Shared-workspace subagents were forced serial regardless of file
disjointness, so every multi-unit plan on harnesses without isolated
workers (Codex, Claude Code without worktree isolation) ran serially.
Replace the absolute rule with a wave contract: clean committed
baseline, exclusive ownership including hidden write surfaces, no
worker Git operations, orchestrator-owned verification, and
abort-on-unowned-writes. Units that cannot meet the contract still
serialize or take isolation.
Also deduplicate the worker close/release conditional to the fresh
worker invariant (#1336's scar is retire-per-unit, which stays at each
action site) and drop rationale tails and a hypothetical-harness note
that restated nothing enforceable.
Claude-Session: https://claude.ai/code/session_01J6MN5Yw34dGR9qFyuUCCde
Trevin Chow
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