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 JosiahSiegel/claude-plugin-marketplace --skill clerk-clock-skew-jwtgit clone --depth 1 https://github.com/JosiahSiegel/claude-plugin-marketplaceWrote 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/josiahsiegel/claude-plugin-marketplace/clerk-clock-skew-jwt)<a href="https://agentmods.dev/skills/josiahsiegel/claude-plugin-marketplace/clerk-clock-skew-jwt"><img src="https://agentmods.dev/badge/skills/josiahsiegel/claude-plugin-marketplace/clerk-clock-skew-jwt/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/josiahsiegel/claude-plugin-marketplace/clerk-clock-skew-jwt"><img src="https://agentmods.dev/badge/skills/josiahsiegel/claude-plugin-marketplace/clerk-clock-skew-jwt.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 | $0.00168 | $0.04679 |
| Opus 5 | $0.00084 | $0.02339 |
| Sonnet 5 | $0.00034 | $0.00936 |
| Haiku 4.5 | $0.00017 | $0.00468 |
Grade B, and why
clerk-clock-skew-jwt scanned grade B 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 12d 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.
Asks for rootmediumPrivilege escalation
A mod that escalates privileges can change anything on the machine, not only the project.
sudo hwclock -s # sync to hardware clock (may also be stale) How it starts
The opening of the file, as written. The whole thing — 381 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Clerk JWT and Clock Skew
Overview
Clerk session tokens and custom JWT templates are signed JSON Web Tokens. The verifier always checks three time-based claims:
| Claim | Meaning | Failure error |
|---|---|---|
iat (issued at) |
When the token was minted | (rare) invalid-iat or iat must be in the past |
nbf (not before) |
Earliest moment the token is valid | token-not-active-yet (Clerk error reason) |
exp (expiration) |
Latest moment the token is valid | token-expired (Clerk error reason) |
Clock skew occurs when the verifier's wall clock is offset from the issuer's wall clock. If the verifier is behind, even freshly minted tokens fail nbf. If the verifier is ahead, long-lived tokens can fail exp early. Both directions break auth.
Clerk's own backend SDK applies a small tolerance (clockSkewInMs default 5000 ms) and the Dashboard exposes a session-level "Allowed Clock Skew" / nbf offset setting for JWT templates. Third-party libraries used to verify Clerk-issued JWTs have their own leeway settings; the safe defaults differ by library.
Time Claims in Clerk Tokens
A decoded Clerk session token typically contains numeric epoch-second values for iat, nbf, and exp. Decode the JWT (without verification) at a tool like jwt.io to inspect them when diagnosing failures.
The verifier must check nbf <= now + leeway and exp >= now - leeway. A token that is "active" must satisfy both bounds. If nbf > now even by one second, the token is rejected with token-not-active-yet. If exp < now, the token is rejected with token-expired.
iat is informational; most libraries do not require it. Clerk's tokens do include iat, and some libraries (for example golang-jwt/jwt v5) require WithIssuedAt() to enable iat validation.
Clerk SDK Clock Skew Configuration
Clerk's verifyToken() (in @clerk/backend, @clerk/nextjs/server, and equivalent) accepts a clockSkewInMs option. Documented default: 5000 (5 seconds). The same option is exposed to authenticateRequest for HTTP requests.
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.
- 12d ago First seen · 381 lines · 168 tokens per session scan B 5a91fb63eb51
clerk-clock-skew-jwt is a skill published in the GitHub repository JosiahSiegel/claude-plugin-marketplace (54 stars, last pushed 2mo ago), licensed MIT. It adds 168 tokens to every session and 4,679 once invoked, about $0.0008 per session on Opus 5. A static security scan graded it B with 1 finding (asks for root). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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