Design Spec

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

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:

1. Normalize the ask into plain mechanism language ("6 DOF gripper" → standalone gripper, wrist+gripper, or arm+gripper).
2. 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.
3. 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.
4. Choose the process posture per the taxonomy above.
5. Pick qualitative levers — duty (light/general/sturdy), scale (compact/medium/large), cost (cheapest/balanced/performance-first) — and translate to family-scoped starter assumptions.
6. 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?"
7. 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.