Borrowing it
Nothing to install: this file belongs to cardstack/boxel. 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/cardstack/boxel/main/.claude/skills/outbound-guard/SKILL.mdgit clone --depth 1 https://github.com/cardstack/boxelWrote 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/skills/cardstack/boxel/outbound-guard)<a href="https://agentmods.dev/skills/cardstack/boxel/outbound-guard"><img src="https://agentmods.dev/badge/skills/cardstack/boxel/outbound-guard/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/skills/cardstack/boxel/outbound-guard"><img src="https://agentmods.dev/badge/skills/cardstack/boxel/outbound-guard.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 | $0.00404 | $0.05853 |
| Opus 5.5 | $0.00162 | $0.02341 |
| Sonnet 5.5 | $0.00081 | $0.01171 |
| Haiku 4.5 | $0.00040 | $0.00585 |
Grade C, and why
outbound-guard scanned grade C with 1 finding 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 5d 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.
Cloud metadata endpointhighServer-side request forgery
One request to 169.254.169.254 can return temporary IAM credentials.
| `address` | the host is, or resolves to, a non-public address: private, loopback, link-local, CGNAT, multicast, reserved, IPv4-mapped/embedded forms of those, and the metadata addresses `169.254.169.254` / `fd00:ec2::2 How it starts
The opening of the file, as written. The whole thing — 412 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Outbound guard (operators)
Every realm-server call that carries a stored credential to an outside vendor
goes through outboundFetch in packages/realm-server/lib/outbound/client.ts.
Today that is POST /_request-forward (including streamed calls),
POST /_openrouter/chat/completions, the generation-cost lookup either of
them makes when a response carried a generation id but no inline cost
(OpenRouterCreditStrategy in lib/credit-strategies.ts), and every callApi
card operation (lib/outbound/call-api-sender.ts; see the call-api skill).
A callApi carries no credential: its credentials are substitutions,
filled into the request after the destination check and never followed across
a redirect to another origin. A substitution may not change the URL's origin,
and one filled into the path stays within its segment: a value holding / or
\, or one the URL would resolve away as a dot segment, is refused
invalid-url (502) before anything is sent. The guard is
always on, in every environment; local development gets a short allowlist, never an off switch.
What it refuses
| Refusal | When | Caller sees |
|---|---|---|
scheme |
the URL (or a redirect hop) is not https:, and its host is not on the development allowlist |
502 Destination refused: Only https URLs … |
address |
the host is, or resolves to, a non-public address: private, loopback, link-local, CGNAT, multicast, reserved, IPv4-mapped/embedded forms of those, and the metadata addresses 169.254.169.254 / fd00:ec2::254 |
502 Destination refused: … private or reserved |
forbidden-header |
a header named X-Boxel-* in the forwarded headers |
400, before any credit slot is taken |
too-many-redirects |
more than 5 redirect hops | 502 |
timeout (connect/total) |
no connection in 10 s, or the whole exchange (every hop and the body) not done in 10 min | 504; mid-stream, an SSE error frame then [DONE] |
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.
- 5d ago First seen · 412 lines · 404 tokens per session scan C f37860b3f4c5
outbound-guard is a skill published in the GitHub repository cardstack/boxel (27 stars, last pushed today), licensed MIT. It adds 404 tokens to every session and 5,853 once invoked, about $0.0016 per session on Opus 5.5. A static security scan graded it C with 1 finding (cloud metadata endpoint). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-10-21.
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