feat(slack): support multiple media files in a single message

Previously, when a user sent multiple images/files in a single Slack
message, only the first file was processed and passed to the agent.

Changes:
- Add resolveSlackMediaList() to process ALL files (not just the first)
- Add buildSlackMediaPayload() helper to build singular + plural fields
- Update prepareSlackMessage() to populate MediaPaths/MediaUrls/MediaTypes
- Keep backwards compatibility via deprecated resolveSlackMedia()

This mirrors the existing Discord implementation which already supports
multiple attachments per message.

Fixes #XXXX

src/slack/monitor/media.ts

@@ -37,16 +37,23 @@ export async function fetchWithSlackAuth(url: string, token: string): Promise<Re
   return fetch(resolvedUrl, { redirect: "follow" });
 }
 
-export async function resolveSlackMedia(params: {
-  files?: SlackFile[];
-  token: string;
-  maxBytes: number;
-}): Promise<{
+export type SlackMediaInfo = {
   path: string;
   contentType?: string;
   placeholder: string;
-} | null> {
+};
+
+/**
+ * Resolves all Slack media files from a message.
+ * Returns an array of successfully downloaded files.
+ */
+export async function resolveSlackMediaList(params: {
+  files?: SlackFile[];
+  token: string;
+  maxBytes: number;
+}): Promise<SlackMediaInfo[]> {
   const files = params.files ?? [];
+  const out: SlackMediaInfo[] = [];
   for (const file of files) {
     const url = file.url_private_download ?? file.url_private;
     if (!url) continue;
@@ -71,16 +78,58 @@ export async function resolveSlackMedia(params: {
         params.maxBytes,
       );
       const label = fetched.fileName ?? file.name;
-      return {
+      out.push({
         path: saved.path,
         contentType: saved.contentType,
         placeholder: label ? `[Slack file: ${label}]` : "[Slack file]",
-      };
+      });
     } catch {
-      // Ignore download failures and fall through to the next file.
+      // Ignore download failures and continue to the next file.
     }
   }
-  return null;
+  return out;
+}
+
+/**
+ * Legacy function for backwards compatibility.
+ * @deprecated Use resolveSlackMediaList instead.
+ */
+export async function resolveSlackMedia(params: {
+  files?: SlackFile[];
+  token: string;
+  maxBytes: number;
+}): Promise<SlackMediaInfo | null> {
+  const list = await resolveSlackMediaList(params);
+  return list[0] ?? null;
+}
+
+/**
+ * Builds the media payload fields for the inbound context.
+ * Provides both singular (MediaPath) and plural (MediaPaths) fields for compatibility.
+ */
+export function buildSlackMediaPayload(mediaList: SlackMediaInfo[]): {
+  MediaPath?: string;
+  MediaType?: string;
+  MediaUrl?: string;
+  MediaPaths?: string[];
+  MediaUrls?: string[];
+  MediaTypes?: string[];
+  placeholder?: string;
+} {
+  if (mediaList.length === 0) return {};
+  const first = mediaList[0];
+  const mediaPaths = mediaList.map((media) => media.path);
+  const mediaTypes = mediaList.map((media) => media.contentType).filter(Boolean) as string[];
+  const placeholders = mediaList.map((media) => media.placeholder);
+  return {
+    MediaPath: first?.path,
+    MediaType: first?.contentType,
+    MediaUrl: first?.path,
+    MediaPaths: mediaPaths.length > 0 ? mediaPaths : undefined,
+    MediaUrls: mediaPaths.length > 0 ? mediaPaths : undefined,
+    MediaTypes: mediaTypes.length > 0 ? mediaTypes : undefined,
+    placeholder: placeholders.length > 0 ? placeholders.join(" ") : undefined,
+  };
 }
 
 export type SlackThreadStarter = {

src/slack/monitor/message-handler/prepare.ts

@@ -44,7 +44,12 @@ import { resolveSlackAllowListMatch, resolveSlackUserAllowed } from "../allow-li
 import { resolveSlackEffectiveAllowFrom } from "../auth.js";
 import { resolveSlackChannelConfig } from "../channel-config.js";
 import { normalizeSlackChannelType, type SlackMonitorContext } from "../context.js";
-import { resolveSlackMedia, resolveSlackThreadStarter } from "../media.js";
+import {
+  buildSlackMediaPayload,
+  resolveSlackMediaList,
+  resolveSlackThreadStarter,
+  type SlackMediaInfo,
+} from "../media.js";
 
 import type { PreparedSlackMessage } from "./types.js";
 
@@ -331,12 +336,13 @@ export async function prepareSlackMessage(params: {
     return null;
   }
 
-  const media = await resolveSlackMedia({
+  const mediaList = await resolveSlackMediaList({
     files: message.files,
     token: ctx.botToken,
     maxBytes: ctx.mediaMaxBytes,
   });
-  const rawBody = (message.text ?? "").trim() || media?.placeholder || "";
+  const mediaPayload = buildSlackMediaPayload(mediaList);
+  const rawBody = (message.text ?? "").trim() || mediaPayload.placeholder || "";
   if (!rawBody) return null;
 
   const ackReaction = resolveAckReaction(cfg, route.agentId);
@@ -453,7 +459,7 @@ export async function prepareSlackMessage(params: {
 
   let threadStarterBody: string | undefined;
   let threadLabel: string | undefined;
-  let threadStarterMedia: Awaited<ReturnType<typeof resolveSlackMedia>> = null;
+  let threadStarterMediaList: SlackMediaInfo[] = [];
   if (isThreadReply && threadTs) {
     const starter = await resolveSlackThreadStarter({
       channelId: message.channel,
@@ -474,15 +480,15 @@ export async function prepareSlackMessage(params: {
       const snippet = starter.text.replace(/\s+/g, " ").slice(0, 80);
       threadLabel = `Slack thread ${roomLabel}${snippet ? `: ${snippet}` : ""}`;
       // If current message has no files but thread starter does, fetch starter's files
-      if (!media && starter.files && starter.files.length > 0) {
-        threadStarterMedia = await resolveSlackMedia({
+      if (mediaList.length === 0 && starter.files && starter.files.length > 0) {
+        threadStarterMediaList = await resolveSlackMediaList({
           files: starter.files,
           token: ctx.botToken,
           maxBytes: ctx.mediaMaxBytes,
         });
-        if (threadStarterMedia) {
+        if (threadStarterMediaList.length > 0) {
           logVerbose(
-            `slack: hydrated thread starter file ${threadStarterMedia.placeholder} from root message`,
+            `slack: hydrated ${threadStarterMediaList.length} thread starter file(s) from root message`,
           );
         }
       }
@@ -492,7 +498,9 @@ export async function prepareSlackMessage(params: {
   }
 
   // Use thread starter media if current message has none
-  const effectiveMedia = media ?? threadStarterMedia;
+  const effectiveMediaList = mediaList.length > 0 ? mediaList : threadStarterMediaList;
+  const effectiveMediaPayload =
+    mediaList.length > 0 ? mediaPayload : buildSlackMediaPayload(threadStarterMediaList);
 
   const ctxPayload = finalizeInboundContext({
     Body: combinedBody,
@@ -519,9 +527,12 @@ export async function prepareSlackMessage(params: {
     ThreadLabel: threadLabel,
     Timestamp: message.ts ? Math.round(Number(message.ts) * 1000) : undefined,
     WasMentioned: isRoomish ? effectiveWasMentioned : undefined,
-    MediaPath: effectiveMedia?.path,
-    MediaType: effectiveMedia?.contentType,
-    MediaUrl: effectiveMedia?.path,
+    MediaPath: effectiveMediaPayload.MediaPath,
+    MediaType: effectiveMediaPayload.MediaType,
+    MediaUrl: effectiveMediaPayload.MediaUrl,
+    MediaPaths: effectiveMediaPayload.MediaPaths,
+    MediaUrls: effectiveMediaPayload.MediaUrls,
+    MediaTypes: effectiveMediaPayload.MediaTypes,
     CommandAuthorized: commandAuthorized,
     OriginatingChannel: "slack" as const,
     OriginatingTo: slackTo,