seek's main agent runs your conversation in one thread. When a task needs parallel work (read 50 files at once, propose three approaches simultaneously, try a refactor in isolation), the model spawns subagents — each with its own session, its own token budget, optionally its own git worktree. Results condense into a single summary fed back to the parent.

Why it exists

Three concrete problems the parent agent can't solve on its own:

• Context window protection — "read 50 files and summarize" would bloat the parent transcript past the prefix-cache reuse threshold. A subagent absorbs the 50 files; only the summary crosses back.
• Real parallelism — three independent research questions take min(t1,t2,t3) wall-clock, not t1+t2+t3.
• Safe experimentation — worktree-isolated subagents can try a refactor without touching your working tree. Discard or keep on completion.

The shape

parent agent
   ├─→ subagent · explore  → read 3 directories → summary
   ├─→ subagent · plan     → draft an approach → summary
   └─→ subagent · worktree → try refactor in isolation → summary
        │
        ▼
   father reads summaries → continues conversation

LLM-facing tools

The model spawns subagents through the agent tool; worktree isolation is opt-in via enter_worktree / exit_worktree:

User-facing surfaces

$ seek
> /agents                    # TUI panel: live + completed subagents
> /worktrees                 # TUI panel: seek-managed git worktrees

$ seek worktree list         # CLI: same data as /worktrees
$ seek worktree gc           # reap orphans from crashed sessions

Safety contracts

• Permission单调收紧 — a subagent's permissions can only be tighter than the parent's, never looser. PrefYolo parent can spawn PrefAsk child; the reverse never happens. PlanAnalyze parent forces all children to PlanAnalyze (no escape via spawn).
• Cost rolled up to parent — subagent token usage adds to the parent's status-bar cost via cache.Tracker.AdoptChild. No hidden bill.
• Spawn depth = 1 — subagents cannot themselves spawn subagents. Prevents runaway fan-out. AdoptChild panics if called on a child with children.
• Worktree ≠ sandbox — worktree isolates file conflicts for parallel work; it does NOT sandbox bash (../outside paths and absolute paths still resolve normally). Don't run untrusted code in a worktree expecting containment.

What lives where

Limits

• MaxConcurrent — at most N (default 3) active subagents at once. Excess spawn returns [agent: failed reason=too_many_subagents]; the parent model decides whether to retry or switch strategy.
• MaxTurns inherited — child inherits parent's MaxTurns (default 200). Exhaustion → [agent: failed reason=max_turns_exceeded].
• No streaming surface yet — subagent output is opaque until the summary fires. Live streaming into a collapsible region is a v0.6.x dot follow-up.