diff --git a/api/monitor/report.py b/api/monitor/report.py
index 7467a2b..10691ce 100644
--- a/api/monitor/report.py
+++ b/api/monitor/report.py
@@ -135,7 +135,11 @@ async def build_report(
     start_ms, _ = day_bounds(start_day)
     _, end_ms = day_bounds(end_day)
 
-    scope = list(task_ids) if task_ids else None
+    # 必须是 `is not None`，不能写 `if task_ids` —— **空列表是假值**，而空列表在这里
+    # 的含义是「这个平台一个任务都没有」，不是「不限制平台」。用真值判断的话，
+    # 切到一个还没有任务的平台，报表会把**所有**任务的数据聚合出来（看起来就是
+    # 「抖音的报表里全是小红书的数据」）。
+    scope = list(task_ids) if task_ids is not None else None
     days = iter_days(start_day, end_day)
 
     # Fetch every snapshot up to the range end: the delta on the first day needs
diff --git a/tests/test_monitor_report.py b/tests/test_monitor_report.py
index f71726b..20c5479 100644
--- a/tests/test_monitor_report.py
+++ b/tests/test_monitor_report.py
@@ -184,9 +184,9 @@ async def db():
     await engine.dispose()
 
 
-async def _seed_task(db: AsyncSession, name: str) -> MonitorTask:
+async def _seed_task(db: AsyncSession, name: str, platform: str = "xhs") -> MonitorTask:
     task = MonitorTask(
-        name=name, platform="xhs", mode=MODE_CREATOR, enabled=True,
+        name=name, platform=platform, mode=MODE_CREATOR, enabled=True,
         interval_minutes=60, max_notes_count=20, enable_comments=True,
         max_comments_count=50, run_timeout_seconds=3600,
         notify_enabled=False, created_at=0, updated_at=0,
@@ -263,6 +263,25 @@ class TestBuildReport:
         assert result["totals"]["liked_count_delta"] == 30
         assert result["task_ids"] is None
 
+    @pytest.mark.asyncio
+    async def test_an_empty_selection_is_not_the_same_as_no_filter(self, db):
+        """空列表 ≠ 不限制。
+
+        ``_resolve_scope`` 在「这个平台一个任务都没有」时返回**空列表**。如果按真值
+        处理（``if task_ids``），报表就会退化成「不限制平台」，把**所有**任务的数据
+        聚合进来 —— 现象就是切到抖音，报表里却全是小红书的数据。
+        """
+        other = await _seed_task(db, "xhs task", platform="xhs")
+        await _seed_note_with_metrics(db, other, "n1", [(_ms(2026, 1, 10, 10), 999)])
+        await db.commit()
+
+        result = await build_report(db, [], date(2026, 1, 10), date(2026, 1, 10))
+
+        assert result["totals"]["liked_count_delta"] == 0
+        assert result["note_count"] == 0
+        # 空列表要原样透出去；None 在 API 里的意思是「全部任务」，两者不能混。
+        assert result["task_ids"] == []
+
     @pytest.mark.asyncio
     async def test_baseline_from_before_the_range_is_used(self, db):
         """Growth is measured against the last value before the window opens."""
diff --git a/tests/test_platforms.py b/tests/test_platforms.py
index f9bc74f..355ab54 100644
--- a/tests/test_platforms.py
+++ b/tests/test_platforms.py
@@ -21,13 +21,15 @@
 import httpx
 import pytest
 import pytest_asyncio
-from sqlalchemy import text
+from sqlalchemy import select, text
+
+from tools.time_util import get_current_timestamp
 
 from api.main import app
 from api.monitor import adapters
 from api.monitor import db as monitor_db
 from api.monitor import platforms
-from api.monitor.models import MonitorTask
+from api.monitor.models import MonitorNote, MonitorNoteMetric, MonitorTask
 
 XHS_TARGET = "5f58bd990000000001003753"
 
@@ -44,6 +46,33 @@ async def client(tmp_path):
     await monitor_db.dispose_engine()
 
 
+async def _seed_note_with_one_snapshot(task_name: str) -> None:
+    """给某个任务塞一条作品和一次指标快照。
+
+    过滤类测试**必须有真数据**才有意义 —— 库里空着的话，过滤有没有生效结果都是 0，
+    测试就变成了空跑（这个坑踩过一次：一个报表串数据的 bug 因此没被拦住）。
+    """
+    async with monitor_db.get_session() as session:
+        task = await session.scalar(select(MonitorTask).where(MonitorTask.name == task_name))
+        now = get_current_timestamp()
+        session.add(
+            MonitorNote(
+                task_id=task.id, note_id="seed-note", title="seed", note_url="",
+                cover="", creator_hash="", source_kind="", published_at=None,
+                first_seen_run_id=1, first_seen_at=now,
+                last_seen_run_id=1, last_seen_at=now,
+            )
+        )
+        session.add(
+            MonitorNoteMetric(
+                task_id=task.id, note_id="seed-note", run_id=1, captured_at=now,
+                liked_count=42, comment_count=0, collected_count=0, share_count=0,
+                raw_liked_count="42", raw_comment_count="0",
+                raw_collected_count="0", raw_share_count="0",
+            )
+        )
+
+
 class TestCapabilityMatrix:
     @pytest.mark.asyncio
     async def test_matrix_is_exposed_to_the_ui(self, client):
@@ -191,8 +220,15 @@ class TestPlatformScoping:
 
     @pytest.mark.asyncio
     async def test_a_platform_with_no_tasks_yields_empty_not_everything(self, client):
-        """An empty task set must not degrade into "no filter"."""
+        """空的任务集合不能退化成「不加过滤」。
+
+        **这里必须真的有数据。** 没有数据时，过滤生效与否结果都是 0 —— 这条测试原先
+        就栽在这个空跑上，所以没能拦下一个报表串数据的 bug（切到抖音，报表里却出现
+        小红书的数据）。最后那段「小红书自己的报表看得到」就是为了证明这些数据确实
+        存在、上面那两个 0 是过滤出来的。
+        """
         await self._seed_two_platforms(client)
+        await _seed_note_with_one_snapshot("小红书任务")
 
         body = (await client.get("/api/monitor/notes", params={"platform": "bili"})).json()
         assert body["notes"] == []
@@ -203,6 +239,11 @@ class TestPlatformScoping:
         assert report["totals"]["liked_count_delta"] == 0
         assert report["note_count"] == 0
 
+        xhs = (
+            await client.get("/api/monitor/report", params={"platform": "xhs"})
+        ).json()
+        assert xhs["note_count"] == 1
+
 
 class TestPerPlatformSettings:
     @pytest.mark.asyncio