One relay adapter fronts N platforms on one WS, but the capability surface
(MAX_MESSAGE_LENGTH / message_len_fn) was a scalar from whichever descriptor
resolved the handshake — and the transport's read loop OVERWROTE it on every
descriptor frame (last-writer-wins). A Discord chat on a gateway whose
applied descriptor was Telegram's inherited the 4,096-char cap and over-sent
into Discord's 2,000-char API 400 (observed live: 2,543/2,641-char replies
silently lost while inbound kept working).
- ws_transport: accumulate one descriptor per platform in
_descriptors_by_platform (exposed via descriptor_for_platform); the FIRST
descriptor of a connection generation stays the session default instead of
last-writer-wins; the map resets on re-dial.
- BasePlatformAdapter: new max_message_length_for_chat /
message_len_fn_for_chat hooks defaulting to the scalar surface (native
single-platform adapters unchanged).
- RelayAdapter: overrides resolve the chat's platform from _platform_by_chat
(the same map per-frame egress uses) and look up that platform's negotiated
descriptor; falls back to the scalar for unknown chats/transports.
- stream_consumer (streaming budget, _raw_message_limit, fallback-continuation
chunking) + run.py tool-progress limit now resolve per-chat.
Tests: tests/gateway/relay/test_relay_per_platform_caps.py (7) — verified
fail-without/pass-with (all 7 fail with the fix stashed). Relay + stream
consumer suites green (213 + 223).
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