_provider_normalize_no_mutate.py
new file mode 100644
index 00000000000..1a37cb6e075
--- /dev/null
+++ b/tests/hermes_cli/test_custom_provider_normalize_no_mutate.py
@@ -0,0 +1,62 @@
+"""Regression: _normalize_custom_provider_entry must never mutate its input.
+
+Provider entries frequently alias cached config dicts (load_config_readonly()
+shares one dict process-wide). The normalizer's alias rewrites
+(api_key_env -> key_env, camelCase -> snake_case) used to write INTO the
+caller's entry, corrupting the shared cache for every subsequent reader.
+"""
+
+import copy
+
+from hermes_cli.config import (
+    _normalize_custom_provider_entry,
+    get_compatible_custom_providers,
+    providers_dict_to_custom_providers,
+)
+
+
+def test_normalizer_does_not_mutate_entry_api_key_env():
+    entry = {"base_url": "https://x.example/v1", "api_key_env": "MY_KEY"}
+    snapshot = copy.deepcopy(entry)
+    out = _normalize_custom_provider_entry(entry, provider_key="p")
+    assert entry == snapshot, "input entry must not gain key_env"
+    assert out is not None and out.get("key_env") == "MY_KEY"
+
+
+def test_normalizer_does_not_mutate_entry_camel_aliases():
+    entry = {"baseUrl": "https://x.example/v1", "apiKey": "sk-test-not-real"}
+    snapshot = copy.deepcopy(entry)
+    _normalize_custom_provider_entry(entry, provider_key="p")
+    assert entry == snapshot, "camelCase aliasing must not write into the input"
+
+
+def test_providers_dict_roundtrip_leaves_cached_config_untouched():
+    config = {
+        "providers": {
+            "myprov": {
+                "base_url": "https://x.example/v1",
+                "api_key_env": "MY_KEY",
+                "models": {"m1": {"context_length": 8192}},
+            }
+        }
+    }
+    snapshot = copy.deepcopy(config)
+    providers_dict_to_custom_providers(config["providers"])
+    assert config == snapshot
+
+    get_compatible_custom_providers(config)
+    assert config == snapshot
+
+
+def test_normalized_models_mapping_is_not_shared_with_input():
+    entry = {
+        "base_url": "https://x.example/v1",
+        "api_key": "sk-test-not-real",
+        "models": {"m1": {}},
+    }
+    out = _normalize_custom_provider_entry(entry, provider_key="p")
+    assert out is not None
+    out["models"]["injected"] = {}
+    assert "injected" not in entry["models"], (
+        "normalized models mapping must not alias the caller's dict"
+    )