fix: normalize Discord autoThread reply target (#8302) (thanks @gavinbmoore)

src/discord/monitor/threading.test.ts

@@ -210,6 +210,31 @@ describe("resolveDiscordAutoThreadReplyPlan", () => {
     );
   });
 
+  it("routes replies to an existing thread channel", async () => {
+    const client = { rest: { post: async () => ({ id: "thread" }) } } as unknown as Client;
+    const plan = await resolveDiscordAutoThreadReplyPlan({
+      client,
+      message: {
+        id: "m1",
+        channelId: "parent",
+      } as unknown as import("./listeners.js").DiscordMessageEvent["message"],
+      isGuildMessage: true,
+      channelConfig: {
+        autoThread: true,
+      } as unknown as import("./allow-list.js").DiscordChannelConfigResolved,
+      threadChannel: { id: "thread" },
+      baseText: "hello",
+      combinedBody: "hello",
+      replyToMode: "all",
+      agentId: "agent",
+      channel: "discord",
+    });
+    expect(plan.deliverTarget).toBe("channel:thread");
+    expect(plan.replyTarget).toBe("channel:thread");
+    expect(plan.replyReference.use()).toBe("m1");
+    expect(plan.autoThreadContext).toBeNull();
+  });
+
   it("does nothing when autoThread is disabled", async () => {
     const client = { rest: { post: async () => ({ id: "thread" }) } } as unknown as Client;
     const plan = await resolveDiscordAutoThreadReplyPlan({

src/discord/monitor/threading.ts

@@ -294,7 +294,9 @@ export async function resolveDiscordAutoThreadReplyPlan(params: {
   agentId: string;
   channel: string;
 }): Promise<DiscordAutoThreadReplyPlan> {
-  const originalReplyTarget = `channel:${params.message.channelId}`;
+  // Prefer the resolved thread channel ID when available so replies stay in-thread.
+  const targetChannelId = params.threadChannel?.id ?? params.message.channelId;
+  const originalReplyTarget = `channel:${targetChannelId}`;
   const createdThreadId = await maybeCreateDiscordAutoThread({
     client: params.client,
     message: params.message,
@@ -391,17 +393,11 @@ export function resolveDiscordReplyDeliveryPlan(params: {
   let deliverTarget = originalReplyTarget;
   let replyTarget = originalReplyTarget;
 
-  // When a new thread was created, route to the new thread
+  // When a new thread was created, route to the new thread.
   if (params.createdThreadId) {
     deliverTarget = `channel:${params.createdThreadId}`;
     replyTarget = deliverTarget;
   }
-  // When in an existing thread (not newly created), ensure we route to the thread
-  // This fixes #8278: autoThread replies sometimes going to root channel
-  else if (params.threadChannel?.id) {
-    deliverTarget = `channel:${params.threadChannel.id}`;
-    replyTarget = deliverTarget;
-  }
   const allowReference = deliverTarget === originalReplyTarget;
   const replyReference = createReplyReferencePlanner({
     replyToMode: allowReference ? params.replyToMode : "off",