Feature Implementation Plan

---
name: feature-implementation-plan
description: >
  Assembling everything established about a feature into one document another engineer could
  execute without re-deriving the architecture: the resources in dependency order, the schema,
  contract, configuration and security changes named individually, the test strategy per
  resource, the migration, deployment and rollback story, and acceptance criteria a test can be
  written from. Use once the decisions are taken and before implementation starts, when a plan
  is a list of file names, when the plan and the code have drifted apart, when a feature is being
  handed to someone else or resumed after a break, or when the rollback story is discovered
  during the rollback. Does not produce the breakdown it orders (feature-decomposition), does not
  execute it (feature-execution), does not track status against it
  (feature-progress-tracking), and does not invent dates or effort estimates
  (estimation-under-uncertainty).
---

# Feature Implementation Plan

## Purpose

The plan is the hand-off artefact. It is read by whoever implements the feature — a different
person, a different agent, or the same agent after the context is gone — and its test is
whether they can proceed without asking what was decided or reading the analysis again.

It is also the artefact that goes stale fastest, so the second half of this skill is about
keeping it true rather than about writing it well the first time.

## Workflow

1. **Assemble, do not re-decide.** Relevant sections use existing artifacts or accepted session evidence:
   scope, impact map, decisions, resources, risks. If a section needs a new decision, the plan
   is incomplete for dependent work — route that decision while assembling independent parts.
2. **Order the resources** from their dependencies, and mark which arrows are forced. The order
   is a dependency graph, not necessarily one chain. Validate IDs/cycles and identify ready work;
   shared-file ownership and integration/release gates may constrain otherwise independent nodes.
3. **Name every change of kind** — schema, contract, configuration, security, observability,
   messaging — in a discoverable section or compact labelled entry. These are the entries
   people search for, and a change buried in a resource description is not found.
4. **State the test strategy per resource kind**, not as a paragraph. What level, against what,
   and what it must establish.
5. **Write migration, deployment and rollback as sequences**, with the ordering constraint
   spelled out. "Deploy then migrate" and "migrate then deploy" are different plans.
6. **Assemble, do not author, acceptance.** Carry accepted `BAC-*` from Product and `TC-*` from
   Engineering, trace both to `RES-*`, and name planned `EV-*`. If a criterion is missing or cannot
   become a check, return it to its owning stage instead of repairing intent in the plan.
7. **Distinguish inapplicable from unresolved.** Use `none, because <reason>` only for established
   inapplicability; unknown decisions/evidence name the affected resources and next action. A Light
   plan can stay inline with scope, resource, validation and relevant constraints.

## Decision rules

```text
IF a section would require a decision to fill
THEN mark dependent work not ready and route the decision. A draft may name the unknown;
     it must not present a placeholder as an accepted design or block unrelated ready work.

IF a resource has no validation
THEN it is not planned yet, whatever else is written about it.

IF the plan names a file that does not exist and is not created by a resource
THEN the file list is wrong. Every path is either created here or exists today.

IF the change touches persisted data
THEN the plan states the compatibility window: what runs against old data, what
     runs against new, and whether they overlap.

IF the rollback story is "revert the commit"
THEN check it against the schema section. Once a migration has run, reverting the
     code is not reverting the change.

IF implementation contradicts the plan
THEN distinguish a defect from evidence invalidating the plan. Fix a defect against the
     accepted contract; otherwise revise affected artifacts and readiness with existing
     authority or obtain a genuinely missing decision. Do not reapprove unchanged work.

IF the feature spans sessions
THEN the plan must be readable cold. Assume the reader has none of the conversation.
```

## Constraints

- **Do not invent dates or effort estimates.** `estimation-under-uncertainty` owns forecasts.
  Preserve operational facts that constrain execution — migration windows, batch limits,
  retention periods, rollout intervals, and externally committed deadlines — and label their
  provenance instead of deleting them from the plan.
- **No design arguments.** The plan says what will be built; why it was chosen lives in the
  decision records, and duplicating it means two documents that disagree later.
- **No aspiration.** Everything in the plan is work someone will do. Nice-to-haves belong in the
  scope table's Optional bucket.
- **The plan is a living artefact.** It is amended during implementation, not preserved as a
  historical curiosity and quietly ignored.
- Cite the implementation baseline: repository/input revisions, relevant working-tree changes,
  compiler/runtime and dependency/contract versions. Preserve accepted compatibility constraints;
  planning does not authorize upgrades. Planned evidence is not an observed test result.

## Keeping it true

Amend the plan whenever any of these happens, and say so in the execution log:

| Event                                    | Amendment                                                                          |
| ---------------------------------------- | ---------------------------------------------------------------------------------- |
| A resource turns out to need another one | Trace to accepted scope or resolve a scope change, then add RES-* and dependencies |
| A resource turns out to be unnecessary   | Mark it cancelled with the reason; do not delete                                   |
| A decision is superseded                 | Update affected `RES-*`/`CT-*`/`TC-*` and the record pointer                       |
| A file turns out not to need changing    | Correct the file list                                                              |
| Implementation reveals a risk            | Add it to the register and, if it needs work, a resource                           |

Keep a current authoritative plan with dated revision-impact entries and preserved history.
Readers must not reconstruct the executable state by mentally applying every old amendment.
Synchronize affected resource links with the existing progress record rather than creating a
competing status source.

## Output

The full section list and the shape of each is in `references/plan-template.md`. The plan is
ready for its declared scope when applicable decisions are resolved, dependencies, paths and
acceptance agree, and planned validation names executable checks and prerequisites. Return draft
status with affected blockers when that cannot yet be established; omit irrelevant template sections.