fino

Fino is a JavaScript runtime for building agentic applications: tools that call models, expose actions, run untrusted or reloadable code, serve HTTP APIs, and keep their capabilities explicit. Instead of scattering those concerns across a pile of services and framework glue, Fino combines ordinary TypeScript with strong execution boundaries — a permissioned module system, isolated realms, sandbox-aware processes, and built-in AI and observability subsystems — in one runtime.

Download the latest Fino release for Linux (x64 or arm64) or macOS (Apple silicon), or browse the source on GitHub.

A Streaming Support Agent

An agent with a validated tool, streaming its answers over server-sent events from an HTTP route:

import { agent, openai, streamText, tool } from 'fino:ai';
import { App } from 'fino:net/http/app';
import { v } from 'fino:validate';

const lookupTicket = tool({
  name: 'lookup_ticket',
  description: 'Read a support ticket by id.',
  parameters: v.object({
    id: v.string().describe('Ticket id, such as T-1001'),
  }),
  execute: async ({ id }: { id: string }) => {
    return JSON.stringify({ id, status: 'open', plan: 'business' });
  },
});

const assistant = agent({
  model: openai({ model: 'gpt-4o' }), // reads OPENAI_API_KEY from the env
  instructions: 'Answer as a concise support engineer.',
  tools: [lookupTicket],
});

const app = new App();

app.route('/chat').sse(async (events, ctx) => {
  const { message } = await ctx.request.json();

  for await (const text of streamText(assistant.stream(message))) {
    await events.write({ data: JSON.stringify(text) });
  }
  await events.write({ event: 'done', data: '{}' });
});

app.listen({ port: 3000 });

Run it and talk to it:

fino run --watch app.ts

curl -N -H 'content-type: application/json' \
  -d '{"message":"What is the status of ticket T-1001?"}' \
  http://127.0.0.1:3000/chat

The model calls the validated tool, and the reply streams back token by token as SSE events. Everything in the example — the agent loop, tool validation, router, and streaming response — is built into the runtime.

Quick Start

Fino builds from source with a Rust toolchain:

git clone https://github.com/Sequins-dev/fino.git
cd fino
cargo build --release
export PATH="$PWD/target/release:$PATH"

Then run a script, a server, or tests:

fino ./hello.ts
fino test tests

Getting Started walks from a first script to a model-calling program.

Requirements

Native libraries for specific features are optional: install only those for the features you use. The current runtime also needs a loadable zlib (libz) at startup because its global compression and fetch modules import fino:compress. A library may already be included in a release binary or provided by the operating system; the table describes the required capability, not an extra install step in every environment.

Most optional bindings can be imported without their library installed; using the affected feature then reports that it is unavailable.

Why Fino

AI is a runtime subsystem, not an afterthought. fino:ai provides provider-neutral model calls, agents, validated tools, durable sessions, MCP adapters, evals, skills, and memory, designed to work with the rest of the runtime: routes, realms, OpenTelemetry, and sandboxed processes. See the AI section.

Imports are permissions. Runtime APIs are imported through public fino:* modules instead of one ambient global bag, so what code can do is visible in review — and enforceable for children. A parent decides exactly what a child realm may import, starting from a deny-all baseline for untrusted code; children cannot grant themselves modules the parent has denied. See Import Capabilities.

import { ImportMap, Realm } from 'fino:realm';

const realm = new Realm({
  entry: './skill.ts',
  overrides: ImportMap.deny([
    { pattern: './skill.ts', directive: 'inherit' },
    { pattern: 'fino:ai/tool', directive: 'inherit' },
    { pattern: 'fino:validate', directive: 'inherit' },
  ]),
});

await realm.run();

Realms are built in. A realm is an isolated JavaScript execution context with its own global object, module graph, and event loop state, and it can run embedded, in a thread, in a process, or remotely — with the same messaging, facade, and import-rule model in every mode. Use realms for plugin hosts, skill execution, and reloadable workers. See Realms.

Sandboxing is part of process execution. Process APIs can request sandbox policies — filesystem, network, process, resource limits — and report what the platform can enforce. Strict mode fails closed when a requested boundary is unavailable rather than pretending it was enforced. See the runtime model.

HTTP apps get a real router. fino:net/http/app provides middleware, validated bodies and schemas, sessions, cookies, WebSocket, SSE, and WebTransport routes, and OpenAPI generation while preserving plain Request and Response. See HTTP.

Observability is expected. A full OpenTelemetry implementation — traces, metrics, logs, propagation, runtime instrumentations — plus benchmark, doc, and profiling commands make agent behavior and runtime performance measurable from the start. See OpenTelemetry.

Beyond those: background jobs, durable workflows, a task model shared by the CLI and AI tools, Arrow and Parquet data tooling, SQLite, and more — indexed in the documentation map.

Documentation

• Getting Started — install to first agent.
• Documentation map — every guide, organized by goal.
• AI — models, tools, agents, sessions, evals, MCP.
• Realms — isolation, capabilities, messaging, pools.
• HTTP — serving, routing, clients, WebSockets, SSE.
• Runtime Model — the concepts behind it all.

On the generated docs site the full API reference sits alongside these guides; build it locally with fino doc build --format html js.

Project Status

Fino is experimental and under active development. Some APIs are broad, some are new, and Node/npm ecosystem compatibility is intentionally partial. Treat documented public fino:* modules and authored guides as the supported application surface, and expect low-level internals to change as the runtime evolves.

For build instructions, repository layout, and development workflow, see CONTRIBUTING.