[BUG] Excessive config file polling during UI render causes full CPU usage when arrow key is pressed to recall previous prompt

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?

When pressing arrow keys to recall previous prompts (history navigation), Claude Code enters a tight polling loop checking ~/.claude/.config.json approximately 15,000 times per second. This causes 100% CPU usage on one core.

The config file is checked via access() + statx() syscalls on every UI render frame without any caching. This happens regardless of whether the file exists or not.

Key finding: This is NOT caused by a missing file (as suggested in #11170). The polling continues at the same rate even after creating the file.

What Should Happen?

Configuration files should be:

1. Read once at startup and cached
2. Optionally watched via inotify for changes
3. NOT re-read on every render frame

Error Messages/Logs

strace output showing the polling pattern (excerpt from ~10ms window):

  read(20, "\33[A", 262144)               = 3    ← Arrow Up key press
  access("/home/user/.claude/history.jsonl", F_OK) = 0
  clone3({flags=CLONE_THREAD...})         = 695753    ← Thread spawned
  access("/home/user/.claude/.config.json", F_OK) = 0
  statx(AT_FDCWD, "/home/user/.claude/.config.json", ...) = 0
  access("/home/user/.claude/.config.json", F_OK) = 0
  statx(AT_FDCWD, "/home/user/.claude/.config.json", ...) = 0
  madvise(0x..., 16384, MADV_DODUMP) = 0
  access("/home/user/.claude/.config.json", F_OK) = 0
  statx(AT_FDCWD, "/home/user/.claude/.config.json", ...) = 0
  [... repeats ~150 times in 10ms ...]
  write(12, "\33[?2026h\33[2K\33[1A...", 2212) = 2212   ← Terminal redraw

  The madvise(MADV_DODUMP) calls interleaved with config checks indicate this is tied to the render/memory allocation cycle.

Steps to Reproduce

1. Start Claude Code in terminal
2. Run strace -f -e trace=access,statx -p $(pgrep -f "claude") in another terminal
3. Press Up Arrow to recall previous prompt
4. Observe thousands of access("/home/user/.claude/.config.json") calls

Alternative verification:
strace -c -p $(pgrep -f "claude")
Run for a few seconds while pressing arrow keys, observe access and statx dominating syscall counts.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.69

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Xterm

Additional Information

Relationship to other issues

• Issue #11170 reported similar symptoms but misdiagnosed the cause as "non-existent config file". Creating the file does NOT fix the problem - the polling frequency remains the same.
• Issue #2906 (fixed in v1.0.53) addressed similar excessive I/O for settings.json, but .config.json polling was not fixed.

Evidence that file existence doesn't matter

Before creating file (polled ~15,000/sec):
access("/home/user/.claude/.config.json", F_OK) = -1 ENOENT
statx(AT_FDCWD, "/home/user/.claude.json", ...) = 0
[repeats thousands of times]
Note: Falls back from ~/.claude/.config.json to ~/.claude.json

After creating file (STILL polled ~15,000/sec):
access("/home/user/.claude/.config.json", F_OK) = 0
statx(AT_FDCWD, "/home/user/.claude/.config.json", ...) = 0
[repeats thousands of times]
Note: No fallback needed, checks same file

The polling frequency is identical in both cases. Creating the file only changes which file gets statx'd, not how often.

Root cause analysis

The config loading appears to be called synchronously within the UI render loop without caching:

Render cycle:
└─ For each component
└─ loadConfig() ← Called every frame, no cache
└─ render()