docs: update agent harnesses for fx support

[tembo]

What changed

Adds "fx" to the Agent harness list on /features/agents (rendered under the #agent-harness anchor). Includes the harness description and its full supported model list, matching the style of the existing Claude Code / Codex / Cursor / Opencode / Amp / Pi entries.

Why

The monorepo added fx as a fully wired, GA agent harness — not documented anywhere in the harness list.

Evidence

• monorepo PR #10736 "feat(agent-runtime): add fx support" (merged 2026-08-25, commit ee978b24e7)
• packages/agent-runtime/src/runtime/harness/fx/definition.ts — defines fxDefinition with name: 'fx', its model list, and default model (gpt-5.5)
• packages/agent-runtime/src/runtime/registry.ts — fx is registered unconditionally in AGENT_CLASSES alongside claudeCode, codex, openCode, cursor, amp, and Pi
• packages/agent-runtime/src/runtime/schema.ts — fx is a first-class value in AGENT_TYPES
• apps/web/components/icons/agent-icon.tsx + apps/web/public/fx.svg — dedicated UI icon shipped for the harness (used to build the docs icon)
• nix/packages/fx/default.nix — upstream package metadata describes fx as "Tiny, open, native coding agent" (fx.sh), used for the one-line description
• Model display names cross-checked against apps/api/src/server/models/index.ts (MODELS registry) for exact labels

Confidence

This is a high-confidence, additive documentation update: fx is merged to main, registered unconditionally (no feature flag) as a peer of the other already-documented harnesses, and ships a production UI icon — the same bar every other harness in this list already met. No wording/positioning judgment calls were needed; the change only adds a missing entry using the same structure as its neighbors.

@benja

Want tembo to make any changes? Add a comment with @tembo and i'll get back to work!

      

---

Note

Low Risk
Additive docs-only change with no runtime, auth, or data impact.

Overview
Documents the fx coding agent harness on the Agents page under Agent harness, alongside Claude Code, Codex, Cursor, and the other entries.

The new accordion uses the same pattern as Pi and Opencode: a short description (“tiny, open, native coding agent with multi-provider model support”), the fx SVG icon, and an inline Models list (Anthropic, OpenAI, Gemini, GLM, Kimi, DeepSeek, MiniMax, etc.). No other sections of features/agents.mdx are changed.

^{Reviewed by Cursor Bugbot for commit 264999a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[mintlify]

Preview deployment for your docs. Learn more about Mintlify Previews.

Project	Status	Preview	Updated (UTC)
tembo	🟢 Ready	View Preview	Aug 26, 2026, 1:09 PM

💡 Tip: Enable Workflows to automatically generate PRs for you.

[tembo]

Requesting review from @benja who has experience with the following files modified in this PR:

• features/agents.mdx

[tembo]

Reviewed and approved for merge. Verified against the released implementation: the fx identifier, unconditional registration, icon, description, and model list match v0.2.828, and that release completed its production deployment successfully. No correctness or release blockers.