jxscout-custom-analyzers

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Create custom jxscout analyzers (regex, derived, or script-based) and retrigger analysis. Use when the user wants to find specific code patterns across all project files, add new match kinds, or extend jxscout's static analysis capabilities.

dreadnode By dreadnode schedule Updated 5/4/2026
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name: jxscout-custom-analyzers description: Create custom jxscout analyzers (regex, derived, or script-based) and retrigger analysis. Use when the user wants to find specific code patterns across all project files, add new match kinds, or extend jxscout's static analysis capabilities. license: proprietary metadata: source: jxscout-pro-v2 author: francisconeves97 origin: ported from jxscout-pro-v2 agent skills

jxscout Custom Analyzers

jxscout supports custom analyzers that you can add to a project's configuration. These run alongside the built-in analyzers and generate new match kinds that appear in the VS Code extension and are queryable via get-matches.

There are three types: regex, derived, and script. All are configured in the project's settings.jsonc under analyzer > custom_analyzers.

Prerequisites

The JXSCOUT_PROJECT_NAME environment variable must be set. The project settings file is at <working_directory>/settings.jsonc. All commands use jxscout-pro-v2 -c (client mode).

Choosing the right analyzer type

For JS/TS code pattern matching (function calls, assignments, data flow, control structures), prefer AST-based analysis via script analyzers (e.g. semgrep) over regex. Regex works for simple literal patterns -- hardcoded URLs, API keys, specific strings -- but is fragile for code constructs because it misses variations in whitespace, formatting, nesting, and comments.

Rule of thumb:

  • Regex: simple string patterns that don't depend on code structure
  • Derived: narrowing down an existing match kind by value
  • Script (semgrep): anything involving code semantics -- function calls, assignments, data flow, missing checks

Analyzer types

Regex analyzer

Pattern-matches source code with one or more regexes. Good for simple literal patterns (URLs, API keys, specific strings) but fragile for code constructs.

{
  "analyzer": {
    "custom_analyzers": {
      "sensitive_fetch": {
        "enabled": true,
        "type": "regex",
        "file_types": ["js", "reversed_source"],
        "match_kind": "sensitive_fetch",
        "regex_list": ["fetch\\s*\\([^)]*\\/api\\/admin", "fetch\\s*\\([^)]*\\/internal"]
      }
    }
  }
}
  • file_types: which files to scan -- js, html, reversed_source
  • match_kind: the kind string for generated matches
  • regex_list: array of regex patterns

Derived analyzer

Filters an existing match kind to create a new, more specific one. Useful for flagging specific values within a broader match kind.

{
  "analyzer": {
    "custom_analyzers": {
      "admin_paths": {
        "enabled": true,
        "type": "derived",
        "file_types": ["js", "html", "reversed_source"],
        "original_match_kind": "path",
        "new_match_kind": "admin_path",
        "regex_list": [".*admin.*", ".*internal.*", ".*debug.*"]
      }
    }
  }
}
  • original_match_kind: the source match kind to derive from
  • new_match_kind: the new match kind to create
  • regex_list: if any regex matches the original value, a derived match is created
  • preprocess_script: optional script that receives the match value on stdin and outputs the transformed value (or nothing to skip)

Script analyzer

Runs an arbitrary script that receives file paths on stdin and outputs matches as JSON. Most flexible -- use for complex analysis like AST-based pattern matching.

{
  "analyzer": {
    "custom_analyzers": {
      "semgrep_postmessage": {
        "enabled": true,
        "type": "script",
        "file_types": ["js", "reversed_source"],
        "script": "$JXSCOUT_PROJECT_DIR/scripts/semgrep_to_jxscout.sh --rule $JXSCOUT_PROJECT_DIR/semgrep/postmessage.yaml --kind postmessage_handler"
      }
    }
  }
}

The script receives file paths on stdin (one per line) and must output a JSON array:

[
  {
    "kind": "postmessage_handler",
    "value": "window.addEventListener('message', function(e) { ... })",
    "start": { "line": 10, "column": 0 },
    "end": { "line": 15, "column": 1 }
  }
]

$JXSCOUT_PROJECT_DIR and $JXSCOUT_PROJECT_NAME are available as environment variables in all scripts.

Using semgrep for JS analysis

When semgrep is available and you need to find JS/TS code patterns, prefer it over regex. Semgrep uses AST-based matching which is far more accurate for code patterns (function calls, assignments, data flow) than regular expressions.

1. Create a semgrep rule

Create a YAML rule file in your project (e.g. semgrep/postmessage_no_origin.yaml):

rules:
  - id: postmessage-no-origin-check
    message: postMessage handler without origin check
    pattern: |
      window.addEventListener("message", function($EVENT) {
        ...
      })
    languages: [javascript, typescript]
    severity: WARNING

2. Create the wrapper script

Create scripts/semgrep_to_jxscout.sh in your project directory. This script converts semgrep JSON output to the jxscout match format:

#!/usr/bin/env bash
set -euo pipefail

RULE_PATH=""
KIND=""

while [[ $# -gt 0 ]]; do
  case $1 in
    -r|--rule) RULE_PATH="$2"; shift 2 ;;
    -k|--kind) KIND="$2"; shift 2 ;;
    *) echo "Usage: $0 -r|--rule <rule_path> -k|--kind <kind> (file paths on stdin)" >&2; exit 1 ;;
  esac
done

[[ -n "$RULE_PATH" ]] || { echo "Missing -r|--rule <path>" >&2; exit 1; }
[[ -n "$KIND" ]] || { echo "Missing -k|--kind <kind>" >&2; exit 1; }

FILES=()
while IFS= read -r line || [[ -n "$line" ]]; do
  [[ -n "$line" ]] && FILES+=("$line")
done

[[ ${#FILES[@]} -gt 0 ]] || { echo "No file paths on stdin" >&2; exit 1; }

SEMGREP_JSON=$(semgrep scan --config "$RULE_PATH" "${FILES[@]}" --json --quiet)

output=""
while IFS= read -r result; do
  path=$(echo "$result" | jq -r '.path')
  start_offset=$(echo "$result" | jq -r '.start.offset')
  end_offset=$(echo "$result" | jq -r '.end.offset')
  start_line=$(echo "$result" | jq -r '.start.line')
  start_col=$(echo "$result" | jq -r '.start.col')
  end_line=$(echo "$result" | jq -r '.end.line')
  end_col=$(echo "$result" | jq -r '.end.col')

  value=$(tail -c +$((start_offset + 1)) "$path" | head -c $((end_offset - start_offset)))

  one=$(jq -cn \
    --arg kind "$KIND" \
    --arg value "$value" \
    --argjson start_line "$start_line" \
    --argjson start_col "$start_col" \
    --argjson end_line "$end_line" \
    --argjson end_col "$end_col" \
    '{kind: $kind, value: $value, start: {line: $start_line, column: $start_col}, end: {line: $end_line, column: $end_col}}')
  output="${output}${one}"$'\n'
done < <(echo "$SEMGREP_JSON" | jq -c '.results[]? // empty')

if [[ -z "$output" ]]; then
  echo "[]"
else
  echo "$output" | jq -cs 'if . == null then [] else . end'
fi

Make it executable: chmod +x scripts/semgrep_to_jxscout.sh

3. Add to project settings

{
  "analyzer": {
    "custom_analyzers": {
      "postmessage_no_origin": {
        "enabled": true,
        "type": "script",
        "file_types": ["js", "reversed_source"],
        "script": "$JXSCOUT_PROJECT_DIR/scripts/semgrep_to_jxscout.sh --rule $JXSCOUT_PROJECT_DIR/semgrep/postmessage_no_origin.yaml --kind postmessage_no_origin"
      }
    }
  }
}

Testing analyzers

Always test every new or modified analyzer before retriggering project-wide. Use the analyze command on a single file:

jxscout-pro-v2 -c analyze --file-type <js|html|reversed_source> <file_path>

This outputs the matches that would be generated but does not store them in the database.

If no file in the project contains the pattern you're targeting, create a small temporary test file with known patterns that should match, run analyze against it, verify the output is correct, and delete the temp file. This ensures the analyzer actually works before committing to a potentially long retrigger across the full project.

Retriggering analysis

After adding or modifying analyzers, retrigger analysis to regenerate matches across the project:

jxscout-pro-v2 -c retrigger-events --subscriber analyzer [options]

Options:

  • --glob <pattern> -- only retrigger for files matching a glob (e.g. *.js, /path/to/*.html)
  • --event-name <name> -- filter by event name (repeatable)
  • --status <status> -- filter by status: done, failed, killed (repeatable)
  • --json -- output result as JSON

Always retrigger after adding or modifying an analyzer -- without this, the new matches won't exist in the database. Only skip retriggering if the user explicitly says they don't want to. On large projects you can mention it may take a while, but proceed unless told otherwise. Use --glob to scope the retrigger to specific files or directories if the user wants a faster, targeted run.

To see what subscribers and events are available:

jxscout-pro-v2 -c list-subscribers --json

Updating the VS Code matches view

You must always add new match kinds to the VS Code matches view. Without this step, the analyzer's results won't be visible in the sidebar and the analyzer is effectively useless to the user.

CRITICAL: Before adding a match kind, you MUST check whether vscode_extension already exists in settings.jsonc. If it does NOT exist, you MUST first copy the entire vscode_extension block from the full project settings into settings.jsonc. If you skip this and only add the new match kind entry, all built-in match kinds will disappear from the VS Code sidebar because the override replaces the defaults entirely.

Step 1: Ensure vscode_extension is in settings.jsonc

Check settings.jsonc for a vscode_extension key. If it is not present:

  1. Run jxscout-pro-v2 -c print-full-project-settings
  2. Copy the entire vscode_extension block from the output into settings.jsonc
  3. Only then proceed to add your new match kind

This preserves all existing built-in match kinds (paths, URLs, secrets, sinks, etc.) in the sidebar. Never skip this step -- adding only the new entry without the existing structure will remove every other match kind from the view.

If vscode_extension > matches_view > structure already exists in settings.jsonc, skip to step 2.

Step 2: Add your match kind to the view

Append an entry like this to the vscode_extension > matches_view > structure array:

{
  "type": "navigation",
  "label": "Sensitive Fetches",
  "icon": "vscode:shield",
  "children": [
    {
      "type": "match",
      "match_kind": "sensitive_fetch",
      "icon": "vscode:shield",
      "dont_collapse_individual_matches": true
    }
  ]
}
  • type: "navigation" for tree nodes, "match" for leaves that show actual matches
  • match_kind: must match the kind string from your analyzer config
  • icon: use jxscout:<name> for built-in icons, vscode:<name> for VS Code icons, or file:/path/to/icon.svg for custom SVGs
  • dont_collapse_individual_matches: controls whether matches with the same value are collapsed into one tree node or shown individually (see below)

Collapsing vs. showing individual matches

By default, matches with the same value are collapsed into a single tree node (e.g. if /api/users appears in 5 files, you see one /api/users node). This is useful for value-oriented match kinds where the value itself is what matters -- paths, URLs, hostnames, query params, secrets.

For source/sink analyzers, collapsing is wrong. Two matches like document.innerHTML = e can have the same textual value but represent completely different code locations with different variables, contexts, and security implications. Set "dont_collapse_individual_matches": true so every match gets its own node.

Rule of thumb:

  • Collapse (default, or false): the match value alone tells you everything -- paths, URLs, hostnames, secrets, query params
  • Don't collapse (true): the match value is a code snippet where the surrounding context matters -- sinks (innerHTML, eval), event handlers (onmessage), assignments, function calls

jxscout auto-reloads when settings.jsonc changes, so the view updates immediately after saving.

Workflow for adding a custom analyzer

  1. Decide the analyzer type: regex for simple literal patterns, derived for filtering existing match kinds, script for code-semantic analysis (prefer semgrep for JS/TS when available)
  2. Add the configuration to settings.jsonc under analyzer > custom_analyzers
  3. Test with analyze on a file that should match (create a temp test file if needed, then delete it)
  4. Always add the new match kind to the VS Code matches view -- first ensure the full vscode_extension block is in settings.jsonc (get it from print-full-project-settings if missing), then append your entry
  5. Always retrigger analysis with retrigger-events --subscriber analyzer (skip only if the user explicitly says not to)
  6. Query results with get-matches --match-kind <your_kind> --json
Install via CLI
npx skills add https://github.com/dreadnode/capabilities --skill jxscout-custom-analyzers
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