Support @ mentions for files and paths in other projects

[dpshade]

Problem

Composer @ mentions are currently oriented around the active project/workspace. That makes it hard to reference files, folders, or relevant context that lives in a different local project when working across repositories.

Desired behavior

Allow @ mentions to target other projects in T3 Code, not just the currently active one.

This could mean being able to:

• Search across multiple known projects/workspaces
• Scope a mention to another project explicitly
• Insert references to files or folders outside the current repo while preserving clear project context

Why this matters

A lot of real workflows span multiple repositories: app + backend, app + design system, SDK + example app, or several related services. If the composer can only mention files from the current project, users lose a fast way to provide cross-repo context to the agent.

Acceptance criteria

• Users can use @ to reference files or folders from another project available in T3 Code
• The mention UI makes it clear which project a result belongs to
• Inserted mentions preserve enough project context to avoid ambiguity between similarly named paths
• The agent receives the referenced cross-project context in a predictable way

Open design questions

• Should project selection be part of the @ query syntax or handled in the picker UI?
• Should search default to the current project first, then expand to others?
• How should mentions render when two projects contain the same path or filename?

[chuks-qua]

Hey,

Cross-project references would solve a real problem for multi-repo workflows. After some thought, the cleaner path here is a workspace model that lets users group related projects.

Without that foundation, extending @ mentions across projects has real downsides:

• The mention picker becomes a filesystem browser, slower and noisier for everyone
• Disambiguation between projects with similar paths (e.g. two repos both having src/utils/auth.ts) falls on the user
• The agent receives files without surrounding project context, which can mislead more than it helps

Workspace support would scope cross-project references within a boundary, keeping the mention picker focused rather than turning it into a file explorer.

We could even add a scope modifier to opt into cross-project search explicitly, something like @:query where : tells the picker to search across projects in the workspace. That keeps the default behavior fast and local.

@maria-rcks what are your thoughts?