Semgrep Agent Skill

Semgrep Security Scan

Run a Semgrep scan with automatic language detection, parallel execution via Task subagents, and merged SARIF output.

Essential Principles

• Always use --metrics=off — Semgrep sends telemetry by default; --config auto also phones home. Every semgrep command must include --metrics=off to prevent data leakage during security audits.
• User must approve the scan plan (Step 3 is a hard gate) — The original "scan this codebase" request is NOT approval. Present exact rulesets, target, engine, and mode; wait for explicit "yes"/"proceed" before spawning scanners.
• Third-party rulesets are required, not optional — Trail of Bits, 0xdea, and Decurity rules catch vulnerabilities absent from the official registry. Include them whenever the detected language matches.
• Spawn all scan Tasks in a single message — Parallel execution is the core performance advantage. Never spawn Tasks sequentially; always emit all Task tool calls in one response.
• Always check for Semgrep Pro before scanning — Pro enables cross-file taint tracking and catches ~250% more true positives. Skipping the check means silently missing critical inter-file vulnerabilities.

When to Use

• Security audit of a codebase
• Finding vulnerabilities before code review
• Scanning for known bug patterns
• First-pass static analysis

When NOT to Use

• Binary analysis → Use binary analysis tools
• Already have Semgrep CI configured → Use existing pipeline
• Need cross-file analysis but no Pro license → Consider CodeQL as alternative
• Creating custom Semgrep rules → Use semgrep-rule-creator skill
• Porting existing rules to other languages → Use semgrep-rule-variant-creator skill

Output Directory

All scan results, SARIF files, and temporary data are stored in a single output directory.

• If the user specifies an output directory in their prompt, use it as OUTPUT_DIR.
• If not specified, default to ./static_analysis_semgrep_1. If that already exists, increment to _2, _3, etc.

In both cases, always create the directory with mkdir -p before writing any files.

# Resolve output directory
if [ -n "$USER_SPECIFIED_DIR" ]; then
  OUTPUT_DIR="$USER_SPECIFIED_DIR"
else
  BASE="static_analysis_semgrep"
  N=1
  while [ -e "${BASE}_${N}" ]; do
    N=$((N + 1))
  done
  OUTPUT_DIR="${BASE}_${N}"
fi
mkdir -p "$OUTPUT_DIR/raw" "$OUTPUT_DIR/results"

The output directory is resolved once at the start of Step 1 and used throughout all subsequent steps.

$OUTPUT_DIR/
├── rulesets.txt                 # Approved rulesets (logged after Step 3)
├── raw/                         # Per-scan raw output (unfiltered)
│   ├── python-python.json
│   ├── python-python.sarif
│   ├── python-django.json
│   ├── python-django.sarif
│   └── ...
└── results/                     # Final merged output
    └── results.sarif

Prerequisites

Required: Semgrep CLI (semgrep --version). If not installed, see Semgrep installation docs.

Optional: Semgrep Pro — enables cross-file taint tracking, inter-procedural analysis, and additional languages (Apex, C#, Elixir). Check with:

semgrep --pro --validate --config p/default 2>/dev/null && echo "Pro available" || echo "OSS only"

Limitations: OSS mode cannot track data flow across files. Pro mode uses -j 1 for cross-file analysis (slower per ruleset, but parallel rulesets compensate).

Scan Modes

Select mode in Step 2 of the workflow. Mode affects both scanner flags and post-processing.

| Mode | Coverage | Findings Reported |

|------|----------|-------------------|

| Run all | All rulesets, all severity levels | Everything |

| Important only | All rulesets, pre- and post-filtered | Security vulns only, medium-high confidence/impact |

Important only applies two filter layers:

• Pre-filter: --severity MEDIUM --severity HIGH --severity CRITICAL (CLI flag)
• Post-filter: JSON metadata — keeps only category=security, confidence∈{MEDIUM,HIGH}, impact∈{MEDIUM,HIGH}

See scan-modes.md for metadata criteria and jq filter commands.

Orchestration Architecture

┌──────────────────────────────────────────────────────────────────┐
│ MAIN AGENT (this skill)                                          │
│ Step 1: Detect languages + check Pro availability                │
│ Step 2: Select scan mode + rulesets (ref: rulesets.md)           │
│ Step 3: Present plan + rulesets, get approval [⛔ HARD GATE]     │
│ Step 4: Spawn parallel scan Tasks (approved rulesets + mode)     │
│ Step 5: Merge results and report                                 │
└──────────────────────────────────────────────────────────────────┘
         │ Step 4
         ▼
┌─────────────────┐
│ Scan Tasks      │
│ (parallel)      │
├─────────────────┤
│ Python scanner  │
│ JS/TS scanner   │
│ Go scanner      │
│ Docker scanner  │
└─────────────────┘

Workflow

Follow the detailed workflow in scan-workflow.md. Summary:

| Step | Action | Gate | Key Reference |

|------|--------|------|---------------|

| 1 | Resolve output dir, detect languages + Pro availability | — | Use Glob, not Bash |

| 2 | Select scan mode + rulesets | — | rulesets.md |

| 3 | Present plan, get explicit approval | ⛔ HARD | AskUserQuestion |

| 4 | Spawn parallel scan Tasks | — | scanner-task-prompt.md |

| 5 | Merge results and report | — | Merge script (below) |

Task enforcement: On invocation, create 5 tasks with blockedBy dependencies (each step blocks the previous). Step 3 is a HARD GATE — mark complete ONLY after user explicitly approves.

Merge command (Step 5):

uv run {baseDir}/scripts/merge_sarif.py $OUTPUT_DIR/raw $OUTPUT_DIR/results/results.sarif

Agents

| Agent | Tools | Purpose |

|-------|-------|---------|

| static-analysis:semgrep-scanner | Bash | Executes parallel semgrep scans for a language category |

Use subagent_type: static-analysis:semgrep-scanner in Step 4 when spawning Task subagents.

Rationalizations to Reject

| Shortcut | Why It's Wrong |

|----------|----------------|

| "User asked for scan, that's approval" | Original request ≠ plan approval. Present plan, use AskUserQuestion, await explicit "yes" |

| "Step 3 task is blocking, just mark complete" | Lying about task status defeats enforcement. Only mark complete after real approval |

| "I already know what they want" | Assumptions cause scanning wrong directories/rulesets. Present plan for verification |

| "Just use default rulesets" | User must see and approve exact rulesets before scan |

| "Add extra rulesets without asking" | Modifying approved list without consent breaks trust |

| "Third-party rulesets are optional" | Trail of Bits, 0xdea, Decurity catch vulnerabilities not in official registry — REQUIRED |

| "Use --config auto" | Sends metrics; less control over rulesets |

| "One Task at a time" | Defeats parallelism; spawn all Tasks together |

| "Pro is too slow, skip --pro" | Cross-file analysis catches 250% more true positives; worth the time |

| "Semgrep handles GitHub URLs natively" | URL handling fails on repos with non-standard YAML; always clone first |

| "Cleanup is optional" | Cloned repos pollute the user's workspace and accumulate across runs |

| "Use . or relative path as target" | Subagents need absolute paths to avoid ambiguity |

| "Let the user pick an output dir later" | Output directory must be resolved at Step 1, before any files are created |

Reference Index

| File | Content |

|------|---------|

| rulesets.md | Complete ruleset catalog and selection algorithm |

| scan-modes.md | Pre/post-filter criteria and jq commands |

| scanner-task-prompt.md | Template for spawning scanner subagents |

| Workflow | Purpose |

|----------|---------|

| scan-workflow.md | Complete 5-step scan execution process |

Success Criteria

• [ ] Output directory resolved (user-specified or auto-incremented default)
• [ ] All generated files stored inside $OUTPUT_DIR
• [ ] Languages detected with file counts; Pro status checked
• [ ] Scan mode selected by user (run all / important only)
• [ ] Rulesets include third-party rules for all detected languages
• [ ] User explicitly approved the scan plan (Step 3 gate passed)
• [ ] All scan Tasks spawned in a single message and completed
• [ ] Every semgrep command used --metrics=off
• [ ] Approved rulesets logged to $OUTPUT_DIR/rulesets.txt
• [ ] Raw per-scan outputs stored in $OUTPUT_DIR/raw/
• [ ] results.sarif exists in $OUTPUT_DIR/results/ and is valid JSON
• [ ] Important-only mode: post-filter applied before merge; unfiltered results preserved in raw/
• [ ] Results summary reported with severity and category breakdown
• [ ] Cloned repos (if any) cleaned up from $OUTPUT_DIR/repos/