Shipping a CAD Agent from Zero: DXF Parsing, Edit Engines, and LLM Planner Interfaces

The Problem A structural engineer named Tony needs to edit CAD drawings. His workflow: open a DXF file, move some entities, update text labels, add block references, save to a new file. He does this hundreds of times. The edits are mechanical and repetitive. The goal: let Tony type "move the column east by 2 feet" and have the software do it. Local-first, no cloud dependency for the core workflow, and never modify the original file. Architecture: Prompt to DXF The pipeline is five steps: User Prompt → LLM Planner → ChangeSet (JSON) → Validator → EditEngine → Save-As DXF The LLM never touches raw DXF. It receives a structured context about the drawing (entities, layers, blocks) and returns a structured changeset (a list of operations). The validator checks every operation against safety rules. The edit engine applies validated operations to an in-memory copy. The original file is never modified. The Planner Provider Interface The most important architectural decision was abstracting the