Accepting the in-app fullscreen suggestion writes tui:fullscreen and makes Claude Code unstartable on Windows ARM64

Summary

Accepting Claude Code's own in-app suggestion to enable fullscreen mode writes "tui": "fullscreen" to ~/.claude/settings.json. On Windows ARM64 this makes Claude Code unstartable — it opens and exits immediately without accepting input — with no error shown and no in-app way to undo it.

The setting persists, so every subsequent launch fails the same way. Recovery requires knowing to hand-edit settings.json, which is not discoverable from the symptom.

Environment

| | |
|---|---|
| Claude Code | 2.1.221 (native install, %USERPROFILE%\.local\bin\claude.exe) |
| Device | Microsoft Surface Laptop 7, Snapdragon X Elite X1E80100 (ARM64) |
| OS | Windows 11 Pro, build 26200 |
| Terminal | Windows Terminal 1.24.11911.0 (Arm64), running PowerShell |

Steps to reproduce

1. Launch Claude Code in Windows Terminal on Windows ARM64
2. Accept the in-app suggestion to use fullscreen mode
3. Exit
4. Relaunch

Claude Code now opens and exits immediately. Every launch after this fails identically.

Actual behaviour

The process starts, initialises fully, and exits without rendering a prompt or accepting input. No error is printed to the terminal.

Crucially, non-TUI modes continue to work perfectly:

claude --version                     -> 2.1.221 (Claude Code)
claude -p 'reply with exactly: PONG' -> PONG

So the binary, credentials, network and API are all healthy. Only the interactive TUI fails.

Debug log

--debug output ends abruptly after writing .claude.json, with no error. The relevant lines earlier in the log:

XTVERSION: no reply (terminal ignored query)
DECRQM(2026): skipped (no XTVERSION reply) → sync unsupported
DECSTBM: gated (TMUX=unset ZELLIJ=unset TERM_PROGRAM=unset TERM=unset)
...
Prior session exited uncleanly: <session-id> (v2.1.221)

The terminal answered none of the capability probes, so fullscreen initialisation appears to fail — but the failure is silent and terminal.

Settings before and after

Before accepting the suggestion (working):

{
  "permissions": { "defaultMode": "auto" },
  "autoUpdatesChannel": "latest",
  "theme": "dark"
}

After accepting (broken on every launch):

{
  "permissions": { "defaultMode": "auto" },
  "autoUpdatesChannel": "latest",
  "theme": "dark",
  "tui": "fullscreen"
}

Workaround

Remove the "tui": "fullscreen" key from ~/.claude/settings.json. Normal operation returns immediately.

Suggested fixes

The specific problem is less that fullscreen is unsupported on this platform, and more that accepting a suggestion writes a persistent setting that prevents the application from starting, with nothing linking cause to effect. Any of these would break that chain:

1. Probe capability before offering. If XTVERSION gets no reply and DECSTBM is gated, don't offer fullscreen in the first place.
2. Fall back rather than exit. If fullscreen initialisation fails, log it and fall back to the standard TUI instead of exiting.
3. Print an actionable error. Exiting silently gives the user nothing to search for.
4. Self-heal. If a session with tui: fullscreen exits uncleanly during TUI init, ignore the setting on the next launch and say so.

Item 2 alone would reduce this from "application is bricked until you find the right file" to a cosmetic disappointment.