Coalesced channels/incoming rejects messages with inline images: 1 MiB content cap counts base64 image data #266

Summary

Since RFC-006 coalescing (#197), a channels/incoming (or push/event) item that carries a coalesce member goes through validateCoalescedContent(content). Both call sites pass no maxBytes, so the default 1 MiB of serialized JSON applies to the whole content array, inlined base64 images included. A message carrying one ordinary screenshot exceeds that. The item is rejected as coalesce_invalid, is never stored, and never wakes the agent. The text that came with the image is lost too.

The host advertises eventCoalescing.channelsIncoming: true (PUSH_COALESCING_SUPPORT), so any coalescing-aware server is affected. discord-mcpl (716bf21) tags every message create with coalesce: { key: "message:<id>", initial: true }, which means every Discord message with a sizeable image is dropped.

Where (main)

• src/mcpl/push-coalescer.ts:243: maxBytes = 1024 * 1024
• src/mcpl/channel-registry.ts:938: validateCoalescedContent(message.content, undefined, …)
• src/mcpl/push-handler.ts:243: validateCoalescedContent(params.payload?.content, undefined, …) (DMs and closed channels)

Only the coalescer's own result.content check reads options.maxContentBytes. The two ingress checks above can't be configured.

Observed (framework 0.21.0, host main 406dcc1, discord-mcpl 716bf21)

Over ~1.5 h on one resident, 27 Discord messages were forwarded via channels/incoming. The 24 without images, plus one with two small images (~260 KB and ~480 KB), were stored. The 3 missing ones were exactly the messages with large screenshots:

discord-mcpl downsamples images before inlining (long edge ≤ 1568 px, re-encoded under a 3.5 MB raw cap; PNGs with alpha stay PNG). So anything that comes out of that above roughly 750 KB raw (~1 MiB base64), or several smaller images in one message, crosses the host's 1 MiB limit. That covers most macOS/Retina screenshots.

The failure is invisible to the user: the rejection only appears as a per-item accepted: false result, there is no console line, and the agent never learns that a message arrived. From the human side the agent "didn't see" a message it was explicitly addressed in. (discord-mcpl also discards the per-item results and logs sent, which is a separate issue on that side.)

Expected

A message with inline images that the provider accepts (~5 MB base64 per image for Anthropic) should be admitted. Possible directions:

• leave image block data out of the byte budget (or count it against a separate, provider-sized per-image limit), and/or
• pass a configurable maxContentBytes at both ingress sites, with a default sized for multimodal content, and/or
• at minimum, log the rejection host-side (console.warn with server, channel, messageId and size), so this can't fail silently.

Repro

Send a channels/incoming item with coalesce: { key: "message:x", initial: true } and one { type: "image", data: <≥ 1 MiB base64>, mimeType: "image/png" } block to a 0.21 host. The result is { accepted: false, reason: "coalesce_invalid" } and nothing is stored.

Diagnosed and filed by Claude Code on behalf of @HO-git (production fleet running 0.21.0).