Connecting Vercel Labs fx over ACP in SuperQode

Vercel Labs released fx, a Zig coding agent distributed as a 7.8 MiB Apache-2.0 binary. SuperQode 0.2.102 registers fx in the harness catalog with three attach routes and a :fx command family.

fx owns its loop, tools, permissions, skills, MCP servers, and sessions. SuperQode launches fx acp and does not replace that loop.

What fx is

fx is a native CLI. Interactive sessions run as fx. One-shot requests use fx ask. Editors and other clients attach with fx acp.

The ACP server implements initialize, session/new, session/load, session/resume, session/list, session/close, session/prompt, session/cancel, session/set_mode, and session/set_config_option. Modes are ask and code. The client can supply filesystem and terminal capabilities and mcpServers. SuperQode uses that surface, so session load and resume follow the ACP contract fx advertises.

fx labels itself experimental. The SuperQode catalog row does the same.

License versus inference path

Every model request from fx goes to Vercel AI Gateway in the AI SDK wire format, not OpenAI /v1/chat/completions. The compiled default is zai/glm-5.2-fast. Model ids come from the Gateway catalog. There is no Ollama, vLLM, or LM Studio path, and no third-party BYOK key.

SuperQode's Open list is for OSI-licensed harnesses that accept a key or a local endpoint. Grok Build, Qwen Code, and OpenCode implement that. fx implements only the Gateway key. Its source is Apache-2.0. Its inference path is one vendor endpoint.

The Open row therefore uses the locked vendor-key attach (vendor-key-acp) used by Closed rows such as Factory Droid and Junie. It reads AI_GATEWAY_API_KEY, injects that variable into the fx acp child only, and does not open the SuperQode model picker. The row text states the Gateway-only constraint.

Harness openness and model-route openness are separate fields in the catalog. They are not merged into one badge.

Three attach routes

:connect fx is the Subscriptions row. Auth is fx login (Vercel OAuth) in ~/.fx/auth.json. SuperQode does not copy that file. Usage is billed as the signed-in Vercel team's AI Gateway credits.

:connect fx-key is the Open row. It requires fx setup or an exported AI_GATEWAY_API_KEY.

:connect acp fx starts the same fx acp process from the ACP catalog.

Subscription billing

fx credential order is VERCEL_OIDC_TOKEN, then AI_GATEWAY_API_KEY, then the saved fx login session. An exported Gateway key therefore takes precedence over the login if SuperQode leaves the environment unchanged.

The Subscriptions child environment strips AI_GATEWAY_API_KEY and prints the variable name plus :connect fx-key. VERCEL_OIDC_TOKEN is not stripped. OIDC is Vercel team identity in the same billing family as fx login.

Demo

Install and connect

Install fx and run fx login. On a headless host, set FX_NO_OPEN_BROWSER=1 so the authorization URL is printed.

:fx status reports whether the binary is on PATH, whether ~/.fx/auth.json exists, and whether AI_GATEWAY_API_KEY is set.

:fx login runs fx login after confirmation in the TUI. SuperQode does not read or copy ~/.fx/auth.json.

Limits

The row is marked experimental. There is no local-model attach, no SuperQode BYOK picker, and no :harness switch fx --fork. fx owns session threads. SuperQode does not replace fx tools, permissions, skills, or MCP configuration. ACP sessions use only mcpServers supplied by the client.

If AI_GATEWAY_API_KEY is set, :connect fx ignores it. Use :connect fx-key to spend that key.

Harness Hub

The Harness Hub lists fx under Coding agents and fx (ACP) under ACP agents. Both records report openness open and license Apache-2.0. The published catalog has 102 records and is generated from one file used by the website and the terminal.

Filtering the Hub by openness shows fx as open source. The row description still names AI Gateway as the only inference path.

Shipped in SuperQode 0.2.102. See the fx provider page, GitHub, or PyPI.