CLI exits 1 with EACCES on /tmp/claude-settings-<hash>.json when multiple macOS users share the host

Summary

On a multi-user macOS host, the Claude Code CLI (bundled with the desktop app, v2.1.128) crashes immediately with exit code 1 whenever any user other than the one who created /tmp/claude-settings-<sha256>.json tries to start a session. The desktop surfaces this as the generic "Claude Code crashed / Claude Code process exited with code 1" banner with no further detail.

Environment

• Claude desktop app: 1.6608.2
• Bundled CLI: 2.1.128
• macOS: Darwin 25.4.0 (arm64)
• Two user accounts on the host (hectortosa, codesharegrow), each with its own Claude desktop install

Reproduction

1. On host with users A and B, both signed in to Claude on their respective accounts.
2. Log in as A, open Claude desktop, start any Claude Code chat. This creates /tmp/claude-settings-44136fa355b3678a.json (content: {}, mode 0644, owner A).
3. Switch to user B, open Claude desktop, start any Claude Code chat.
4. Desktop reports: "Claude Code crashed / Claude Code process exited with code 1".

Root cause

Captured by wrapping claude.app/Contents/MacOS/claude and recording argv, stdin, stderr:

• Desktop spawns CLI with --settings '{}' (literal empty JSON inline override).
• CLI hashes {} → sha256 prefix 44136fa355b3678a… and tries to open /tmp/claude-settings-44136fa355b3678a.json for read+write.
• File exists, owned by user A, mode 0644. User B can read but cannot open O_RDWR. Open fails with EACCES.
• CLI emits Error processing settings: EACCES: permission denied, open '/tmp/claude-settings-44136fa355b3678a.json' on stderr and exits 1.
• The desktop's child-process error handler reports only the exit code; the stderr line never reaches the UI.

/tmp has the sticky bit, so user B cannot delete or chmod the file without sudo.

Workaround

sudo rm /tmp/claude-settings-<hash>.json

(or have the owning user delete it; or chmod 666 it). The CLI recreates the file on next launch, owned by whichever user got there first, and the same collision recurs.

Suggested fixes

1. Skip the cache entirely when --settings is an empty object — there is nothing to cache.
2. Namespace the cache path by uid, e.g. /tmp/claude-settings-\$UID-<hash>.json, so users on a shared host don't collide.
3. Forward the CLI's stderr line to the desktop's crash banner so users see the actual cause instead of a bare exit code.

Diagnostic capture (redacted)

argv:

--output-format stream-json --verbose --input-format stream-json
--effort xhigh --model claude-opus-4-7[1m]
--permission-prompt-tool stdio
--allowedTools mcp__computer-use,mcp__ccd_session__spawn_task,...
--setting-sources=user,project,local --permission-mode auto
--allow-dangerously-skip-permissions --include-partial-messages
--plugin-dir <...> [...]
--replay-user-messages --settings {}

stderr:

Error processing settings: EACCES: permission denied, open '/tmp/claude-settings-44136fa355b3678a.json'

stat of the colliding file:

-rw-r--r--  1 hectortosa  wheel  2  May 11 15:39  /tmp/claude-settings-44136fa355b3678a.json
content: {}