Coasts

T3 Code

Quick setup

Requires the Coast CLI. Copy this prompt into your agent's chat to set up Coasts automatically:

You can also get the skill content from the CLI: coast skills-prompt.

After setup, restart T3 Code for the skill and rules changes to take effect.

Note: T3 Code may not load project-level skills from .agents/skills/ or .claude/skills/ yet. The setup prompt also places the skill in ~/.codex/skills/coasts/ so it is available globally to the Codex provider. The Coast Runtime rules in AGENTS.md and CLAUDE.md still apply on every task regardless.

T3 Code creates git worktrees at ~/.t3/worktrees/<project-name>/, checked out on named branches.

T3 Code wraps Codex, so it uses AGENTS.md for always-on rules and .agents/skills/coasts/SKILL.md for the reusable /coasts workflow.

Because these worktrees live outside the project root, Coasts 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"]

Coasts 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 (Coasts reads the new Coastfile), but assigning to a T3 Code worktree requires the bind mount inside the container.

Where Coasts guidance goes

Use this layout for T3 Code:

  • put the short Coast Runtime rules in AGENTS.md
  • put the reusable /coasts workflow in .agents/skills/coasts/SKILL.md
  • do not add a separate T3-specific project command or slash-command layer for Coasts
  • if this repo uses multiple harnesses, see Multiple Harnesses and Skills for Host Agents.

What Coasts does

  • Run — coast run <name> creates a new Coast instance from the latest build. Use coast run <name> -w <worktree> to create and assign a T3 Code worktree in one step. See Run.
  • Bind mount — At container creation, Coasts 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 Coasts 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, Coasts 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"
  • .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)

Troubleshooting

  • Worktree not found — If Coasts expects a worktree to exist but cannot find it, verify that the Coastfile's worktree_dir includes ~/.t3/worktrees/<project-name> and that <project-name> matches the actual folder name under ~/.t3/worktrees/. See Worktree Directories for syntax and path types.

Limitations

  • Avoid relying on T3 Code-specific environment variables for runtime configuration inside Coasts. Coasts manages ports, workspace paths, and service discovery independently — use Coastfile [ports] and coast exec instead.