Context compaction drops media_type from base64 images, causing API 400 errors

Bug

After context compaction in a long session, the API returns:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.106.content.2.image.source.base64.media_type: Field required"}}

Steps to Reproduce

1. Start a long Claude Code session
2. Paste/share a screenshot during the conversation (e.g. a Vercel error screenshot)
3. Continue working until context compaction triggers
4. After compaction, subsequent messages fail with the 400 error above

Expected Behavior

Context compaction should either:

• Preserve the media_type field when retaining image content
• Or strip the entire image block (base64 + media_type) if it's being removed from context

Actual Behavior

Compaction appears to retain the base64 image data but drops the required media_type field (image/png, image/jpeg, etc.), causing all subsequent API calls to fail with invalid_request_error.

Workaround

Starting a fresh session (/clear or new terminal) resolves the issue since the corrupted image reference is discarded.

Environment

• Claude Code (latest)
• macOS Darwin 25.3.0 (ARM64)
• Model: claude-opus-4-6
• Long session with multiple screenshots shared earlier in conversation