Differential Review: Security-Focused Code Change Analysis for Agents

Where this fits

You're a developer who just pushed a PR that changes a core authentication function. You ask an agent to review it for security issues, but the agent skims the diff and misses the re-introduced null-pointer check that was fixed last month. Or it flags an innocuous refactor but doesn't notice the untested error path. The differential-review skill from Trail of Bits solves this by giving agents a structured, security-first approach to analyzing any code change — whether it's a PR, a single commit, or a raw diff.

Why agents benefit

• Adaptive depth: The skill automatically scales its analysis from SMALL to LARGE codebases, so you don't waste time on deep dive when a quick scan suffices — or miss critical context in a huge repo.
• Blast radius calculation: It counts callers of each modified function, so the agent can tell you exactly what else could break — not just guess.
• Test coverage check: It identifies which modified lines have no test covering them, pointing out high-risk untested paths before they become production bugs.
• Git blame context: It uses history to see if a change re-introduces a previously fixed bug — a common and dangerous oversight.
• Always produces a markdown report: The agent outputs a structured, evidence-based report with line numbers, attack scenarios, and confidence levels — ready to paste into your PR review.

Practical scenarios

Re-introduced bug in a login flow

A developer reverts a password validation change to fix a performance issue. The agent runs differential-review and uses git blame to see that the exact check was removed two months ago to address a timing attack. It flags the re-introduction with a concrete proof-of-concept.

High-risk refactor of a shared utility

A utility function is rewritten to be more generic. The skill counts its 47 callers and maps which ones are in authentication and payment modules. The agent warns that changing the return type silently breaks three critical call sites.

Untested edge case in an error handler

A commit adds a new exception path to an API endpoint. The report shows that 80% of the modified lines have no test coverage. The agent suggests specific test cases and highlights the exact branches that are unprotected.

Add it to your agent workflow

Install the skill using the CLI (requires Node.js):

npx skills add trailofbits/skills --skill differential-review

Then, in your agent prompt, simply ask:

Review the last commit in this repo for security issues using differential-review.

The agent will automatically produce a differential-review-report.md in the workspace, which you can attach to the PR.

Compare before adopting

Not all review skills are equal. Compare differential-review against alternatives like generic “code review” prompts or other security-focused skills. Look for:

• Evidence quality: Does it cite line numbers and git history, or just give generic advice?
• Adaptivity: Does it adjust depth based on codebase size, or treat every file with the same lens?
• Maintenance: Check the GitHub repo (trailofbits/skills) for recent commits and issue responsiveness.
• Workflow fit: Does it output a report you can directly use in your CI or code review tool? This one does.

Also, verify the skill’s allowed tools (Read, Write, Grep, Glob, Bash) match your security policy.

Why it is worth tracking

This skill comes from Trail of Bits, a respected security firm, and the parent repo has over 6,800 stars — a strong signal of community trust. As AI agents become more autonomous in code review, having a skill that explicitly focuses on security and produces verifiable artifacts is a must. If you routinely merge code that touches authentication, crypto, or external calls, you should evaluate this now. For more details and community updates, see the skill page.