Coasts

T3 Code

T3 Code is an open-source coding agent harness from Ping. Each workspace is a git worktree stored at ~/.t3/worktrees/<project-name>/, checked out on a named branch.

Because these worktrees live outside the project root, Coast needs explicit configuration to discover and mount them.

Setup

Add ~/.t3/worktrees/<project-name> to worktree_dir. T3 Code nests worktrees under a per-project subdirectory, so the path must include the project name. In the example below, my-app must match the actual folder name under ~/.t3/worktrees/ for your repo.

[coast]
name = "my-app"
worktree_dir = [".worktrees", "~/.t3/worktrees/my-app"]

Coast expands ~ at runtime and treats any path starting with ~/ or / as external. See Worktree Directories for details.

After changing worktree_dir, existing instances must be recreated for the bind mount to take effect:

coast rm my-instance
coast build
coast run my-instance

The worktree listing updates immediately (Coast reads the new Coastfile), but assigning to a T3 Code worktree requires the bind mount inside the container.

What Coast does

  • Bind mount — At container creation, Coast mounts ~/.t3/worktrees/<project-name> into the container at /host-external-wt/{index}.
  • Discovery — git worktree list --porcelain is repo-scoped, so only worktrees belonging to the current project appear.
  • Naming — T3 Code worktrees use named branches, so they appear by branch name in the Coast UI and CLI.
  • Assign — coast assign remounts /workspace from the external bind mount path.
  • Gitignored sync — Runs on the host filesystem with absolute paths, works without the bind mount.
  • Orphan detection — The git watcher scans external directories recursively, filtering by .git gitdir pointers. If T3 Code removes a workspace, Coast auto-unassigns the instance.

Example

[coast]
name = "my-app"
compose = "./docker-compose.yml"
worktree_dir = [".worktrees", ".claude/worktrees", "~/.codex/worktrees", "~/.t3/worktrees/my-app"]
primary_port = "web"

[ports]
web = 3000
api = 8080

[assign]
default = "none"
[assign.services]
web = "hot"
api = "hot"
  • .worktrees/ — Coast-managed worktrees
  • .claude/worktrees/ — Claude Code (local, no special handling)
  • ~/.codex/worktrees/ — Codex (external, bind-mounted)
  • ~/.t3/worktrees/my-app/ — T3 Code (external, bind-mounted; replace my-app with your repo folder name)

Limitations

  • Coast discovers and mounts T3 Code worktrees but does not create or delete them.
  • New worktrees created by coast assign always go in the local default_worktree_dir, never in an external directory.
  • Avoid relying on T3 Code-specific environment variables for runtime configuration inside Coasts. Coast manages ports, workspace paths, and service discovery independently — use Coastfile [ports] and coast exec instead.