Subagent Conflict Detection

Native mechanism

Claude Code's own isolation primitives are the Subagents feature ("each subagent runs in its own context window with a custom system prompt, specific tool access, and independent permissions") and isolation:"worktree"'s base-branch selection. Neither one checks whether a NEW dispatch's file scope overlaps an in-flight one, or whether the worktree base is stale — that pre-flight discipline is what this skill adds on top.

• Official sources: when verifying or updating a factual or version-sensitive claim, read references/official-docs.md.

When to invoke

Before dispatching a new subagent via the Agent tool — especially with isolation: "worktree" — if ANY other subagent is currently running or has an active worktree.

Trigger phrases / situations:

• "派 subagent" / "dispatch a subagent" / "再派一個" while a prior subagent is in flight
• About to call Agent tool when git worktree list shows non-main worktrees
• Multiple [in_progress] tasks in TaskList that involve subagent work

Skip when: dispatching the first subagent in a session, or all prior subagents have completed AND their worktrees are cleaned/merged.

The pattern (3-step pre-dispatch check)

Step 1 — inventory in-flight subagents

git worktree list --porcelain | grep '^worktree ' | cut -d' ' -f2- | tail -n +2

Plain git worktree list is fine for a human to eyeball, but don't parse it: filtering on [main] breaks when the default branch isn't named main, and splitting on whitespace breaks on a path containing a space — cut -d' ' -f2- keeps everything after the first space instead of splitting on every space, so it doesn't have that problem. --porcelain sidesteps the [main]-name issue — tail -n +2 drops the first (main-checkout) worktree line. For each remaining worktree path, capture:

• Branch checked out
• Dirty files: (cd <path> && git status --short)
• Most recent commit subject: (cd <path> && git log -1 --format=%s)

Expect one permission prompt per worktree for these two checks, whichever form you write them in. Per the permissions docs, a Bash rule "must match each subcommand independently"; git -C <path> status is a different invocation form that Bash(git status *) doesn't match; and cd combined with git "prompts when the cd changes into a different directory, since running git in a new directory can execute that directory's hooks". So this skill's allowed-tools covers the Step-1 listing (Bash(git worktree *)) but not the per-worktree checks — the prompt is expected; approve it.

Step 2 — enumerate the NEW dispatch's likely file scope

Read the planned subagent's task prompt. Extract:

• Explicit file paths it'll edit (usually under the prompt's ## Task scope and ## Inputs sections — see leader-developer-handoff-contract)
• Likely-touched files via the task domain (e.g. "Settings redesign" → Sources/.../Settings/)
• Test files it'll add or modify

Step 3 — compute intersection

For each in-flight subagent's dirty-file set vs the new dispatch's likely scope:

• Direct overlap (same file path): BLOCK dispatch, surface to user. Options: serialize (wait for in-flight to merge) OR carve scopes (rewrite new dispatch's prompt to exclude overlapping files).
• Module overlap (same target directory but different files): WARN but allow with isolation: "worktree". Note in dispatch prompt: "in-flight subagent X is editing target Y; do not touch files Z."
• No overlap: dispatch safely.

Non-file exclusive resources also conflict

The intersection check above only reasons about file paths, but a booted Simulator is just as exclusive a resource as a file: pre-assign a UDID per subagent in the dispatch prompt rather than letting each agent boot/pick one implicitly. Some simctl settings are device-global, not per-app — xcrun simctl ui <udid> appearance|content_size changes the whole device's state, so agent A switching to dark mode or Dynamic Type contaminates agent B's screenshots if they share a simulator (see apple-dev-skills:interactive-simulator-ux-audit for the driving pattern this protects).

Pre-dispatch base correctness (verify the worktree base before you dispatch)

isolation: "worktree" does not always branch from your current local HEAD — the base depends on worktree.baseRef ("fresh" vs "head"), and a resumed agent can drift off its starting branch. Before dispatching, confirm the base:

git rev-parse --abbrev-ref HEAD          # on the branch you think you are?
git log --oneline -3                     # does it include the commit/PR this work depends on?
git merge-base --is-ancestor <dep-sha> HEAD && echo "base OK" || echo "STALE BASE"

For the baseRef decision table, the sync-before-dispatch recovery recipe, the real incident that motivated this check, and the two resumed-agent traps (stale pwd/branch, ghost index entries), see references/worktree-base-and-recovery.md.

Coexisting with another live agent / session on the same repo

When ANOTHER Claude session (or human) is actively editing the same working checkout — or a git submodule vendored into your repo (e.g. a shared .claude/skills/<plugin> submodule) — do NOT edit that shared checkout in place. Two writers on one working tree clobber each other's uncommitted edits, fight over branch HEAD, and produce confusing diffs.

Instead, collaborate through isolation + PR:

1. Add your own worktree of THAT repo, branched from its origin/main (not the shared checkout's possibly-dirty local state): git -C <shared-repo-or-submodule-path> fetch origin && git -C <…> worktree add /tmp/<name> -b <branch> origin/main
2. Make your edits in the isolated worktree.
3. Commit, push the branch, open a PR on that repo. Let the normal review/merge flow integrate it.
4. For a submodule: after the upstream PR merges (and is tagged, if the consumer pins tags), bump the submodule pointer in the consuming repo via a SEPARATE PR — never hand-edit the submodule's checked-out files from the parent repo.

This is the cross-session mirror of the within-session conflict check: same goal (no two writers on one tree), different scope (independent sessions / submodules rather than your own in-flight subagents). When unsure whether another agent is on a path, treat it as occupied and use the worktree+PR path — it's cheap insurance.

Output format

When overlap detected, present to user:

⚠️ Conflict detected between new dispatch and in-flight subagent:

In-flight: <subagent-id> editing:
  - <file 1>
  - <file 2>

New dispatch would touch:
  - <file 1>  ← OVERLAP
  - <file 3>

Options:
1. Serialize — wait for in-flight to merge, then dispatch
2. Carve — rewrite new dispatch prompt to exclude overlapping files
3. Proceed anyway — risk: subagent commits compete on push

Anti-patterns this prevents

• Parallel-dispatch race: Two subagents on isolated worktrees edit the same file. --force-with-lease does NOT silently overwrite — it rejects the push when the remote ref has moved since the client last fetched. The real footgun is a different one: worktree B rebases onto a stale base (e.g. the main SHA from before worktree A pushed), producing a divergent history; resolving it then requires a force-push that can drop worktree A's commits. Prevent this by serializing or carving scopes before dispatch.
• Lost-work on worktree wipe: Subagent A's worktree wipes without commit; subagent B's dispatch reuses the path or branch name; A's work is unrecoverable.
• Code Reviewer confusion: CR sees a PR whose diff includes changes from a parallel subagent that's not yet merged; verdict is on wrong baseline.

Pre-flight discipline this skill adds

If your Leader runs a pre-dispatch pre-flight (process cleanup, rebase onto main, tool trust), insert this conflict-detection step before it. For why mise trust is required before mise install/mise exec take effect in a fresh worktree or CI checkout, see apple-dev-skills:mise-tool-management.

False-positive handling

If git worktree list shows stale entries (worktree dir gone but git registration alive), they are NOT a conflict source — they just need git worktree prune. Don't block dispatch on stale registrations; check ls <worktree-path> to confirm the directory actually exists before computing dirty-file intersection.

Example application

Leader is about to dispatch: "Developer for error funnel refactor — files: Sources/App/Composition/Live.swift, Sources/App/Root/RootViewModel.swift, Tests/RootViewModelTests.swift"

`git worktree list` shows in-flight subagent `agent-abc123` editing:
  M Sources/App/Components/BannerController.swift

Intersection: Module overlap (same `Sources/App/`, different files) → WARN.

Verdict: dispatch with `isolation: "worktree"`. Note in prompt: "in-flight subagent on BannerController.swift — do not touch that file; module Sources/App/ is shared."

Related skills

• github-contribution-workflow — routes worktree/submodule collision questions here; that skill owns PR/branch mechanics, this one owns pre-dispatch and cross-session conflict checks.