[Bug]: Images over 5 MB but under 2000px still stop a Claude turn when sent mid-turn #20106

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web and apps/mobile (attachment preparation); the failure surfaces in the Claude provider.

Steps to reproduce

1. Start a Claude thread and send a prompt that keeps Claude busy for a while (for example "run sleep 20, then say done").
2. While that turn is running, attach a PNG or JPEG that is at or under 2000px on its long edge but larger than ~3.75 MB (for example a photo pasted as PNG, or a 1800×1200 PNG of 6.5 MB), and send it.

Expected behavior

The image reaches Claude and the turn continues, as it does when the same image starts a turn.

Actual behavior

Claude Code rejects the steer before calling the API and ends the turn with terminal_reason: "image_error":

Image base64 size (8.2MB) exceeds API limit (5MB). Please resize the image before sending.

T3 shows "Claude stopped: an image in the conversation could not be processed." In our case the T3 session then stopped, and the image stays in the conversation.

Why

Anthropic caps each image at 5 MB of base64 (5,242,880 characters, about 3,932,160 bytes of file). Claude Code shrinks the images of a message that starts a turn, but sends images queued into a running turn as they are (same mechanism as #13647).

#19364 fixed the dimension half of this by scaling PNG/JPEG over 2000px at attach time. The byte half is still open on main (bb4e4e7ec): an image within 2000px passes through byte for byte as long as it fits the 10 MB PROVIDER_SEND_TURN_MAX_IMAGE_BYTES wire cap.

• Web/desktop: compressImageToByteLimit returns the original when fitImageToMaxEdge has nothing to do and file.size <= maxBytes, and ChatComposer / SnapShotCoordinator pass the 10 MB cap.
• Mobile: fitComposerImage only re-renders when the long edge is over 2000px, so the photo library, clipboard, native paste and share sheet keep originals up to 10 MB.
• Server: ClaudeAdapterV2 base64-encodes the stored attachment as is.

So anything between ~3.75 MB and 10 MB that is already ≤2000px breaks a running Claude turn.

Evidence

Reproduced directly against Claude Code 2.1.296 with claude -p --input-format stream-json, sending the image as a second user message while a sleep 20 tool call was running:

We first hit it from the iPhone app on v0.0.45 with a 1206×2622 screenshot (5.3 MB, 16-bit PNG); a second screenshot of 4.5 MB failed the same way. #19364 now scales that particular screenshot, but the byte limit is independent of dimensions.

Impact

Major degradation or frequent failure: a normal paste while Claude is working stops the session.

Version or commit

Observed on desktop v0.0.45 (Claude Code 2.1.296); code paths confirmed unchanged on main @ bb4e4e7ec.

Environment

macOS, T3 Code desktop + iOS app, Claude provider (Claude Code 2.1.296).

Workaround

Send images only when Claude is idle, or shrink them below ~3.75 MB first.