Mattermost

Mattermost provides a full team messaging platform — channels, DMs, threads, posts, reactions, file uploads — accessible via a comprehensive REST API that covers nearly every UI action. For agents, the primary integration patterns are: bot accounts (sending notifications and responding to messages), incoming webhooks (one-way automated posts to channels), outgoing webhooks (trigger agent actions from channel messages), and slash commands. The REST API is well-documented and consistent. Self-hostable for data sovereignty; Mattermost Cloud for managed deployment. Strong in environments where Slack is blocked (air-gapped networks, regulated industries). Confidence is docs-derived.

REST API at {host}/api/v4. Resources: posts, channels, users, teams, files, webhooks. POST /api/v4/posts creates a message. GET /api/v4/channels/{id}/posts fetches channel history (paginated). POST /api/v4/hooks/incoming/{hookId} sends a webhook post. POST /api/v4/users (bot) for bot account management. WebSocket at {host}/api/v4/websocket for real-time event subscription (new messages, user status changes, etc.). Response payloads are clean JSON. Pagination via page and per_page params.

Bearer token auth via Authorization: Bearer {token} header. Token types: personal access tokens (per user, long-lived), session tokens (from login, expire on logout), and bot account tokens. Bot accounts created via API or admin console. Token management: POST /api/v4/users/{userId}/tokens to create PATs. Team-level and channel-level permissions enforced by server roles (system admin, team admin, channel admin, member). HTTPS enforced; self-hosted requires operator TLS configuration.

Standard HTTP status codes with JSON error bodies including id (error code string), message, and detailed_error. 401 for invalid/expired token, 403 for insufficient permissions, 400 for validation errors. Rate limiting configurable by server admin (default: 10 req/sec per user). WebSocket reconnect logic required in client implementations. Mattermost Cloud uptime at status.mattermost.com. Self-hosted reliability depends on deployment (PostgreSQL backend, operator-managed HA). Audit logging available for compliance.

api.mattermost.com provides the full interactive API reference (OpenAPI-based) with all endpoints, request/response schemas, and auth documentation. Getting started is fast: deploy with Docker (docker.mattermost.com) or use Mattermost Cloud. Bot creation guide well-documented. Integration guides for webhooks, slash commands, and interactive message buttons. SDKs not officially maintained; community-maintained clients for Go and Python. Community via Mattermost community server.