## Summary
Two improvements to session `.in/append`:
- Oversize-body 413 responses now carry CORS headers, so browser fetches
see a readable status instead of an opaque `TypeError: Failed to fetch`.
App-side retry-on-disconnect loops no longer spin forever on a
permanently-rejected payload.
- The per-record cap is now computed precisely against S2's actual
ceiling instead of the conservative 512 KiB floor. Legitimate ~600-900
KiB tool outputs (search results, file content) now succeed;
pathological all-quote content that would double under JSON escape still
rejects cleanly.
## Design
S2 enforces a per-record metered size of `8 + 2*H + Σ(header name +
value) + body ≤ 1048576` bytes. With no record headers (our case), the
budget reduces to `body ≤ 1048568`. Verified empirically against cloud
S2 — append succeeds at metered=1048576 and 422s at 1048577 with `record
must have metered size less than 1 MiB`.
The old `MAX_APPEND_BODY_BYTES = 512 KiB` was derived by assuming
worst-case JSON escape doubling (every byte becomes `\"` or `\\`),
giving `(1 MiB - overhead) / 2`. Safe, but rejects ~half the legitimate
input space.
The new flow:
1. Pre-cap the HTTP body at 1 MiB (DoS guard against reading arbitrary
garbage before we can compute the wrap).
2. After reading, `S2RealtimeStreams.#appendPartByName` computes
`Buffer.byteLength(JSON.stringify({data: part, id: partId}), "utf8") +
8` and throws `S2RecordTooLargeError` (a `ServiceValidationError` with
status 413) if it would exceed S2's ceiling. The route's existing error
branch maps the throw to a 413 with a descriptive message.
The 413 CORS fix is a single-line change in `apiBuilder.server.ts` —
`wrapResponse` was being skipped on the body-too-large branch; every
other error branch wraps; the 413 was the exception.
## Test plan
- Empirically verified against cloud S2 with a boundary scan across
`[1048568, 1048569, ..., 1048576]` and across H ∈ {0, 1×5 hdr bytes,
1×14 hdr bytes} — the formula matches exactly
- Browser-side fetch on a 700 KiB POST now resolves with a readable
`status: 413` (no `TypeError: Failed to fetch`)
- A 900 KiB ASCII tool output now passes (would have 413'd at 512 KiB
pre-fix)
Eric Allam
c0365d36fb