diff --git a/backend/open_webui/main.py b/backend/open_webui/main.py
index 2761b7f81b..8780b87015 100644
--- a/backend/open_webui/main.py
+++ b/backend/open_webui/main.py
@@ -899,7 +899,7 @@ app.state.config.ENABLE_EVALUATION_ARENA_MODELS = ENABLE_EVALUATION_ARENA_MODELS
 app.state.config.EVALUATION_ARENA_MODELS = EVALUATION_ARENA_MODELS
 
 # Migrate legacy access_control → access_grants on boot
-from open_webui.utils.access_control import migrate_access_control
+from open_webui.utils.access_control import migrate_access_control, check_base_model_access
 
 connections = app.state.config.TOOL_SERVER_CONNECTIONS
 if any('access_control' in c.get('config', {}) for c in connections):
@@ -1682,9 +1682,16 @@ async def chat_completion(
 
             # Re-run the access check against the resolved base model. Access on
             # the user-facing wrapper does not extend to the chain target, so
-            # each link must be authorized independently.
-            if not BYPASS_MODEL_ACCESS_CONTROL and (user.role != 'admin' or not BYPASS_ADMIN_ACCESS_CONTROL):
-                await check_model_access(user, request.app.state.MODELS[resolved_base_model_id])
+            # each link must be authorized independently. check_base_model_access
+            # raises HTTPException(403) — which the outer except HTTPException: raise
+            # preserves — rather than the bare Exception raised by the utils/models.py
+            # helper, so authorization failures reach the client as 403, not 400.
+            await check_base_model_access(
+                user,
+                resolved_base_model_id,
+                bypass_filter=BYPASS_MODEL_ACCESS_CONTROL
+                or (user.role == 'admin' and BYPASS_ADMIN_ACCESS_CONTROL),
+            )
 
         # Chat Params
         stream_delta_chunk_size = form_data.get('params', {}).get('stream_delta_chunk_size')
@@ -1763,6 +1770,11 @@ async def chat_completion(
         request.state.metadata = metadata
         form_data['metadata'] = metadata
 
+    except HTTPException:
+        # Preserve intentional HTTP statuses — e.g. the 403 raised by the
+        # base-model access check and the 404 from the chat-ownership check —
+        # so authorization failures are not remapped to 400.
+        raise
     except Exception as e:
         log.debug(f'Error processing chat metadata: {e}')
         raise HTTPException(
diff --git a/backend/open_webui/utils/access_control/__init__.py b/backend/open_webui/utils/access_control/__init__.py
index 2d76dfa7d0..e8fa86f605 100644
--- a/backend/open_webui/utils/access_control/__init__.py
+++ b/backend/open_webui/utils/access_control/__init__.py
@@ -316,13 +316,41 @@ async def check_base_model_access(
     access decision. Call this before any code path that resolves or dispatches
     to the base, and before persisting a base_model_id chosen by the caller.
 
+    Unlike the generic check_model_access (which only enforces grants for
+    user.role == 'user' and lets admins through unconditionally), this helper
+    enforces the grant check for every role when bypass_filter is False.
+    Callers are expected to opt admins out explicitly by passing
+    bypass_filter=True when BYPASS_ADMIN_ACCESS_CONTROL is set — otherwise an
+    admin creating a chain or dispatching through one still needs read access
+    to the base, since the wrapper's ownership does not transfer to the base.
+
     Raises HTTPException(403) if not authorized. Does nothing if bypass_filter
     is True or base_model_id is falsy.
     """
+    from fastapi import HTTPException
+
     if bypass_filter or not base_model_id:
         return
 
     from open_webui.models.models import Models
+    from open_webui.models.access_grants import AccessGrants
 
     base_model_info = await Models.get_model_by_id(base_model_id, db=db)
-    await check_model_access(user, base_model_info, bypass_filter)
+    if base_model_info is None:
+        raise HTTPException(status_code=403, detail='Model not found')
+
+    if user.id == base_model_info.user_id:
+        return
+
+    user_group_ids = {group.id for group in await Groups.get_groups_by_member_id(user.id, db=db)}
+    if await AccessGrants.has_access(
+        user_id=user.id,
+        resource_type='model',
+        resource_id=base_model_info.id,
+        permission='read',
+        user_group_ids=user_group_ids,
+        db=db,
+    ):
+        return
+
+    raise HTTPException(status_code=403, detail='Model not found')