[BUG] Parallel Task agents return full transcript instead of summary, causing parent context explosion

Preflight Checklist

• [x] I have searched existing issues and this has not been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

When running multiple Task tool (subagent) invocations in parallel, some subagents return their entire transcript (jsonl-like data) to the parent instead of just the final summary text. This causes the parent's context to explode, forcing compaction.

Observed behavior:

• 4-5 parallel general-purpose subagents were launched for research tasks
• Each subagent performed multiple WebSearch/WebFetch/Read operations
• Upon completion, approximately 900KB of data was returned to the parent
• Parent context hit 0% and triggered compaction
• The returned data appeared to be the full transcript, not the expected summary

When running a single subagent with the same general-purpose type, only the expected text summary + agentId is returned (correct behavior).

What Should Happen?

According to documentation and observed single-agent behavior, subagents should return:

• Final text summary
• agentId

Subagents should NOT return:

• Full transcript/conversation history
• Internal tool call results

The transcript should only be saved to agent-{agentId}.jsonl file, not returned to the parent.

Steps to Reproduce

1. Launch 4-5 general-purpose subagents in parallel using the Task tool
2. Each subagent should perform substantial work (multiple WebSearch, WebFetch, Read operations)
3. Wait for all subagents to complete
4. Observe that parent context is consumed by massive return data (~900KB for 4-5 agents)
5. Parent is forced into compaction

Contrast with working case:

1. Launch a single general-purpose subagent
2. Have it perform similar work or just respond with a greeting
3. Observe that only text summary + agentId is returned (correct)

Is this a regression?

I don't know

Claude Code Version

2.1.5

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code integrated terminal (devcontainer)

Additional Information

Relation to #14055:
This may be related to the race condition described in #14055 (parallel Task agents intermittently lose output content). Both issues involve parallel subagent execution causing unexpected behavior:

• #14055: Output is lost (agentId only, no content)
• This issue: Output is too large (full transcript returned)

These could be two different symptoms of the same underlying race condition in parallel subagent handling.

Workaround found:
Instructing subagents to write their results to a file (e.g., tmp/research.md) provides a failsafe. Even when the parent's context explodes and compaction occurs, the files persist and the post-compaction parent can Read them to reconstruct the output.

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✍️ Author: Claude Code (DevContainer) with @carrotRakko

Note: This issue was written and submitted by an AI agent (Claude Code), with human review and approval.