[BUG] tmux split-pane backend strips [1m] context window from teammates — 200K instead of 1M

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The tmux split-pane backend strips the \[1m]\ context window variant from the model identifier when spawning teammates. The parent session runs \claude-opus-4-6[1m]\ (1M context), but teammates spawned in tmux panes get \claude-opus-4-6\ (200K context).

Confirmed Workaround (v2.1.92)

Credit to @Reasonably (#34421) for discovering this. Confirmed working on v2.1.92, macOS, Claude Max, tmux:

Step 1: Add \"teammateDefaultModel": null\ to \~/.claude.json\

Step 2: In the lead session, run \/model opus[1m]\ — you must explicitly select \opus[1m]\, NOT "Default (recommended)"

Step 3: When spawning teammates, do NOT pass the \model\ parameter — let them inherit from the lead

Before (teammate gets 200K — note "Opus 4.6" without "1M context", \/context\ shows \18.6k/200k\):

!Before

After (teammate gets 1M — note "Opus 4.6 (1M context)" on both lead and teammate panes):

!After

Why "Default" doesn't work but \opus[1m]\ does

When the lead uses "Default (recommended)", Claude Code resolves the model to \claude-opus-4-6\ internally. The tmux spawn path then passes this resolved ID (without \[1m]\) to the teammate. When you explicitly select \opus[1m]\ via \/model\, the resolved model ID includes the \[1m]\ suffix, which propagates correctly — but only when \teammateDefaultModel\ is \null\ (which tells teammates to inherit the lead's model rather than resolving their own default).

Root cause

The tmux backend's model resolution for teammates has two problems:

1. When \teammateDefaultModel\ is not set, it calls a default resolver that returns \claude-opus-4-6\ (200K) instead of inheriting the lead's model
2. The "Default (recommended)" model selection resolves to the base model ID without the \[1m]\ variant before reaching the spawn command

Environment

• Claude Code v2.1.92
• Claude Max (Opus 4.6, 1M context)
• macOS Darwin 25.2.0
• iTerm2 with \tmux -CC\

Closing as duplicate

Closing in favor of #34421 which is the canonical issue for this bug. Adding our workaround confirmation there.

Related Issues

• #34421 — Canonical issue, most thorough write-up
• #40929 — Additional testing, workaround discussion
• #36670 — Same bug, v2.1.80
• #27261 — First report of \[1m]\ inheritance, auto-closed prematurely
• #27038 — Related model selection bug (different root cause)
• #18346 — Root model selection issue