v0.6.4: Large image base64 payload causes Internal Error (-32603) via ACP

[chaodu-agent]

Problem

When sending images via ACP to kiro-cli (v0.6.4), large images encoded as base64 in the JSON-RPC payload trigger an Internal Error (code: -32603). This is likely due to the payload exceeding the message size limit of the ACP transport layer.

Steps to Reproduce

1. Send a large image (e.g. high-res photo) through Discord to the ACP bridge
2. The bridge encodes the image as base64 and sends it via JSON-RPC to kiro-cli
3. kiro-cli returns Internal Error (code: -32603)

Small images may work fine; the issue appears with larger payloads.

Survey: How other ACP agents handle images

CLI Agent	ACP Support	Image Input	Large Image Issues
Claude Code	✅ Native ACP	base64 ImageContent	Multiple issues: empty base64, format errors (#34338, #4534, #6300)
Codex CLI	⚠️ Via codex-acp bridge	Native --image flag reads local file; ACP uses base64	Model-dependent vision support
Gemini CLI	❌ Not ACP	Local file / paste base64	1MB+ base64 hangs (#3955)

Key findings:

• All ACP agents use base64 — no file path standard exists in the ACP spec
• Large images are a shared pain point — Gemini CLI hangs at 1MB+, Claude Code has multiple base64-related bugs
• Compression before encoding is necessary regardless of which agent is used

Decision

Follow OpenClaw's approach: resize + compress images before base64 encoding on the openab side.

• Resize: longest side capped at 1200px (matching OpenClaw's DEFAULT_IMAGE_MAX_DIMENSION_PX)
• Compress: re-encode as JPEG at quality 75
• Result: ~200-400KB after base64, well within transport limits
• GIFs: pass through unchanged to preserve animation

This is ACP spec-compliant and works across all agents without requiring changes to kiro-cli or any other ACP agent.

Related

• PR: feat: resize and compress images before base64 encoding #210
• Original feature: feat: support Discord image attachments via ACP ImageContent blocks #158 by @the3mi

[chaodu-agent]

Related PR: #158 (feat: support Discord image attachments via ACP ImageContent blocks) by @the3mi

[chaodu-agent]

Reference: How other projects handle image transfer

Hermes Agent (WhatsApp bridge)

• Downloads image to local filesystem (~/.hermes/image_cache/)
• Passes local file path to the agent, not base64
• Source: scripts/whatsapp-bridge/bridge.js

OpenClaw

• Pre-processes images before sending: resize to max 1200px, max 5MB
• Progressive quality reduction (85 → 75 → 65 → 55 → 45 → 35) using sharp
• Source: src/agents/image-sanitization.ts, src/media/image-ops.ts

Takeaway

Neither project sends raw base64 in the JSON-RPC payload. The recommended approach for openab is to download Discord attachments to /tmp and pass the file path via ACP.

[chaodu-agent]

Survey: How CLI agents handle image input via ACP

ACP Spec (authoritative)

ACP ImageContent requires base64 data (required) + mimeType (required). There is an optional uri field but it's for reference only — file path is NOT a standard ACP mechanism for image transfer.

Source: https://agentclientprotocol.com/protocol/content

Agent Comparison

CLI Agent	ACP Support	Image Input	Large Image Issues
kiro-cli	✅ Native ACP	base64 ImageContent	-32603 Internal Error on large images
Claude Code	✅ Native ACP	base64 ImageContent	Multiple issues: empty base64 data, format errors (#34338, #4534, #6300)
Codex CLI	⚠️ Via codex-acp bridge	Native --image flag reads local file; ACP uses base64	Model-dependent vision support
Gemini CLI	❌ Not ACP	Local file / paste base64	1MB+ base64 hangs (#3955)

How other projects solve this

OpenClaw (ACP-compatible approach):

• Pre-processes images before sending: resize to max 1200px, max 5MB
• Progressive JPEG quality reduction (85 → 75 → 65 → 55 → 45 → 35) using sharp
• Source: src/agents/image-sanitization.ts, src/media/image-ops.ts

Hermes Agent (non-ACP approach):

• WhatsApp bridge downloads image to ~/.hermes/image_cache/
• Passes local file path to agent (bridge + agent share filesystem)
• Source: scripts/whatsapp-bridge/bridge.js
• ⚠️ Not applicable to ACP — file path is not part of the ACP spec

Conclusion

Since ACP mandates base64 data, the correct fix for openab is to resize + compress images before base64 encoding (OpenClaw's approach). This works across all ACP agents.

Implemented in PR #210: resize to max 1200px + JPEG quality 75.