feat(discord): add forum channel thread (post) support

Adds support for creating threads in Discord forum channels via the
message tool's thread-create action.

Forum channels require an initial message object when creating a post,
unlike regular text channel threads. This change:

- Adds 'message' parameter for forum post content (required for forums)
- Adds 'appliedTags' parameter for forum tag IDs
- Updates DiscordThreadCreate type with new optional fields
- Updates createThreadDiscord to include message/tags in API payload
- Updates action handlers to pass through new parameters

Usage:
  message action=thread-create channelId=<forum-id> \
    threadName='Post Title' \
    message='Initial post content' \
    appliedTags='["tag-id-1", "tag-id-2"]'

Fixes: Discord forum thread creation was not possible via message tool

src/agents/tools/discord-actions-messaging.ts

@@ -284,13 +284,33 @@ export async function handleDiscordMessagingAction(
         typeof autoArchiveMinutesRaw === "number" && Number.isFinite(autoArchiveMinutesRaw)
           ? autoArchiveMinutesRaw
           : undefined;
+
+      // Forum channel support: initial message content
+      const messageContent = readStringParam(params, "content");
+      const appliedTags = readStringArrayParam(params, "appliedTags");
+      const embeds =
+        Array.isArray(params.embeds) && params.embeds.length > 0 ? params.embeds : undefined;
+
+      const payload: Parameters<typeof createThreadDiscord>[1] = {
+        name,
+        messageId,
+        autoArchiveMinutes,
+      };
+
+      // Add forum-specific fields if content is provided
+      if (messageContent) {
+        payload.message = {
+          content: messageContent,
+          embeds,
+        };
+      }
+      if (appliedTags?.length) {
+        payload.appliedTags = appliedTags;
+      }
+
       const thread = accountId
-        ? await createThreadDiscord(
-            channelId,
-            { name, messageId, autoArchiveMinutes },
-            { accountId },
-          )
-        : await createThreadDiscord(channelId, { name, messageId, autoArchiveMinutes });
+        ? await createThreadDiscord(channelId, payload, { accountId })
+        : await createThreadDiscord(channelId, payload);
       return jsonResult({ ok: true, thread });
     }
     case "threadList": {

src/channels/plugins/actions/discord/handle-action.ts

@@ -183,6 +183,10 @@ export async function handleDiscordMessageAction(
     const autoArchiveMinutes = readNumberParam(params, "autoArchiveMin", {
       integer: true,
     });
+    // Forum channel support
+    const content = readStringParam(params, "message");
+    const appliedTags = readStringArrayParam(params, "appliedTags");
+    const embeds = Array.isArray(params.embeds) ? params.embeds : undefined;
     return await handleDiscordAction(
       {
         action: "threadCreate",
@@ -191,6 +195,9 @@ export async function handleDiscordMessageAction(
         name,
         messageId,
         autoArchiveMinutes,
+        content,
+        appliedTags,
+        embeds,
       },
       cfg,
     );

src/discord/send.messages.ts

@@ -97,6 +97,22 @@ export async function createThreadDiscord(
   if (payload.autoArchiveMinutes) {
     body.auto_archive_duration = payload.autoArchiveMinutes;
   }
+
+  // Forum channels require a message object for the initial post
+  if (payload.message) {
+    body.message = {
+      content: payload.message.content,
+      ...(payload.message.embeds?.length ? { embeds: payload.message.embeds } : {}),
+    };
+  }
+
+  // Forum tags
+  if (payload.appliedTags?.length) {
+    body.applied_tags = payload.appliedTags;
+  }
+
+  // For forum channels without messageId, use the channel threads endpoint
+  // For regular channels with messageId, use the message-based thread endpoint
   const route = Routes.threads(channelId, payload.messageId);
   return await rest.post(route, { body });
 }

src/discord/send.types.ts

@@ -66,10 +66,19 @@ export type DiscordMessageEdit = {
   content?: string;
 };
 
+export type DiscordForumMessage = {
+  content: string;
+  embeds?: unknown[];
+};
+
 export type DiscordThreadCreate = {
   messageId?: string;
   name: string;
   autoArchiveMinutes?: number;
+  /** Initial message content for forum posts (required for forum channels) */
+  message?: DiscordForumMessage;
+  /** Tag IDs to apply to forum posts */
+  appliedTags?: string[];
 };
 
 export type DiscordThreadList = {