Vercel Labs has open-sourced fx, an experimental coding agent and command-line tool written in Zig. Rather than running as a full-screen terminal app, it writes its conversation into ordinary scrollback and exposes the same engine through a CLI and a library. Announced in August 2026, it ships as a 6.3 MiB native binary, and Vercel states a single-digit-megabyte memory baseline and a 10-microsecond cold start, figures the article notes have not been independently verified.
For automation, fx ask --json returns structured output that a script or CI job can parse, and fx acp lets editors attach over the Agent Client Protocol. The libfx library exposes the engine to JavaScript, including a WebAssembly build. Release 0.0.11 added named OpenAI-compatible connections for local servers and alternative gateways, and the README lists Vercel AI Gateway, ChatGPT and Grok subscriptions, Ollama and OpenRouter.
Vercel has placed fx in its AI SDK harness layer alongside Claude Code, Cline, Codex, Cursor, OpenCode and Pi. Release 0.0.13, dated October 4, added an Ultrafast mode on supported OpenAI models, but newly written libfx checkpoints cannot be restored by older versions. A GitHub issue notes that the name overlaps with the established fx terminal JSON viewer and its Homebrew formula, and packaging is under review.
Why it matters if you build agents
Developers embedding a coding agent can call fx from scripts, CI jobs or JavaScript and parse its JSON output without a terminal UI. Anyone upgrading to 0.0.13 or later should check the libfx migration guide first, since older versions cannot restore newly written checkpoints. Because fx is pre-1.0 and changing quickly, the createFxAgent() call now maps to createFxEngine() in current documentation.
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