fx inside a Vercel Function

Runs fx (Vercel's coding agent) inside a Vercel Function using the WebAssembly build from libfx, with just-bash as an in-memory bash workspace for the agent's run_command tool.

How it works

• api/ask.mjs — the Vercel Function handler. It resolves an AI Gateway credential (AI_GATEWAY_API_KEY or the deployment's OIDC token) and spawns a child Node process with --experimental-wasm-jspi, since JSPI cannot be enabled via NODE_OPTIONS on the function's main process.
• api/_worker.mjs — the child process. It runs the fx terminal wasm (fx-term.wasm) headlessly: only createFxTerminal() supports the workspace adapter that provides the run_command tool (the headless createFxAgent() exposes no tools in libfx 0.0.4). The TUI is rendered into @xterm/headless, the prompt is typed in as keystrokes, turn completion is detected by screen stability, and the transcript is scraped from the emulator buffer.
• Every shell command fx runs goes through just-bash (a pure-JS in-memory bash — the same approach fx.sh/try uses), so the agent gets a real, sandboxed filesystem per request with no native binaries.

The native libfx addon does not work on Vercel's runtime (needs GLIBC 2.36), which is why the WebAssembly backend + JSPI child process is used.

Usage

GET /api/ask?prompt=<url-encoded prompt>

Browsers get streamed plain text; non-HTML clients (or ?format=ndjson) get an NDJSON event stream:

curl 'https://your-deployment.vercel.app/api/ask?prompt=Run%20ls%2C%20create%20hello.txt%20containing%20hi%2C%20then%20cat%20it.'

{"type":"command","command":"ls","exitCode":0}
{"type":"command","command":"printf 'hi' > hello.txt","exitCode":0}
{"type":"command","command":"cat hello.txt","exitCode":0}
{"type":"chunk","text":"┃ Run ls, create hello.txt containing hi, then cat it.\n\n● 3 tool calls · 3 commands\n├ Ran ls\n├ Ran printf 'hi' > hello.txt\n└ Ran cat hello.txt\n\n  Done. ls listed the directory (exit 0), hello.txt was created with hi, and cat returned hi."}
{"type":"done","stopReason":"end_turn"}

The command events are emitted directly from the just-bash workspace adapter, so they reflect commands that actually executed.

Deploy

npm install
vercel deploy --prod

Requires Node.js 24.x on the function runtime (set in package.json engines). The function needs a long maxDuration (see vercel.json) since a single agent turn can take a while. Note: the endpoint is unauthenticated as written — add your own auth/rate limiting before exposing it.

Known limitations

• fx's noninteractive mode (fx ask) is blocked in the wasm build (WasmTerminalInteractiveLaunchRequired), hence the headless-TUI + keystroke approach.
• Turn completion is detected heuristically (screen stops changing), capped at 4 minutes.
• One prompt turn per request; sessions are not persisted between invocations.