[Bug][P1] Multiple Coding Plan subscribers unable to log into ZCode — login fails across all browsers

Bug Description

Multiple paid Coding Plan subscribers are unable to log into the ZCode application using their Z.AI accounts. This has been an ongoing issue reported in Discord since June 20, 2026 and is still unresolved as of July 15, 2026.

Affected Users (from Discord)

1. Tang Wang (WUMP | Mango) — reported June 20, 2026 with screenshot evidence
2. haywire — reported July 10, 2026; confirmed Coding Plan subscriber, cannot login
3. jackwealth — reported July 15, 2026; Coding Plan tied to Proton Mail account; login fails across ALL browsers; noted that signing in via Gmail works but plan is tied to a different email

Steps to Reproduce

1. Navigate to ZCode application
2. Attempt to log in with Z.AI account (Coding Plan subscriber)
3. Login fails or gets stuck
4. Observed across: Chrome, Firefox, and other browsers (all browsers tested by affected users)

Key Observations

• Gmail OAuth login works but the user's Coding Plan is tied to a different email provider (e.g., Proton Mail). This suggests the issue may be related to:
  • OAuth token exchange failing for non-Google email providers
  • Account-plan association not resolving correctly during authentication
  • Session state not persisting after OAuth callback for certain providers
• Multiple users affected — this is NOT an isolated incident
• Current support response ("try a different browser") does not resolve the issue

Expected Behavior

Coding Plan subscribers should be able to log into ZCode using the email account associated with their subscription, regardless of email provider.

Actual Behavior

Login fails for paid subscribers, effectively blocking access to the service they paid for. The issue persists across all browsers and appears related to the email provider / OAuth flow.

Severity Assessment: P1 (Critical)

• Revenue impact: Paying customers cannot access their purchased service
• Scope: Multiple users affected, not isolated
• Duration: Unresolved for 25+ days
• Trust impact: Users are expressing frustration publicly in Discord

Recommended Investigation Areas

1. OAuth callback handling for non-Google email providers (Proton Mail, etc.)
2. Account-plan association resolution during authentication
3. Whether the recent model picker bug (#130) and this login issue share a common OAuth root cause
4. Server-side logs for failed authentication attempts from June 20 onward

Current Support Response (from Discord)

Agnes|Z.ai — 6/21/26: "Please email the support team user_feedback@z.ai"
Agnes|Z.ai — 7/13/26: "plz try to change a browser."

The browser-change suggestion has been confirmed ineffective by affected users.

Regards,
Roman

• http://www.rommark.dev
• Vibe Coders Telegram Channel: https://t.me/VibeCodePrompterSystem

No source files or tests are named. Start by reproducing the login failure with a Coding Plan account using a non-Google provider, then inspect OAuth callback handling, account-plan association, and server-side authentication logs from June 20 onward. Done means affected subscribers can authenticate and retain access to the plan across supported browsers.