Calling GPT-5 from a plain Ruby script using the official OpenAI library

openai.rb

require "openai"

client = OpenAI::Client.new   # assumes valid key in OPENAI_API_KEY

begin
  resp = client.chat.completions.create(
    model: :"gpt-5-nano",
    reasoning_effort: :high,
    verbosity: :low,
    messages: [{ role: "user", content: "Tell me a joke" }]
  )
  puts resp.choices[0].message.content
  puts "#{resp.usage.completion_tokens} (of which #{resp.usage.completion_tokens_details.reasoning_tokens} were reasoning tokens)"
rescue => e
  puts e.body[:error][:message]
end

# reasoning_effort can be :minimal, :low, :medium, :high
# verbosity can be :low, :medium, :high

IMPORTANT UPDATE! RubyLLM does support GPT-5, I'm just an idiot who didn't read all the documentation. RubyLLM has a registry of models that stands distinct from the specific version you have installed and you can update it! More info at https://rubyllm.com/models/#refreshing-the-registry https://x.com/paolino/status/1953591077480665217

Thanks to @crmne for bringing this to my attention. My little "hack" above will force the issue, however, if for some reason you don't/can't/won't update the registry.