_write_claude_code_credentials wrote ~/.claude/.credentials.json via
Path.write_text + replace + post-write chmod(0o600). Both the temp file
and the destination briefly inherited the process umask (commonly 0o644
= world-readable) between create/replace and chmod, exposing the OAuth
access/refresh tokens to other local users on multi-user hosts.
Use os.open with O_WRONLY|O_CREAT|O_EXCL and an explicit S_IRUSR|S_IWUSR
mode so the temp file is created atomically at 0o600. After os.replace,
the destination inherits the temp's mode, so the post-write chmod is no
longer needed. The temp name also gains a per-process random suffix to
avoid collisions between concurrent writers and stale leftovers from a
crashed prior write.
Parent dir (~/.claude/) is owned by Claude Code itself and shared with
its native auth, so we deliberately don't tighten its mode here (unlike
the mcp_oauth fix which owns its own subtree under HERMES_HOME).
Mirrors the fix shipped for agent/google_oauth.py in #19673 and the
parallel fix for tools/mcp_oauth.py in #21148.
Adds a regression test in TestWriteClaudeCodeCredentials asserting the
resulting file mode is 0o600 (skipped on Windows where POSIX mode bits
aren't enforced).
Guts
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