ActionController::UnknownFormat

ActionController::UnknownFormat is raised when a request asks for a format (like JSON or XML) that the controller action doesn't support via respond_to.

Common causes

• Request with Accept header for unsupported format
• URL with unsupported extension (.json, .xml)
• Missing respond_to block in controller
• API endpoint missing JSON response
• Bots requesting unusual formats

How to fix it

• Add respond_to block with supported formats
• Use respond_to with any format fallback
• Rescue and return appropriate error
• Add default format to route
• Implement proper content negotiation

Example

# Error when requesting /users.xml without XML support
# => ActionController::UnknownFormat

# Fix with respond_to
def show
  @user = User.find(params[:id])
  respond_to do |format|
    format.html
    format.json { render json: @user }
    format.any { head :not_acceptable }
  end
end

# Or rescue globally
rescue_from ActionController::UnknownFormat do
  head :not_acceptable
end

Related errors