Agent teams regression: teammates not spawned after TeamCreate (v2.1.42+)

Description

Agent teams stopped working between v2.1.37 and v2.1.42. TeamCreate succeeds, but the model never calls the Task tool with team_name to actually spawn teammates into tmux panes. No error is logged — the spawn step simply never happens.

Reproduction

1. Set teammateMode: "tmux" in Claude Code settings
2. Set CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1
3. Start Claude Code in a tmux session
4. Ask it to create a team and spawn teammates

Expected behavior

After TeamCreate, the model should call the Task tool with team_name and name parameters to spawn teammates into tmux panes, producing debug log entries like:

[TmuxBackend] Created teammate pane for build-checker: %1
[TmuxBackend] Created teammate pane for bugfix-auditor: %2

This worked correctly on v2.1.37 (Feb 9, 2025).

Actual behavior

TeamCreate succeeds (team config written to ~/.claude/teams/{name}/config.json with only the lead agent as a member), but the model immediately calls ExitPlanMode instead of spawning teammates via the Task tool. The team config shows tmuxPaneId: "" and only one member (the lead). Zero [TmuxBackend] entries appear in the debug log.

This started on v2.1.42 and persists on v2.1.44.

Evidence

Working (v2.1.37, Feb 9) — debug log:

[TmuxBackend] Created teammate pane for build-checker: %1
[TmuxBackend] Created teammate pane for bugfix-auditor: %2

Team config showed multiple members with populated tmuxPaneId values.

Broken (v2.1.44, Feb 16) — team config after TeamCreate:

{
  "members": [
    {
      "agentId": "team-lead@gamestate-protocol",
      "name": "team-lead",
      "agentType": "coordinator",
      "tmuxPaneId": "",
      "subscriptions": []
    }
  ]
}

No other members spawned. No [TmuxBackend] log entries.

Configuration

• teammateMode: "tmux" (set in settings)
• CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS: "1" (env var)
• defaultMode: "plan" was enabled for both working and broken versions — not the cause
• macOS, tmux session active

Analysis

This appears to be a model behavior regression caused by changes to the system prompt or tool orchestration between v2.1.37 and v2.1.42. The model no longer follows through with the Task tool call (with team_name parameter) after TeamCreate. The infrastructure (tmux backend, team config) works — it's never invoked.

Versions

• Worked: v2.1.37
• Broken: v2.1.42, v2.1.44