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 NOMARJ/sigil --skill differential-reviewgit clone --depth 1 https://github.com/NOMARJ/sigilWrote 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/nomarj/sigil/differential-review)<a href="https://agentmods.dev/skills/nomarj/sigil/differential-review"><img src="https://agentmods.dev/badge/skills/nomarj/sigil/differential-review.svg" alt="Measured on agentmods" 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 | $0.00072 | $0.01037 |
| Opus 5 | $0.00036 | $0.00518 |
| Sonnet 5 | $0.00014 | $0.00207 |
| Haiku 4.5 | $0.00007 | $0.00104 |
Grade A, and why
differential-review 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 4d 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 — 103 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Differential Security Review
Adapted from Trail of Bits' differential-review skill. Layer 2 analysis — runs on PRs and merges, examining not just what changed but what the change means in context.
How This Differs from Layer 1
Layer 1 scans look at code in isolation — "is this line dangerous?" Layer 2 looks at the change in context — "does this change create a new attack surface, alter the security model, or introduce a vulnerability path that didn't exist before?"
Review Methodology
1. Change Classification
Before diving into code, classify the change:
- Security-critical: Touches auth, authz, crypto, PII, payment, or infrastructure
- Security-adjacent: Touches input handling, API endpoints, data storage, or external integrations
- Security-neutral: UI changes, documentation, refactoring with no behavioral change
This classification determines review depth. Security-critical changes get line-by-line analysis. Security-neutral changes get a quick scan for accidental security regressions.
2. Attack Surface Analysis
For every change, ask:
- New endpoints? Every new route/endpoint is a new entry point for attackers
- New data flows? Does data move to a new destination? Does sensitive data reach a new component?
- Changed trust boundaries? Does this change what code is trusted vs. untrusted?
- Changed permissions? Does this alter who can do what?
- New dependencies? (Feeds into supply-chain audit)
3. Contextual Analysis
Read the diff in context of surrounding code:
- Does the changed function handle user input upstream?
- Are the changed values used in security-sensitive operations downstream?
- Does this change alter assumptions that other code relies on?
- Does the git history show this area has been fixed for security issues before? (Recurrence pattern)
4. Common Differential Vulnerabilities
Changes that look innocent but aren't:
- Adding a new API field that exposes internal data
- Changing a default value from deny to allow
- Adding a code path that bypasses existing validation
- Refactoring that removes a security check as "dead code"
- Adding logging that captures sensitive data
- Changing error handling from fail-closed to fail-open
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.
- 4d ago First seen · 103 lines · 72 tokens per session scan A 7bc8616e371a
differential-review is a skill published in the GitHub repository NOMARJ/sigil (5 stars, last pushed 2d ago), licensed Apache-2.0. It adds 72 tokens to every session and 1,037 once invoked, about $0.0004 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 2026-09-04.
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