Sync unsent composer drafts across devices

[SimonLiu423]

Problem

I start writing a prompt on my Mac, then have to leave (commuting, etc.). I want to pick it up on my phone and keep going. Right now the draft is stuck on the first device.

Current behavior (v0.1.5)

Drafts never leave the device they were typed on:

• Web/desktop: browser localStorage (apps/web/src/composerDraftStore.ts, key t3code:composer-drafts:v1)
• Mobile: a local file (apps/mobile/src/state/use-composer-drafts.ts, composer-drafts/)
• The server has no draft concept. New, unsent threads also exist only on the device that created them.

Proposal

Store drafts on the server that owns the thread, so every client connected to it sees the same draft:

• Scope: all drafts. That covers drafts on existing threads and new, not-yet-sent threads (project, environment, model selection included).
• Saves are debounced, and other connected clients get them live.
• Conflict rule: last writer wins, with a revision number, so a stale device can't overwrite newer text without knowing it.
• Attachments sync once uploaded. Local-only items can stay device-local at first.
• Sending or discarding a draft clears it everywhere.

Open questions for maintainers

• Should it be on by default, or a setting?
• Should synced drafts include context items (terminal context, review comments, preview annotations), or just text and attachments to start?
• With multiple environments, does a draft live on the thread's environment server? (I'd assume yes.)

Related

#17422 and #21097 are mobile draft-saving bugs on a single device. This is different: it's about moving drafts between devices.

Happy to implement this if the direction and scope are approved.

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Drafted by Claude Opus 5.5 (claude-opus-5-5) via t3 triage in T3 Code.