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)