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Agent skill

Legacy Migration Planner

by tech-leads-club in tech-leads-club/agent-skills

Produces evidence-based migration plans using the Strangler Fig pattern for monolith splits, rewrites and framework upgrades, without implementing them.

CC-BY-4.0Auto-check passedDevelopment

Install Legacy Migration Planner

skills CLI
$ npx skills add tech-leads-club/agent-skills --skill legacy-migration-planner -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install tech-leads-club/agent-skills legacy-migration-planner --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/tech-leads-club/agent-skills.git skills-src && mkdir -p .claude/skills && cp -r skills-src/'packages/skills-catalog/skills/(architecture)/legacy-migration-planner' .claude/skills/legacy-migration-planner && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
legacy-migration-planner
GitHub stars
7k
Token cost
~1.8k tokens
SKILL.md length
636 words
Files
7 (incl. references)
Skills in repo
74
Repo updated
First seen
Licence
CC-BY-4.0

At a glance

Produces evidence-based migration plans using the Strangler Fig pattern for monolith splits, rewrites and framework upgrades, without implementing them.

  • Works in 5 steps: Never assume. If you encounter an… → Always cite evidence. Every claim in… → Always research before recommending.… → …
  • Planning a gradual move off a legacy system without a big-bang rewrite
  • SKILL.md covers Core Principles, Workflow, Output Structure and Reference Guide, plus 1 more section
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

The agent acts as a migration architect that writes plans and leaves execution to other agents or developers. Work always runs in two phases: a research phase that analyzes the codebase, maps bounded contexts and looks up the current and target stacks, then a plan phase that defines the migration direction and designs seams and facades.

Findings must cite a file and line from your code or a verified external URL, and the agent checks technologies with web search and context7 when available instead of relying on memory. It does not guess what an unfamiliar acronym means and asks you instead. Output goes into separate files per domain. Reference files cover the assessment framework, strangler fig patterns, frontend and backend strategies, and testing safety nets.

Any direction is allowed, including monolith to microservices, microservices back to a modular monolith, microfrontends to a single-page app and cross-language moves.

When your agent uses it

  • Planning a gradual move off a legacy system without a big-bang rewrite
  • Deciding how to break a monolith into services, or merge services back
  • Staging a cross-language or cross-framework rewrite
  • Producing a refactoring roadmap backed by evidence from the codebase

Example prompts

  • “Plan a strangler fig migration of our PHP monolith to a set of Node services.”
  • “Write a migration plan for moving our Angular microfrontends into one SPA.”
  • “Assess this codebase and tell me how to consolidate our microservices.”

Requirements

  • Web search access, plus context7 when available

Workflow steps

5 steps, taken from the first numbered list in SKILL.md.

  1. Never assume. If you encounter an acronym, term, pattern, or technology you are not 100% certain about, stop and either research it (web…
  2. Always cite evidence. Every claim in your output must reference either a specific file:line from the user's codebase or a verified…
  3. Always research before recommending. Before suggesting any technology, pattern, or approach, use web search and context7 (when available)…
  4. Minimize token consumption. Write output files per domain. Never dump entire file contents — reference by file:line ranges. Keep each…
  5. Direction-agnostic. This skill handles ANY migration direction: monolith to microservices, microservices to modular monolith…

What it can do on your machine

Read from SKILL.md and the folder at commit 6df68d5. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md.

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    No URLs in SKILL.md.

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Legacy Migration Planner loads about 1.8k tokens when it runs, and up to ~16k if it reads all its reference files. Until then it costs about 117 tokens; SKILL.md has 636 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~117
When it runs · the whole SKILL.md, loaded when a task matches
~1.8k
With references · SKILL.md plus every file in references/, read only if the agent opens them
~16k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from tech-leads-club/agent-skills at commit 6df68d5, republished under its CC-BY-4.0 licence (© tech-leads-club). 636 words, ~1,821 tokens.

Download SKILL.mdSave it as .claude/skills/legacy-migration-planner/SKILL.md (or your agent's skills folder). This skill also uses 6 other files; get the full folder from GitHub.
name
legacy-migration-planner
description
Use when planning legacy system migrations, codebase modernization, monolith decomposition, microservices consolidation, cross-language rewrites, or framework upgrades. Invoke for strangler fig pattern, incremental migration strategy, or refactoring roadmaps. Do NOT use for domain analysis (use domain-analysis), component sizing (use component-identification-sizing), or step-by-step decomposition plans (use decomposition-planning-roadmap).
license
CC-BY-4.0
metadata.author
Felipe Rodrigues - github.com/felipfr
metadata.version
1.0.0

Legacy Migration Planner

Senior migration architect that produces comprehensive, evidence-based migration plans using the Strangler Fig pattern. You create plans — you do not implement them. Other agents or developers execute the plan you produce.

Core Principles

These are non-negotiable. Violating any of these invalidates your output.

  1. Never assume. If you encounter an acronym, term, pattern, or technology you are not 100% certain about, stop and either research it (web search, context7) or ask the user. Say "I don't know what X means — can you clarify?" rather than guessing.
  2. Always cite evidence. Every claim in your output must reference either a specific file:line from the user's codebase or a verified external URL. No unreferenced assertions.
  3. Always research before recommending. Before suggesting any technology, pattern, or approach, use web search and context7 (when available) to verify it is current, maintained, and appropriate. Never recommend based solely on training data.
  4. Minimize token consumption. Write output files per domain. Never dump entire file contents — reference by file:line ranges. Keep each output file focused on one bounded context.
  5. Direction-agnostic. This skill handles ANY migration direction: monolith to microservices, microservices to modular monolith, microfrontends to SPA, cross-language, cross-framework, or any combination.

Workflow

Every engagement follows two mandatory phases. Never skip RESEARCH. Never start PLAN without completing RESEARCH.

RESEARCH (mandatory)                    PLAN (mandatory)
├─ 1. Codebase deep analysis            ├─ 5. Define migration direction
├─ 2. Domain/bounded context mapping    ├─ 6. Design seams and facades
├─ 3. Stack research (web + context7)   ├─ 7. Per-domain migration files
└─ 4. Risk and dependency mapping       └─ 8. Consolidated roadmap
│                                        │
└─ Output: ./migration-plan/research/   └─ Output: ./migration-plan/domains/
RESEARCH Phase

Load references/research-phase.md for detailed instructions.

  1. Analyze the codebase — Read the project structure, entry points, configuration files, and dependencies. Map every module and its responsibility. Cite every finding as file:line.
  2. Identify bounded contexts — Group related modules into candidate domains. Load references/assessment-framework.md for the domain identification method.
  3. Research current and target stacks — Use web search and context7 to gather up-to-date documentation on both the current stack and the target stack (if migrating cross-framework/language). Document version compatibility, migration guides, and known pitfalls.
  4. Map risks and dependencies — Identify integration points, shared databases, circular dependencies, and external service couplings. Load references/assessment-framework.md for the risk matrix method.

Output: Write findings to ./migration-plan/research/ with one file per concern (e.g., dependency-map.md, domain-candidates.md, stack-research.md, risk-assessment.md).

Show full SKILL.md (298 more words)Show less
PLAN Phase

Load references/plan-phase.md for detailed instructions.

  1. Define migration direction — Based on RESEARCH findings, determine the appropriate strategy. Load references/strangler-fig-patterns.md for pattern selection.
  2. Design seams and facades — Identify where to cut the system. Define the facade/router layer that will enable incremental migration. Load references/frontend-backend-strategies.md for stack-specific patterns.
  3. Write per-domain migration plans — One file per bounded context in ./migration-plan/domains/. Each file contains: current state (with file:line refs), target state, migration steps, testing strategy (load references/testing-safety-nets.md), rollback plan, and success metrics.
  4. Write consolidated roadmap — ./migration-plan/00-roadmap.md with phase sequencing, dependencies between domains, risk mitigation timeline, and success criteria.

Output Structure

./migration-plan/
├── 00-roadmap.md                    # Consolidated roadmap, phases, timeline
├── research/
│   ├── dependency-map.md            # Module dependencies with file:line refs
│   ├── domain-candidates.md         # Identified bounded contexts
│   ├── stack-research.md            # Current + target stack analysis
│   └── risk-assessment.md           # Risk matrix with mitigations
└── domains/
    ├── 01-domain-{name}.md          # Per-domain migration plan
    ├── 02-domain-{name}.md
    └── ...

Reference Guide

Load references based on the current phase and need. Do not preload all references.

TopicReferenceLoad When
Research methodologyreferences/research-phase.mdStarting RESEARCH phase
Plan methodologyreferences/plan-phase.mdStarting PLAN phase
Strangler Fig patternsreferences/strangler-fig-patterns.mdChoosing migration pattern, designing seams
Assessment and risksreferences/assessment-framework.mdMapping dependencies, scoring risks, identifying domains
Testing strategiesreferences/testing-safety-nets.mdDesigning safety nets for each domain
Stack-specific patternsreferences/frontend-backend-strategies.mdFrontend or backend migration specifics

Constraints

MUST DO
  • Research every technology recommendation via web search before including it
  • Use context7 for library documentation when available
  • Cite file:line for every codebase observation
  • Ask the user when encountering unknown terms, acronyms, or ambiguous requirements
  • Produce one output file per domain to keep context manageable
  • Include rollback strategy for every migration step
  • Validate that current stack versions match what is actually in the codebase (package.json, requirements.txt, etc.)
MUST NOT DO
  • Guess the meaning of acronyms, internal terms, or business logic
  • Recommend technologies without web search verification
  • Write implementation code (this skill produces plans, not code)
  • Assume migration direction without evidence from RESEARCH
  • Skip the RESEARCH phase or combine it with PLAN
  • Reference files or lines that were not actually read
  • Include unreferenced claims in any output file

© tech-leads-club, CC-BY-4.0. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 6 other files (references) in packages/skills-catalog/skills/(architecture)/legacy-migration-planner of tech-leads-club/agent-skills.

  • SKILL.md
  • references/assessment-framework.md
  • references/frontend-backend-strategies.md
  • references/plan-phase.md
  • references/research-phase.md
  • references/strangler-fig-patterns.md
  • references/testing-safety-nets.md

Open the folder on GitHubat commit 6df68d5

Compare with similar skills

Legacy Migration Planner next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

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Categories

Questions about Legacy Migration Planner

What does Legacy Migration Planner do?

Produces evidence-based migration plans using the Strangler Fig pattern for monolith splits, rewrites and framework upgrades, without implementing them. The agent acts as a migration architect that writes plans and leaves execution to other agents or developers. Work always runs in two phases: a research phase that analyzes the codebase, maps bounded contexts and looks up the current and target stacks, then a plan phase that defines the migration direction and designs seams and facades.

When should I use Legacy Migration Planner?

Legacy Migration Planner fits situations like: planning a gradual move off a legacy system without a big-bang rewrite; deciding how to break a monolith into services, or merge services back; staging a cross-language or cross-framework rewrite; producing a refactoring roadmap backed by evidence from the codebase.

How do I install Legacy Migration Planner in Claude Code?

Run `npx skills add tech-leads-club/agent-skills --skill legacy-migration-planner -a claude-code`. Or copy the skill folder (packages/skills-catalog/skills/(architecture)/legacy-migration-planner in tech-leads-club/agent-skills) into .claude/skills/legacy-migration-planner in your project. Claude Code loads it when a task matches its description.

How do I install Legacy Migration Planner in Codex?

Run `npx skills add tech-leads-club/agent-skills --skill legacy-migration-planner -a codex`. Or copy the skill folder (packages/skills-catalog/skills/(architecture)/legacy-migration-planner in tech-leads-club/agent-skills) into .agents/skills/legacy-migration-planner in your project. Codex loads it when a task matches its description.

Can I use Legacy Migration Planner in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add tech-leads-club/agent-skills --skill legacy-migration-planner -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/legacy-migration-planner, .gemini/skills/legacy-migration-planner, .github/skills/legacy-migration-planner and .opencode/skills/legacy-migration-planner in your project.

What does Legacy Migration Planner need to run?

SKILL.md names no scripts, command-line tools or credentials: Legacy Migration Planner is instructions for the agent only. Our summary lists: Web search access, plus context7 when available.

Does Legacy Migration Planner access the network?

SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.

Is Legacy Migration Planner safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Legacy Migration Planner use?

Legacy Migration Planner is published under the CC-BY-4.0 licence (declared in SKILL.md). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Legacy Migration Planner use?

About 1.8k tokens (SKILL.md is roughly 7.3k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 15k tokens, read only when the agent opens those files.

What are the alternatives to Legacy Migration Planner?

Skills that share tags, products or a category with Legacy Migration Planner: Convert Internal Package to TypeScript (TryGhost/Ghost, 56k stars), jscpd Code Migration Tracker (kucherenko/jscpd, 6.4k stars), Designing Architecture (CloudAI-X/claude-workflow-v2, 1.4k stars) and Rails Upgrade Assistant (ombulabs/claude-code_rails-upgrade-skill, 391 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Legacy Migration Planner?

tech-leads-club (a GitHub organization) maintains it in tech-leads-club/agent-skills, which has 7,045 GitHub stars. The repository holds 74 skills in this directory. The repository was last updated on October 9, 2026.

Source: tech-leads-club/agent-skills on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.