Stable account identity check: SideNav + Profile href must match before Reply/Post

[Trace824]

Summary

On x.com, the visible SideNav account and the account that actually publishes can diverge under multi-account session switching. Automation can click Reply/Post while the wrong account is active, producing an unintended author on the published post.

Repro

• Context: Multiple X accounts signed in; AccountSwitcher available.
• Steps:
  1. Open AccountSwitcher ([data-testid=SideNav_AccountSwitcher_Button]) and select account A; confirm Profile link path.
  2. Open a reply compose; wait or allow other UI activity.
  3. Click [data-testid=tweetButtonInline] / reply send without re-checking identity.
• Observed: published author can be account B even when SideNav briefly showed A.
• Example pattern: wrong-account status URL under a different handle than intended.

Code pointers (verified against /workspace/FSB)

• extension/site-guides/social/twitter.js — SideNav_AccountSwitcher_Button, Profile/account menu notes (~L63, ~L152, ~L218); tweet buttons (~L139–143).
• extension/utils/agent-tab-resolver.js — tab ownership (TAB_NOT_OWNED) — related multi-agent contention, not identity.
• Likely area: new pre-send assert in content actions or a site-guide helper (no existing dedicated identity-guard symbol found).

Proposed fix

Before send: read SideNav account label + /<handle> Profile href (and optionally draft permalink context) and require match to a caller-supplied expected handle. Abort send on mismatch. Root cause is missing identity gate, not the tweet button itself.

Acceptance / regression

• Mismatch aborts before click; match allows send.
• Works across AccountSwitcher transitions when sessions are present.

Owner

extension (primary); mcp-server if expected-handle is a tool param

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Priority: P1 · Owner: extension