Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add instructions/provos/ironcurtain/sandboxgit clone --depth 1 https://github.com/provos/ironcurtainWrote 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/provos/ironcurtain/sandbox)<a href="https://agentmods.dev/instructions/provos/ironcurtain/sandbox"><img src="https://agentmods.dev/badge/instructions/provos/ironcurtain/sandbox.svg" alt="Measured on agentmods" 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 | $0.00450 | $0.00450 |
| Opus 5 | $0.00225 | $0.00225 |
| Sonnet 5 | $0.00090 | $0.00090 |
| Haiku 4.5 | $0.00045 | $0.00045 |
Grade A, and why
ironcurtain sandbox.instructions.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 6d 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 — 19 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Sandbox Review Rules
The sandbox registers a custom UTCP communication protocol (ironcurtain-protocol.ts) that routes tool calls in-process to the ToolCallCoordinator. It is the integration point between the agent's V8 isolate and the security boundary.
Mandatory Checks
- The sandbox MUST route tool calls through the
ToolCallCoordinatorvia the custom IronCurtain UTCP protocol (ironcurtain-protocol.ts). Never register real MCP servers directly with UTCP — that bypasses the entire policy engine. - MCP proxy subprocesses (
mcp-proxy-server.ts) are pure relay transports spawned byMCPClientManager. Security is enforced upstream in the coordinator, not in the proxy.PROXY_COMMANDandPROXY_ARGSstill exist for starting relay subprocesses. - Each backend MCP server gets its own relay subprocess via
SERVER_FILTERenv var. This ensures credential isolation (OAuth tokens, sandbox-runtime wrapping). - The per-sandbox unique manual name (e.g.,
tools_<uuid>) in the UTCP registration prevents cross-sandbox collision when multiple sandboxes run in the same process (daemon mode). Never hardcode the manual name. - The
Protocol.requesttimeout monkey-patch must execute beforeCodeModeUtcpClient.create(). Moving it later leaves client instances with the wrong timeout. - Tool functions inside the sandbox are synchronous. Do not add
awaitto sandbox tool calls. @utcp/mcpmust be imported (side-effect import) to register the MCP call template type. Removing this import breaks MCP functionality.- Strip internal fields at the sandbox boundary: when returning coordinator responses to the V8 isolate (or any untrusted execution context), explicitly filter to the public contract (
{ content, isError }). Do not rely on a specific transport's serialization behavior (e.g., the MCP SDK dropping unknown keys during stdio framing) to keep internal fields out — a new transport may not inherit that behavior. Internal-only fields like_policyDecisionmust never reach code running inside the isolate, where they leak policy-rule names and reasons to untrusted LLM-generated code.
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.
- 6d ago First seen · 19 lines · 450 tokens per session scan A 92b71de1c341
ironcurtain sandbox.instructions.md is an instructions file published in the GitHub repository provos/ironcurtain (600 stars, last pushed 2d ago), licensed Apache-2.0. It adds 450 tokens to every session, about $0.0022 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 2026-08-30.
Other instructions, from other repositories
agentos CLAUDE.md
Claude Code instructions for rivet-dev/agentos, covering agentos, boundaries, security model, sqlite schema ownership and runtime and registry.
agent-sandbox AGENTS.md
AGENTS.md instructions for kubernetes-sigs/agent-sandbox, covering agents.md, project summary, repository layout, agent skills and build, test, lint.
agent-sandbox copilot-instructions.md
Copilot instructions for kubernetes-sigs/agent-sandbox: Project Context & Architecture: Refer to AGENTS.md for full project background, module layout, toolchain versions, and core conventions.
mcp-server-starrocks CLAUDE.md
Claude Code instructions for StarRocks/mcp-server-starrocks, covering claude.md, project overview, development commands, run the server directly for testing and run with test mode to verify table overview functionality.
after-effects-mcp AGENTS.md
AGENTS.md instructions for JUNKDOGE-JOE/after-effects-mcp, covering repository development and delivery rules, 0.0 read the current architecture direction first, 0. user authorization is the scope boundary, 1. measure outcomes, not activity and 2. prioritize by dependency and user value.
Symbiont AGENTS.md
AGENTS.md instructions for ThirdKeyAI/Symbiont, covering symbiont — agent instructions, project structure, build and test, code style and commit guidelines.