markus41/plugins/claude-code-expert/skills/channels

Channels Reference

Build MCP servers that push webhooks, alerts, and chat messages into a Claude Code session. Requires Claude Code v2.1.80+ (permission relay requires v2.1.81+). Research preview — requires claude.ai login; Console/API key auth not supported.

What Is a Channel

A channel is an MCP server that pushes events into a Claude Code session so Claude can react to things happening outside the terminal. Claude Code spawns it as a subprocess and communicates over stdio.

One-way channels: Forward alerts, webhooks, monitoring events for Claude to act on. Two-way channels: Also expose a reply tool so Claude can send messages back. Permission relay: Trusted two-way channels can forward tool approval prompts to remote devices.

Architecture

External System → Your Channel Server (local) ←stdio→ Claude Code Session

• Chat platforms (Telegram, Discord): Plugin polls platform API, forwards messages to Claude
• Webhooks (CI, monitoring): Server listens on local HTTP port, pushes POSTs to Claude

Built-in Channels (Research Preview)

Telegram, Discord, iMessage, and fakechat are included. Custom channels require --dangerously-load-development-channels flag.

Building a Channel

Requirements

• @modelcontextprotocol/sdk package
• Node.js-compatible runtime (Bun, Node, Deno)
• stdio transport (Claude Code spawns as subprocess)

Minimal One-Way Channel (Webhook Receiver)

#!/usr/bin/env bun
import { Server } from '@modelcontextprotocol/sdk/server/index.js'
import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js'

const mcp = new Server(
  { name: 'webhook', version: '0.0.1' },
  {
    // This key makes it a channel — Claude Code registers a listener
    capabilities: { experimental: { 'claude/channel': {} } },
    // Added to Claude's system prompt
    instructions: 'Events from the webhook channel arrive as <channel source="webhook" ...>. Read them and act, no reply expected.',
  },
)

await mcp.connect(new StdioServerTransport())

// HTTP server forwards every POST to Claude
Bun.serve({
  port: 8788,
  hostname: '127.0.0.1',
  async fetch(req) {
    const body = await req.text()
    await mcp.notification({
      method: 'notifications/claude/channel',
      params: {
        content: body,
        meta: { path: new URL(req.url).pathname, method: req.method },
      },
    })
    return new Response('ok')
  },
})

Registration (.mcp.json)

{
  "mcpServers": {
    "webhook": { "command": "bun", "args": ["./webhook.ts"] }
  }
}

Testing

# Start with development flag
claude --dangerously-load-development-channels server:webhook

# In another terminal, send a test event
curl -X POST localhost:8788 -d "build failed on main: https://ci.example.com/run/1234"

Events arrive as <channel> tags:

<channel source="webhook" path="/" method="POST">build failed on main: https://ci.example.com/run/1234</channel>

Server Options

| Field | Type | Description | |-------|------|-------------| | capabilities.experimental['claude/channel'] | object | Required. Always {}. Registers the notification listener. | | capabilities.experimental['claude/channel/permission'] | object | Optional. Enables permission relay for remote tool approval. | | capabilities.tools | object | Two-way only. Always {}. Enables MCP tool discovery. | | instructions | string | Recommended. Added to Claude's system prompt. Describe events, reply behavior, and routing. |

Notification Format

Push events via mcp.notification() with method notifications/claude/channel:

| Field | Type | Description | |-------|------|-------------| | content | string | Event body — becomes body of <channel> tag | | meta | Record<string, string> | Optional. Each entry becomes a tag attribute (e.g., chat_id, severity). Keys: letters, digits, underscores only. |

await mcp.notification({
  method: 'notifications/claude/channel',
  params: {
    content: 'build failed on main',
    meta: { severity: 'high', run_id: '1234' },
  },
})
// Arrives as: <channel source="your-channel" severity="high" run_id="1234">build failed on main</channel>

Two-Way Channels: Reply Tool

Add a reply tool so Claude can send messages back:

1. Add tools: {} to capabilities
2. Register ListToolsRequestSchema and CallToolRequestSchema handlers
3. Update instructions to tell Claude when/how to reply

import { ListToolsRequestSchema, CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js'

mcp.setRequestHandler(ListToolsRequestSchema, async () => ({
  tools: [{
    name: 'reply',
    description: 'Send a message back over this channel',
    inputSchema: {
      type: 'object',
      properties: {
        chat_id: { type: 'string', description: 'The conversation to reply in' },
        text: { type: 'string', description: 'The message to send' },
      },
      required: ['chat_id', 'text'],
    },
  }],
}))

mcp.setRequestHandler(CallToolRequestSchema, async req => {
  if (req.params.name === 'reply') {
    const { chat_id, text } = req.params.arguments as { chat_id: string; text: string }
    send(`Reply to ${chat_id}: ${text}`)
    return { content: [{ type: 'text', text: 'sent' }] }
  }
  throw new Error(`unknown tool: ${req.params.name}`)
})

Sender Gating (Security)

An ungated channel is a prompt injection vector. Always check sender identity before emitting notifications:

const allowed = new Set(loadAllowlist())

// Gate on sender identity, NOT room/chat identity
if (!allowed.has(message.from.id)) {
  return  // drop silently
}
await mcp.notification({ ... })

Gate on message.from.id, not message.chat.id — in group chats these differ, and gating on room would let anyone in an allowlisted group inject messages.

Pairing Flows

• Telegram/Discord: User DMs bot → bot sends pairing code → user approves in Claude Code → platform ID added to allowlist
• iMessage: Detects user's own addresses from Messages DB at startup, auto-allows

Permission Relay

Requires Claude Code v2.1.81+. Lets remote users approve/deny tool use from another device.

How It Works

1. Claude Code generates 5-letter request ID, notifies your server
2. Server forwards prompt + ID to chat app
3. User replies yes <id> or no <id>
4. Server parses reply into verdict notification back to Claude Code

Local terminal dialog stays open — first answer (local or remote) wins.

Request ID Format

Five lowercase letters from a-z excluding l (avoids confusion with 1/I).

Permission Request Fields

| Field | Description | |-------|-------------| | request_id | Five-letter ID to echo in verdict | | tool_name | Tool name (e.g., Bash, Write) | | description | Human-readable summary of tool call | | input_preview | Tool args as JSON, truncated to 200 chars |

Implementation

import { z } from 'zod'

// 1. Declare capability
capabilities: {
  experimental: {
    'claude/channel': {},
    'claude/channel/permission': {},  // opt in
  },
  tools: {},
},

// 2. Handle incoming permission requests
const PermissionRequestSchema = z.object({
  method: z.literal('notifications/claude/channel/permission_request'),
  params: z.object({
    request_id: z.string(),
    tool_name: z.string(),
    description: z.string(),
    input_preview: z.string(),
  }),
})

mcp.setNotificationHandler(PermissionRequestSchema, async ({ params }) => {
  send(
    `Claude wants to run ${params.tool_name}: ${params.description}\n\n` +
    `Reply "yes ${params.request_id}" or "no ${params.request_id}"`,
  )
})

// 3. Parse verdict from inbound messages
const PERMISSION_REPLY_RE = /^\s*(y|yes|n|no)\s+([a-km-z]{5})\s*$/i

const m = PERMISSION_REPLY_RE.exec(body)
if (m) {
  await mcp.notification({
    method: 'notifications/claude/channel/permission',
    params: {
      request_id: m[2].toLowerCase(),
      behavior: m[1].toLowerCase().startsWith('y') ? 'allow' : 'deny',
    },
  })
  return  // don't also forward as chat
}

Publishing as a Plugin

Wrap your channel in a plugin and publish to a marketplace:

• Users install with /plugin install
• Enable per session with --channels plugin:<name>@<marketplace>
• Custom marketplace channels still need --dangerously-load-development-channels
• Submit to official marketplace for security review and allowlisting
• Team/Enterprise admins can add to allowedChannelPlugins list instead

Troubleshooting

| Symptom | Diagnosis | |---------|-----------| | curl succeeds but event doesn't reach Claude | Run /mcp to check server status. Check ~/.claude/debug/<session-id>.txt for stderr | | curl fails with "connection refused" | Port not bound or stale process. lsof -i :<port> to check, kill stale process | | "blocked by org policy" | Team/Enterprise admin must enable channels first | | Permission relay verdict ignored | ID doesn't match open request — check format (5 lowercase letters, no l) |

Key Points

• Channels are MCP servers with claude/channel experimental capability
• Events arrive as <channel source="name" ...>content</channel> tags
• Two-way channels add tools: {} capability and reply tool handlers
• Always gate inbound messages on sender identity to prevent prompt injection
• Permission relay requires claude/channel/permission capability and v2.1.81+
• Local terminal dialog and remote relay both stay live — first answer wins
• Package as plugin for distribution via marketplace

See Also

• Channels user guide — install and use built-in channels
• Official channel implementations — Telegram, Discord, iMessage, fakechat
• MCP documentation — underlying protocol
• Plugins — package channels for distribution