Mattermost

The Mattermost adapter targets the current Mattermost Server REST API v4 and official WebSocket protocol. REST calls, an actively managed reliable WebSocket, externally owned sockets, and low-level ingest(rawEvent) share one MattermostClient; the SDK never opens another listener.

Resource mapping

• Direct channels (D) map to direct.
• Group Message channels (G) map to group.
• Public and private channels (O / P) map to channel.
• Teams map to guild.
• Posts, threads, files, and reactions map to canonical messages, thread segments, file segments, and reaction notices.

Mattermost fixes GDM membership when the channel is created, so the adapter does not emulate nonexistent GDM membership mutation. Team and channel member methods traverse every REST page instead of silently stopping at the 200-item server limit.

Delivery and recovery

websocket connects to the instance's /api/v4/websocket, sends the official authentication_challenge, and retries forever with bounded exponential backoff. Reconnect URLs include connection_id and the last observed sequence_number; gaps raise the typed missed event. manual performs REST identity verification without opening a socket, then accepts an existing socket through acceptSocket() or decoded official envelopes through ingest().

An AbortSignal remains attached after the initial handshake and ends the complete lifecycle. An external socket with owned: false is unbound but not closed. Stringified event entities are parsed and runtime-validated, downstream failures remain retryable, and unknown plugin or future events retain the complete raw_event in a canonical custom notice.

High-value native actions cover post search, ephemeral and scheduled posts, DM/GDM, channel/team/member management, status and typing, channel bookmarks, custom emoji, bot accounts, slash commands, plus a constrained relative-path call_mattermost_api. Administrative actions depend on token permissions; Scheduled Posts depend on server features and licensing. Account capabilities are narrowed by configured event filters, receive mode, and current socket availability.

See Mattermost configuration. Official references: REST API, WebSocket API, and Bot Accounts.