SDK-spawned subagent sessions are persisted as independent top-level sessions (flood the resume picker)

Environment

• Claude Code version: 2.1.211
• Platform: macOS (darwin 25.5.0)
• Entrypoint of affected sessions: sdk-py (subagents / review fan-outs spawned via the Python SDK)

Bug

Subagent runs spawned through the SDK (Agent tool, /security-review fan-out agents, etc.) are persisted as independent top-level sessions in ~/.claude/projects/<project>/. Each agent gets its own <uuid>.jsonl with:

• isSidechain: false
• userType: "external"
• parentUuid: null on the first user message
• no parent-session pointer anywhere in the file

The only hint that they are agent transcripts is entrypoint: "sdk-py" / promptSource: "sdk" on the message records.

Because isSidechain is false and there is no parent link, the resume/session picker lists every subagent as if it were a real interactive session.

Impact

In one active project directory, 267 of ~440 session files are SDK-spawned agent transcripts — 265 of them are security-review fan-out agents whose first message is "Review this change for security vulnerabilities…". The session list is ~60% agent noise, and real sessions are hard to find in claude --resume.

Sample record (first user message of one such file)

{
  "parentUuid": null,
  "isSidechain": false,
  "type": "user",
  "permissionMode": "default",
  "promptSource": "sdk",
  "userType": "external",
  "entrypoint": "sdk-py",
  "cwd": "/Users/<user>/<project>",
  "sessionId": "4825e1eb-...",
  "version": "2.1.211",
  "gitBranch": "main"
}

Expected

Subagent transcripts should either be marked as sidechains (isSidechain: true), carry a parent-session reference, or otherwise be excluded/grouped in the session picker — not listed as standalone user sessions.

Repro

1. In any project, run a command that fans out SDK subagents (e.g. /security-review, or spawn agents via the Agent tool).
2. ls ~/.claude/projects/<project>/ — one new top-level <uuid>.jsonl per agent.
3. claude --resume — each agent appears as an independent session.