What is simplewebauthn?

A TypeScript library for adding passkeys, which let users sign in with a device or security key instead of a password, to Node.js applications.

Use it when building passkey registration and sign-in with the SimpleWebAuthn server package.

Passkey setup requires several registration, sign-in, data-format, and verification details. This guidance helps avoid using the wrong types or stored credential fields.

How GoogleChromeLabs/passkey-skills-demo configures Claude Code and Co

Reuse

Borrowing it

Nothing to install: this file belongs to GoogleChromeLabs/passkey-skills-demo. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file

curl -O https://raw.githubusercontent.com/GoogleChromeLabs/passkey-skills-demo/main/.agents/skills/simplewebauthn/SKILL.md

Clone the repo

git clone --depth 1 https://github.com/GoogleChromeLabs/passkey-skills-demo

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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/googlechromelabs/passkey-skills-demo/simplewebauthn/github.svg)](https://agentmods.dev/skills/googlechromelabs/passkey-skills-demo/simplewebauthn)

Your own site

<a href="https://agentmods.dev/skills/googlechromelabs/passkey-skills-demo/simplewebauthn"><img src="https://agentmods.dev/badge/skills/googlechromelabs/passkey-skills-demo/simplewebauthn/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.

Your own site · 80×15

<a href="https://agentmods.dev/skills/googlechromelabs/passkey-skills-demo/simplewebauthn"><img src="https://agentmods.dev/badge/skills/googlechromelabs/passkey-skills-demo/simplewebauthn.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 43 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 319 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.

Model	Per session	Once invoked
Fable 5.1	$0.00043	$0.00319
Opus 5	$0.00022	$0.00160
Sonnet 5	$0.00009	$0.00064
Haiku 4.5	$0.00004	$0.00032

Measured 11d ago against content hash a0b666db7de6, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-11, from the pricing page.

Security

Grade A, and why

simplewebauthn 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 11d 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.

.agents/skills/simplewebauthn/SKILL.md · 13 lines

What it actually says

Look up on generateRegistrationOptions.d.ts, verifyRegistrationResponse.d.ts, generateAuthenticationOptions.d.ts and verifyAuthenticationResponse.d.ts for types.
Use binary value for userID when invoking generateRegistrationOptions().
Use requireUserVerification: false for verifyRegistrationResponse() unless authenticatorSelection.residentKey: "required" on the frontend.
registrationInfo.credentialID doesn't exist in verifyRegistrationResponse() result. Use registrationInfo.credential.id instead, which represents string.

credentialDeviceType and credentialBackUp are VerifiedRegistrationResponse.registrationInfo properties.
Use isoBase64URL.fromBuffer and isoBase64URL.toBuffer to convert between base64url string and Buffer.
Append verifyBrowserAutofillInput: false to startAuthentcation() when the form is consisted of web components.
When implementing verifyAuthenticationResponse(), pass in credential object instead of authenticator object, which contains: id for the credential ID as a buffer, publicKey for the public key as a buffer, counter for the counter as a number, transports for the transports as an array of strings.

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.

11d ago First seen · 13 lines · 43 tokens per session scan A a0b666db7de6

Subscribe to this mod's changes

simplewebauthn is a skill published in the GitHub repository GoogleChromeLabs/passkey-skills-demo (5 stars, last pushed today), licensed Apache-2.0. It adds 43 tokens to every session and 319 once invoked, about $0.0002 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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passkey-skills-demo: Skill for Claude Code