Problem

Attaching multiple photos from the iOS app's chat composer — particularly modern iPhone 48MP photos — causes one or more of:

• Attachments silently fail to deliver
• Memory pressure during base64 encoding on the MainActor
• Per-attachment payload exceeds gateway/model limits (typical ceiling ~5 MB)
• Stack overflows during image decode when multiple full-resolution assets are loaded at once

Tested on iPhone 16 Pro Max (iOS 26.3.1) against gateway 2026.4.15 over WAN WebSocket. Multi-image send in a single message reliably degrades the experience.

Expected

iOS client resizes/compresses attachments before transport:

• Progressive resize to a budget (e.g. 3.5 MB per image) targeting Anthropic's 5 MB limit with headroom
• Decode + resize off MainActor
• Aspect-ratio preserving
• Pixel-count cap to avoid 48MP decode bombs
• Per-message total budget across N attachments

Context

This was partially prototyped locally in the branch anker3/image-resize-chat-attachments (not yet upstreamed):

• 9 commits
• 22 tests
• 4 adversarial review rounds (8/8 pass)
• Fixes: 48MP decode stack overflow, base64 off MainActor, progressive resize to 3.5 MB budget

Related but distinct:

• Feature Request: Auto-resize images exceeding 5MB limit instead of rejecting #61041 — Feature Request: Auto-resize images exceeding 5MB limit (general)
• OpenAI compat gateway images bypass sanitization/resize, hit Anthropic 5MB limit #59913 — OpenAI compat gateway images bypass sanitization/resize
• tool-images resizer bypassed on context replay: images re-injected at original size exceed 5MB limit #53924 — tool-images resizer bypassed on context replay
• [Bug]: Control UI reprocesses historical images on session:history (repeated agents/tool-images “Image resized to fit limits” logs) #33179 — Control UI reprocesses historical images on session:history
• [Bug]: bug(gateway): offloadedRefs metadata lost in transcript for iOS share/node path #60339 — gateway offloadedRefs metadata lost in iOS share/node path

This issue is specifically about the iOS client composer resizing before transport, not server-side resize.

Repro

1. Open iOS app, Control UI or node mode
2. Attach 3–6 recent Camera Roll photos (HEIC/JPEG, default iPhone capture)
3. Send
4. Observe partial failure, memory spike, or payload rejection

Proposed fix

Upstream the prototype from anker3/image-resize-chat-attachments. If desired I can prep a PR with the 9-commit stack rebased onto current main.