Langfuse Ruby SDK

Ruby SDK for Langfuse - Open-source LLM observability and prompt management.

Installation

gem "langfuse-rb"

Quick Start

Langfuse.configure do |config|
  config.public_key = ENV["LANGFUSE_PUBLIC_KEY"]
  config.secret_key = ENV["LANGFUSE_SECRET_KEY"]
  config.base_url = ENV.fetch("LANGFUSE_BASE_URL", "https://cloud.langfuse.com")

  # Optional: sample traces and trace-linked scores deterministically
  config.sample_rate = 1.0
end

message = Langfuse.client.compile_prompt(
  "greeting",
  variables: { name: "Alice" }
)

Langfuse tracing is isolated by default. Langfuse.configure stores configuration only; it does not replace OpenTelemetry.tracer_provider.

sample_rate is applied to traces and trace-linked scores. Rebuild the client with Langfuse.reset! before expecting runtime sampling changes to take effect.

Trace an LLM Call

Langfuse.observe("chat-completion", as_type: :generation) do |gen|
  gen.model = "gpt-4.1-mini"
  gen.input = [{ role: "user", content: "Hello!" }]

  response = openai_client.chat(
    parameters: {
      model: "gpt-4.1-mini",
      messages: [{ role: "user", content: "Hello!" }]
    }
  )

  gen.update(
    output: response.dig("choices", 0, "message", "content"),
    usage_details: {
      prompt_tokens: response.dig("usage", "prompt_tokens"),
      completion_tokens: response.dig("usage", "completion_tokens")
    }
  )
end

Start Here

License

MIT