PR Fix — Parallel Disjoint-Lane Remediation Skill

PR Fix — Parallel Disjoint-Lane Remediation

The execution arm: take a set of verified findings and fix them all, fast, by running multiple subagents in parallel — safely, because their file sets never overlap.

Pairs with pr-audit-angles (the find arm, which produces the verified findings). Verify before you fix: spending agents on phantom or already-closed findings is the main failure mode.

When to invoke

You hold verified findings (from pr-audit-angles, bot review, or your own pass) and want them all fixed in one push.
The remediation spans several files across 2+ subsystems — enough that serial fixing is slow and naive parallel fixing would corrupt the tree.

Skip for: a 1–3 file fix (just do it inline with a failing test first), or when findings are unverified — verify first so you don't fix non-bugs.

The core idea: lanes of exclusive file ownership

Two agents editing the same file at the same time corrupt each other. So partition the findings into lanes whose file sets are disjoint. Then:

Independent lanes run fully in parallel.
A lane whose findings all touch one hot file runs as sequential stages within that lane (thread each stage's result into the next), while the other lanes run alongside it.

parallel([
  laneA = stage1 → (thread result) → stage2 → ...   // shared hot file: serial stages
  laneB,                                              // disjoint files: concurrent
  laneC,
])
→ orchestrator runs the ONE final lint → build → test gate
→ report; do NOT commit unless the user asks

Recipe

Group findings into lanes by the files they touch. Draw boundaries so file sets are disjoint. Two findings that must edit the same file go in the same lane (as stages), never in two parallel lanes.
Confirm a clean starting tree before dispatch. Run git status --short. If the working tree has changes unrelated to the campaign, stop and ask the user to commit or stash them first — otherwise pre-existing edits get co-mingled with the fix and the final gate mis-attributes any breakage. The lane preamble promises agents a clean tree; this is the step that makes that promise true.
Warm the build once before dispatching (if the repo has a build/compile step). Parallel agents that compile/test on a cold tree can race on shared build outputs.
Give every lane the shared binding-rules preamble (below) + its lane-specific section: exclusive file list, forbidden files, required reading, and the per-finding fix spec.
Force structured output (completed / deferred / testsRun / filesTouched / notes) so you merge results mechanically — see EXAMPLES.md.
Within a shared-file lane, thread context: pass stage 1's result JSON into stage 2's prompt ("STAGE-1 RESULT, already applied to the tree: …") so the next stage edits current state, not the base.
The orchestrator owns the gates. After all lanes return, you run the repo's full lint → build → test. Agents never run the full suite.
Report, then stop. Summarize completed/deferred per lane and the gate result. Never commit or stage without an explicit request — and when asked, stage explicit paths, never a blanket git add -A (it sweeps in unrelated working-tree files).

The shared lane preamble (binding rules, verbatim into every lane)

You are one lane of a coordinated fix campaign on <PR/branch>. The tree is checked out and clean; the
build is already warm. A verified-findings audit produced the items below; you fix YOUR lane's findings.

HARD RULES:
- Use the repo's own commands (from package.json / Makefile / CI). For focused iteration, run only the
  narrowest test selection for the files you touched — NEVER the full suite (the orchestrator owns that).
- Bug fixes use a failing test FIRST: write it, run it, OBSERVE it fail (proves it catches the bug), then
  fix, then re-run green. Never weaken an assertion to make a test pass.
- Zero new warnings in files you touch. Fix the root cause; never reshape code just to dodge a lint/guard
  rule — the rule is a proxy for an intent, so satisfy the intent.
- Follow THIS repo's conventions (read the required docs/configs below); do not import patterns from elsewhere.
- NEVER assume the shape of an artifact you depend on — read the authoritative source (schema, type, an
  existing call site) before writing code against it.
- Do NOT git add/commit/stage anything. Leave all changes in the working tree.
- LANE DISCIPLINE: edit ONLY the files listed for your lane. Other lanes run in parallel on disjoint files;
  touching an out-of-lane file causes merge corruption. If a correct fix genuinely needs an out-of-lane
  file, DO NOT edit it — record it in `deferred` with the exact change needed.
- Cite file:line in every `completed` entry. Your structured output is consumed by the orchestrator.

Per-finding fix spec — hand the agent the failing test, don't delegate "TDD it"

The biggest quality lever: write the failing-test design into the prompt. Don't say "fix the bug, TDD it" — specify:

RED: the exact setup/seeding, the exact failure to observe, and a fallback if the harness can't reproduce it ("if the test can't trigger X, assert the underlying state instead and say so in notes").
GREEN: the fix direction + the existing pattern/function to route through ("reuse <helper> at <file:line> — read its signature first") and the invariants to preserve.
VERIFY: the exact narrow test command, and "report every test you added by name."

For each fix also give: YOUR FILES (exclusive lock), FORBIDDEN (files another lane owns), and READ FIRST (the 2–4 docs + exemplar file:line that define the established pattern).

Schema & contract safety inside a lane

If a fix needs a schema/migration change, make that one lane's exclusive job; require the repo's migration/codegen step so generated artifacts stay in sync. Other lanes must not touch those generated files.
Freeze shared wire contracts for the campaign unless a finding is explicitly about them: "do NOT add/rename/renumber the enum members / DTO fields — the contract is frozen; if a change seems unavoidable, defer it with your design." This stops two lanes diverging a shared contract.

Pre-step: verify findings before fixing (find → adversarially verify)

If the findings aren't already verified, run that first: candidate-finders emit findings → one skeptical verifier per finding ("Adversarially VERIFY against current code. Default to 'not real' unless you can prove it with file:line. Check whether a recent commit already closed it."). Keep only isReal && reachable && !alreadyFixed. This is "a review is a lead, not a verdict" mechanized — it stops the campaign burning agents on phantom or already-closed findings. pr-audit-angles already does this in its synthesis; reuse its output when you have it.

Common pitfalls

Overlapping lanes → merge corruption. If two parallel lanes list the same file, re-partition or merge them into one staged lane.
Wrong parallelism — shared hot file ⇒ sequential stages; disjoint files ⇒ concurrent. Don't barrier disjoint lanes.
Weak-model dispatch — subtle invariants need your most capable model; a cheaper tier wastes a round-trip.
Agents running the full suite or committing — both are the orchestrator's job; agents use narrow test selections and leave the tree dirty.
Weakening an assertion to go green — if a test exposes a bug, fix the bug.
Committing unasked, or git add -A — stage explicit paths, only on request.

See EXAMPLES.md for a ready-to-adapt orchestration script (disjoint lanes + sequential stages + the fix schema).

Triggering

/pr-fix-angles                 # fix the verified findings for the current branch
/pr-fix-angles <PR#>           # fix findings for a specific PR

Or naturally: "fix all the audit findings, blocker to minor", "patch everything and make it merge-ready", "run a fix campaign on this PR". Typically chained right after pr-audit-angles.

Agent Skills: PR Fix — Parallel Disjoint-Lane Remediation

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