docs(sync): record Slack DM empty-thread_ts stream() divergence (#94)

docs/UPSTREAM_SYNC.md

@@ -616,6 +616,7 @@ stay explicit instead of being rediscovered in code review.
 | Google Chat heading rendering | `#`-headings emit as `*text*` (bold) so they're visually distinct | Falls through to default node-to-text (plain concatenation) | Google Chat has no heading syntax; emitting plain text loses the visual hierarchy. Bold is the closest approximation the platform supports. |
 | Google Chat image rendering | Images emit as `{alt} ({url})` or bare `url` | No image branch — falls through to default which concatenates children only, dropping the URL | Upstream silently drops image URLs when rendering to Google Chat text. We preserve the URL so the message content isn't lost. |
 | Fallback streaming stream-exception capture (non-Teams adapters) | `_fallback_stream` captures exceptions from the stream iterator, flushes whatever content was already rendered, awaits `pending_edit`, and re-raises after cleanup | `try/finally` only — exception propagates immediately, `pendingEdit` is un-awaited, and the placeholder is stranded as `"..."` | Upstream leaves a hard UX failure when streams crash mid-flight (common: LLM connection drops): placeholder visible forever, orphan background task. We flush + clean up before re-raising so the caller still sees the original error and users see the partial content instead of a spinner. As above, this divergence no longer applies to Teams after vercel/chat#416: `_stream_via_emit` cancels the session on iterator exception so `_close_stream_session` skips the final-message activity, and the original exception still propagates to the caller. |
+| Slack `stream()` to a top-level DM (empty `thread_ts`) | Normalizes the empty `thread_ts` to `None` and degrades to a single accumulated `post_message` call so the streamed reply still lands (chat-sdk-python#94) | Passes the empty `thread_ts` straight to `chat.startStream` (`adapter-slack/src/index.ts` `stream()`), which Slack rejects (`invalid_thread_ts`) — the streamed DM reply is silently dropped | Top-level DM messages intentionally encode `threadTs=""` on both sides (`_handle_message_event` / `handleMessageEvent`, "matches openDM subscriptions") — that part is faithful to upstream and **not** a bug. The bug is that upstream's `stream()` never reconciled that legitimate value with `startStream`'s requirement for a non-empty `thread_ts`; `postMessage` accepts no `thread_ts` for DMs, so we degrade instead of erroring. Tracked for contribution upstream — remove this divergence once vercel/chat fixes `stream()` to handle empty-`thread_ts` DM thread ids. |
 | Fallback streaming final SentMessage content (non-Teams adapters) | SentMessage + final edit carry `final_content` (remend'd — inline markers auto-closed) | SentMessage + final edit carry raw `accumulated` | Narrow UX refinement. If a stream ends with an unclosed `*`/`~~`/etc., upstream ships the unclosed marker; we run `_remend` so the user sees a clean final message. Not observable in the common case where streams close their own markers. Teams native streaming and the Teams accumulate-and-post path both ship raw `accumulated`, matching upstream after #416; this divergence applies only to the remaining adapters that still route through `_fallback_stream`. |
 | Teams group-chat / channel streaming via accumulate-and-post | `TeamsAdapter.stream` accumulates the full text and issues a single `post_message` instead of post+edit, even for group chats and channel threads | Same after vercel/chat#416 (`if (activeStream && !activeStream.canceled) … else { accumulate; postMessage }`) — no divergence at the adapter level | Documented for clarity: the Python port matches upstream's post-#416 behavior of avoiding the post+edit flicker where Teams doesn't support native streaming. The adapter no longer touches `_teams_update` from the streaming path. |
 | Teams native streaming hand-rolled wire format (DMs) — **transitional** | `_stream_via_emit` and `_close_stream_session` build Bot Framework streaming payloads (`channelData`/`entities`/`streamSequence`/`streamId`) by hand and post them via `_teams_send`, including a per-emit throttle gate (`native_stream_min_emit_interval_ms`, default 1500ms) honoring caller-supplied `StreamOptions.update_interval_ms` | `IStreamer.emit(text)` from `@microsoft/teams.apps` (npm) handles the wire format and throttling under the hood. The equivalent Python SDK (`microsoft-teams-apps`) only went GA on 2026-05-01 — too late for the 4.27 sync window. | Hand-rolled streaming primitives are tracked for migration to `microsoft-teams-apps` (Python) in a follow-up release (0.4.28 / future Python-only). Once we migrate, this row, the divider-rendering row, and the `_active_streams` accounting all simplify or disappear. Until then we own the wire format ourselves — including the 1500ms emit throttle that protects against Teams' ~1 req/sec streaming-endpoint quota — and surface send failures by re-raising (so `Thread.stream`'s outer accumulator can't record text Teams rejected). |