Session JSONL files grow to multi-GB size due to bloated "progress" entries

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Session JSONL files can grow to extreme sizes (5+ GB) due to progress type entries accumulating large amounts of duplicated data.

In my case, a session file grew to 5.27 GB with 1,739 "progress" entries averaging ~3 MB each. The actual conversation (user/assistant messages) was only 0.67 MB.

This caused:

• Session disappeared from Past Conversations
• Claude Code became unresponsive
• ~5 GB of unexpected disk usage

What Should Happen?

Progress entries should be lightweight streaming updates, not accumulate the full conversation context repeatedly. A typical session should remain under 10 MB.

Error Messages/Logs

No explicit error - the session simply stopped appearing in Past Conversations.

File analysis showed:
| Message Type | Count | Size |
|--------------|-------|------|
| progress | 1,739 | 5,396 MB (99.6%) |
| user | 56 | 0.39 MB |
| assistant | 110 | 0.25 MB |

Steps to Reproduce

1. Start a conversation using multiple sub-agents via the Task tool (e.g., AWS Infrastructure agent + Datadog agent in parallel)
2. Have a lengthy back-and-forth troubleshooting session
3. Check session file size in: ~/.claude/projects/<project>/
4. Observe progress entries growing to MB+ sizes each

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.9

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

Workaround: Filtering out "progress" type entries from the JSONL file restored functionality:

• Before: 5.27 GB (1,927 lines)
• After: 0.67 MB (188 lines)

The session involved parallel Task tool calls with sub-agents investigating a performance issue. This may be related to how agent progress is logged/accumulated.

Possibly related to #9890, #10107, #10505 - but those focus on FileHistory/.claude.json while this bug is specifically about "progress" type entries in session JSONL files accumulating context.