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 skills add hookdeck/webhook-skills --skill circleci-webhooksgit clone --depth 1 https://github.com/hookdeck/webhook-skillsWrote 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/hookdeck/webhook-skills/circleci-webhooks)<a href="https://agentmods.dev/skills/hookdeck/webhook-skills/circleci-webhooks"><img src="https://agentmods.dev/badge/skills/hookdeck/webhook-skills/circleci-webhooks/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/hookdeck/webhook-skills/circleci-webhooks"><img src="https://agentmods.dev/badge/skills/hookdeck/webhook-skills/circleci-webhooks.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 | $0.00150 | $0.04305 |
| Opus 5.5 | $0.00060 | $0.01722 |
| Sonnet 5.5 | $0.00030 | $0.00861 |
| Haiku 4.5 | $0.00015 | $0.00430 |
Grade A, and why
circleci-webhooks 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 18d 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 — 301 lines — stays where its author put it; the contents beside it link to each section on GitHub.
CircleCI Webhooks
CircleCI (circleci.com) is a CI/CD platform. Its outbound webhooks POST JSON to your endpoint when a workflow or a job reaches a terminal state.
Not Circle. Circle (circle.com) is the USDC / Circle Payments Network / Circle Mint payments company. It signs with ECDSA and an
X-Circle-Signatureheader. CircleCI signs with HMAC-SHA256 and acircleci-signatureheader. Two unrelated companies, two entirely different schemes — never mix their headers or events.
Not CircleCI custom webhooks. Those go the opposite direction: a third party POSTs to CircleCI to trigger a pipeline (docs). This skill is about CircleCI POSTing to you.
The signing secret is not your CircleCI API token.
Circle-Tokenauthenticates you calling CircleCI's API. The webhook "Secret token" (API fieldsigning-secret) verifies CircleCI calling you.
When to Use This Skill
- How do I receive CircleCI webhooks?
- How do I verify a CircleCI webhook signature?
- Why is my
circleci-signatureverification failing? - How do I handle
workflow-completedorjob-completedevents? - How do I get the branch and commit SHA out of a CircleCI webhook payload?
- How do I deduplicate CircleCI webhook deliveries?
- Does CircleCI send a challenge/handshake when I add a webhook endpoint?
Verification (core)
CircleCI signs the raw request body bytes only — no timestamp, no delivery id,
no prefix, no delimiter — with HMAC-SHA256 keyed on the webhook's Secret token
used as UTF-8 bytes directly (no base64 decode). The digest is lowercase
hex (64 chars) and arrives in circleci-signature as a comma-separated list
of <version>=<signature> pairs. CircleCI's docs: "Only check the latest
signature type to prevent downgrade attacks." Today v1 is the latest and only
version, so take the v1 entry, ignore everything else, and reject if there is
no v1 entry — never fall back to another version.
What ships with it
21 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
- examples/express/.env.example 835 B
- examples/express/package.json 483 B
- examples/express/README.md 6.9 KB
- examples/express/src/index.js 10 KB runs code
- examples/express/test/webhook.test.js 20 KB runs code
- examples/fastapi/.env.example 878 B
- examples/fastapi/main.py 11 KB runs code
- examples/fastapi/README.md 7.6 KB
- examples/fastapi/requirements.txt 82 B
- examples/fastapi/test_webhook.py 20 KB runs code
- examples/nextjs/.env.example 872 B
- examples/nextjs/app/webhooks/circleci/route.ts 12 KB runs code
- examples/nextjs/package.json 561 B
- examples/nextjs/README.md 7.5 KB
- examples/nextjs/test/webhook.test.ts 20 KB runs code
- examples/nextjs/tsconfig.json 562 B
- examples/nextjs/vitest.config.ts 140 B runs code
- references/overview.md 11 KB
- references/setup.md 8.4 KB
- references/verification.md 10 KB
- TODO.md 1.1 KB
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.
- 18d ago First seen · 301 lines · 150 tokens per session scan A d78d3d33fe3e
circleci-webhooks is a skill published in the GitHub repository hookdeck/webhook-skills (92 stars, last pushed yesterday), licensed MIT. It adds 150 tokens to every session and 4,305 once invoked, about $0.0006 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 2026-09-29.
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