Agent skill

Strangler Fig Patterns

Detailed implementation guide for the Strangler Fig pattern and related migration patterns for incrementally replacing legacy systems.

Apache-2.0Auto-check passedDevelopment

Install Strangler Fig Patterns

skills CLI

$ npx skills add revfactory/harness-100 --skill strangler-fig-patterns -a claude-code

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

GitHub CLI

$ gh skill install revfactory/harness-100 strangler-fig-patterns --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/revfactory/harness-100.git skills-src && mkdir -p .claude/skills && cp -r skills-src/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns .claude/skills/strangler-fig-patterns && 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/

Paste into Claude Code

Install the "strangler-fig-patterns" agent skill from https://github.com/revfactory/harness-100/tree/main/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns into .claude/skills/strangler-fig-patterns/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "strangler-fig-patterns", then confirm the skill loads.

Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

$skill-installer

$skill-installer install https://github.com/revfactory/harness-100/tree/main/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns

Type this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.

skills CLI

$ npx skills add revfactory/harness-100 --skill strangler-fig-patterns -a codex

Project install goes to .agents/skills/; add -g for ~/.codex/skills/.

GitHub CLI

$ gh skill install revfactory/harness-100 strangler-fig-patterns --agent codex

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/revfactory/harness-100.git skills-src && mkdir -p .agents/skills && cp -r skills-src/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns .agents/skills/strangler-fig-patterns && rm -rf skills-src

Use ~/.agents/skills/ instead of .agents/skills for a personal install.

Codex skills documentation · loads skills from .agents/skills/

Paste into Codex

Install the "strangler-fig-patterns" agent skill from https://github.com/revfactory/harness-100/tree/main/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns into .agents/skills/strangler-fig-patterns/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "strangler-fig-patterns", then confirm the skill loads.

Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add revfactory/harness-100 --skill strangler-fig-patterns -a cursor

Project install goes to .agents/skills/; add -g for ~/.cursor/skills/.

GitHub CLI

$ gh skill install revfactory/harness-100 strangler-fig-patterns --agent cursor

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/revfactory/harness-100.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns .cursor/skills/strangler-fig-patterns && rm -rf skills-src

Use ~/.cursor/skills/ instead of .cursor/skills for a personal install.

Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/

Paste into Cursor

Install the "strangler-fig-patterns" agent skill from https://github.com/revfactory/harness-100/tree/main/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns into .cursor/skills/strangler-fig-patterns/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "strangler-fig-patterns", then confirm the skill loads.

Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

gemini skills install

$ gemini skills install https://github.com/revfactory/harness-100.git --path en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns

--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.

skills CLI

$ npx skills add revfactory/harness-100 --skill strangler-fig-patterns -a gemini-cli

Project install goes to .agents/skills/; add -g for ~/.gemini/skills/.

GitHub CLI

$ gh skill install revfactory/harness-100 strangler-fig-patterns --agent gemini-cli

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/revfactory/harness-100.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns .gemini/skills/strangler-fig-patterns && rm -rf skills-src

Use ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.

Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/

Paste into Gemini CLI

Install the "strangler-fig-patterns" agent skill from https://github.com/revfactory/harness-100/tree/main/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns into .gemini/skills/strangler-fig-patterns/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "strangler-fig-patterns", then confirm the skill loads.

Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

GitHub CLI

$ gh skill install revfactory/harness-100 strangler-fig-patterns

Installs for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).

skills CLI

$ npx skills add revfactory/harness-100 --skill strangler-fig-patterns -a github-copilot

Project install goes to .agents/skills/; add -g for ~/.copilot/skills/.

Manual copy

$ git clone --depth 1 https://github.com/revfactory/harness-100.git skills-src && mkdir -p .github/skills && cp -r skills-src/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns .github/skills/strangler-fig-patterns && rm -rf skills-src

Use ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.

GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/

Paste into GitHub Copilot

Install the "strangler-fig-patterns" agent skill from https://github.com/revfactory/harness-100/tree/main/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns into .github/skills/strangler-fig-patterns/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "strangler-fig-patterns", then confirm the skill loads.

GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add revfactory/harness-100 --skill strangler-fig-patterns -a opencode

OpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.

GitHub CLI

$ gh skill install revfactory/harness-100 strangler-fig-patterns --agent opencode

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/revfactory/harness-100.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns .opencode/skills/strangler-fig-patterns && rm -rf skills-src

Use ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.

OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/

Paste into OpenCode

Install the "strangler-fig-patterns" agent skill from https://github.com/revfactory/harness-100/tree/main/en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns into .opencode/skills/strangler-fig-patterns/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "strangler-fig-patterns", then confirm the skill loads.

OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

Facts

Skill name: strangler-fig-patterns
GitHub stars: 1.3k
Token cost: ~1.4k tokens
SKILL.md length: 285 words
Files: 1
Skills in repo: 464
Repo updated: Mar 22, 2026
First seen: Oct 7, 2026
Licence: Apache-2.0

At a glance

Detailed implementation guide for the Strangler Fig pattern and related migration patterns for incrementally replacing legacy systems.

Works in 5 steps: Strangler Fig Pattern → Branch by Abstraction → Parallel Run (Scientist Pattern) → …
Incremental migration
SKILL.md covers Pattern Catalog, Pattern Selection Decision Tree and Warning Signs and Responses
Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Strangler Fig Patterns is an agent skill from revfactory/harness-100. Detailed implementation guide for the Strangler Fig pattern and related migration patterns for incrementally replacing legacy systems. Use this skill for 'strangler fig', 'incremental migration', 'refactoring patterns', 'branch by abstraction', 'parallel run', 'gradual replacement', 'migration pattern selection', and other legacy migration pattern applications. Enhances pattern selection and implementation for refactoring-strategist and migration-engineer. Note: full team orchestration and project management are…

Its SKILL.md is about 1.4k tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.

It sits in Development, covering Legacy modernization, Project management and Refactoring. The licence is Apache-2.0.

When your agent uses it

Incremental migration
Refactoring patterns
Branch by abstraction
Gradual replacement

Example prompts

“strangler fig”
“incremental migration”
“refactoring patterns”
“/strangler-fig-patterns”

Requirements

Python 3

Workflow steps

5 steps, taken from the step headings in SKILL.md.

Strangler Fig Pattern
Branch by Abstraction
Parallel Run (Scientist Pattern)
Anti-Corruption Layer (ACL)
Feature Toggle Strategy

What it can do on your machine

Read from SKILL.md and the folder at commit 8e8d35c. 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 (its code samples are python and yaml).
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

Strangler Fig Patterns loads about 1.4k tokens when it runs. Until then it costs about 143 tokens; SKILL.md has 285 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it: ~143
When it runs · the whole SKILL.md, loaded when a task matches: ~1.4k

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 revfactory/harness-100 at commit 8e8d35c, republished under its Apache-2.0 licence (© revfactory). 285 words, ~1,420 tokens.

Download SKILL.mdSave it as .claude/skills/strangler-fig-patterns/SKILL.md (or your agent's skills folder).

name: strangler-fig-patterns
description: Detailed implementation guide for the Strangler Fig pattern and related migration patterns for incrementally replacing legacy systems. Use this skill for 'strangler fig', 'incremental migration', 'refactoring patterns', 'branch by abstraction', 'parallel run', 'gradual replacement', 'migration pattern selection', and other legacy migration pattern applications. Enhances pattern selection and implementation for refactoring-strategist and migration-engineer. Note: full team orchestration and project management are outside the scope of this skill.

Strangler Fig Patterns — Incremental Migration Pattern Guide

Detailed implementation methodologies for proven patterns for replacing legacy systems.

Pattern Catalog

1. Strangler Fig Pattern

Applicable Conditions: When incremental transition from a monolith to a new system is needed

+---------------------------------------------+
|              Router/Proxy                    |
|  +----------+    +----------------------+   |
|  | Legacy   | <-> | New System          |   |
|  | (shrink- |    | (expanding)          |   |
|  |  ing)    |    |                      |   |
|  +----------+    +----------------------+   |
+---------------------------------------------+

Implementation Steps:

Step	Task	Verification Criteria	Rollback Method
1. Insert Interceptor	Add router/proxy layer	Confirm 100% existing traffic routes through legacy	Remove interceptor
2. Extract by Feature	Start with most independent feature into new service	Verify identical input/output (Shadow Test)	Restore routing
3. Traffic Migration	Canary -> gradual percentage increase	Error rate, latency, business metrics	Revert to 0%
4. Legacy Removal	Delete fully migrated code	Confirm 100% migration complete	N/A

Extraction Priority Matrix:

High Business Value
       |
  (3) Quick | (1) Top Priority
    Wins    |  Extraction Targets
  ----------+-----------> Low Coupling
  (4) Defer | (2) Gradual
    Hold    |  Decouple then Extract
       |
Low Business Value

2. Branch by Abstraction

Applicable Conditions: When replacing internal components (within the same codebase)

python

# Step 1: Insert abstraction layer
class PaymentGateway(ABC):
    @abstractmethod
    def process(self, amount: Decimal) -> PaymentResult: ...

# Step 2: Wrap legacy implementation
class LegacyPayment(PaymentGateway):
    def process(self, amount):
        return self.old_system.charge(amount)

# Step 3: New implementation
class ModernPayment(PaymentGateway):
    def process(self, amount):
        return self.stripe_client.create_charge(amount)

# Step 4: Switch via feature flag
gateway = ModernPayment() if feature_flag('new_payment') else LegacyPayment()

3. Parallel Run (Scientist Pattern)

Applicable Conditions: When verifying the accuracy of a new system before replacement

python

class Experiment:
    def run(self, input_data):
        control_result = self.legacy.execute(input_data)
        try:
            candidate_result = self.modern.execute(input_data)
            if not self.compare(control_result, candidate_result):
                self.report_mismatch(input_data, control_result, candidate_result)
        except Exception as e:
            self.report_error(input_data, e)
        return control_result  # Always return legacy result

Comparison Strategies:

Comparison Level	Method	Tolerance
Exact match	`==`	0% difference
Structural match	Schema comparison	Field presence/type match
Semantic match	Business rule-based	Domain-specific tolerance
Statistical match	Aggregate/distribution comparison	>= 99.9% match rate

4. Anti-Corruption Layer (ACL)

Applicable Conditions: When legacy and new systems must coexist, preventing domain contamination

ACL Components:

Component	Role	Implementation
Translator	Domain model conversion	DTO <-> Domain Object mapping
Adapter	Interface conversion	Legacy API -> Modern interface
Facade	Complexity hiding	Multiple legacy calls -> single method

5. Feature Toggle Strategy

yaml

toggles:
  # Phase 1: Dev team only
  new_user_service: { type: permission, users: ["dev-team"] }
  # Phase 2: Canary (10%)
  new_user_service: { type: gradual-rollout, percentage: 10, sticky: true }
  # Phase 3: Full rollout
  new_user_service: { type: release, enabled: true }
  # Phase 4: Remove toggle (delete legacy code)

Pattern Selection Decision Tree

Is legacy replacement needed?
+-- External system/service replacement -> Strangler Fig
+-- Internal component replacement
|   +-- Pre-replacement verification needed -> Parallel Run + Branch by Abstraction
|   +-- Confident in replacement -> Branch by Abstraction
+-- Long-term legacy-new coexistence -> Anti-Corruption Layer
+-- All cases -> Feature Toggle recommended in combination

Warning Signs and Responses

Warning Sign	Meaning	Response
Module to extract has 10+ dependencies	Excessive coupling	Insert ACL then incrementally resolve dependencies
Parallel Run mismatch rate > 5%	Excessive behavioral differences	Re-verify business rules, collect edge cases
> 20 toggles active simultaneously	Toggle debt	Introduce regular toggle cleanup sprints
Migration ongoing for 6+ months	Increasing parallel operation costs	Reduce scope or consider partial Big Bang

Files

Just SKILL.md in en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns of revfactory/harness-100.

Open the folder on GitHubat commit 8e8d35c

Compare with similar skills

Strangler Fig Patterns 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.

Strangler Fig Patterns compared with similar skills
Skill	Stars	Used in	Tokens	Auto-check	Licence	Repo updated
Strangler Fig Patterns this skillrevfactory/harness-100	1.3k	—	~1.4k	Automated safety check: Pass	Apache-2.0	6 mo ago
Legacy Code Summarizermajiayu000/claude-skill-registry	666	1 repos	~5.2k	Automated safety check: Pass	MIT	today
Code Refactoring Workflowluongnv89/claude-howto	42k	—	~3.1k	Automated safety check: Pass	MIT	7 days ago
Modern JavaScript Patternswshobson/agents	40k	11 repos	~548	Automated safety check: Pass	MIT	2 days ago
Fowler-Style Refactoringlhfer/claude-howto-zh-cn	2.3k	—	~156	Automated safety check: Pass	MIT	2 mo ago
Extract Command from Git::Libruby-git/ruby-git	1.8k	—	~4.4k	Automated safety check: Pass	MIT	5 days ago

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Questions about Strangler Fig Patterns

What does Strangler Fig Patterns do?

Detailed implementation guide for the Strangler Fig pattern and related migration patterns for incrementally replacing legacy systems. Strangler Fig Patterns is an agent skill from revfactory/harness-100. Detailed implementation guide for the Strangler Fig pattern and related migration patterns for incrementally replacing legacy systems.

When should I use Strangler Fig Patterns?

Strangler Fig Patterns fits situations like: incremental migration; refactoring patterns; branch by abstraction; gradual replacement.

How do I install Strangler Fig Patterns in Claude Code?

Run `npx skills add revfactory/harness-100 --skill strangler-fig-patterns -a claude-code`. Or copy the skill folder (en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns in revfactory/harness-100) into .claude/skills/strangler-fig-patterns in your project. Claude Code loads it when a task matches its description.

How do I install Strangler Fig Patterns in Codex?

Run `npx skills add revfactory/harness-100 --skill strangler-fig-patterns -a codex`. Or copy the skill folder (en/22-legacy-modernizer/.claude/skills/strangler-fig-patterns in revfactory/harness-100) into .agents/skills/strangler-fig-patterns in your project. Codex loads it when a task matches its description.

Can I use Strangler Fig Patterns 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 revfactory/harness-100 --skill strangler-fig-patterns -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/strangler-fig-patterns, .gemini/skills/strangler-fig-patterns, .github/skills/strangler-fig-patterns and .opencode/skills/strangler-fig-patterns in your project.

What does Strangler Fig Patterns need to run?

SKILL.md names no scripts, command-line tools or credentials: Strangler Fig Patterns is instructions for the agent only. Our summary lists: Python 3.

Does Strangler Fig Patterns 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 Strangler Fig Patterns 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 Strangler Fig Patterns use?

Strangler Fig Patterns is published under the Apache-2.0 licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Strangler Fig Patterns use?

About 1.4k tokens (SKILL.md is roughly 5.7k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to Strangler Fig Patterns?

Skills that share tags, products or a category with Strangler Fig Patterns: Legacy Code Summarizer (majiayu000/claude-skill-registry, 666 stars), Code Refactoring Workflow (luongnv89/claude-howto, 42k stars), Modern JavaScript Patterns (wshobson/agents, 40k stars) and Fowler-Style Refactoring (lhfer/claude-howto-zh-cn, 2.3k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Strangler Fig Patterns?

revfactory (a GitHub user) maintains it in revfactory/harness-100, which has 1,290 GitHub stars. The repository holds 464 skills in this directory. The repository was last updated on March 22, 2026.

Source: revfactory/harness-100 on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.

Install Strangler Fig Patterns

Facts

At a glance

What it does

When your agent uses it

Example prompts

Requirements

Workflow steps

What it can do on your machine

Tool permissions

Runs code

Network

Credentials

Context cost

Safety

SKILL.md

Strangler Fig Patterns — Incremental Migration Pattern Guide

Pattern Catalog

1. Strangler Fig Pattern

2. Branch by Abstraction

3. Parallel Run (Scientist Pattern)

4. Anti-Corruption Layer (ACL)

5. Feature Toggle Strategy

Pattern Selection Decision Tree

Warning Signs and Responses

Files

Compare with similar skills

Similar skills

More from revfactory/harness-100

Related

Questions about Strangler Fig Patterns