Inside the Skill Library: How chatCAD Achieves 90%+ Accuracy

When we first built chatCAD using a raw GPT-4 call, accuracy on standard parts was around 60%. The model knew about hex bolts but didn't reliably produce the right ISO tolerances, thread pitches, or chamfer conventions. Every second or third model needed manual correction.

The Skill Library fixed that.

What is a Skill?

A Skill is a structured Markdown document that teaches the agent a specific CAD domain. Each skill contains:

Geometry conventions — exact formulas for derived dimensions (e.g., hex nut height = 0.8 × pitch diameter)

Standard tables — DIN/ISO/ANSI values for every common size variant

FreeCAD-specific pitfalls — e.g., how to correctly set ScaleType = "Custom" before modifying a TechDraw view scale

build() function template — a parametric Python skeleton the agent fills in

Here's a simplified excerpt from the hex-fasteners skill:

def build(params):
    """
    params = {
        "nominal_diameter": 8,   # M8
        "pitch": 1.25,           # mm
        "length": 30,            # mm
        "standard": "DIN_933",   # full thread
    }
    """
    d = params["nominal_diameter"]
    s = 1.5 * d          # wrench size (across flats)
    e = s / cos(30°)     # circumscribed circle
    k = 0.63 * d         # head height
    # ... FreeCAD Part API calls

How the agent uses Skills

At inference time the LangGraph skill_loader node:

1. Embeds the user's description

2. Finds the top-3 most relevant skills by cosine similarity 3. Appends the full skill text to the system prompt

This gives the LLM exact domain knowledge without fine-tuning. Because the knowledge lives in plain Markdown, updating a tolerance table or adding a new standard takes minutes, not a retraining run.

Results

After adding 24 skills covering the most common part categories:

CategoryBefore SkillsAfter Skills --------- Standard fasteners58%96% Rotational parts61%91% Sheet metal44%78% Assemblies30%72%

The remaining errors are almost all edge cases at the boundary of standard ranges (very large diameters, non-standard materials, exotic thread forms).

What we're building next

We're expanding the Skill Library to cover springs, O-ring grooves, keyways, and press-fit tolerances in the next release. If you have a part category where accuracy is lower than you'd expect, tell us — we'll add a skill for it.