diff --git a/backend/open_webui/main.py b/backend/open_webui/main.py
index f5c182eb3f..de48336799 100644
--- a/backend/open_webui/main.py
+++ b/backend/open_webui/main.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import asyncio
+import copy
 import logging
 import mimetypes
 import os
@@ -239,6 +240,7 @@ from open_webui.utils.middleware import (
     process_chat_payload,
     process_chat_response,
 )
+from open_webui.utils.misc import merge_model_params
 from open_webui.utils.model_ids import strip_provider_model_prefix
 from open_webui.utils.models import (
     check_model_access,
@@ -1096,17 +1098,15 @@ async def chat_completion(
             await _set_direct_model(request, model, user)
 
         # Model params: global defaults as base, per-model overrides win
-        default_model_params = await Config.get('models.default_params', {}) or {}
-        model_info_params = {
-            **default_model_params,
-            **(model_info.params.model_dump() if model_info and model_info.params else {}),
-        }
+        # Deep copy: downstream mutates params in place and the config default is shared
+        default_model_params = copy.deepcopy(await Config.get('models.default_params', {}) or {})
+        model_info_params = merge_model_params(
+            default_model_params,
+            model_info.params.model_dump() if model_info and model_info.params else {},
+        )
         request_params = {key: value for key, value in (form_data.get('params') or {}).items() if value is not None}
         if model_info_params or request_params:
-            form_data['params'] = {
-                **model_info_params,
-                **request_params,
-            }
+            form_data['params'] = merge_model_params(model_info_params, request_params)
 
         # Check base model existence for custom models
         if model_info and model_info.base_model_id:
diff --git a/backend/open_webui/utils/misc.py b/backend/open_webui/utils/misc.py
index 4e0f756cb8..39608ccf5c 100644
--- a/backend/open_webui/utils/misc.py
+++ b/backend/open_webui/utils/misc.py
@@ -29,6 +29,16 @@ def deep_update(d, u):
     return d
 
 
+def merge_model_params(base: dict, override: dict) -> dict:
+    # Params override wholesale, except custom_params: a user-defined bag merged per key
+    params = {**base, **override}
+    base_custom = base.get('custom_params')
+    override_custom = override.get('custom_params')
+    if isinstance(base_custom, dict) and (override_custom is None or isinstance(override_custom, dict)):
+        params['custom_params'] = {**base_custom, **(override_custom or {})}
+    return params
+
+
 def _strip_filter_entry(entry):
     # Compose list-form env syntax passes surrounding quotes through verbatim
     return (entry or '').strip().strip('"\'').strip()