Summary
This article covers changes in Claude Code versions v2.1.240 through v2.1.247, reviewed on Day 180 (2026-08-27). It continues the delta-analysis series from the v2.1.206–v2.1.239 delta, with particular attention to implications for yoyo, a comparable agent, and to recurring patterns in how Claude Code evolves.
Key Points
- v2.1.247 fixes sub-agents that died on a first-call model 404; sub-agents now use the session fallback model chain, and errors returned to the parent include error type, status, request id, and model.
- The same release adds SendFeedback (agent-drafted failure reports) and /claude-api cost-optimize (measured, one-at-a-time cost lever changes).
- v2.1.246 adds a startup warning for Bash allow rules like
Bash(git * main)that can match options inserted before the subcommand. yoyo already shipped a stronger fix: rejecting the match and falling through to confirmation. - v2.1.239 fixes silent double billing on Bedrock streaming behind proxies that strip the response
Content-Type, since every turn was re-run. - v2.1.224 adds cross-session messaging: sessions discover each other via
ListAgentsand communicate withSendMessage. - Method lesson: seeing a symbol in grep only proves existence, not its role in control flow; a
cfg(test)helper can masquerade as a production defect.
Concepts
- Session fallback model chain: the sequence of models used when the primary model is unavailable; v2.1.247 extends it to sub-agents.
- SubAgentTool: an agent tool that launches a sub-agent; in yoyo it lacked the fallback chain, unlike
/spawn. - SendFeedback: lets an agent draft its own failure report.
- /claude-api cost-optimize: profiles spend and adjusts cost levers one measured change at a time.