ActionController::UnknownFormat Ruby / Rails Origin: Application & API Last updated 5 Nov 2026

Rails: ActionController::UnknownFormat — UsersController#show is missing a template for this request format and variant

The controller has no template or respond_to handler for the requested format (HTML/JSON/etc.).

Exact message you may see ActionController::UnknownFormat in UsersController#showUsersController#show is missing a template for this request format and variantActionController::UnknownFormat (ActionController::UnknownFormat)missing a template for this request format and variant
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Seen on: Ruby

Meaning

Add the template (show.html.erb/show.json.jbuilder), or handle the format in respond_to. Often a browser hits a JSON-only action, an API client sends Accept: */* incorrectly, or a Turbo request needs turbo_stream. Check the Accept header and the URL extension.

Common causes

  • Missing template for the format
  • JSON endpoint requested as HTML (or reverse)
  • respond_to lacking the format
  • Turbo/Hotwire turbo_stream template missing
  • Wrong Accept header

⚡ Quick fix

  1. Add show.json.jbuilder or render json:
  2. Use respond_to do |format| format.html; format.json { render json: @user } end
  3. Send Accept: application/json

Detailed fix by platform

Rails

  1. curl -H "Accept: application/json" http://localhost:3000/users/1.json

How to diagnose

  1. Format — Requested?
  2. Template — Exists?
  3. respond_to — Handles it?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

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