impl-new

name	impl-new
disable-model-invocation	true
description	Scaffold a new candidate implementation in the spec-driven `implementations/<version>/<n>-<name>/` tree, to stress-test the spec against a fresh stack.
argument-hint	[slug]

Impl New Skill

You are scaffolding a new candidate implementation in a spec-driven project. The candidate's job is to stress-test the spec — to surface gaps, contradictions, and overly prescriptive requirements by being built against them. Treat it as an instrumented experiment, not a competitor or a draft of "the real thing."

%%{ init: { 'look': 'handDrawn' } }%%
flowchart TD
    Start(["/sextant:impl-new [slug]"]) --> Locate

    subgraph "Step 1: Locate"
        Locate["Find SPEC.md + implementations/ root"] --> Version["Resolve active spec version"]
        Version --> Siblings["List existing impl siblings"]
    end

    subgraph "Step 2: Gather"
        Siblings --> Slug["Resolve slug (arg or asked)"]
        Slug --> Stack["Stack + key libraries"]
        Stack --> UXCheck{"UX work?"}
        UXCheck -->|Yes| Aesthetic["Aesthetic direction"]
        UXCheck -->|No| Constraints
        Aesthetic --> Constraints["Hard constraints + non-goals"]
    end

    subgraph "Step 3: Read"
        Constraints --> ReadSpec["Read SPEC.md"]
        ReadSpec --> LoadBearing["Identify load-bearing requirements for this stack"]
    end

    subgraph "Step 4: Propose"
        LoadBearing --> Plan["Numbered plan: layout, build, first slice, requirement strategy"]
        Plan --> Confirm["User reviews"]
    end

    subgraph "Step 5: Scaffold"
        Confirm --> Mkdir["Create implementations/<v>/<slug>/"]
        Mkdir --> Justfile["Write justfile stub"]
        Justfile --> Status["Seed STATUS.md (all Missing)"]
        Status --> Handoff(["Hand off to user / greenfield recipe"])
    end

Step 1: Locate the spec-driven tree

Find the active spec using the shared discovery order in references/locate-spec.md (the source of truth every sextant skill uses — do not use a bespoke order here). In brief, first hit wins: STATUS.md spec-pointer → spec/ directory (incl. vnext/, exploration/, migration/) → justfile spec variable → CURRENT_SPEC_VERSION → root SPEC.md (or docs/spec.md).

If no SPEC.md is found, stop and ask. Don't scaffold an implementation tree before there's a contract to build against.

Resolve the implementations root from the spec location:

• If spec lives at spec/<v>/SPEC.md, implementations live at implementations/<v>/.
• If spec lives at the root, ask whether to scaffold implementations/v1/ or just implementations/.
• If the project lives inside a subfolder (vnext/spec/<v>/...), all paths are relative to that subfolder.

Once located, list existing sibling implementations (implementations/<v>/*/) so the user and you both know what number this candidate is and what stacks are already covered.

Step 2: Gather inputs conversationally

The goal is to make the user's choices explicit before scaffolding, so the implementation starts with intentional constraints rather than convention defaults.

Slug

The skill's positional argument is the slug. If absent, ask. Naming convention: <n>-<name> where <n> is the next sequence number across existing siblings and <name> is a short, kebab-case label. Examples: 1-python, 2-js, 3-hybrid, 4-rust.

Validate the slug:

• Lowercase, kebab-case.
• Starts with the next sequence number — refuse to scaffold 2-x if 1-y already exists with no 2-* siblings; suggest 3-x (or whatever the next gap is).
• Doesn't collide with an existing sibling.

Stack and key libraries

Ask the user to name:

• Primary language and framework — "Python + FastAPI", "TypeScript + Vite + React", "Go stdlib only", etc.
• Key libraries that shape the architecture — the ones whose choice constrains everything downstream. Database driver, UI framework, state management, HTTP client. Don't ask about every transitive dep — just the load-bearing ones.

If the user can't articulate a stack yet, present the existing siblings and suggest a deliberately different choice. The point of multiple implementations is breadth — don't build a second Python+FastAPI candidate when a JS or Go one would surface different blind spots.

Aesthetic direction (UX work only)

Skip this step for CLI tools, libraries, and headless services — the spec dictates output format and there's no aesthetic to choose.

For UX-bearing work, ask for an explicit aesthetic anchor: "Pico CSS classless," "warm darkroom," "Swiss Neobrutalist," "iOS dark mode." This is the single biggest lever against AI-aesthetic convergence — without a deliberate direction, multiple UI implementations will drift toward the same "generic AI dashboard" look and stop being useful for comparison.

If you can't tell whether the work is UX-bearing, look at SPEC.md for keywords ("page", "render", "click", "view", "panel", "screen") or ask.

Hard constraints and non-goals

Ask for:

• Constraints to honor — must-use libraries (e.g., "the team's existing ORM"), forbidden patterns (e.g., "no global state"), performance budgets (e.g., "render under 100ms"), platform targets.
• Out-of-scope for this candidate — what this implementation deliberately won't cover. The candidate doesn't have to address every requirement — sometimes the point is to test whether a subset feels right.

These are the spec deltas for this implementation. Capture them — they shape the plan in Step 4 and the seeded STATUS.md in Step 5.

Step 3: Read SPEC.md

Read the full SPEC.md (or just the active version if multi-versioned). Build a working model of:

• Categories and requirement counts — CM: 4, OP: 7, etc.
• The core contract — the one or two ubiquitous requirements that anchor everything else (usually the first few entries; sometimes called out explicitly).
• Load-bearing requirements for this stack — requirements that the chosen stack will need explicit work to satisfy, versus requirements that fall out naturally. Example: in a JS+React implementation, [OP-3] system shall render under 100ms needs explicit work (memoization, virtualization); in a Go+stdlib HTTP server, the same requirement falls out of the stack.

This identification is the key value-add over the user just running mkdir. The plan in Step 4 builds on it.

Step 4: Propose a plan

Present a numbered plan, ~6-10 items, that the user can review and edit before any files are touched. Cover:

1. Directory layout — what subdirs and entry-point files this implementation needs given the stack (e.g., src/, tests/, package.json + tsconfig.json for TS; pyproject.toml + src/<name>/ for Python; etc.).
2. Build / run / test commands — what just build, just run, just test will invoke. Each implementation owns its own justfile.
3. First slice to implement — the core contract from SPEC.md, end-to-end. Resist the urge to scaffold ten requirements at once; the first slice should exercise the one invariant that the candidate's main job is to verify.
4. Requirement coverage strategy — for each category, which requirements likely fall out of the stack vs. need explicit work. Reference IDs explicitly.
5. Out-of-scope acknowledgments — restate what the user said this candidate deliberately won't cover, with the relevant requirement IDs (e.g., FUT-1, FUT-3 deferred per user direction).
6. Anti-port reminder — do not copy from sibling implementations. If you need to look at one for inspiration, stop and consult the spec instead. The candidate's value comes from building from the spec, not from porting.

Present the plan, then wait for sign-off. If the user edits the plan, redo the relevant sections. If the user changes stack or constraints, loop back to Step 2 and gather again.

Step 5: Scaffold

On sign-off, create:

implementations/<v>/<slug>/
├── justfile         # build / run / test stubs
├── STATUS.md        # requirement coverage, all 'Missing' by default
└── (other entry-point files per Step 4's plan)

justfile

Minimal stub — the user fills in the body during implementation. Three recipes at a minimum: build, run, test. Include a header comment naming the slug and the spec version it's built against:

# implementations/<v>/<slug>/justfile
# Spec version: <v>
# Stack: <stack>

default:
    @just --list

build:
    # TODO

run:
    # TODO

test:
    # TODO

STATUS.md

Seed with every non-FUT requirement ID from SPEC.md, each marked Missing (nothing is built yet — see the shared status vocabulary below). The user (and /sextant:spec-status) will flip entries to Covered / Partial as implementation progresses.

# STATUS — <slug>

**Spec:** spec/<v>/SPEC.md
**Stack:** <stack from Step 2>
**Constraints:** <constraints from Step 2>
**Out of scope:** <FUT IDs and any user-declared deferrals from Step 2>
**Last audit:** <today>
**Coverage:** 0 Covered, 0 Partial, N Missing/Contradicts<, plus K deferred (FUT-…)>

## Coverage

| ID    | Requirement (abbreviated) | Status  | Location |
|-------|---------------------------|---------|----------|
| CM-01 | <text>                    | Missing | —        |
| CM-02 | <text>                    | Missing | —        |
| ...   | ...                       | ...     | ...      |

Use the shared status vocabulary — Covered / Partial / Missing / Contradicts (capitalized), the same terms spec-status and spec-sync classify with, so a later ledger refresh reads these rows without a vocabulary mismatch. A fresh candidate seeds every row Missing.

Use abbreviated requirement text in the table (first ~10-12 words) — full text lives in SPEC.md, the table is for tracking, not for re-reading. This per-implementation shape (candidate metadata block + per-requirement table) is a first-class variant that /sextant:spec-status refreshes in place — it updates the **Last audit:**/**Coverage:** lines and each row's Status/Location while preserving the Stack/Constraints/Out-of-scope block.

Hand off

Report back to the user:

Scaffolded implementations/<v>/<slug>/:
  → justfile stub (build/run/test)
  → STATUS.md with N Missing requirements seeded from spec/<v>/SPEC.md

Next: start implementing the first slice — the core contract. See your plan, item 3.
Start a greenfield dev session for the implementation phase (e.g. /recipe greenfield, if you use the recipe plugin).

Do not start implementing yourself. The skill's job is to scaffold and frame the work; the user (or a fresh greenfield session) drives the build.

Anti-Patterns

• Scaffolding without a spec — if SPEC.md doesn't exist or doesn't have requirement IDs, stop and ask the user to draft one first (/sextant:spec-req init bootstraps a fresh spec; /sextant:spec-req new adds requirements). An implementation without a spec to verify against isn't a candidate, it's just code.
• Skipping the conversational gather — slug + "default Python project" is not enough. The plan in Step 4 depends on stack + constraints; jumping straight to scaffolding produces a generic skeleton that gets thrown away the moment the user tries to add their actual stack choices.
• Porting from siblings — if existing implementations have files, do not copy them. The candidate's job is to build from the spec, not to recapitulate prior work. If a sibling solved an interesting problem worth referencing, note it in the plan for the user — don't pre-seed code from it.
• Filling STATUS.md with optimistic guesses — every requirement starts Missing. Resist marking entries Covered based on stack convention ("React will handle accessibility") — let /sextant:spec-sync verify after the implementation exists.
• Numbering off-by-one — 1-python already exists, user asks for 1-rust. Refuse and propose the next available number, or ask whether to retire 1-python first.

Related

• /sextant:spec-req — useful before scaffolding if SPEC.md is missing requirement IDs or needs a new requirement captured first.
• /sextant:spec-sync — run after the first slice to verify the candidate is on track. The seeded STATUS.md is the input its analysis reads against.
• /sextant:impl-select — the inverse operation. When one candidate has earned the right to be the only one, impl-select retires the others and flattens the winner to the repo root.