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 formstack-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/formstack-webhooks)<a href="https://agentmods.dev/skills/hookdeck/webhook-skills/formstack-webhooks"><img src="https://agentmods.dev/badge/skills/hookdeck/webhook-skills/formstack-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/formstack-webhooks"><img src="https://agentmods.dev/badge/skills/hookdeck/webhook-skills/formstack-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 | $0.00108 | $0.04299 |
| Opus 5.5 | $0.00043 | $0.01720 |
| Sonnet 5.5 | $0.00022 | $0.00860 |
| Haiku 4.5 | $0.00011 | $0.00430 |
Grade A, and why
formstack-webhooks scanned grade A with 1 finding against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 21d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Makes network callslowCapability
Not a fault in itself. Listed so you know the mod talks to something, and to what.
curl -X POST "https://www.formstack.com/api/v2025/forms/$FORM_ID/webhooks" \ How it starts
The opening of the file, as written. The whole thing — 332 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Formstack Webhooks
Formstack Forms (formstack.com) sends submission data to your endpoint through a WebHook submit action — labelled "Send Data to an External URL (WebHook)" in the form builder. One HTTP POST per form submission.
Scope. This skill is about Formstack Forms only. It does not cover Formstack Documents (formerly WebMerge, which has its own separate "Webhook Delivery" feature), Formstack Sign (formerly InsureSign), or the Salesforce-packaged products. Those are different surfaces with different payloads. A Documents delivery is easy to tell apart: it posts
merge_id,handshake,file_nameand a base64file_contents, with aWebMergeuser-agent and no signature header. A Forms delivery postsFormIDandUniqueIDwith aFormstackWebhook/1.0user-agent.
X-FS-Signatureis a shared header name, not a shared scheme. FastSpring — an unrelated e-commerce company — uses the same header name with a base64 digest and anevents[]payload envelope. Formstack uses lowercase hex and has no envelope. If you are writing.digest('base64')or looking forevents, you have the wrong provider.
When to Use This Skill
- How do I receive Formstack form submissions as a webhook?
- How do I verify the
X-FS-Signatureheader from Formstack? - Why is my Formstack HMAC verification failing on a urlencoded body?
- What event types does Formstack send? (None — see below.)
- What fields will a Formstack webhook POST to my endpoint?
- How do I configure a Formstack WebHook's HMAC Key / Custom HMAC Header?
- What is the Formstack "WebHook Shared Secret" / Handshake Key and should I use it?
There Are No Event Types
A Formstack WebHook fires on exactly one thing: a form submission.
- There is no event-type header and no event-type body field.
- There is no list of event names and no subscribe-to-events model.
form.submitted,submission.created,form_submission— none of these exist. Do not write code that switches on them.
What ships with it
20 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 1.7 KB
- examples/express/package.json 437 B
- examples/express/README.md 6.4 KB
- examples/express/src/index.js 9.4 KB runs code
- examples/express/test/webhook.test.js 13 KB runs code
- examples/fastapi/.env.example 1.3 KB
- examples/fastapi/main.py 8.5 KB runs code
- examples/fastapi/README.md 6.9 KB
- examples/fastapi/requirements.txt 82 B
- examples/fastapi/test_webhook.py 13 KB runs code
- examples/nextjs/.env.example 1.4 KB
- examples/nextjs/app/webhooks/formstack/route.ts 8.1 KB runs code
- examples/nextjs/package.json 515 B
- examples/nextjs/README.md 6.7 KB
- examples/nextjs/test/webhook.test.ts 14 KB runs code
- examples/nextjs/vitest.config.ts 162 B runs code
- references/overview.md 10 KB
- references/setup.md 10 KB
- references/verification.md 13 KB
- TODO.md 2.6 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.
- 21d ago Changed · +16 lines a59c75fc02b0
- 1mo ago First seen · 316 lines · 108 tokens per session scan A 7cc717fbdd57
formstack-webhooks is a skill published in the GitHub repository hookdeck/webhook-skills (92 stars, last pushed yesterday), licensed MIT. It adds 108 tokens to every session and 4,299 once invoked, about $0.0004 per session on Opus 5.5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-15.
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