Add Vercel fx as a provider #16556

fx is Vercel's terminal coding agent. It has its own sign-in, a model catalog served through AI Gateway, Codex, or Grok, a permission system, and skills. It ships fx acp, an Agent Client Protocol server. T3 Code can't run fx today. I'd like maintainer approval of the direction and scope below before I open a PR.

Proposal

Add fx as a built-in provider, shaped like Grok: a thin flavor on makeAcpAdapterV2 and AcpSessionRuntime that launches fx acp. The shared ACP runtime and packages/effect-acp don't change.

Why not the ACP Registry path

fx needs agent-specific handling, and the registry path is designed to stay agent-agnostic:

• session/new lists fx's backend option provider before model, and both have category: "model". The generic path writes the selected model into the first such option, so T3 has to write config id model only.
• initialize advertises no auth methods. Sign-in state comes from fx status --json.
• fx 0.0.13 reports permission mode ask on a new session but enforces its global default (auto) until the client writes the mode. T3 has to write the mode every time, or Supervised threads run without approvals. I've reported this to the fx team.

Scope

• Off by default. Users add it from Settings > Providers > Add provider.
• The health check runs fx --version, fx status --json, and fx models --json. It opens no ACP session.
• Models come from fx's catalog. Effort offers Auto through Max, with Medium as the default.
• Runtime modes: Supervised and Auto-accept edits run fx in ask mode, and Auto runs it in code mode. Full access isn't offered, because fx acp has no way to turn off fx's own permission checks.
• Skills: the $ and / menus list skills from fx's documented skill folders. A picked skill reaches fx as $name, fx's own syntax.
• Threads resume through session/load, and Stop uses ACP cancel.
• The fx icon on web, desktop, and mobile from fx's brand assets, a user guide in docs/user, and fx on the marketing page.

Out of scope for the first PR

• Mid-turn steering. fx has a steering extension, but T3's ACP runtime has no path for a second prompt during a turn.
• Plan mode.
• Choosing the fx backend (AI Gateway, Codex, or Grok) in T3. fx keeps that setting.
• Commit message and title generation. Every fx acp session is saved under ~/.fx, and ACP can't delete it, so each helper call would add a session to the user's fx history.
• The usage page.

Implementation and evidence

• Branch on current main, one commit: main...agadzik:t3code:feat/fx-provider-upstream
• Earlier PR on my fork, with its review history: feat(providers): add Vercel fx as a provider agadzik/t3code#12
• On that branch, 10 server test suites (95 tests) and settings.test.ts (160 tests) pass, and typecheck passes for server, contracts, web, mobile, and marketing.
• In the desktop app built from that branch, with fx 0.0.13, I added fx in the wizard, ran a turn, listed skills with $ and /, declined a Supervised file write (fx asked first, and no file was written), and stopped a running turn.
• This video was recorded on the fork branch, before the port to current main:

Questions

1. Is a built-in fx provider in scope, with this shape and off by default?
2. Is leaving out Full access acceptable?
3. Are the out-of-scope items fine for a first PR?
4. Should fx appear on the marketing page, or would you rather handle that?

If you approve the direction and scope, I'll open the PR from that branch and link your approval.

Built with the fx coding agent.