Paths: File paths (shared/, references/, ../ln-*) are relative to skills repo root. If not found at CWD, locate this SKILL.md directory and go up one level for repo root.

Open Source Replacer

L3 Worker that discovers custom modules, analyzes their purpose, and finds battle-tested open-source replacements via MCP Research.

Purpose & Scope

• Discover significant custom modules (>=100 LOC, utility/integration type)
• Analyze PURPOSE of each module by reading code (goal-based, not pattern-based)
• Search open-source alternatives via WebSearch, Context7, Ref
• Evaluate alternatives: stars, maintenance, license, CVE status, API compatibility
• Score replacement confidence (HIGH/MEDIUM/LOW)
• Generate migration plan for viable replacements
• Output: file-based report to docs/project/.audit/

Out of Scope:

• Pattern-based detection of known reinvented wheels (custom sorting, hand-rolled validation)
• Package vulnerability scanning (CVE/CVSS for existing dependencies)
• Story-level optimality checks via OPT- prefix

Input

- codebase_root: string        # Project root
- tech_stack: object            # Language, framework, package manager, existing dependencies
- output_dir: string            # e.g., "docs/project/.audit/ln-640/{YYYY-MM-DD}"

# Domain-aware (optional, from coordinator)
- domain_mode: "global" | "domain-aware"   # Default: "global"
- current_domain: string                   # e.g., "users", "billing" (only if domain-aware)
- scan_path: string                        # e.g., "src/users/" (only if domain-aware)

Workflow

MANDATORY READ: Load shared/references/two_layer_detection.md for detection methodology.

Phase 1: Discovery + Classification

scan_root = scan_path IF domain_mode == "domain-aware" ELSE codebase_root

# Step 1: Find significant custom files
candidates = []
FOR EACH file IN Glob("**/*.{ts,js,py,rb,go,java,cs}", root=scan_root):
  IF file in node_modules/ OR vendor/ OR .venv/ OR dist/ OR build/ OR test/ OR __test__/:
    SKIP
  line_count = wc -l {file}
  IF line_count >= 100:
    candidates.append(file)

# Step 2: Filter to utility/library-like modules
utility_paths = ["utils/", "lib/", "helpers/", "common/", "shared/", "pkg/", "internal/"]
name_patterns = ["parser", "formatter", "validator", "converter", "encoder",
                 "decoder", "serializer", "logger", "cache", "queue", "scheduler",
                 "mailer", "http", "client", "wrapper", "adapter", "connector",
                 "transformer", "mapper", "builder", "factory", "handler"]

modules = []
FOR EACH file IN candidates:
  is_utility_path = any(p in file.lower() for p in utility_paths)
  is_utility_name = any(p in basename(file).lower() for p in name_patterns)
  export_count = count_exports(file)  # Grep for export/module.exports/def/class
  IF is_utility_path OR is_utility_name OR export_count > 5:
    modules.append(file)

# Step 3: Pre-classification gate
FOR EACH module IN modules:
  # Read first 30 lines to classify
  header = Read(module, limit=30)
  classify as:
    - "utility": generic reusable logic (validation, parsing, formatting, HTTP, caching)
    - "integration": wrappers around external services (email, payments, storage)
    - "domain-specific": business logic unique to project (scoring, routing, pricing rules)

  IF classification == "domain-specific":
    no_replacement_found.append({module, reason: "Domain-specific business logic"})
    REMOVE from modules

# Cap: analyze max 15 utility/integration modules per invocation
modules = modules[:15]

Phase 2: Goal Extraction

FOR EACH module IN modules:
  # Read code (first 200 lines + exports summary)
  code = Read(module, limit=200)
  exports = Grep("export|module\.exports|def |class |func ", module)

  # Extract goal: what problem does this module solve?
  goal = {
    domain: "email validation" | "HTTP retry" | "CSV parsing" | ...,
    inputs: [types],
    outputs: [types],
    key_operations: ["validates email format", "checks MX records", ...],
    complexity_indicators: ["regex", "network calls", "state machine", "crypto", ...],
    summary: "Custom email validator with MX record checking and disposable domain filtering"
  }

Phase 3: Alternative Search (MCP Research)

FOR EACH module WHERE module.goal extracted:
  # Strategy 1: WebSearch (primary)
  WebSearch("{goal.domain} {tech_stack.language} library package 2026")
  WebSearch("{goal.summary} open source alternative {tech_stack.language}")

  # Strategy 2: Context7 (for known ecosystems)
  IF tech_stack.package_manager == "npm":
    WebSearch("{goal.domain} npm package weekly downloads")
  IF tech_stack.package_manager == "pip":
    WebSearch("{goal.domain} python library pypi")

  # Strategy 3: Ref (documentation search)
  ref_search_documentation("{goal.domain} {tech_stack.language} recommended library")

  # Strategy 4: Ecosystem alignment — check if existing project dependencies
  # already cover this goal (e.g., project uses Zod → check zod plugins first)
  FOR EACH dep IN tech_stack.existing_dependencies:
    IF dep.ecosystem overlaps goal.domain:
      WebSearch("{dep.name} {goal.domain} plugin extension")

  # Collect candidates (max 5 per module)
  alternatives = top_5_by_relevance(search_results)

Phase 4: Evaluation

MANDATORY: Security Gate and License Classification run for EVERY candidate before confidence assignment.

FOR EACH module, FOR EACH alternative:
  # 4a. Basic info
  info = {
    name: "zod" | "email-validator" | ...,
    version: "latest stable",
    weekly_downloads: N,
    github_stars: N,
    last_commit: "YYYY-MM-DD",
  }

  # 4b. Security Gate (mandatory)
  WebSearch("{alternative.name} CVE vulnerability security advisory")
  IF unpatched HIGH/CRITICAL CVE found:
    security_status = "VULNERABLE"
    → Cap confidence at LOW, add warning to Findings
  ELIF patched CVE (older version):
    security_status = "PATCHED_CVE"
    → Note in report, no confidence cap
  ELSE:
    security_status = "CLEAN"

  # 4c. License Classification
  license = detect_license(alternative)
  IF license IN ["MIT", "Apache-2.0", "BSD-2-Clause", "BSD-3-Clause", "ISC", "Unlicense"]:
    license_class = "PERMISSIVE"
  ELIF license IN ["GPL-2.0", "GPL-3.0", "AGPL-3.0", "LGPL-2.1", "LGPL-3.0"]:
    IF project_license is copyleft AND compatible:
      license_class = "COPYLEFT_COMPATIBLE"
    ELSE:
      license_class = "COPYLEFT_INCOMPATIBLE"
  ELSE:
    license_class = "UNKNOWN"

  # 4d. Ecosystem Alignment
  ecosystem_match = alternative.name IN tech_stack.existing_dependencies
                    OR alternative.ecosystem == tech_stack.framework
  # Prefer: zod plugin over standalone if project uses zod

  # 4e. Feature & API Evaluation
  api_surface_match = HIGH | MEDIUM | LOW
  feature_coverage = percentage  # what % of custom module features covered
  migration_effort = S | M | L   # S=<4h, M=4-16h, L=>16h

  # 4f. Confidence Assignment
  # HIGH: >10k stars, active (commit <6mo), >90% coverage,
  #       PERMISSIVE license, CLEAN security, ecosystem_match preferred
  # MEDIUM: >1k stars, maintained (commit <1yr), >70% coverage,
  #         PERMISSIVE license, no unpatched CRITICAL CVEs
  # LOW: <1k stars OR unmaintained OR <70% coverage
  #      OR COPYLEFT_INCOMPATIBLE OR VULNERABLE

Phase 5: Write Report + Migration Plan

MANDATORY READ: Load shared/references/audit_worker_core_contract.md and shared/templates/audit_worker_report_template.md.

Build report in memory, write to {output_dir}/645-open-source-replacer[-{domain}].md.

# Open Source Replacement Audit Report

<!-- AUDIT-META
worker: ln-645
category: Open Source Replacement
domain: {domain_name|global}
scan_path: {scan_path|.}
score: {X.X}
total_issues: {N}
critical: 0
high: {N}
medium: {N}
low: {N}
status: complete
-->

## Checks

| ID | Check | Status | Details |
|----|-------|--------|---------|
| module_discovery | Module Discovery | passed/warning | Found N modules >= 100 LOC |
| classification | Pre-Classification | passed | N utility, M integration, K domain-specific (excluded) |
| goal_extraction | Goal Extraction | passed/warning | Extracted goals for N/M modules |
| alternative_search | Alternative Search | passed/warning | Found alternatives for N modules |
| security_gate | Security Gate | passed/warning | N candidates checked, M clean, K vulnerable |
| evaluation | Replacement Evaluation | passed/failed | N HIGH confidence, M MEDIUM |
| migration_plan | Migration Plan | passed/skipped | Generated for N replacements |

## Findings

| Severity | Location | Issue | Principle | Recommendation | Effort |
|----------|----------|-------|-----------|----------------|--------|
| HIGH | src/utils/email-validator.ts (245 LOC) | Custom email validation with MX checking | Reuse / OSS Available | Replace with zod + zod-email (28k stars, MIT, 95% coverage) | M |

## Migration Plan

| Priority | Module | Lines | Replacement | Confidence | Effort | Steps |
|----------|--------|-------|-------------|------------|--------|-------|
| 1 | src/utils/email-validator.ts | 245 | zod + zod-email | HIGH | M | 1. Install 2. Create schema 3. Replace calls 4. Remove module 5. Test |

<!-- DATA-EXTENDED
{
  "modules_scanned": 15,
  "modules_with_alternatives": 8,
  "reuse_opportunity_score": 6.5,
  "replacements": [
    {
      "module": "src/utils/email-validator.ts",
      "lines": 245,
      "classification": "utility",
      "goal": "Email validation with MX checking",
      "alternative": "zod + zod-email",
      "confidence": "HIGH",
      "stars": 28000,
      "last_commit": "2026-02-10",
      "license": "MIT",
      "license_class": "PERMISSIVE",
      "security_status": "CLEAN",
      "ecosystem_match": true,
      "feature_coverage": 95,
      "effort": "M",
      "migration_steps": ["Install zod + zod-email", "Create validation schema", "Replace validate() calls", "Remove custom module", "Run tests"]
    }
  ],
  "no_replacement_found": [
    {"module": "src/lib/domain-scorer.ts", "reason": "Domain-specific business logic", "classification": "domain-specific"}
  ]
}
-->

Phase 6: Return Summary

Report written: docs/project/.audit/ln-640/{YYYY-MM-DD}/645-open-source-replacer[-{domain}].md
Score: X.X/10 | Issues: N (C:0 H:N M:N L:N)

Scoring

Uses standard penalty formula from shared/references/audit_scoring.md:

penalty = (critical x 2.0) + (high x 1.0) + (medium x 0.5) + (low x 0.2)
score = max(0, 10 - penalty)

Severity mapping:

• HIGH: HIGH confidence replacement for module >200 LOC
• MEDIUM: MEDIUM confidence, or HIGH confidence for 100-200 LOC
• LOW: LOW confidence (partial coverage only)
• Exception: Custom module with domain-specific logic not covered by OSS package → skip. Feature parity <80% → skip recommendation. Layer 2: Verify replacement has full feature parity before recommending

Critical Rules

MANDATORY READ: Load shared/references/audit_worker_core_contract.md.

• Goal-based, not pattern-based: Read code to understand PURPOSE before searching alternatives
• MCP Research mandatory: Always search via WebSearch/Context7/Ref, never assume packages exist
• Security gate mandatory: WebSearch for CVEs before recommending any package; never recommend packages with unpatched HIGH/CRITICAL CVEs
• License classification mandatory: Permissive (MIT/Apache/BSD) preferred; copyleft only if project-compatible
• Ecosystem alignment: Prefer packages from project's existing dependency tree (e.g., zod plugin over standalone if project uses zod)
• Pre-classification gate: Categorize modules before analysis; exclude domain-specific business logic
• No auto-fix: Report only, never install packages or modify code
• Effort realism: S = <4h, M = 4-16h, L = >16h (migration effort is larger than simple fixes)
• Cap analysis: Max 15 modules per invocation to stay within token budget
• Evidence always: Include file paths + line counts for every finding

Definition of Done

MANDATORY READ: Load shared/references/audit_worker_core_contract.md.

• Custom modules discovered (>= 100 LOC, utility/integration type)
• Pre-classification gate applied: domain-specific modules excluded with documented reason
• Goals extracted for each module (domain, inputs, outputs, operations)
• Open-source alternatives searched via MCP Research (WebSearch, Context7, Ref)
• Security gate passed: all candidates checked for CVEs via WebSearch
• License classified: Permissive/Copyleft/Unknown for each candidate
• Ecosystem alignment checked: existing project dependencies considered
• Confidence scored for each replacement (HIGH/MEDIUM/LOW)
• Migration plan generated for HIGH/MEDIUM confidence replacements
• Report written to {output_dir}/645-open-source-replacer[-{domain}].md
• Summary returned to coordinator

Reference Files

• Scoring algorithm: shared/references/audit_scoring.md
• MANDATORY READ: shared/references/research_tool_fallback.md

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Version: 1.0.0 Last Updated: 2026-02-26