PROBLEM: Mattermost threads can become invalid or enormous, exposing two failure modes: internal scratch/reasoning/commentary displays could leak into persistent Mattermost threads via global display toggles, while rejected threaded user-visible replies could disappear unless every failed send fell back flat. A broad flat fallback would pollute channels with tool/status/progress noise.
SOLUTION: Require explicit Mattermost platform opt-in for scratch displays, keep using the existing notify=True metadata marker for user-visible final text/media/file replies, and allow the Mattermost plugin adapter to flat-fallback only notify-worthy sends whose threaded POST failure looks like a broken root/thread. Keep tool/status/progress and other non-notify sends thread-strict. Add regression tests for display opt-in, notify-only broken-thread fallback, generic API failure suppression, and stream notify metadata.
Verification: tests/gateway/test_mattermost.py tests/gateway/test_stream_consumer.py tests/gateway/test_stream_consumer_thread_routing.py tests/gateway/test_stream_consumer_fresh_final.py tests/gateway/test_stream_consumer_draft.py; tests/gateway/test_session_api.py tests/gateway/test_status_command.py tests/gateway/test_resume_command.py tests/hermes_cli/test_commands.py; py_compile touched gateway files; git diff --check.
Session: Mattermost thread 6qg8e9dd1pd9pkhi74xyaa1mry, 2026-06-01.
