Account switches from subscription (i16@i16.jp) to API billing (old account) when tools are used in VS Code extension

Problem

After logging in with a subscription account (i16@i16.jp) in the Claude Code VS Code extension, Account and Usage shows the correct subscription state ("Not Authenticated" = subscription mode). However, after running any tool operations (Bash commands, file reads/writes, etc.), the Account and Usage switches to showing an old API billing account (previously used email address, now deactivated).

Steps to Reproduce

1. Log out of Claude Code VS Code extension
2. Log back in with subscription account (i16@i16.jp via OAuth)
3. Verify Account and Usage shows subscription mode ("Not Authenticated")
4. Run any tool operation (e.g., a Bash command via Claude Code)
5. Check Account and Usage again → shows old API account with usage-based billing

Expected Behavior

Account and Usage should consistently show the subscription account throughout the session, regardless of whether tools are used.

Actual Behavior

After tool use, the account display switches to an old API billing account.

Investigation Notes

• Checked: No ANTHROPIC_API_KEY in environment variables, shell profile, or project config files
• Checked: No hooks configured in ~/.claude/settings.json or project settings
• Found: subscriptionType: null and rateLimitTier: null in the Claude Code-credentials keychain entry (created at login)
• The claudeAiOauth token exists in the keychain but subscription info fields are null

Environment

• OS: macOS
• Claude Code: VS Code extension
• Account type: Subscription (Max plan)

Impact

User is concerned about unexpected API billing charges when tools are used.