coder/docs/ai-coder/agents/extending-agents.md

Extending Agents

Workspace templates can extend the agent with custom skills and MCP tools. These mechanisms let platform teams provide repository-specific instructions, domain expertise, and external tool integrations without modifying the agent itself.

Skills

Skills are structured, reusable instruction sets that the agent loads on demand. They live in the workspace filesystem and are discovered automatically when a chat attaches to a workspace.

How skills work

Place skill directories under .agents/skills/ relative to the workspace working directory. Each directory contains a required SKILL.md file and any supporting files the skill needs.

On the first turn of a workspace-attached chat, the agent scans .agents/skills/ and builds an <available-skills> block in its system prompt listing each skill's name and description. Only frontmatter is read during discovery — the full skill content is loaded lazily when the agent calls a tool.

Two tools are registered when skills are present:

Tool	Parameters	Description
read_skill	name (string)	Returns the SKILL.md body and a list of supporting files
read_skill_file	name (string), path (string)	Returns the content of a supporting file

Directory structure

.agents/skills/
├── deep-review/
│   ├── SKILL.md
│   └── roles/
│       ├── security-reviewer.md
│       └── concurrency-reviewer.md
├── pull-requests/
│   └── SKILL.md
└── refine-plan/
    └── SKILL.md

SKILL.md format

Each SKILL.md starts with YAML frontmatter containing a name and an optional description, followed by the full instructions in markdown:

---
name: deep-review
description: "Multi-reviewer code review with domain-specific reviewers"
---

# Deep Review

Instructions for the skill go here...

Naming and size constraints

• Names must be kebab-case (^[a-z0-9]+(-[a-z0-9]+)*$) and match the directory name exactly.
• SKILL.md has a maximum size of 64 KB.
• Supporting files have a maximum size of 512 KB. Files exceeding the limit are silently truncated.

Path safety

read_skill_file rejects absolute paths, paths containing .., and references to hidden files. All paths are resolved relative to the skill directory.

Workspace MCP tools

Workspace templates can expose custom MCP tools by placing a .mcp.json file in the workspace working directory. The agent discovers these tools automatically when it connects to a workspace and registers them alongside its built-in tools.

Configuration

Define MCP servers in .mcp.json at the workspace root. Each entry under mcpServers describes a server. The transport type is inferred from whether command or url is present, or you can set it explicitly with type:

{
  "mcpServers": {
    "github": {
      "command": "github-mcp-server",
      "args": ["--token", "..."]
    },
    "my-api": {
      "type": "http",
      "url": "http://localhost:8080/mcp",
      "headers": { "Authorization": "Bearer ..." }
    }
  }
}

Stdio transport — set command, and optionally args and env. The agent spawns the process in the workspace.

HTTP transport — set url, and optionally headers. The agent connects to the HTTP endpoint from the workspace.

How discovery works

The agent reads .mcp.json via the workspace agent connection on each chat turn. Discovery uses a 5-second timeout. Servers that fail to respond are skipped — partial success is acceptable. Empty results are not cached because the MCP servers may still be starting.

Tool naming

Tool names are prefixed with the server name as serverName__toolName to avoid collisions between servers and with built-in tools.

Timeouts

• Discovery: 5-second timeout.
• Tool calls: 60 seconds per invocation.