fix(gateway): backfill Discord thread context

Discord threads where the bot has already participated bypass mention gating by default, but the backfill check was still tied to the mention-needed condition. That meant follow-up thread messages could trigger a response without providing recent thread history to the session.

Run history backfill for thread messages whenever backfill is enabled, while keeping DMs skipped and channel mention backfill behavior unchanged. Add a regression test for a known thread follow-up without an explicit mention.

Fixes #33666

Co-authored-by: Cursor <cursoragent@cursor.com>

plugins/platforms/discord/adapter.py

@@ -4818,7 +4818,7 @@ class DiscordAdapter(BasePlatformAdapter):
                 and not in_bot_thread
             )
             _backfill_enabled = self._discord_history_backfill()
-            if _needed_mention and _backfill_enabled:
+            if _backfill_enabled and (_needed_mention or is_thread):
                 _backfill_text = await self._fetch_channel_context(
                     message.channel, before=message,
                 )

scripts/release.py

@@ -52,6 +52,7 @@ AUTHOR_MAP = {
     "270604154+superearn-fisher@users.noreply.github.com": "superearn-fisher",
     "3540493+kpadilha@users.noreply.github.com": "kpadilha",
     "40378218+chaconne67@users.noreply.github.com": "chaconne67",
+    "Pluviobyte@users.noreply.github.com": "Pluviobyte",
     "sanghyuk_seo@nexcubecorp.com": "sanghyuk-seo-nexcube",
     "subrtt@gmail.com": "Brixyy",
     "wangpuv@hotmail.com": "wangpuv",

tests/gateway/test_discord_free_response.py

@@ -851,6 +851,27 @@ async def test_discord_per_user_channel_backfills_too(adapter, monkeypatch):
     assert event.channel_context == "[Recent channel messages]\n[Alice] context"
 
 
+@pytest.mark.asyncio
+async def test_discord_participated_thread_backfills_without_mention(adapter, monkeypatch):
+    """Known threads still need recent thread context when mention gating is bypassed."""
+    monkeypatch.setenv("DISCORD_REQUIRE_MENTION", "true")
+    monkeypatch.delenv("DISCORD_FREE_RESPONSE_CHANNELS", raising=False)
+    monkeypatch.delenv("DISCORD_THREAD_REQUIRE_MENTION", raising=False)
+    adapter.config.extra["history_backfill"] = True
+    adapter._fetch_channel_context = AsyncMock(return_value="[Recent channel messages]\n[Alice] thread context")
+
+    thread = FakeThread(channel_id=456, name="follow-up")
+    adapter._threads.mark("456")
+
+    message = make_message(channel=thread, content="follow-up without mention")
+    await adapter._handle_message(message)
+
+    adapter._fetch_channel_context.assert_awaited_once()
+    event = adapter.handle_message.await_args.args[0]
+    assert event.text == "follow-up without mention"
+    assert event.channel_context == "[Recent channel messages]\n[Alice] thread context"
+
+
 @pytest.mark.asyncio
 async def test_discord_dm_does_not_backfill(adapter, monkeypatch):
     """DMs skip backfill — every DM triggers the bot, so there's no mention gap."""