test_flat_key_wins_over_subblock asserted the OPPOSITE of its name (the
sub-block wins, matching _relay_slack_extra). Rename to what it proves.
Also note in _resolve_cron_surface_mode why its fallback differs from
_relay_slack_extra's all-or-nothing sub-dict: the flat key is the legacy
staging shape, and a flat knob applies to every fronted platform, gated
only by the per-platform D6 capability check.
RelayAdapter.supports_inchannel_continuable is a scalar adopted from the
PRIMARY identity's handshake descriptor, but one RelayAdapter fronts N
platforms and the connector advertises the bit per platform. Reading the
scalar for every logical platform both leaked a Slack-primary True onto
other fronted platforms (activating the flat surface their descriptor
never advertised) and suppressed a non-primary platform's advertised
True (forcing thread mode on capable Slack behind a Discord primary).
Add supports_inchannel_continuable_for_platform(platform): resolves the
platform's own negotiated descriptor via descriptor_for_platform (the
same Phase 1.5 seam max_message_length uses), scalar fallback only when
the per-platform descriptor is unavailable. The scheduler's D6 gate
prefers the query when the adapter provides it; native adapters keep
the class-attribute path byte-identically.
Tests: two-platform descriptor matrix (primary-True no-leak,
non-primary-True honored, unknown-platform scalar fallback).
Field report (enterprise side-by-side, 2026-08-18, finding 1 — the relay-only
blocker): on relay-fronted Slack, cron briefs always deliver into a dedicated
thread; the flat continuable surface (cron_continuable_surface: in_channel)
that native Slack supports is inert, so plain DM replies never continue the
job and the main conversation never sees the brief.
Three gaps closed:
- CapabilityDescriptor gains supports_inchannel_continuable (default False,
additive within contract_version 1; from_json ignores it from old
connectors, old gateways filter it as unknown). The connector advertises
it per platform at handshake.
- RelayAdapter maps the bit onto the adapter capability surface in both the
constructor and _apply_descriptor (renegotiation), so the scheduler's D6
fail-safe gate sees it exactly like native Slack's class attribute.
- _resolve_cron_surface_mode replaces the scheduler's inline flat-key read:
native keeps the shipped flat shape; the relay lane reads the same
per-logical-platform sub-block as the documented relay Slack knobs
(platforms.relay.extra.slack.cron_continuable_surface), sub-block wins,
scoped so a slack block cannot leak onto other fronted platforms.
The seed path needs no changes: RelayAdapter inherits set_session_store
(wired by the generic adapter boot loop) and _seed_cron_channel_session
keys the flat session off the logical platform_name.
12 new tests: descriptor default/from_json/legacy-absence, adapter mapping
constructor + renegotiation, and the surface-knob matrix (native flat key,
relay sub-block, per-platform scoping, precedence, defaults).
Ben Barclay