What
Claude Code now includes this error message: "Claude Design cannot be signed in from inside a Claude Code session. Start the editor outside the session and try again." It sits beside an existing refusal that applies when CLAUDE_CODE_CHILD_SESSION is set, which Claude Code sets for programs it starts itself.
Why
If you see this message, start the editor from a normal terminal rather than from inside Claude Code, then sign in again.
Names in the bundleCLAUDE_CODE_CHILD_SESSION
Documented inclaude-code/agent-sdk/typescript
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
Where Claude Code actually shows this new message could not be found.