Rich message composer

The rich message composer turns one announcement into a reviewable workflow. Use messages_compose to create a local preview, messages_publish to send the approved draft, and messages_update to edit one published message.

The preview is read-only. It shows the final layout, the message parts that will be sent, and file metadata including filename, size, and hash. File bytes are not included in the preview. Files must be supplied as bounded data_uri values; the composer does not fetch URLs or read paths from the host filesystem. This is a structured payload preview, not a rendered Discord visual preview. Each draft accepts at most ten files with a combined decoded size of 2 MiB. New files use simple filenames containing letters, numbers, dots, underscores, or hyphens. Adding files during an update must keep the total attachment count at ten or fewer.

Draft a tournament announcement

Ask the client to call messages_compose with a structured message. This read-only preview has no Discord target; the channel is supplied later to messages_publish. A small draft can look like this:

{
  "content": "## Weekend tournament\nRegistration closes Friday at 18:00 KST.",
  "embeds": [
    {
      "title": "Saturday bracket",
      "description": "Check in at 19:30 KST. Matches begin at 20:00.",
      "color": 5793266
    }
  ],
  "poll": {
    "question": { "text": "Which format should we use?" },
    "answers": [
      { "poll_media": { "text": "Single elimination" } },
      { "poll_media": { "text": "Round robin" } }
    ],
    "duration": 24,
    "allow_multiselect": false
  },
  "files": [
    {
      "filename": "tournament-rules.txt",
      "data_uri": "data:text/plain;base64,aGVsbG8=",
      "description": "Tournament rules"
    }
  ],
  "allowed_mentions": { "parse": [] }
}

The composer splits content longer than Discord’s message limit into at most ten parts. Classic content, embeds, and polls remain classic message parts. A Components V2 layout is kept as its own V2 part and is not mixed into a classic part. allowed_mentions.parse defaults to an empty array, so a draft does not accidentally ping a role, user, or everyone.

Review the preview and its file hashes before publishing. A preview does not write to Discord and does not create a confirmation credential.

Publish the approved draft

The compose result contains a local draft_hash. This identifies the offline preview; it is different from the target-bound payload_hash issued by messages_publish. To publish, first call messages_publish with the same draft plus channel_id. That first call performs no Discord write and returns a bounded review, payload_hash, and one-time approval_id. After approval, repeat the call with __confirm: true, the unchanged __confirm_hash and __confirm_id, and the configured MCP_DRY_RUN=false and MCP_WRITE_MODE=allow.

The confirmed call sends every planned part, then reads the messages back and returns receipts with message IDs, URLs, and readback status. If a send is retried, the exact-hash and one-time approval checks prevent a different draft from being substituted.

Before the first messages_publish call, configure MCP_DRY_RUN=false and MCP_WRITE_MODE=allow. The write policy runs before the payload approval layer; otherwise the call can stop at the write-policy preview and never issue the target-bound approval described above.

Publishing is a confirmed write. Keep the returned receipts as the record of what Discord accepted; a successful request is not a claim that custom button actions are running.

Update the announcement

Use messages_update for one existing message after reading it back. Omitted fields are preserved; when new files are added, existing attachments are kept, so an update can add a revised bracket without silently dropping the original rules file. New filenames must differ from retained attachments. The tool returns a receipt and independent readback status.

If the response is lost after an upload edit, the tool avoids automatically repeating the upload and returns the existing message link with an unknown edit outcome. sent_count: 0 means Discord did not confirm the edit; check the receipt’s readback status before approving another attempt.

Polls and TTS cannot be edited through this tool. A Components V2 message cannot be converted to classic, and a classic message cannot be converted to Components V2 through this tool; create a new message when the layout family must change. The update operation is also a confirmed write.

Discord’s Message object lists attached files that are not referenced by an embed or component. A file used through attachment://... can therefore be absent from attachments[] on readback, so its receipt may be unverified even when Discord accepted the message. If an existing embed or component references an attachment absent from the returned list, the composer refuses to add new files because retention cannot be proven. Publish a new message in that case.

Scope boundary

This composer handles drafting, preview, publishing, file metadata, polls, and single-message updates. Buttons and select menus that need a custom runtime still require the separate interaction runtime work: the composer can publish the components, but it does not provide a long-running handler for their custom actions.