Installation

Prerequisites

Bun below the floor fails before anything else runs:

X_BUN_VERSION: bun version is below the framework floor
  cause: Bun 1.4.3 is older than the required 1.4.4
  fix:   bun upgrade

x doctor --json checks the runtime, the embedded Postgres, ports, and the resolved env. Outside an app, x is on no PATH and bunx x fetches an unrelated npm package named x: spell it bunx @ultimat3/cli doctor --json there, and start an app with bunx create-ultimate myapp.

Create an app

bunx create-ultimate myapp && cd myapp && bun run setup && bun run dev

create-ultimate is a thin front for x new. Inside an existing workspace, use x new directly.

PowerShell and cmd run the same commands. bun run setup runs the app's bin/setup.ts — TypeScript that Bun executes, no bash and no WSL — and bun run dev and bun run verify are x dev and x verify. Windows PowerShell 5.1 has no &&: type the commands one per line there (PowerShell 7 and cmd take the line as written).

bun run setup is not optional. x new writes files and installs nothing, so cd myapp && x dev stops on X_BUILD_FAILED naming bun install (measured As of 2026-08-23; this page said otherwise until then). bun run setup is six steps, idempotent and safe to re-run:

Every flag, from the registry — bun run x -- help new, or x help new in an app. There are six, each with a default, so x new asks nothing: an agent cannot answer a prompt and is never given one.

There is no --admin, --locale, --tz, --currency, --no-install or --yes — this table listed all six until 2026-08-23 and the parser refuses each with X_CLI_BAD_FLAG. apps/admin/ is always written; locale, timezone and currency are app.config.ts fields you edit after scaffolding (Configuration).

What x new writes, and who owns it afterwards:

Typed env, validated at boot

Declared at module scope in app.config.ts, next to defineConfig. There is no env.ts — app.config.ts is the one file the CLI and the runtime both load, so it is the one place the env gate can run before anything binds.

// app.config.ts
import { defineConfig, defineEnv } from '@ultimat3/core';

export const env = defineEnv({
  DATABASE_URL: { type: 'url' },
  NATS_URL:     { type: 'url', required: false },
  S3_BUCKET:    { type: 'string' },
  VAPID:        { type: 'string', required: false },
  STRIPE_KEY:   { type: 'string', pattern: /^sk_/, secret: true },
});

export const config = defineConfig({ name: 'myapp' /* … */ });

Boot parses this before any listener binds. Failure costs ~40ms and exit 1 — not a 3am undefined in a payment handler.

X_ENV_MISSING: required environment variables are missing or invalid
  cause: STRIPE_KEY missing; NATS_URL is not a URL ("nats:4222")
  fix:   add STRIPE_KEY, NATS_URL to .env (copy .env.example), then run: x env check

The fix: is per-key and built from your own declarations (packages/core/src/env.ts:237); defineEnv({ KEY: { fix } }) overrides it for one key.

What Bun natives replace

Each row is a dependency subtree that never enters the lockfile. Target: under 40 direct dependencies for the whole framework.

One more subtree dies with no Bun native behind it — a pure-TypeScript framework pipeline, written because native addons are blocked:

Stated cost: no native-addon packages, and Bun's long-running-process maturity is less proven than Node's.

Editor and agent setup

Two MCP servers, and an app ships both. The dev tools are x mcp serve — x dev does not start them, and nothing listens on 9229 until this command runs. A new app's .mcp.json already names the stdio form, as bunx @ultimat3/cli mcp serve --transport stdio, so a client that reads that file starts it with no setup; MCP and AI says what to give one that does not. The app's own tools are POST /mcp on the app (apps/web/mcp.ts), mounted by x dev and the web role, refusing any request without a bearer token. The HTTP form below prints its token:

bunx x mcp serve --transport stdio              # a client that spawns the server
bunx x mcp serve --transport http --port 9229   # POST /mcp; the bearer token is printed at boot

stdio and http are the two transports (x help mcp) — there is no ws transport, and this page told readers to configure one until 2026-08-23. Register the URL and the printed token with whatever MCP client you use; a request without the token is answered 401 with no catalog.

The catalog is bunx x mcp tools --json: every tool's exact name and description, read off the server itself rather than counted here. The core of it:

Read tools are unrestricted in dev; write tools are scoped to branch environments. The dev server is never exposed under ROLE=web. db.query refuses a batch, a write keyword anywhere at statement level (a data-modifying CTE included), a locking clause, EXPLAIN ANALYZE, and the banned function families matched by prefix (pg_read_*, pg_ls_*, lo_*, pg_advisory_*, pg_sleep*, set_config, …) — X_MCP_QUERY_REJECTED, before the host sees the string. Use x mcp for a standalone server (CI, remote agents). Editor config: Biome is the only formatter/linter — one binary, one config, no ESLint or Prettier.

Upgrading and removal

Framework ranges: an app declares every @ultimat3/* package at ^<version>, as x new writes it, and bun.lock pins the exact build. Keep them on one version: the packages move in lockstep, and x upgrade moves them together. A third-party wrapper declares them as peerDependencies at ^<major> (Publish it).

Version pinning: As of 2026-11, the Bun floor is the one stated at the top of this page and 2.0 the target; SolidJS 2 is in beta and is pinned exactly, and its upgrade is framework work, not app work. There is no ArkType or Drizzle pin to carry — @ultimat3/schema ships its own dependency-free validators and @ultimat3/entity its own postgresDriver(). Details and codemod inventory in Upgrading. Boot failures and port conflicts in Troubleshooting.