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Codex Issues / #36225

Windows unified app 26.721.11231.0 crashes on startup: “Invalid weekday string: MON”

Open 💬 4 comments Opened Jul 30, 2026 by kevinwingz

Description

Immediately after updating the Codex Windows app to the new unified
ChatGPT/Work/Codex desktop application, the application became unusable.

The main process crashes during every launch with:

SyntaxError: Invalid weekday string: MON

The system-tray process may remain running, but selecting “Open chat” does
nothing. Quitting through the tray and relaunching reproduces both error
dialogs.

Environment

  • Platform: Windows x64
  • Package/build visible in error stack:

OpenAI.Codex_26.721.11231.0_x64__2p2nqsd0c76g0

  • Regional format at failure: English (United States)
  • ChatGPT plan: Pro

Steps to reproduce

  1. Update the previous Codex desktop app to the unified ChatGPT desktop app.
  2. Launch ChatGPT/Codex.
  3. Observe “ChatGPT failed to start — Invalid weekday string: MON.”
  4. Dismiss that dialog.
  5. Observe “A JavaScript error occurred in the main process.”
  6. If “Check for Updates” is selected, the tray process may remain open, but

the interface still cannot be opened.

Troubleshooting already attempted

  • Checked for additional updates.
  • Fully quit and relaunched the tray process.
  • Restarted Windows.
  • Changed Regional format to English (United States).
  • Verified that account automations use valid RFC weekday codes such as

TU and SA; none contains MON.

None resolved the problem.

Expected behavior

The unified ChatGPT desktop app should launch and provide access to existing
Codex projects and conversations.

Actual behavior

The main application window cannot open. Local Codex access is completely
blocked, and Codex is not selectable through the ChatGPT website.

Related issue

Possibly related at the parser level to #30411, but this is a distinct
Windows/English-locale regression in build 26.721.11231.0.

Additional information

The failing stack appears to pass the three-letter value MON into recurrence
parsing that expects the RFC 5545 two-letter value MO.

View original on GitHub ↗

4 Comments

kevinwingz · 28 days ago

<img width="752" height="597" alt="Image" src="https://github.com/user-attachments/assets/bc9f956c-3fd6-4d86-9d7a-d5eef44ef960" />
<img width="1123" height="528" alt="Image" src="https://github.com/user-attachments/assets/fc547958-9d6b-40e8-8326-844f984c24db" />

kevinwingz · 27 days ago

Root cause confirmed and recovery verified

I isolated the exact local record causing this Windows startup crash.

Offending record
%USERPROFILE%\.codex\automations\
weekly-wingspan-labs-website-viability-sweep\automation.toml

Relevant fields:

kind = "cron"
status = "ACTIVE"
rrule = "FREQ=WEEKLY;BYDAY=MON;BYHOUR=8;BYMINUTE=30;BYSECOND=0"

The three-letter weekday MON is invalid for this RFC-style recurrence format; the expected value is MO. The exception SyntaxError: Invalid weekday string: MON matches this stored value exactly.

Diagnostic findings
  • There were 13 file-based automation records.
  • The other 12 used valid two-letter weekday values such as MO, TU, WE, TH, FR, SA, and SU.
  • This was the only standalone MON match in the bounded application-state search.
  • The legacy SQLite automation table contained 12 records, all valid.
  • The offending automation was the single file-only record and was absent from the SQLite automation table.
  • The malformed TOML SHA-256 was:
7BFC37EC712EA23702F3972A4D7F3BD57BD8392F6825170009A58E6638E5F313
Verified repair

I preserved an unchanged evidence copy, then changed only:

BYDAY=MON

to:

BYDAY=MO

The repaired file SHA-256 is:

8D5CA9DA65E158B4518D33DDCC00FF2B5BA7E12D8EC5FD1E672910FF8BC1

A byte comparison confirmed that the sole change was removal of the ASCII N.

After restoring the repaired .codex directory, ChatGPT/Codex desktop build 26.721.11231.0 launched successfully with the prior projects and conversations restored. The startup crash is resolved.

Suggested application fixes
  1. Normalize three-letter weekday inputs when automation records are created or migrated: MON→MO, TUE→TU, WED→WE, THU→TH, FRI→FR, SAT→SA, and SUN→SU.
  2. Validate recurrence rules before writing automation.toml.
  3. Do not allow one malformed automation record to terminate the Electron main process.
  4. Quarantine or disable an invalid automation and allow the application to open with a recoverable warning.
  5. Reconcile file-only automation records with the SQLite automation table so malformed or partially created records do not remain startup-fatal.

This confirms that the Windows/English-locale failure was caused by malformed local automation state, not Windows regional settings or the account-side ChatGPT scheduled tasks.

Lei-Zhang1227 · 27 days ago

Thank you so much for documenting this so thoroughly — your post absolutely saved me!

I had the exact same startup crash, except my error was Invalid weekday string: THU. I found the offending automation.toml, changed BYDAY=THU to BYDAY=TH, restarted the app… and it opened immediately! 🎉

I had already tried restarting and checking for updates, so seeing the app finally launch again was such a huge relief. I was genuinely so happy when it worked.

I really appreciate how clearly you identified the root cause, preserved the original file, and explained the repair step by step. You didn’t just report the bug — you gave the rest of us a working rescue path.

Huge thanks. You seriously made my day!

kevinwingz · 26 days ago

I am happy to hear this helped you and others!