diff --git a/backend/open_webui/main.py b/backend/open_webui/main.py
index bde28c1af1..6269315d32 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
@@ -240,6 +241,7 @@ 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,
@@ -1097,17 +1099,14 @@ 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 {}),
-        }
+        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/middleware.py b/backend/open_webui/utils/middleware.py
index 1daccff6ad..1c6f1fc510 100644
--- a/backend/open_webui/utils/middleware.py
+++ b/backend/open_webui/utils/middleware.py
@@ -1976,14 +1976,14 @@ def apply_params_to_form_data(form_data, model):
 
     if model.get('owned_by') == 'ollama':
         # Ollama specific parameters
-        form_data['options'] = params
+        form_data['options'] = {**params, **(form_data.get('options') or {})}
     else:
         if isinstance(params, dict):
             for key, value in params.items():
-                if value is not None:
+                if value is not None and key not in form_data:
                     form_data[key] = value
 
-        if 'logit_bias' in params and params['logit_bias'] is not None:
+        if 'logit_bias' in params and params['logit_bias'] is not None and 'logit_bias' not in form_data:
             try:
                 logit_bias = convert_logit_bias_input_to_json(params['logit_bias'])
 
diff --git a/backend/open_webui/utils/misc.py b/backend/open_webui/utils/misc.py
index 5fd1f8f3cb..f0d9235876 100644
--- a/backend/open_webui/utils/misc.py
+++ b/backend/open_webui/utils/misc.py
@@ -29,6 +29,15 @@ def deep_update(d, u):
     return d
 
 
+def merge_model_params(base: dict, override: dict) -> dict:
+    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()
diff --git a/backend/open_webui/utils/payload.py b/backend/open_webui/utils/payload.py
index 8cc8324ac9..30ca698be0 100644
--- a/backend/open_webui/utils/payload.py
+++ b/backend/open_webui/utils/payload.py
@@ -67,7 +67,7 @@ def apply_model_params_to_body(params: dict, form_data: dict, mappings: dict[str
         return form_data
 
     for key, value in params.items():
-        if value is not None:
+        if value is not None and key not in form_data:
             if key in mappings:
                 cast_func = mappings[key]
                 if isinstance(cast_func, Callable):