refactor(discord): centralize autoThread reply plan (#856)

src/discord/monitor/threading.test.ts

@@ -1,6 +1,11 @@
 import { describe, expect, it } from "vitest";
 import { buildAgentSessionKey } from "../../routing/resolve-route.js";
-import { resolveDiscordAutoThreadContext } from "./threading.js";
+import type { Client } from "@buape/carbon";
+import {
+  resolveDiscordAutoThreadContext,
+  resolveDiscordAutoThreadReplyPlan,
+  resolveDiscordReplyDeliveryPlan,
+} from "./threading.js";
 
 describe("resolveDiscordAutoThreadContext", () => {
   it("returns null when no createdThreadId", () => {
@@ -42,3 +47,88 @@ describe("resolveDiscordAutoThreadContext", () => {
   });
 });
 
+describe("resolveDiscordReplyDeliveryPlan", () => {
+  it("uses reply references when posting to the original target", () => {
+    const plan = resolveDiscordReplyDeliveryPlan({
+      replyTarget: "channel:parent",
+      replyToMode: "all",
+      messageId: "m1",
+      threadChannel: null,
+      createdThreadId: null,
+    });
+    expect(plan.deliverTarget).toBe("channel:parent");
+    expect(plan.replyTarget).toBe("channel:parent");
+    expect(plan.replyReference.use()).toBe("m1");
+  });
+
+  it("disables reply references when autoThread creates a new thread", () => {
+    const plan = resolveDiscordReplyDeliveryPlan({
+      replyTarget: "channel:parent",
+      replyToMode: "all",
+      messageId: "m1",
+      threadChannel: null,
+      createdThreadId: "thread",
+    });
+    expect(plan.deliverTarget).toBe("channel:thread");
+    expect(plan.replyTarget).toBe("channel:thread");
+    expect(plan.replyReference.use()).toBeUndefined();
+  });
+
+  it("always uses existingId when inside a thread", () => {
+    const plan = resolveDiscordReplyDeliveryPlan({
+      replyTarget: "channel:thread",
+      replyToMode: "off",
+      messageId: "m1",
+      threadChannel: { id: "thread" },
+      createdThreadId: null,
+    });
+    expect(plan.replyReference.use()).toBe("m1");
+  });
+});
+
+describe("resolveDiscordAutoThreadReplyPlan", () => {
+  it("switches delivery + session context to the created thread", 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: null,
+      baseText: "hello",
+      combinedBody: "hello",
+      replyToMode: "all",
+      agentId: "agent",
+      channel: "discord",
+    });
+    expect(plan.deliverTarget).toBe("channel:thread");
+    expect(plan.replyReference.use()).toBeUndefined();
+    expect(plan.autoThreadContext?.SessionKey).toBe(
+      buildAgentSessionKey({
+        agentId: "agent",
+        channel: "discord",
+        peer: { kind: "channel", id: "thread" },
+      }),
+    );
+  });
+
+  it("does nothing when autoThread is disabled", 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: false } as unknown as import("./allow-list.js").DiscordChannelConfigResolved,
+      threadChannel: null,
+      baseText: "hello",
+      combinedBody: "hello",
+      replyToMode: "all",
+      agentId: "agent",
+      channel: "discord",
+    });
+    expect(plan.deliverTarget).toBe("channel:parent");
+    expect(plan.autoThreadContext).toBeNull();
+  });
+});