diff --git a/tests/test_stage6_exclude_flags.py b/tests/test_stage6_exclude_flags.py
new file mode 100644
index 0000000..6ed53c3
--- /dev/null
+++ b/tests/test_stage6_exclude_flags.py
@@ -0,0 +1,86 @@
+# flake8: noqa
+# pylint: disable=missing-function-docstring, redefined-outer-name
+"""Stage-6: unit lock for the `exclude_flags` feed filter (issue #33, epic #34).
+
+The filter (rss_generator._render_messages_groups) is a user-facing query-param
+that drops posts from the feed by flag. It is NOT exercised by the stage-0 golden
+oracle (golden_replay runs without exclude_flags), and stage 6 rewrote it from a
+`for/continue` loop into a list-comprehension. These tests pin the membership
+semantics so the rewrite (and any future one) stays honest — this is the
+"dedicated unit tests" that golden_replay.py's header comment refers to.
+
+Flag shapes under the test config: a plain text post carries ['no_image']; a
+merged group additionally carries 'merged'. (Every rendered post carries at least
+one flag, so the `"all"` filter drops every real post; the guard's flagless-keep
+branch — `and post['flags']` — is a media-path concern proven separately by the
+equivalence check in the PR, not reproducible with plain fixtures here.)
+"""
+from datetime import datetime, timezone
+from types import SimpleNamespace
+
+import pytest
+
+from rss_generator import _render_messages_groups
+from post_parser import PostParser
+from tests.test_stage4_eventloop import make_message
+
+
+@pytest.fixture
+def parser():
+    return PostParser(SimpleNamespace())
+
+
+def _plain(mid, date=None):
+    # Plain text post -> flags == ['no_image'].
+    return make_message(mid, text="plain", date=date)
+
+
+def _merged_group(mid, date=None):
+    # Merged group -> flags == ['no_image', 'merged']; message_id is the main id.
+    main = make_message(mid, text="m", date=date)
+    main.media_group_id = f"g{mid}"
+    other = make_message(mid + 1, text="o", date=date)
+    other.media_group_id = f"g{mid}"
+    return [main, other]
+
+
+def _ids(posts):
+    return [p["message_id"] for p in posts]
+
+
+def test_exclude_flags_all_drops_flagged_posts(parser):
+    # `"all"` special case: any post carrying >=1 flag is dropped.
+    posts = _render_messages_groups(
+        [[_plain(10)], [_plain(11)]], parser, exclude_flags="all"
+    )
+    assert _ids(posts) == []  # both ['no_image'] -> dropped
+
+
+def test_exclude_flags_specific_drops_matching_keeps_nonmatching(parser):
+    # Exclude a concrete flag: the post carrying it is dropped, one without it kept.
+    posts = _render_messages_groups(
+        [_merged_group(20), [_plain(22)]], parser, exclude_flags="merged"
+    )
+    ids = _ids(posts)
+    assert 22 in ids  # ['no_image'] has no 'merged' -> kept
+    assert 20 not in ids  # ['no_image', 'merged'] -> dropped
+
+
+def test_exclude_flags_none_or_nonmatching_keeps_all(parser):
+    base = [[_plain(30)], [_plain(31)]]
+    assert len(_render_messages_groups(base, parser)) == 2  # no filter
+    assert (
+        len(_render_messages_groups(base, parser, exclude_flags="nonexistent")) == 2
+    )  # flag matches nothing -> all kept
+
+
+def test_exclude_flags_preserves_survivor_order(parser):
+    # Distinct descending dates so the trailing date-sort is deterministic;
+    # dropping the middle (merged) post must not reorder the survivors.
+    a = make_message(40, text="a", date=datetime(2024, 1, 3, tzinfo=timezone.utc))
+    mid = _merged_group(41, date=datetime(2024, 1, 2, tzinfo=timezone.utc))
+    c = make_message(43, text="c", date=datetime(2024, 1, 1, tzinfo=timezone.utc))
+    posts = _render_messages_groups(
+        [[a], mid, [c]], parser, exclude_flags="merged"
+    )
+    assert _ids(posts) == [40, 43]  # merged(41) dropped; a before c by date-desc