Slackpit - Slack message testing for developers

Slackpit is a small, zero-configuration Slack message testing tool & API for developers.

It captures Slack API calls during development and displays them in a web UI with full Block Kit rendering. No messages are sent to real Slack — everything stays local.

Slackpit is inspired by Mailpit which does the same for email.

Features

• Runs entirely from a single static binary
• Modern web UI to view captured Slack messages with full Block Kit rendering (headers, sections, actions, context, dividers, markdown formatting)
• REST API for automated integration testing
• Real-time web UI updates using polling for new messages
• Optional browser notifications for new messages
• Message type badges (DM, Channel, Webhook)
• Dark theme matching Slack's interface
• Configurable port via CLI flag or environment variable

Installation

The Slackpit web UI listens by default on http://0.0.0.0:9025 and the API on the same port.

Slackpit runs as a single binary and can be installed in different ways:

Install via Homebrew (Mac & Linux)

brew install moritzbeck93/tap/slackpit

To run automatically in the background:

brew services start slackpit

Build from source

git clone https://github.com/moritzbeck93/slackpit.git
cd slackpit
bun install
npm run build:release
./dist/slackpit

Usage

slackpit [options]

Options:
  -p, --port <port>  Port for UI and API (default: 9025, env: SLACKPIT_PORT)
  -v, --version      Show version
  -h, --help         Show help

If installed using Homebrew, you may run brew services start slackpit to always run slackpit automatically in the background.

Configuration

Flag / Env Variable	Default	Description
-p, --port / SLACKPIT_PORT	9025	Port for both UI and API

Sending messages to Slackpit

Configure your application to POST Slack messages to Slackpit instead of the real Slack API:

curl -X POST http://localhost:9025/api/v1/messages \
  -H "Content-Type: application/json" \
  -d '{
    "type": "dm",
    "to": "user@example.com",
    "text": "Hello!",
    "blocks": [
      { "type": "section", "text": { "type": "mrkdwn", "text": "*Bold* message" } }
    ]
  }'

API

POST /api/v1/messages

Send a message to Slackpit.

{
  "type": "dm",
  "to": "user@example.com",
  "text": "Hello!",
  "blocks": [...],
  "metadata": {}
}

Field	Required	Description
type	Yes	"dm", "channel", or "webhook"
to	Yes	Recipient (email, channel name, or webhook URL)
text	No	Plain text fallback
blocks	No	Slack Block Kit blocks array
metadata	No	Additional metadata object

GET /api/v1/messages

List all captured messages.

GET /api/v1/messages/:id

Get a single message by ID.

DELETE /api/v1/messages

Clear all messages.

GET /api/v1/health

Health check endpoint.

{ "status": "ok", "version": "0.1.0", "messages": 5 }

Supported Block Kit elements

• Header blocks
• Section blocks (with text, fields, and accessories)
• Divider blocks
• Actions blocks (buttons, selects, overflow menus)
• Context blocks (text and images)
• Image blocks
• Mrkdwn formatting (bold, italic, strikethrough, code, links, blockquotes, lists, emoji)

Programmatic usage

Slackpit can also be used as a library:

import { createServer } from "slackpit";

const { app, storage, port } = createServer({ port: 9025 });
app.listen(port);

Comparison with Mailpit

	Mailpit	Slackpit
Captures	Emails (SMTP)	Slack messages (HTTP)
Protocol	SMTP server	REST API
UI	HTML email viewer	Slack Block Kit renderer
Storage	SQLite	In-memory
Background	brew services start mailpit	brew services start slackpit

License

MIT