| name | forgecad-design-spec |
| description | Create a ForgeCAD design brief, HLD, or LLD before coding by walking through use, assembly, interfaces, decisions, and verification. |
| forgecad-public | true |
Design Spec
The design document — a git-committed, diff-reviewed markdown file — is the source of truth. Not the code, not the chat, not your head. You iterate it by commit-and-review, and the git diff is the review artifact.
Validate the design by mentally operating the thing, step by step. Walk it as someone assembles, uses, or moves it. The step with no answer — "how does the servo get inside the housing?" — is the real design gap. A spec that reads complete on the page can still hide a hole that only surfaces when you try to put it together in your head. State each gap falsifiably: not "tolerances might be tight" but "the 12mm arm cantilevers under gripping load, may flex >0.5mm."
Every number has a reason; the narrative comes before the numbers. Describe the object as if over the phone, then derive each value and show its math: wallThickness = 2.4mm = 6 × 0.4mm nozzle. The design is implementation-blind — shaped by the object, never by what the ForgeCAD API makes easy. Manufacturing process is one of those reasons — a design decision you weigh, never a default you inherit (never assume FDM/printing).
A vague request is a set of decisions you make honestly, not information to extract. No placeholders ("appropriate motor"); choose a defensible value, show why, continue. The Decisions table fills only after user review, so the loop stays in the document.
Altitude — three phases, one document trail
| Phase | When | Output |
|---|
| Intake | request is fuzzy / process unspecified | engineering brief + master prompt |
| HLD | design is wrong in approach, alternatives exist | <name>-hld.md |
| LLD | decisions locked, or a simple single-body part | <name>-lld.md |
The HLD carries only decision-driving dimensions and genuinely-different alternatives; the LLD carries enough that someone builds from it alone. Speccing every tolerance in an HLD, or revisiting locked decisions in an LLD, is an altitude error — back up. Simple parts skip straight from HLD to code, or from a request to an LLD.
Phase 1 — Intake (fuzzy request → concrete brief)
Use when the user wants something physically real but the ask is vague ("make me a robot gripper", "make it production ready", "pick sensible numbers"). This phase owns intake; once the brief is concrete, continue to HLD or hand off to the forgecad skill.
Manufacturing is a design decision, not a default. Derive the process stack from artifact family, load path, scale, safety expectations, material, production intent, and operating story — never assume printing/plastic. If the user names a process, honor it but warn when it is unsafe or dishonest for the duty. Family→process anchors live in references/default-profiles.md.
Default posture: manufacture-realistic prototype — real materials, purchased-part boundaries, assembly logic, validation; no claims of production tooling or certification. Other postures only when justified: production-realistic, printable, visual-CAD, or a specific process posture (sheet-metal, CNC-machined, laser-cut, welded-tube, injection-molded, cast, hybrid purchased-hardware). Pick the posture honest for the artifact, not the easiest CAD surface.
Family-scoped numbers. Every starter assumption is scoped to one artifact family; never reuse numbers across families.
Workflow:
- Normalize the ask into plain mechanism language ("6 DOF gripper" → standalone gripper, wrist+gripper, or arm+gripper).
- Build a specific operating story — invented (non-famous) org, named program, prototype revision, review moment, mission pressure (pilot gate, demo date, investor milestone), and the generic failure mode to avoid. Prefer bold high-agency stories over modest lab exercises. Never assert the user works for a named real company; use real products only as public comparison anchors; never clone proprietary designs.
- Classify the artifact family (
references/default-profiles.md); use the no-family-fits escape rather than forcing one. Rideables route to human-vehicles, never chassis.
- Choose the process posture per the taxonomy above.
- Pick qualitative levers — duty (
light/general/sturdy), scale (compact/medium/large), cost (cheapest/balanced/performance-first) — and translate to family-scoped starter assumptions.
- Close only critical gaps — at most 3 grouped questions, always choice menus, never raw engineering inputs unless the architecture truly depends on them. Good: "light desk demo, useful hobby tool, or sturdier bench mechanism?" Bad: "What payload mass?"
- Write the engineering brief: artifact + family + normalized interpretation; operating story + production reason + test setting + failure mode to avoid; output posture; intended loads, size envelope, motion/DOF; process stack + material defaults; purchased-part (BOM) boundary; validation standard; variant policy (versions are selectable params, one rendered at a time); file organization (
main.forge.js entry for multi-file); explicit uncertainty policy.
- Emit one master prompt — fill
references/master-prompt.md; return the finished prompt, not notes about it. It must demand exactly BUILD-READY or BEST-EFFORT BUILD CANDIDATE (human-bearing furniture and rideables usually end the latter).
Defaults if the user stays vague: general-duty / medium / balanced, invent the operating story, use family starter assumptions.
Phase 2 — High-Level Design (HLD)
Aligns user and agent on what to build before how. Brevity is a readability tool, not a metric — include whatever evidence, diagrams, and dimensions a good decision needs. Write the sections top to bottom; the order is the workflow.
# [Name] — High-Level Design
## Problem
What must this do? Hard requirements (grip 40-90mm objects, fit a 60mm
housing, use purchased bearings). State the problem without implying a
solution. Unspecified process choice is an open design dimension.
## Approach
How it works conceptually. ASCII diagram of key elements and their
spatial relationships — diagram labels stay in this markdown, never
carried into CAD geometry unless the real artifact needs markings.
## Key Interfaces
Every point where this touches another part or the outside world:
mating surfaces, shared dimensions, coordination points. These are the
contracts that constrain the design.
## Dictionary
| Term | What it is |
Define every domain term in plain words, with dimensions where relevant.
Write for a developer without a mechanical-engineering background.
## Alternatives
| Option | Description | Tradeoff |
2-3 genuinely different strategies, not minor variations. Mark one
recommended and say why. If there is honestly one approach, say so.
## Usage Guide
Work backwards from how someone uses, assembles, or operates the thing,
step by step. If a step doesn't make sense ("how does the servo get
inside?"), flag it inline with ⚠️ and promote it to Concerns.
## Concerns
1. Numbered, falsifiably specific — a reviewer must be able to say "real
problem" or "fine, because…".
## Decisions
| # | Decision | Rationale |
Filled ONLY after user review — never pre-decide. Each row resolves a
concern or alternative.
Rules: if you're speccing every part, formula, and tolerance, you're writing an LLD — back up. If you can't draw it, you don't understand it yet.
Phase 3 — Low-Level Design (LLD)
Implements the HLD's locked Decisions table; it never revisits those decisions. Simple single-body parts skip the HLD and start here. Complex assemblies split into a numbered directory: overview, global constraints, per-component files, assembly, verification.
An LLD is narrative-first (reads like describing the object over the phone), authoritative (the single source code implements), implementation-blind, and shows every number's rationale.
Required structure:
- Narrative — what it is, how it behaves and interacts, why it exists. Concrete comparisons ("about the size of a deck of cards"); no ungrounded vague terms.
- Technical — typed parameter table (length / angle / count / boolean / choice / ratio / clearance — design-document vocabulary, not the runtime
Param.* API), always with units (mm, degrees default) and a rationale for every default and range; derived dimensions shown as math; geometry and constraints, each constraint with a rationale.
- Verification — mandatory checklist: dimensional, functional, printability/process checks.
Don'ts: never open with a parameter list (story before numbers), never leave a constraint implicit, never skip verification. Completeness gate before presenting: can someone build from this alone? Does it implement every HLD decision? Is every constraint explicit with a rationale?
Review via git
HLDs and LLDs iterate through git, not conversation:
- Commit every version. No drafts floating in chat. After writing, commit and tell the user it's ready for review.
- Feedback arrives as file edits (inline comments, strikethroughs) or chat — check both. Read
git diff: the diff is the review artifact.
- Update, commit, repeat until the Decisions table is filled and the user says "go."
Pipeline
| Stage | This skill's phase | Output | Next |
|---|
| Explore a fuzzy ask | Intake | engineering brief + master prompt | HLD |
| Decide what to build | HLD | *-hld.md (Decisions filled) | LLD |
| Detail how to build | LLD | *-lld.md | forgecad-build-model + forgecad → .forge.js |
