SwaggerUI requests favicon files with relative paths causing 500 errors

[gtolarc]

Description

When accessing SwaggerUI at /api/v1/swagger, the browser automatically requests favicon files using relative paths, which results in requests to /api/v1/favicon-16x16.png and similar endpoints.
These requests are then forwarded to the application router, causing 500 errors.

Environment

• OpenApiSpex version: 3.21.5
• Phoenix version: 1.8.0-rc.4
• Elixir version: 1.18.4
• Browser: Chrome 138.0.0.0

Steps to Reproduce

1. Set up a Phoenix application with OpenApiSpex SwaggerUI:
   scope "/api/v1" do
   pipe_through [:api]

forward "/swagger", OpenApiSpex.Plug.SwaggerUI,
  path: "/api/v1/open_api",
  default_model_expand_depth: 4,
  operations_sorter: "alpha",
  tags_sorter: "alpha"

forward "/", MyApp.ApiRouter

end

2. Navigate to /api/v1/swagger in a browser
3. Check the browser's network tab and server logs

Expected Behavior

SwaggerUI should either:

• Use absolute paths for favicon requests (e.g., /favicon-16x16.png instead of favicon-16x16.png)
• Not request favicon files at all
• Include its own favicon handling

Actual Behavior

The browser requests:

• /api/v1/favicon-16x16.png
• /api/v1/favicon-32x32.png
• /api/v1/favicon.ico

These requests are forwarded to the application's API router, causing errors like:
[error] ** (UndefinedFunctionError) function MyApp.Domain.json_api_match_route/2 is undefined or private

Error Log

2025-07-30 10:17:57.849 request_id=GFbinxHb7U2mY28AABkB remote_ip=127.0.0.1 user_agent=Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/138.0.0.0
Safari/537.36 [error] ** (UndefinedFunctionError) function MyApp.Accounts.json_api_match_route/2 is undefined or private
(my_app 0.1.0) MyApp.Accounts.json_api_match_route("GET", ["favicon-16x16.png"])
...