Compaction fails on sessions with pasted screenshots (base64 images)

Description

When a Claude Code session contains pasted screenshots (which are stored as base64-encoded images in the session transcript), compaction fails with "Error compacting conversation".

Steps to reproduce

1. Start a Claude Code session
2. Paste 2-3 screenshots during the conversation (Cmd+V with image in clipboard)
3. Continue conversation until context fills up and auto-compaction triggers
4. Observe "Error compacting conversation" error

Expected behavior

Compaction should handle sessions with embedded images gracefully, either by:

• Stripping images during compaction (preserving text summary)
• Handling larger message payloads without failing
• Warning user about image-heavy sessions before compaction fails

Actual behavior

Compaction fails silently. Manual /compact also fails. Session becomes unusable until user manually deletes the session file from ~/.claude/projects/.

Root cause analysis

Session files grow very large because pasted screenshots are stored as base64 in the message JSON:

{
  "type": "image",
  "source": {
    "type": "base64",
    "media_type": "image/png",
    "data": "iVBORw0KGgo..."  // ~1MB per screenshot
  }
}

A session with 2-3 screenshots can easily reach 4-5MB. Compaction appears to fail when processing these large payloads.

Workaround

• Use Read /path/to/screenshot.png instead of pasting (Claude Code reads image files directly)
• Manually delete bloated session files from ~/.claude/projects/<project>/

Environment

• Claude Code version: 2.0.76
• macOS: Sequoia 15.x / Tahoe 26
• Images: PNG screenshots, ~1MB each base64-encoded