diff --git a/backend/src/module/downloader/rules.py b/backend/src/module/downloader/rules.py
index 4187b181..4d199e53 100644
--- a/backend/src/module/downloader/rules.py
+++ b/backend/src/module/downloader/rules.py
@@ -14,10 +14,18 @@ def build_rss_rule(data: Bangumi | BangumiUpdate, save_path: str) -> dict:
     affected_feeds = (
         data.rss_link if isinstance(data.rss_link, str) else ",".join(data.rss_link)
     )
+    # filter is a comma-separated string of regex terms; qB wants them as a
+    # single alternation. join() would split the string into characters
+    # ("720,480" -> "7|2|0|,|4|8|0"); replace keeps whole terms ("720|480").
+    must_not_contain = (
+        data.filter.replace(",", "|")
+        if isinstance(data.filter, str)
+        else "|".join(data.filter)
+    )
     return {
         "enable": True,
         "mustContain": data.title_raw,
-        "mustNotContain": "|".join(data.filter),
+        "mustNotContain": must_not_contain,
         "useRegex": True,
         "episodeFilter": "",
         "smartFilter": False,
diff --git a/backend/src/test/test_download_client.py b/backend/src/test/test_download_client.py
index 0e1e0620..85d6a62c 100644
--- a/backend/src/test/test_download_client.py
+++ b/backend/src/test/test_download_client.py
@@ -123,8 +123,8 @@ class TestSetRule:
         call_kwargs = mock_qb_client.rss_set_rule.call_args[1]
         rule = call_kwargs["rule_def"]
         assert rule["mustContain"] == "Mushoku Tensei"
-        # filter string is joined char-by-char with "|" (this is how the code works)
-        assert rule["mustNotContain"] == "|".join("720,480")
+        # comma-separated filter terms become a regex alternation for qB
+        assert rule["mustNotContain"] == "720|480"
         assert rule["enable"] is True
         assert "Season 2" in rule["savePath"]