[Bug]: Switching accounts mid-thread shows the handoff as "Claude Opus 5.5 → Claude Opus 5.5" #15476

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web, apps/mobile

Steps to reproduce

1. Set up two accounts of one provider, for example two Claude or two Codex instances. Give them accent colors, or not: with two instances of one provider the badge shows either way.
2. In an existing thread, pick the other account from the model picker (Switch accounts in an existing thread) and send a message.
3. Look at the Context handoff divider in the timeline, on web or mobile.

Expected behavior

Each side of the divider shows its account's badge, like the sidebar, thread rows, composer and model picker do, so you can see which account the thread moved from and to.

Actual behavior

Both sides show the same bare glyph and model: Claude Opus 5.5 → Claude Opus 5.5. The switch looks like a no-op, and the divider is the only record in the thread of which account took over. That's usually what you want to check after moving off an account that hit its usage limit. The account name only shows in a hover tooltip on web, or after a tap on mobile.

A real handoff between two Claude accounts, renamed "Personal" and "Work".

Cause

HandoffEndpoint in apps/web/src/components/chat/V2LifecycleRow.tsx and in apps/mobile/src/features/threads/thread-handoff-row.tsx draws the plain provider icon. It never applies shouldShowInstanceBadge, the shared rule every other provider icon uses to badge the account.

The same gap was fixed as a bug on the other surfaces: #5977 → #5980 (web sidebar) and #9898 → #9899 (mobile thread rows).

Impact

Minor bug or occasional failure

Version or commit

t3@0.0.46-nightly.20261004.2644 (main 8fb068c). Still present on main at a1d9d72.

Environment

macOS desktop client, Linux host. Two Claude accounts with accent colors.

Workaround

Hover each endpoint on web, or tap it on mobile.