fx is a command-line coding agent written in Zig, designed to run like a Unix tool and to be embedded in larger systems. Developers use it to work with language models through local or cloud inference providers. The catalogue instruction extends its coding-agent workflow.
Borrowing it
Nothing to install: this file belongs to vercel-labs/fx. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/vercel-labs/fx/main/AGENTS.mdgit clone --depth 1 https://github.com/vercel-labs/fxWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/instructions/vercel-labs/fx/agents-md)<a href="https://agentmods.dev/instructions/vercel-labs/fx/agents-md"><img src="https://agentmods.dev/badge/instructions/vercel-labs/fx/agents-md/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/instructions/vercel-labs/fx/agents-md"><img src="https://agentmods.dev/badge/instructions/vercel-labs/fx/agents-md.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.08325 | $0.08325 |
| Opus 5.5 | $0.03330 | $0.03330 |
| Sonnet 5.5 | $0.01665 | $0.01665 |
| Haiku 4.5 | $0.00833 | $0.00833 |
Grade A, and why
fx AGENTS.md scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 497 lines — stays where its author put it; the contents beside it link to each section on GitHub.
AGENTS.md
Instructions for AI coding agents working with this codebase.
Declaring Work Ready
Do not say the work is "ready", "done", "good to go", "complete", or similar until you have personally run the binary and exercised the change on its happy path. A passing test suite is necessary, not sufficient — tests in this repo do not always construct the full runtime, attach a TTY, or spawn background threads, so they will not catch startup crashes, render regressions, or thread-lifetime bugs.
Before reporting the work as ready:
- Build succeeds.
- Focused tests for the changed path pass locally.
- CI passes for the exact current commit, including the macOS arm64 check. See CI on Pull Requests.
- Run the built binary locally and drive at least one real interaction that exercises the change end to end.
- Confirm the process did not abort, stderr is clean, and the behavior matches what you are about to tell the user.
If you cannot run the binary in your environment, say so explicitly and ask the user to verify. Do not silently skip this step and declare the work ready. "The tests pass" is not a substitute for running the app.
Always use the built binary in this repo
When running fx for verification, always use the freshly-built binary at ./zig-out/bin/fx from this checkout. Never run fx from PATH, never rely on the installed copy at ~/.local/bin/fx, and never assume an installed copy reflects your change.
-
The user may have an older
fxon their PATH (e.g. installed viafx upgradeor the install script). Running that one will not exercise your edits. -
When a user reports "still not working" after you believe you fixed something, do not assume they are running the wrong binary. Assume your fix is incomplete and investigate further. If you genuinely suspect a PATH mismatch, ask — do not silently copy binaries onto their
PATH. -
In any shell invocation — tmux, direct run, scripts — reference fx as
/Users/<you>/path/to/repo/zig-out/bin/fx(absolute) or./zig-out/bin/fx(when cwd is the repo root). Barefxis always wrong for dev verification.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2d ago Changed · +4 lines · +158 tokens per session 120f1637febd
- 3d ago Changed · +9 tokens per session c910e9fa5ae9
- 10d ago Changed · +6 lines · +221 tokens per session bc27c02297dd
- 11d ago Changed · +6 lines · +458 tokens per session 15954f9da441
- 12d ago Changed · +3 lines · +349 tokens per session f04e1b2db39f
- 19d ago Changed · +1 lines · +628 tokens per session 1587757f208c
- 22d ago Changed · +2 lines · +43 tokens per session 375845555626
- 23d ago Changed · +2 lines · +92 tokens per session 5c9cd7f888e1
- 25d ago Changed · +2 lines · +71 tokens per session 8b6573283c56
- 1mo ago First seen · 471 lines · 6,296 tokens per session scan A 9fb684b76b28
fx AGENTS.md is an instructions file published in the GitHub repository vercel-labs/fx (3,598 stars, last pushed today), licensed Apache-2.0. It adds 8,325 tokens to every session, about $0.0333 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
Other instructions, from other repositories
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next.js AGENTS.md
AGENTS.md instructions for vercel/next.js, covering next.js development guide, codebase structure, monorepo overview, core package: packages/next and other important packages.
vscode oss-third-party-notices.instructions.md
Instructions for microsoft/vscode, covering vs code oss third-party-notices pipeline, architecture, pipeline flow in ci, cache eligibility and rollout and applying the notice (cutover).
langchain AGENTS.md
AGENTS.md instructions for langchain-ai/langchain, covering global development guidelines for the langchain monorepo, corridor security analysis, project architecture and context, monorepo structure and development tools & commands.
vscode copilot-instructions.md
Copilot instructions for microsoft/vscode, covering vs code copilot instructions, project overview, root folders, core architecture (src/ folder) and built-in extensions (extensions/ folder).
deepseek-harness AGENTS.md
AGENTS.md instructions for deepseek-ai/deepseek-harness, covering agents.md, pre-stable apis and released session data, repository layout, commands and host sandbox failures.