Global-state atomic writes leak tmp files truncated at exactly 512 KiB

Environment

• Codex app version: 26.903.71938 (macOS, Darwin 25.6.0)

Observed

~/.codex/ accumulates orphaned temp files named ..codex-global-state.json.tmp-<epoch_ms>-<uuid> — 17 of them between Jun 28 and Aug 8 2026, and every single one is exactly 524,288 bytes (512 KiB), truncated mid-JSON (e.g. tail ends ...{"activePermissionProfile":n).

The live .codex-global-state.json is currently ~2.6 MB, so it exceeds the apparent cap by ~5x.

Hypothesis

The atomic-write path for global state appears to hit a fixed 512 KiB write/buffer limit on the temp file. Once the serialized state exceeds 512 KiB, the tmp write truncates or aborts and the file is never renamed or cleaned up — leaving one orphaned 512 KiB file per failed write.

Impact

• Cosmetic disk litter in ~/.codex/ (17 files ≈ 8.7 MB so far, growing over time)
• More concerning: it suggests global-state writes are failing or being abandoned once state grows past 512 KiB — worth confirming the live .codex-global-state.json write path does not share the same cap.

Repro

Long-running Codex app usage until .codex-global-state.json exceeds 512 KiB; tmp files appear without any user action.

Start by locating the Rust atomic-write path for global state and reproduce the issue with .codex-global-state.json larger than 512 KiB. Verify whether the live state write is capped, ensure failed temporary writes are cleaned up, and confirm successful writes leave no orphaned tmp files.