The impact-surface skill

PILOT BOOK — THE IMPACT-SURFACE SKILL

Every plan has a footprint. Derive it the same way, every time.

A passage plan is only as good as the footprint it declares. This skill is the shared discipline for deriving that footprint — from a PRD, spec, ticket, or plan-mode output — so every agent and human on a team declares work the same way and the traffic advisories stay comparable. It ships as skills/impact-surface/SKILL.md in the Farled repo: agents whose harness loads skills install it; everyone else pastes it into AGENTS.md or their system prompt. Dev-agnostic by construction.

Step 1 — Derive the impact surface

Walk the categories; for each, ask does this work touch it? Most tasks hit three or four — a task that hits all of them should probably be split.

Category	Typical paths	Often forgotten?
Primary code	the feature's own packages/modules	—
Shared contracts	types, schemas, protocol files, API clients	★ the #1 collision source
Data model	migrations, ORM models, seeds	★ ordering conflicts
API surface	routes, handlers, OpenAPI specs
Tests	unit/integration files beside the code
Localization	locale/message files	★ every-PR files — expect overlap
Docs & records	README, docs pages, ADRs, changelogs
Config	CI, build, env, feature flags	★ high blast radius

Only what you will edit. Files read for context are not footprint — reading collides with nothing.
Narrowest honest globs. src/checkout/** if you own the area; the exact file if you touch one. Never src/** — over-broad plans generate advisories against you.
Name symbols when known — symbol advisories are sharper than path advisories.
Set confidence honestly: high = spec'd, medium = mostly known, low = exploratory. Low is fine; lying isn't.

Step 2 — File it

farled_file_plan {
  "intended_paths":  ["src/checkout/**", "src/lib/currency.ts"],
  "intended_symbols": ["renderSummary"],
  "summary":         "ISO-460: per-org MFA policy",
  "confidence":      "high"
}

Step 3 — Act on the advisory

Advisory	Your move
clear	Proceed.
sequence_after	Someone filed overlapping water first. Rescope out of the overlap, wait for their plan to complete, or coordinate explicitly — never silently proceed into it.
coordinate	Someone is already editing inside your footprint. Check farled_query_active_work for the concrete paths and talk first.

Step 4 — Keep the plan honest

Scope grew? Withdraw and refile the real footprint — a plan_drift signal means other traffic can no longer trust your reservation.
Shipped? Complete it (automatic when your reporter session closes or your dispatched assignment completes).
Abandoned? Withdraw. Free the water.

For PRD authors

Write your PRD's "Files to edit" section as paths and globs, one per line. An agent implementing the PRD derives its footprint by copying that section — the PRD becomes machine-readable intent for free, and reviewers can sanity-check the blast radius before a line of code exists.