semgrep-rule-variant-creator
by trailofbitssemgrep-rule-variant-creator helps port existing Semgrep rules into target languages with applicability analysis, test-first validation, and separate rule/test outputs. Use the semgrep-rule-variant-creator skill when you need a reliable guide for Semgrep rule expansion across polyglot codebases, not a brand-new rule from scratch.
This skill scores 78/100, which means it is a solid listing candidate for directory users who need to port existing Semgrep rules into new target languages. The repository gives enough workflow detail to reduce guesswork versus a generic prompt, though adopters should expect a specialized, test-driven porting workflow rather than a broad Semgrep authoring guide.
- Clear trigger and scope: it is explicitly for porting existing Semgrep rules to specified target languages, not for creating rules from scratch.
- Strong operational guidance: the body includes applicability analysis, language syntax translation guidance, and a phased workflow with test-first validation.
- Good install decision value: the repo documents inputs, outputs, and when not to use the skill, helping agents decide fit quickly.
- It is experimental/test-tagged, so users should treat it as a specialized workflow aid rather than a fully polished general-purpose skill.
- No install command or supporting automation is provided, so execution depends on the agent following the documented steps manually.
Overview of semgrep-rule-variant-creator skill
The semgrep-rule-variant-creator skill helps you port an existing Semgrep rule into one or more target languages, with applicability analysis and test-driven validation built into the workflow. It is best for security engineers, AppSec teams, and rule authors who already have a working rule and need reliable language variants instead of a fresh detection from scratch.
The real job-to-be-done is not “write a Semgrep rule,” but “determine whether the same vulnerability pattern still matters in another language, then translate the detection without breaking intent.” That makes semgrep-rule-variant-creator especially useful for Semgrep rule maintenance, language expansion, and semgrep-rule-variant-creator for Security Audit work across polyglot codebases.
What this skill does well
It separates analysis from translation: first it checks whether the pattern applies, then it creates a language-specific rule and matching test file. That reduces false ports, especially for vulnerabilities whose sinks, sources, or syntax change across languages.
Best fit for this skill
Use the semgrep-rule-variant-creator skill when you already know the source rule, the target language(s), and the security pattern is expected to have a meaningful analogue. It fits teams that want independent rule/test directories for each language rather than a one-off prompt answer.
When it is not a good fit
If you need a brand-new rule, use a rule-creation skill instead. If the target language cannot realistically express the vulnerability, or you only want to run existing rules against code, semgrep-rule-variant-creator is the wrong tool.
How to Use semgrep-rule-variant-creator skill
Install and open the source files
For semgrep-rule-variant-creator install, add the skill from the skills repo and then inspect the core files first:
npx skills add trailofbits/skills --skill semgrep-rule-variant-creator
Start with SKILL.md, then read references/applicability-analysis.md, references/language-syntax-guide.md, and references/workflow.md. Those files explain the decision path, syntax translation cautions, and the phase-by-phase workflow.
Give the skill enough input to work
The semgrep-rule-variant-creator usage pattern expects two essentials: the original Semgrep rule and the target language list. A weak request is “port this rule to Java.” A stronger request is: “Port python/sql-injection to Go and Java, keep taint semantics where possible, and skip any language where the sink is not meaningfully comparable.”
Follow the workflow in the right order
Use this guide as a three-step loop: confirm applicability, create tests first, then write the rule and validate it. The repository explicitly recommends independent cycles per language, so do not batch multiple languages if one needs deeper AST or sink research.
Tips that improve output quality
Provide the original YAML, any known test file, and the security intent in plain English. If the source rule depends on framework-specific calls, mention the framework and the expected equivalent APIs in the target language. That lets the skill preserve detection intent instead of copying syntax that will not parse.
semgrep-rule-variant-creator skill FAQ
What problem does semgrep-rule-variant-creator solve?
It turns one Semgrep rule into language variants with validation, so you can expand coverage without guessing whether the pattern still applies. For semgrep-rule-variant-creator guide users, the key value is controlled translation, not generic rule brainstorming.
Is this better than a normal prompt?
Yes, when the task involves applicability decisions, AST-aware translation, and test files. A normal prompt often misses language-specific semantics; this skill is designed to catch cases where a pattern must change or should not be ported at all.
Can beginners use it?
Yes, if they can provide a source rule and a target language. The main barrier is not Semgrep syntax alone, but understanding whether the vulnerability class and sink/source pattern exist in the new language.
When should I not use it?
Do not use semgrep-rule-variant-creator for minor syntax tweaks within the same language, or when the security issue does not transfer cleanly. If the pattern is language-agnostic only in theory, the applicability step should stop the port before you spend time on a bad variant.
How to Improve semgrep-rule-variant-creator skill
Start with a tighter source rule brief
The best inputs name the rule ID, the original language, the vulnerability class, and the target languages. For example: “Port django-sqli from Python to PHP and Ruby; preserve taint flow and identify non-applicable languages before writing tests.” That helps semgrep-rule-variant-creator focus on the right sink/source relationships.
Share the parts that usually break
If the rule depends on framework helpers, builder APIs, string interpolation, reflection, or query execution, say so up front. Those are the places where a direct syntax translation fails and where the skill’s applicability analysis matters most.
Validate the first output against your real repo
Use the first generated variant to check whether the test file matches your project style, whether the rule is too broad, and whether the sink is the one your reviewers expect. If the output feels close but noisy, refine the source rule or narrow the target language scenario before expanding further.
Iterate on precision, not volume
The most useful improvement is usually narrowing the rule intent, not asking for more variants. If the first pass detects the right issue but overmatches, provide a more specific sink, a stronger source constraint, or a concrete code example from your codebase so the next semgrep-rule-variant-creator usage pass can tighten the pattern.