subagent-conflict-detection

name

subagent-conflict-detection

description

Use before dispatching a subagent (especially `isolation:"worktree"`) to avoid three dispatch hazards — file-scope overlap with an in-flight subagent, a stale dispatch base, and collisions with another live agent/session editing the same checkout or git-submodule path. Invoke when about to call the Agent tool with `isolation:"worktree"`; when another subagent is running; right after a merge or branch switch (verify the dispatch base first); or when another Claude session is editing a shared repo/submodule path.

Subagent Conflict Detection

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 | grep -v "\[main\]" | awk '{print $1}'

For each worktree path, capture:

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

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 "Mission" / "Required reads to edit" sections)
• 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.

Pre-dispatch base correctness (verify HEAD before you dispatch)

isolation: "worktree" branches the subagent's worktree from the current HEAD of the dispatching checkout at dispatch time. So WHAT HEAD points at is part of the dispatch contract — get it wrong and the agent silently works against the wrong base.

The classic failure: you merge PR A to main, then immediately dispatch a subagent for follow-up work that depends on A. If HEAD is not actually on the post-merge main (e.g. a background build left it detached, you forgot to git checkout main && git reset --hard origin/main, or the branch you're on predates A), the worktree branches from a pre-A base. The agent then can't see A's new symbols/files — it fails to compile against APIs that "should exist", or worse, re-implements against the old shape. Code review may not catch it (the agent's local build can pass on the stale base); it surfaces only when you try to build the integrated result.

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"

If the work depends on a just-merged PR, sync first (git checkout main && git fetch && git reset --hard origin/main — reset --hard discards uncommitted local changes, so stash them first) THEN dispatch. <dep-sha> above is the commit your work depends on (e.g. the merged PR's commit on main). State the expected base SHA in the dispatch prompt and tell the agent to verify it (git log --oneline -5; confirm a key file/symbol exists) before coding.

Real incident: a DEBUG test-hook subagent was dispatched right after a fix merged to main, but the dispatching HEAD was a pre-merge commit. The worktree branched from the stale base, so the new code referenced an init parameter and a file that only existed post-merge → 2 compile errors that the agent's own package build hadn't surfaced. Cost a full cherry-pick-onto-correct-base + rebuild cycle.

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 (methodology.md §Anti-patterns): 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.

Integration with methodology.md

This skill operationalizes docs/methodology.md §派發契約 item 9 (Leader pre-flight). The pre-flight checklist explicitly lists "kill orphan procs" + "rebase WIP onto main" + "mise trust" — this skill adds the conflict-detection step before those.

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: "Senior Developer for error funnel refactor — files: AppComposition/Live.swift, AppUI/Root/RootViewModel.swift, Tests/RootViewModelTests.swift"

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

Intersection: NONE (different AppUI subdir).

Verdict: dispatch safely. Note in prompt: "in-flight subagent on MonetizationStateController — do not touch that file."