Parallel subagents and worktree isolation

The task tool runs one subagent and waits. The tasks tool runs several at once. The agent chooses it when subtasks are independent — investigate three modules, try two fixes, review each service — and the model's own prompt decides that, not a flag.

{
  "tasks": [
    {"prompt": "…", "description": "audit pkg/auth", "agent_type": "review"},
    {"prompt": "…", "description": "audit pkg/store", "agent_type": "review"}
  ],
  "isolation": "worktree"
}

Up to eight per call. Concurrency is bounded by limits.max_parallel_subagents (zero means all at once), and each subagent still counts against limits.max_subagents and the token budget exactly as a single task does.

Isolation

The interesting case is when subagents write. Two agents editing one checkout overwrite each other, and the parent cannot tell whose change survived. With "isolation": "worktree" each subagent gets:

• its own git worktree under .abhed/worktrees/<id> — a separate checkout of HEAD, so its files are its own;
• its own branch, abhed/<id>;
• its own scoping boundary: the child's file and shell tools are rooted in the worktree and cannot reach the parent's tree or a sibling's.

Nothing touches the main tree. When the tasks return, the parent receives each subagent's summary and, for each worktree, what changed and how to take it:

## Task 1 — audit pkg/auth
<summary>

Worktree .abhed/worktrees/k3f9q2 (branch abhed/k3f9q2): 3 file(s) changed, UNCOMMITTED.
 internal/auth/local.go | 12 ++++---
 …
To take these changes: review with `git -C .abhed/worktrees/k3f9q2 diff`, commit
there, then `git merge abhed/k3f9q2` from the main tree. To discard:
`git worktree remove --force .abhed/worktrees/k3f9q2 && git branch -D abhed/k3f9q2`.

Changes are left uncommitted in the worktree, on purpose. Merging is a decision, and the point of isolation is that a person — or the parent agent, with the diff in front of it — makes that decision rather than discovering it.

Worktrees are created up front and serially before any subagent starts, so a git failure stops the whole call before a token is spent. The directory is added to .git/info/exclude so the main tree's git status stays clean without touching the repository's tracked .gitignore.

Requirements and limits

• Isolation needs the workspace to be a git repository, and git on the path. Asking for it elsewhere is refused with that reason, before spawning.
• Worktrees are not removed automatically. They are small (a checkout, shared object store) and the parent's report says how to remove each one.
• Without isolation, subagents share the parent's workspace and boundary — fine for read-only work, and the default, because most parallel work is investigation.

What it is not

It is not a multi-agent framework with roles and message passing. Abhed's model is simpler and stays simple: a subagent is a fresh loop with a complete prompt, it returns a summary, and the parent decides what to do with it. Running several at once, in separate checkouts, is the whole of what this adds.