A Ruby server implementation of the AG-UI protocol — plus the CopilotKit runtime surface its React client expects — so you can drive a CopilotKit frontend from Ruby (Falcon/Rails) with any LLM.

require "ag_ui"
require "ag_ui/terminals/ruby_llm"

RubyLLM.configure { |c| c.anthropic_api_key = ENV.fetch("ANTHROPIC_API_KEY") }

terminal = AgUi::Terminals::RubyLLM.new(model: "anthropic/claude-sonnet-4-5")

# AgUi.agent gives you a Rack app serving /info, /agent/:id/run (SSE), /connect,
# /stop. The block is the per-run handler: open the SSE stream and drive a
# STANDARD Brute agent — the AG-UI middleware stack over your terminal.
app = AgUi.agent(agent_id: "default") do |env|
  input = env["ag_ui.input"]

  agent = Brute.agent
               .use(AgUi::Middleware::SystemPrompt,
                    prompt: "You are a helpful assistant.", context: input.context)
               .use(AgUi::Middleware::ForwardedProps, props: input.forwarded_props)
               .use(Brute::Middleware::Loop::ToolResult)
               .use(Brute::Middleware::MaxIterations, max_iterations: 10)
               .use(AgUi::Middleware::ToolRouter, tools: input.tools, server_tools: [])
               .run(terminal)

  env["ag_ui.stream"].open(thread_id: input.thread_id, run_id: input.run_id) do |stream|
    stream.run_started
    agent.start(AgUi::Messages.to_brute(input.messages), events: AgUi::EventBridge.new(stream))
    stream.run_finished
  rescue => e
    stream.run_error(message: e.message, code: e.class.name)
  end
end

# mount it wherever the client's runtimeUrl points.
run app

• Schema-exact wire protocol — every event/type definition is generated from the reference Python SDK's models (data/generate-ag-ui-schema.py), validated with json_schemer before it hits the wire, and conformance-tested byte-for-byte against the official SDK test expectations.
• Falcon-native SSE streaming — Protocol::HTTP::Body::Writable bodies with Async fibers: true streaming with backpressure, no buffering.
• The CopilotKit runtime surface — GET /info (capability envelope), POST /agent/:id/run, /connect (replay + live attach), /stop (run cancellation), and the bare-run root HttpAgent expects.
• The client-tool multi-run model — frontend tools (useFrontendTool, useHumanInTheLoop) are advertised schema-only, emitted as TOOL_CALL_*, and the run ends cleanly for the browser to execute and re-run.
• Server tools with an agentic turn loop — {name:, description:, parameters:, handler:} tools execute inline and the model continues the same run (brute's Loop::ToolResult + MaxIterations).
• A2UI generative UI — render_a2ui injection, catalog fetch with retry/degrade, the tool-call → ACTIVITY_SNAPSHOT transform, semantic component validation (a port of the official a2ui_toolkit), and in-run regeneration on invalid surfaces.
• Extended thinking → REASONING_*, multimodal attachments, context injection, and dynamic suggestions (forwardedProps.toolChoice).
• LLM-agnostic core — the LLM lives in a terminal callable at the bottom of a brute middleware pipeline; a ruby_llm terminal ships as the reference adapter (require "ag_ui/terminals/ruby_llm").

bundle add ag-ui

The gem requires Ruby ≥ 3.3. The reference terminal additionally needs ruby_llm; the server runs on any Rack 3 host, with Falcon recommended for native streaming.

HTTP/SSE   Triage → SSEStream → dispatch          (Rack, Falcon-native)
turn       SystemPrompt → ForwardedProps →
           Loop::ToolResult → MaxIterations →
           A2ui → ToolRouter → terminal           (brute pipeline)
terminal   your LLM call                          (ruby_llm adapter included)

One run = one RunAgentInput POST. The pipeline seeds the conversation from the request (history, tools, context), streams events through an EventBridge into the SSE body as they happen, and ends the run per the AG-UI contract (RUN_FINISHED / RUN_ERROR). A pluggable RunStore (in-memory included) records every run for /connect replay, live mid-run attach, and /stop cancellation.

Runnable servers in examples/ (served with ratalada):

• echo.rb — transport smoke test, no LLM
• claude.rb — full runtime surface: Claude streaming, a demo server tool, optional extended thinking (THINKING_BUDGET=2048)
• bare.rb — the bare HttpAgent endpoint shape
• a2ui_analyst.rb — A2UI agent for the vendored showcase canvas

Beyond the unit + conformance suite (bin/test), the repo vendors two real CopilotKit apps as browser-driven regression harnesses — see verification/README.md for the runbook: chat, frontend tools, human-in-the-loop, and an A2UI canvas, all driven against this server with the Node runtime both in and out of the path.

bin/setup        # bundle install (nix users: `nix develop` provides the shell)
bin/test         # scampi — specs are co-located in each file's __END__ block
bin/console      # irb with the gem loaded

Releasing (builds, pushes to rubygems, tags v<version> and pushes the tag):

bin/increment-version <major|minor|patch>
bin/release-gem

MIT — see LICENSE.