forgecad-lld

name	forgecad-lld
description	Write a Low-Level Design (LLD) for a CAD model — exact dimensions, constraints, parameters, and verification criteria. Use after a High-Level Design (HLD) exists and decisions are locked, or for simple parts that don't need an HLD. The detailed design document that code implements.
forgecad-public	true

Low-Level Design (LLD)

Create detailed, authoritative design documents for ForgeCAD models. This is the construction document — exact dimensions, constraints, parameters, and verification criteria.

Prerequisites

Before writing an LLD, a High-Level Design (/forgecad-high-level-spec) should exist with its Decisions table filled. The HLD defines what to build and why. The LLD defines exactly how.

For simple parts (single body, no alternatives to explore), skip the HLD and write the LLD directly.

Philosophy

An LLD should read like you're describing the object to someone over the phone — vivid, narrative, conveying shape, behavior, purpose, and intent. But it must also be authoritative — the single source of truth that code will implement.

Key principles:

• Story-first: Lead with the "what it is" narrative before technical details
• Sensory richness: Describe appearance, proportions, materials, feel
• Behavioral clarity: How it functions, moves, interacts
• Technical precision: Exact constraints, dimensions, and relationships
• No implementation assumptions: The LLD knows nothing about ForgeCAD's capabilities

Output Location

LLDs go next to the model files. Name: <name>-lld.md. For complex assemblies, use a directory:

<project>-lld/
├── 00-overview.md
├── 01-global-constraints.md
├── 02-components/
│   ├── base.md
│   └── bracket.md
├── 03-assembly.md
└── 04-verification.md

Document Template

1. Narrative Section

Lead with vivid description — what it is, purpose, visual character, how it behaves.

2. Technical Section

Parameters table, derived dimensions, geometry description, constraints with rationale.

3. Verification

Dimensional checks, functional checks, printability checks — as a checklist.

(See examples in the sections below.)

Workflow

Phase 1: Verify HLD decisions are locked

If an HLD exists, check that its Decisions table is filled. The LLD implements the decisions — it doesn't revisit them.

Phase 2: Write the document

1. Capture the narrative (what it is, how it works)
2. Extract parameters and constraints
3. Define verification criteria

Phase 3: Commit and present for review

Commit the LLD to git. Tell the user it's ready for review.

Phase 4: Iterate via git

Same as HLD: user reviews the file (edits or verbal feedback), agent reads the diff, updates, commits, repeats.

Agent writes LLD → git commit → User reviews
→ Agent reads diff → updates LLD → git commit → repeat until approved

Phase 5: Review for completeness

• Can someone build from this alone?
• Does it implement every HLD decision?
• Are all constraints explicit with rationale?

Git Workflow

LLDs iterate through git, not conversation. The document is the single source of truth.

• Every version gets committed. No unsaved drafts in conversation.
• User feedback goes in the file (inline comments, strikethroughs) or in chat — agent checks both.
• The diff is the review artifact.

Writing Guidelines

Do:

• Use concrete comparisons ("about the size of a deck of cards")
• Show the math: wallThickness = 2.4mm
• Provide rationale for every constraint
• Start with the story, then the numbers

Don't:

• Start with a parameter list
• Leave constraints implicit
• Skip verification criteria
• Use vague terms without grounding

Parameter Types

• length — mm default
• angle — degrees default
• count — integer
• boolean — true/false
• choice — enumerated options
• ratio — dimensionless
• clearance — fit tolerances, mm default

Always specify units. Always provide rationale for defaults and ranges.

Relationship to Other Skills

Stage	Skill	Output
1. Explore the problem space	/forgecad-high-level-spec	*-hld.md
2. Detailed design	/forgecad-lld (this skill)	*-lld.md
3. Implementation	/forgecad-make-a-model + /forgecad	.forge.js files