From 4006ca072fb91dfc137e92caadc32cbaa652c480 Mon Sep 17 00:00:00 2001
From: FanouZeng-TT <124567600+FanouZeng-TT@users.noreply.github.com>
Date: Wed, 30 Sep 2026 10:17:46 +0800
Subject: [PATCH] fix(channels): keep the delimiter newline when splitting long
 Discord replies (#6062)

* fix(channels): preserve Discord split boundaries

* test(channels): cover Discord split at leading remainder newline

Signed-off-by: Willem Jiang <willem.jiang@gmail.com>

---------

Signed-off-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
---
 backend/app/channels/discord.py |  6 +++++-
 backend/tests/test_channels.py  | 24 ++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/backend/app/channels/discord.py b/backend/app/channels/discord.py
index 35cf228406..32699216fb 100644
--- a/backend/app/channels/discord.py
+++ b/backend/app/channels/discord.py
@@ -876,8 +876,12 @@ class DiscordChannel(Channel):
             split_at = remaining.rfind("\n", 0, _DISCORD_MAX_MESSAGE_LEN)
             if split_at <= 0:
                 split_at = _DISCORD_MAX_MESSAGE_LEN
+            else:
+                # Keep the delimiter on this chunk's tail so consecutive
+                # Discord messages reconstruct the original text exactly.
+                split_at += 1
             chunks.append(remaining[:split_at])
-            remaining = remaining[split_at:].lstrip("\n")
+            remaining = remaining[split_at:]
 
         if remaining:
             chunks.append(remaining)
diff --git a/backend/tests/test_channels.py b/backend/tests/test_channels.py
index f045a54e3c..5ffdfe6606 100644
--- a/backend/tests/test_channels.py
+++ b/backend/tests/test_channels.py
@@ -6131,6 +6131,30 @@ class TestHandleChatWithArtifacts:
 
 
 class TestDiscordChannel:
+    @pytest.mark.parametrize(
+        "text",
+        [
+            "x" * 1990 + "\n\n" + "y" * 30,
+            "x" * 2000 + "\n\n\n" + "y" * 30,
+            "x" * 2000 + "\n" + "y" * 2000,
+            ("第一段\n\n" + "句" * 1990 + "\n\n最后一段") * 3,
+            "x" * 4500,
+        ],
+    )
+    def test_split_text_preserves_content_within_discord_limit(self, text: str):
+        from app.channels.discord import _DISCORD_MAX_MESSAGE_LEN, DiscordChannel
+
+        chunks = DiscordChannel._split_text(text)
+
+        assert all(0 < len(chunk) <= _DISCORD_MAX_MESSAGE_LEN for chunk in chunks)
+        assert "".join(chunks) == text
+
+    def test_split_text_preserves_empty_and_short_messages(self):
+        from app.channels.discord import DiscordChannel
+
+        assert DiscordChannel._split_text("") == [""]
+        assert DiscordChannel._split_text("hello\n\nworld") == ["hello\n\nworld"]
+
     def test_stop_prevents_queued_typing_starter_from_creating_task(self):
         from app.channels.discord import DiscordChannel