From e25ae55879360e58f22ba9757c6274d5b0f9d47a Mon Sep 17 00:00:00 2001
From: CHISEN Kaoru <cordx56@cordx.cx>
Date: Sat, 7 Feb 2026 08:10:58 +0000
Subject: [PATCH] fix(discord): replyToMode first behaviour

---
 src/auto-reply/reply/formatting.test.ts | 23 ++++++++++++++++++++++-
 src/auto-reply/reply/reply-reference.ts | 12 +++++-------
 src/discord/monitor/threading.test.ts   | 15 +++++++++++++++
 src/slack/monitor/replies.ts            |  5 ++++-
 4 files changed, 46 insertions(+), 9 deletions(-)

diff --git a/src/auto-reply/reply/formatting.test.ts b/src/auto-reply/reply/formatting.test.ts
index 38729bf9a49..e6fb0689881 100644
--- a/src/auto-reply/reply/formatting.test.ts
+++ b/src/auto-reply/reply/formatting.test.ts
@@ -200,14 +200,35 @@ describe("createReplyReferencePlanner", () => {
     expect(planner.use()).toBe("parent");
   });
 
-  it("prefers existing thread id regardless of mode", () => {
+  it("respects replyToMode off even with existingId", () => {
     const planner = createReplyReferencePlanner({
       replyToMode: "off",
       existingId: "thread-1",
       startId: "parent",
     });
+    expect(planner.use()).toBeUndefined();
+    expect(planner.hasReplied()).toBe(false);
+  });
+
+  it("uses existingId once when mode is first", () => {
+    const planner = createReplyReferencePlanner({
+      replyToMode: "first",
+      existingId: "thread-1",
+      startId: "parent",
+    });
     expect(planner.use()).toBe("thread-1");
     expect(planner.hasReplied()).toBe(true);
+    expect(planner.use()).toBeUndefined();
+  });
+
+  it("uses existingId on every call when mode is all", () => {
+    const planner = createReplyReferencePlanner({
+      replyToMode: "all",
+      existingId: "thread-1",
+      startId: "parent",
+    });
+    expect(planner.use()).toBe("thread-1");
+    expect(planner.use()).toBe("thread-1");
   });
 
   it("honors allowReference=false", () => {
diff --git a/src/auto-reply/reply/reply-reference.ts b/src/auto-reply/reply/reply-reference.ts
index 5a3427e832e..903f011f9f6 100644
--- a/src/auto-reply/reply/reply-reference.ts
+++ b/src/auto-reply/reply/reply-reference.ts
@@ -32,20 +32,18 @@ export function createReplyReferencePlanner(options: {
     if (options.replyToMode === "off") {
       return undefined;
     }
-    if (existingId) {
-      hasReplied = true;
-      return existingId;
-    }
-    if (!startId) {
+    const id = existingId ?? startId;
+    if (!id) {
       return undefined;
     }
     if (options.replyToMode === "all") {
       hasReplied = true;
-      return startId;
+      return id;
     }
+    // "first": only the first reply gets a reference.
     if (!hasReplied) {
       hasReplied = true;
-      return startId;
+      return id;
     }
     return undefined;
   };
diff --git a/src/discord/monitor/threading.test.ts b/src/discord/monitor/threading.test.ts
index e0a5c537d8a..2b59bc45362 100644
--- a/src/discord/monitor/threading.test.ts
+++ b/src/discord/monitor/threading.test.ts
@@ -93,7 +93,22 @@ describe("resolveDiscordReplyDeliveryPlan", () => {
       threadChannel: { id: "thread" },
       createdThreadId: null,
     });
+    // "all" returns the reference on every call.
     expect(plan.replyReference.use()).toBe("m1");
+    expect(plan.replyReference.use()).toBe("m1");
+  });
+
+  it("uses existingId only on first call with replyToMode first inside a thread", () => {
+    const plan = resolveDiscordReplyDeliveryPlan({
+      replyTarget: "channel:thread",
+      replyToMode: "first",
+      messageId: "m1",
+      threadChannel: { id: "thread" },
+      createdThreadId: null,
+    });
+    // "first" returns the reference only once.
+    expect(plan.replyReference.use()).toBe("m1");
+    expect(plan.replyReference.use()).toBeUndefined();
   });
 });
 
diff --git a/src/slack/monitor/replies.ts b/src/slack/monitor/replies.ts
index c759ca0b500..550bb9c66b2 100644
--- a/src/slack/monitor/replies.ts
+++ b/src/slack/monitor/replies.ts
@@ -89,8 +89,11 @@ function createSlackReplyReferencePlanner(params: {
   messageTs: string | undefined;
   hasReplied?: boolean;
 }) {
+  // When already inside a Slack thread, always stay in it regardless of
+  // replyToMode — thread_ts is required to keep messages in the thread.
+  const effectiveMode = params.incomingThreadTs ? "all" : params.replyToMode;
   return createReplyReferencePlanner({
-    replyToMode: params.replyToMode,
+    replyToMode: effectiveMode,
     existingId: params.incomingThreadTs,
     startId: params.messageTs,
     hasReplied: params.hasReplied,