agext-cli

CLI tool to extend VS Code Copilot agent plugins with repo-specific overrides.

Why?

VS Code installs agent plugins in a user-level directory, and adding any customization means modifying the external source directory and these changes are overwritten by any future updates.

agext-cli lets you layer repo-specific overrides on top of any installed agent plugin — without forking or modifying the originals — so every agent behaves exactly the way your project needs.

Features

• Scaffold — Interactively generate a .extend.yaml override file from any installed agent plugin
• Apply — Merge all .extend.yaml overrides with base agents and write repo-local .md agent files
• Subagent Remap — Automatically update subagent and handoff references when extended agents are renamed, creating or editing .extend.yaml files as needed
• Additive merge — Tools and agents lists are unioned (no duplicates); descriptions and models are replaced
• Prepend / append instructions — Inject repo-specific markdown before or after the agent body
• AI editor pass — Optionally rewrite the merged output with a Copilot SDK model
• Cycle detection — Warns about circular subagent references during remap

Install

Requires Node.js >= 18.

You can install the global cli agext

npm install -g agext-cli
agext -h

or run directly with npx.

npx agext-cli

Usage

agext scaffold

Interactively create a .extend.yaml file from an installed agent plugin:

agext scaffold

1. Lists installed plugins from ~/.vscode/agent-plugins/
2. Select a plugin, then an agent
3. Generates .agext/extend/{plugin-name}/agents/{agent-name}.extend.yaml

agext apply

Apply all extensions to generate repo-local agent files:

agext apply

Scans .agext/extend/ for .extend.yaml files, merges each with its base agent, and writes the result to .github/agents/{agent-name}.md.

agext remap

Update subagent references across .extend.yaml files after agents are renamed:

agext remap

1. Reads existing .extend.yaml files to determine agent renames (via name-suffix)
2. Builds a dependency map of subagent and handoff references for each plugin
3. Detects and reports circular references
4. Creates or updates .extend.yaml files to remap subagent/handoff names so they point to the renamed agents

Run agext apply afterwards to regenerate the agent files.

Extension YAML format

# Suffix appended to the agent name (default: " - Repo Extended")
name-suffix: " - Repo Extended"

# Replaces the base agent description
description: "Custom description"

# Additional tools (merged with base, no duplicates)
tools:
  - browser

# Additional agents (merged with base, no duplicates)
agents:
  - MyCustomAgent

# Replaces the base agent model
model:
  - Claude Sonnet
  - claude-sonnet

# Markdown inserted before the agent body
prepend-instructions: |
  Setup instructions here.

# Markdown inserted after the agent body
append-instructions: |
  Your repo-specific instructions here.

# Optional: AI edit pass after merge (uses GitHub Copilot SDK)
# ai-editor-instructions: |
#   Rewrite the identity section to be more concise.
# ai-editor-model: gpt-5

Merge rules

Field	Behavior
tools, agents	Additive (union, no duplicates)
description, model	Replacement
prepend-instructions	Inserted before agent body
append-instructions	Inserted after agent body
ai-editor-instructions	AI edit pass on final content (Copilot SDK)
ai-editor-model	Model for AI edit (default: gpt-5)
name-suffix	Appended to agent name

Development

npm run dev -- scaffold   # run without compiling
npm test                  # run tests
npm run build             # compile TypeScript to dist/