Desktop: name the session from a prompt prefix ("My Title: rest of prompt")

Feature description

In the Claude Code Desktop app, let the first user prompt set the session title via a simple prefix convention: if the opening prompt starts with Some Title: , use Some Title as the session name instead of the auto-generated summary.

Example — starting a new session with:

Claude Repo Idea Suggestion: Check to see if this exists currently

would name the session "Claude Repo Idea Suggestion" in the sidebar, and the prompt sent to the model would be the full text (or optionally just the part after the colon).

Why desktop specifically

The desktop app is where this convention pays off most: sessions live in a sidebar identified by their titles, and the only rename affordance today is clicking the title in the toolbar of the active session, after the fact. Users who run many parallel sessions end up with a sidebar of look-alike auto-generated summaries and a manual rename chore per session.

A prompt-prefix convention makes naming part of the act of starting the work — zero extra clicks, and the session is findable in the sidebar from the very first second.

Suggested behavior / options

• Trigger only on the first prompt of a session, only when it matches something like ^[^:\n]{3,60}:\s (short title, colon, space), to avoid false positives on code-ish prompts.
• Could be opt-in via a setting (e.g. "sessionTitleFromPromptPrefix": true).
• A user-set title from this convention should win over later auto-retitling.

Related (CLI-side analogues, not duplicates)

• #2112 — --session-name flag / /session-name command (terminal CLI)
• #35945 — auto-rename sessions based on first user prompt (CLI, closed)
• #32548 — session title templates via settings/CLAUDE.md
• #35185 / #46155 — desktop rename UX requests (covered by toolbar rename)

None of these propose deriving the title from an explicit user-authored prefix in the prompt itself, and none target the desktop sidebar workflow.