feat(web): continue conversations in new threads

[aflekkas]

Problem

Once a T3 Code thread has conversation state, moving the work to another coding harness means manually reconstructing its context. The existing New thread on branch action carries workspace state, but intentionally starts with an empty conversation.

Solution

Add Continue in new thread to the shared thread action menu on web and desktop. It:

• loads the source thread's recent completed user/assistant conversation
• creates an editable draft in the same workspace
• initially carries the source model, permission mode, and interaction mode
• leaves the provider/model selector available so the user can choose another configured harness before sending
• bounds the portable handoff to the provider input limit while retaining the newest context
• keeps attachment names as reminders without attempting to copy provider-local attachment data

This is deliberately a first, provider-agnostic slice. It does not clone a provider's private session, tool state, or image data, and it does not yet add the action to mobile. Native same-provider forks and provider-specific session capabilities can build on this separately.

Verification

• pnpm --dir apps/web exec vp test run --passWithNoTests --project unit src/threadHandoff.test.ts src/threadContinuation.test.ts src/components/threadActionMenu.logic.test.ts (20 tests passed)
• pnpm --filter @t3tools/web typecheck
• targeted vp lint for the changed TypeScript/TSX files
• vp fmt --check for all changed files
• git diff --check
• integrated browser pass using an isolated T3 home and browser profile

Screenshots

Thread action	Editable continued draft

Model: GPT-5 | Harness: Codex

Note

Add continue-in-new-thread action to thread context menus

• Adds a continue-in-new-thread menu item to thread context menus in Sidebar.tsx and useThreadActionMenu.ts. The item is disabled when the thread has no conversation content or is running.
• Introduces continueThreadInNewDraft in threadContinuation.ts to create a new draft populated with the source thread's provider, model, runtime, and interaction settings.
• Adds buildThreadHandoffPrompt in threadHandoff.ts to generate a bounded, portable prompt from the thread's recent messages, ignoring system, streaming, and empty messages.
• Behavioral Change: Thread context menus now render a new continue-in-new-thread item at the top.

^{Macroscope summarized 30e33a3.}

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on this repository. Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: e2f97663-dc78-4f9c-afb1-9f60f88fc0dd

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

_{Comment @coderabbitai help to get the list of available commands.}

[t3dotgg]

Note

🤖 GPT-5.6 Sol responding on behalf of Theo

We're closing this PR as we clean up the T3 Code backlog. Thank you for taking the time to put this together.

We are not adding a separate action to continue a conversation in a new thread. The current thread controls remain available, and this extra interaction path is not a priority.

If you believe we closed this in error, please reopen the PR and leave a comment explaining what we missed.