Field report (enterprise side-by-side, 2026-08-18, finding 2): identical
agent output renders native rich_text lists, Block Kit tables, and
highlighted code on native Slack, but literal '-' bullets and code-fence
tables on the relay lane. Native reads platforms.slack.extra.rich_blocks /
markdown_blocks and renders Block Kit locally; relay frames carried no
formatting signal, so the connector had no way to know the operator wants
block rendering.
Contract (additive, v1): the connector advertises supports_block_formatting
in its capability descriptor. When it does AND the operator enables
platforms.relay.extra.slack.rich_blocks / markdown_blocks (same per-platform
sub-block and same _coerce_flag semantics as the other relay Slack knobs),
the gateway stamps format_hints into outbound metadata on BOTH text egress
lanes — send and edit (a streamed reply's final edit carries the finished
markdown, so it must signal too or streams seal as plain text). The
connector renders blocks and keeps plain text as the fallback.
Old connector: never advertises -> no dead metadata ever sent. Old gateway:
never stamps -> connector renders plain text as today. Knobs default OFF,
matching native's opt-in posture.
8 new tests: descriptor default/from_json, hint stamping (capable+enabled),
capability-absent suppression, knobs-off suppression, YAML-quoted-false
coercion, partial knobs, edit-lane parity.
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