diff --git a/litellm/proxy/_experimental/mcp_server/db.py b/litellm/proxy/_experimental/mcp_server/db.py
index be30f866eee..72875b5cf8d 100644
--- a/litellm/proxy/_experimental/mcp_server/db.py
+++ b/litellm/proxy/_experimental/mcp_server/db.py
@@ -500,6 +500,24 @@ async def store_user_oauth_credential(
     if scopes:
         payload["scopes"] = scopes
 
+    # Guard against silently overwriting a BYOK credential with an OAuth token.
+    # BYOK credentials lack a "type" field (or use a non-"oauth2" type).
+    existing = await prisma_client.db.litellm_mcpusercredentials.find_unique(
+        where={"user_id_server_id": {"user_id": user_id, "server_id": server_id}}
+    )
+    if existing is not None:
+        try:
+            raw = json.loads(base64.urlsafe_b64decode(existing.credential_b64).decode())
+            if raw.get("type") != "oauth2":
+                raise ValueError(
+                    f"A non-OAuth2 credential already exists for user {user_id} "
+                    f"and server {server_id}. Refusing to overwrite."
+                )
+        except (ValueError, KeyError):
+            raise
+        except Exception:
+            pass  # Malformed existing record — allow overwrite
+
     encoded = base64.urlsafe_b64encode(json.dumps(payload).encode()).decode()
     await prisma_client.db.litellm_mcpusercredentials.upsert(
         where={"user_id_server_id": {"user_id": user_id, "server_id": server_id}},
diff --git a/litellm/proxy/_experimental/mcp_server/discoverable_endpoints.py b/litellm/proxy/_experimental/mcp_server/discoverable_endpoints.py
index 1e368787dbf..ad1dadb1222 100644
--- a/litellm/proxy/_experimental/mcp_server/discoverable_endpoints.py
+++ b/litellm/proxy/_experimental/mcp_server/discoverable_endpoints.py
@@ -341,8 +341,19 @@ async def authorize(
         mcp_server = _resolve_oauth2_server_for_root_endpoints()
     if mcp_server is None:
         raise HTTPException(status_code=404, detail="MCP server not found")
-    # Use server's stored client_id when caller doesn't supply one
+    # Use server's stored client_id when caller doesn't supply one.
+    # Raise a clear error instead of passing an empty string — an empty
+    # client_id would silently produce a broken authorization URL.
     resolved_client_id: str = mcp_server.client_id or client_id or ""
+    if not resolved_client_id:
+        raise HTTPException(
+            status_code=400,
+            detail={
+                "error": "client_id is required but was not supplied and is not "
+                "stored on the MCP server record. Provide client_id as a query "
+                "parameter or configure it on the server."
+            },
+        )
     return await authorize_with_server(
         request=request,
         mcp_server=mcp_server,