Resolves the two remaining findings from the Phase 1-4 retrospective
review (the Python-side counterparts to phase 5a):
1. Important — inbox_uploads.fetch_and_stage blocked the inbox poll
loop synchronously per row. A user dragging 4 files into chat at
once would stall the poller for 4× per-fetch latency before the
chat message reached the agent. Add BatchFetcher: a thread-pool
wrapper (default 4 workers) that submits fetches concurrently and
exposes wait_all() as the barrier the inbox loop calls before
processing the chat-message row that references the uploads.
The drain barrier is the correctness invariant: rewrite_request_body
must observe a populated URI cache when it walks the chat-message
row's parts. _poll_once now drains the BatchFetcher inline before
the first non-upload row, AND at end-of-batch (case: batch contains
only upload rows; the corresponding chat message arrives in a later
poll, but the future-poll-races-current-fetch race is closed).
2. Nit — fetch_and_stage created two httpx.Client instances per row
(one for GET /content, one for POST /ack). Refactor so a single
client serves both calls. When called from BatchFetcher, the
batch-shared client serves every row's GET + ack — so the second
fetch reuses the TCP+TLS handshake from the first.
Comprehensive tests:
- 13 new inbox_uploads tests:
- fetch_and_stage with supplied client: zero httpx.Client
constructions, GET+POST through the same client, caller's client
not closed (lifecycle owned by caller).
- fetch_and_stage without supplied client: exactly one
httpx.Client constructed (was 2 pre-fix), closed on the way out.
- BatchFetcher: 3 rows × 120ms = parallel completion < 250ms
(vs. ~360ms serial), URI cache hot when wait_all returns,
per-row failure isolation, single-client reuse across all
submits, idempotent close, submit-after-close raises,
owned-vs-supplied client lifecycle, no-op wait_all on empty
batch, graceful httpx-missing degradation.
- 3 new inbox tests:
- poll_once drains uploads before processing the chat-message row
(in-place mutation of row['request_body'] proves the URI was
rewritten BEFORE message_from_activity returned).
- poll_once with only upload rows still drains at end-of-batch.
- poll_once with no upload rows never constructs a BatchFetcher
(zero overhead on the no-upload happy path).
133 total inbox + inbox_uploads tests pass; 0 regressions.
Closes the chat-upload poll-mode-perf gap end-to-end.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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