opencode-slack-mcp-proxy

Local TypeScript + Hono facade for Slack MCP that only intercepts the Slack OAuth-specific surfaces it must rewrite, while proxying MCP traffic to Slack and preserving the incoming Slack clientId from opencode requests.

Motivation

Slack MCP is designed to work with Claude Code's OAuth flow. opencode uses a fixed local callback port for OAuth, and Slack MCP does not authenticate correctly against that flow out of the box.

Because of that mismatch, Slack MCP authentication fails in opencode unless the OAuth endpoints and callback flow are adapted. This project acts as a small local proxy that rewrites only the Slack OAuth-specific pieces, keeps the rest of the MCP traffic pointed at Slack, and makes Slack MCP usable from opencode.

Related MCP server: slack-mcp

Installation

See docs/installation.md for setup, background-run instructions, Docker usage, verification, and troubleshooting.

Useful commands:

pnpm lint
pnpm format
pnpm format:check
pnpm typecheck
pnpm check
pnpm test
pnpm test:all

Source lives in src/, and the main route implementation is in src/app.ts.

For production-style runs, the app can also be compiled to build/ and started with pnpm start.

Optional environment variables:

• FACADE_HOST - facade host, default 127.0.0.1

• FACADE_PORT or PORT - facade port, default 3120

• CALLBACK_HOST - callback listener host, default 127.0.0.1

• CALLBACK_PORT - callback port, default 3118

• LOG_LEVEL - pino log level, default info

• OPENCODE_HOST - opencode callback host, default 127.0.0.1

• OPENCODE_PORT - opencode callback port, default 19876

• OPENCODE_CALLBACK_PATH - opencode callback path, default /mcp/oauth/callback

• REQUEST_TIMEOUT_MS - timeout for Slack and local proxy requests, default 10000

All listener hosts are restricted to loopback values (127.0.0.1 or localhost) to avoid exposing the callback bridge on external interfaces.

For containerized runs, listener hosts may also use 0.0.0.0, and OPENCODE_HOST may be set to a reachable host such as host.docker.internal.

opencode config

Use this in place of the remote Slack MCP URL after the local facade is already running:

{
  "slack": {
    "type": "remote",
    "url": "http://127.0.0.1:3120/mcp",
    "oauth": {
      "clientId": "YOUR_SLACK_CLIENT_ID"
    }
  }
}

Flow

1. opencode connects to the local MCP facade at http://127.0.0.1:3120/mcp

2. the facade serves local OAuth discovery metadata

3. opencode uses the facade's local authorize and token endpoints

4. the authorize endpoint redirects to Slack with redirect_uri=http://localhost:3118/callback

5. the callback bridge forwards Slack's callback to http://127.0.0.1:19876/mcp/oauth/callback

6. the token endpoint forwards to Slack and forces the same redirect_uri=http://localhost:3118/callback

Logs

The server logs:

• OAuth discovery responses

• authorize forwarding and rewritten redirect_uri

• token forwarding and rewritten redirect_uri

• Slack token response status and sanitized body

• callback forwarding to opencode

Tests

The unit tests cover the highest-risk rewrite paths:

• authorize redirect rewriting

• token redirect rewriting

• callback proxy failure handling

• WWW-Authenticate resource metadata rewriting

Use pnpm test for unit tests and pnpm test:all to run both static checks and tests together.