refactor: enforce plugin-owned channel boundaries

extensions/mattermost/src/channel.ts

@@ -288,6 +288,13 @@ export const mattermostPlugin: ChannelPlugin<ResolvedMattermostAccount> = create
     messaging: {
       defaultMarkdownTableMode: "off",
       normalizeTarget: normalizeMattermostMessagingTarget,
+      resolveDeliveryTarget: ({ conversationId, parentConversationId }) => {
+        const parent = parentConversationId?.trim();
+        const child = conversationId.trim();
+        return parent && parent !== child
+          ? { to: `channel:${parent}`, threadId: child }
+          : { to: normalizeMattermostMessagingTarget(`channel:${child}`) };
+      },
       resolveOutboundSessionRoute: (params) => resolveMattermostOutboundSessionRoute(params),
       targetResolver: {
         looksLikeId: looksLikeMattermostTargetId,

extensions/slack/src/channel.ts

@@ -333,6 +333,13 @@ export const slackPlugin: ChannelPlugin<ResolvedSlackAccount, SlackProbe> = crea
     },
     messaging: {
       normalizeTarget: normalizeSlackMessagingTarget,
+      resolveDeliveryTarget: ({ conversationId, parentConversationId }) => {
+        const parent = parentConversationId?.trim();
+        const child = conversationId.trim();
+        return parent && parent !== child
+          ? { to: `channel:${parent}`, threadId: child }
+          : { to: normalizeSlackMessagingTarget(`channel:${child}`) };
+      },
       resolveSessionTarget: ({ id }) => normalizeSlackMessagingTarget(`channel:${id}`),
       parseExplicitTarget: ({ raw }) => parseSlackExplicitTarget(raw),
       inferTargetChatType: ({ to }) => parseSlackExplicitTarget(to)?.chatType,

extensions/whatsapp/src/session-contract.test.ts

@@ -1,5 +1,9 @@
 import { describe, expect, it } from "vitest";
-import { canonicalizeLegacySessionKey, isLegacyGroupSessionKey } from "./session-contract.js";
+import {
+  canonicalizeLegacySessionKey,
+  deriveLegacySessionChatType,
+  isLegacyGroupSessionKey,
+} from "./session-contract.js";
 
 describe("whatsapp legacy session contract", () => {
   it("canonicalizes legacy WhatsApp group keys to channel-qualified agent keys", () => {
@@ -16,6 +20,12 @@ describe("whatsapp legacy session contract", () => {
 
   it("does not claim generic non-WhatsApp group keys", () => {
     expect(isLegacyGroupSessionKey("group:abc")).toBe(false);
+    expect(deriveLegacySessionChatType("group:abc")).toBeUndefined();
     expect(canonicalizeLegacySessionKey({ key: "group:abc", agentId: "main" })).toBeNull();
   });
+
+  it("derives chat type for legacy WhatsApp group keys", () => {
+    expect(deriveLegacySessionChatType("123@g.us")).toBe("group");
+    expect(deriveLegacySessionChatType("whatsapp:123@g.us")).toBe("group");
+  });
 });

extensions/whatsapp/src/session-contract.ts

@@ -28,6 +28,10 @@ export function isLegacyGroupSessionKey(key: string): boolean {
   return extractLegacyWhatsAppGroupId(key) !== null;
 }
 
+export function deriveLegacySessionChatType(key: string): "group" | undefined {
+  return isLegacyGroupSessionKey(key) ? "group" : undefined;
+}
+
 export function canonicalizeLegacySessionKey(params: {
   key: string;
   agentId: string;

extensions/whatsapp/src/shared.ts

@@ -32,7 +32,11 @@ import {
   unsupportedSecretRefSurfacePatterns,
 } from "./security-contract.js";
 import { applyWhatsAppSecurityConfigFixes } from "./security-fix.js";
-import { canonicalizeLegacySessionKey, isLegacyGroupSessionKey } from "./session-contract.js";
+import {
+  canonicalizeLegacySessionKey,
+  deriveLegacySessionChatType,
+  isLegacyGroupSessionKey,
+} from "./session-contract.js";
 
 export const WHATSAPP_CHANNEL = "whatsapp" as const;
 
@@ -245,6 +249,7 @@ export function createWhatsAppPluginBase(params: {
     config: base.config!,
     messaging: {
       defaultMarkdownTableMode: "bullets",
+      deriveLegacySessionChatType,
       resolveLegacyGroupSessionKey,
       isLegacyGroupSessionKey,
       canonicalizeLegacySessionKey: (params) =>