developing-yolo-jail

developing-yolo-jail is a skill for Claude Code, Codex from mschulkind-oss/yolo-jail. It costs 62 tokens per session (3,082 once invoked), scanned A, original, Apache-2.0.

A development guide for changing the Go source code and Nix configuration of yolo-jail, a tool that runs work inside an isolated environment. It explains how building, installing, deploying and testing the jail relate to each other.

In plain words
What is it for?
Use it when editing cmd/, internal/ or flake.nix, rebuilding or deploying yolo-jail, and verifying changes in a nested jail.
Why use it?
It prevents edited code from being mistaken for the code actually running in the jail. It also highlights the differences between cross-compiling files for shipping and compiling the live checkout for testing.

Skill for Claude CodeCodex

Written for no agent in particular: nothing here depends on one. Also seen: mentions AGENTS.md.

Not installable: its command points at a path on the author’s own machine, so it runs nowhere else. The line is /home/agent/.claude.

Good fit Use it when editing cmd/, internal/ or flake.nix, rebuilding or deploying yolo-jail, and verifying changes in a nested jail.

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Install

Getting it into your agent

There is no command for this one: it runs only inside a plugin, and the catalogue could not identify which plugin ships it. The source is linked below.

Made for: Claude Code, Codex.

Wrote 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.

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README.md
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Per session 62 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 3,082 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found in the catalogue.
Token cost

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.

ModelPer sessionOnce invoked
Fable 5.1 $0.00062 $0.03082
Opus 5 $0.00031 $0.01541
Sonnet 5 $0.00012 $0.00616
Haiku 4.5 $0.00006 $0.00308

Measured today against content hash 72408f70227c, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-10, from the pricing page.

Security

Grade A, and why

developing-yolo-jail 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 today.

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.

internal/jailcontent/builtinskills/developing-yolo-jail/SKILL.md · 206 lines

How it starts

The opening of the file, as written. The whole thing — 206 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Developing yolo-jail

This jail is running against the yolo-jail source tree itself. /workspace is bind-mounted live.

The image does not contain yolo (since 2026-09-06). /opt/yolo-jail is two :ro bind mounts the LAUNCH supplies — the linux binaries at bin/, the flake bundle at share/yolo-jail/ — and the image bakes only those mountpoints plus the /bin/<name> symlinks that point into them. Two things follow, and they pull in opposite directions so keep both: a commit touching only cmd/ or internal/ no longer moves the image at all (no rebuild, no image delivery — the Go source left the image derivation), and there is still no dev-override wrapper, so you cannot live-patch the running jail's binaries either. A nested jail builds the live checkout's own .#installPrefix and mounts THAT — but only when you pass YOLO_REPO_ROOT=/workspace, and only from a throwaway workspace (see "The flake a launch builds from" and "Never launch a nested jail on /workspace" below). Then your edited Go code is what runs. These are the build/deploy traps that have no yolo --help home — the authoritative version lives in /workspace/AGENTS.md (bind-mounted, always current); read it for the full detail.

Verifying a change on macos-user

A macos-user jail cannot launch a jail, so the nested-jail loop does not exist there. Two kernel rules, not a missing feature: sudo cannot exec inside any Seatbelt sandbox, and sandbox_apply refuses a profile that differs from the active one. Nesting is unavailable however the code is written.

What DOES work from inside, and covers most of the backend:

  • yolo run --dry-run — the whole plan, invariants included.
  • yolo internal darwin-bootstrap with HOME/JAIL_HOME pointed at a temp dir — the real generators, the real shims, the real overlay install.
  • the staging argv from the plan, with sudo dropped and the destination redirected — the copies, the modes, and the fresh-inode rule.

Both of the last two are automated (internal/entrypoint/darwinbootstrap_darwin_test.go, internal/macosuser/staging_darwin_test.go) and run on any Mac with no privilege.

Read the full file on GitHub · 206 lines

Changes

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.

  1. today Changed · +39 lines 72408f70227c
  2. 5d ago Changed · +21 lines ddd52cb647ef
  3. 6d ago Changed · +55 lines 5da2514e9537
  4. 10d ago First seen · 91 lines · 62 tokens per session scan A 7e84e0a287e3

Subscribe to this mod's changes

developing-yolo-jail is a skill published in the GitHub repository mschulkind-oss/yolo-jail (2 stars, last pushed today), licensed Apache-2.0. It adds 62 tokens to every session and 3,082 once invoked, about $0.0003 per session on Opus 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 2026-08-31.

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