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Upgrade GH1 to GH2

Upgrade Grasshopper components to GH2

Voxelizer GH1 component and its GH2 equivalent solving side by side

Rhino 9 ships Grasshopper 2 alongside Grasshopper 1. If you have a GH1 plugin and you want to try upgrading to GH2, this is a perfect use of an AI agent and the Rhino MCP server. The MCP exposes parallel g1_* and g2_* tool families, so the assistant can place a GH1 component and its new GH2 counterpart on their respective canvases in the same session and compare what they solve to.

We’ll use Voxelizer as the running example: a small, single-component GH1 plugin that’s easy to follow end to end.

ytakzk/Voxelizer

The loop

With both canvases reachable, the assistant can:

  • Read each GH1 component’s inputs, outputs, and solve logic.
  • Scaffold a GH2 equivalent in a new folder, leaving the GH1 sources untouched.
  • Place the GH1 component on a GH1 canvas with sample inputs, place the new GH2 component on a GH2 canvas with the same inputs, and compare outputs through the MCP.
  • Iterate until both solve the same way, then move to the next component.

The side-by-side check matters: GH2’s parameter and data-tree conventions aren’t a one-for-one match with GH1, so “it compiles and places” isn’t enough on its own.

A prompt to start with

Prompt

Add a GH2 port of this plugin in a folder called gh2/ in this repo. The GH1 project must build untouched.

Use McNeel’s official template, not hand-rolled scaffolding: dotnet new install Rhino.Templates dotnet new gh2 –IncludeSample false -n Gh2 -o .gh2

Reference docs while porting: Guide: https://developer.rhino3d.com/guides/grasshopper2/your-first-component-mac/ Template: https://github.com/mcneel/RhinoVisualStudioExtensions/tree/main/Rhino.Templates/content/CSGrasshopper2

Discover, don’t assume:

  • Find the GH1 csproj and list every GH_Component subclass — these are what you port.
  • Identify any helper classes that only depend on RhinoCommon (no Grasshopper.Kernel.*). These should be SHARED, not copied: If there are no such helpers, skip this step.
  • Note each component’s inputs and outputs (types, access mode) — you’ll need this for both the port and the comparison.

Port one component at a time. For each:

  1. Generate or write the GH2 component shell (Component subclass, [IoId], Nomen, AddInputs/AddOutputs/Process). Preserve the original GH1 ComponentGuid by reusing it as the [IoId] value — this keeps any existing GH1 documents that reference it identifiable for future upgrade tooling.
  2. Build (.rhp output).
  3. Validate via rhino-mcp:
    • spawn one Rhino WIP
    • place the GH1 component on the GH1 canvas
    • place the GH2 component on the GH2 canvas
    • feed both the same fixture: minimal, deterministic, picked based on the component’s input types (e.g. a slider for numbers, Mesh Without Normals for meshes, native geometry components for curves/surfaces). If a fixture cannot be expressed natively, commit it as a .3dm under tests/fixtures/ and load via the MCP.
    • solve both, then compare outputs by type: lists/trees: structure (branch + leaf counts) must match exactly numeric: within abs tolerance (default 1e-9) or rel tolerance the agent justifies inline geometry: bbox + element count must match exactly; per-element drift within tolerance is warn, not fail bools/ints: exact equality
    • Mismatch in structure or count = hard fail. Per-element drift within tolerance = warn-only.

Constraints:

  • Don’t modify any file under the GH1 source folder.
  • If anything is unclear before you start, ask. Don’t infer.

What to review

  • Parameter types. GH2’s type system isn’t identical to GH1’s; check that the assistant picked the right GH2 type rather than the closest-named one.
  • Data-tree handling. If your GH1 component does anything non-trivial with branches or paths, look at how that translates. This is the most common place for “solves but wrong” bugs.
  • Component metadata. Names, categories, icons, GUIDs: easy to get wrong, hard to fix later once users have files referencing them.

When the assistant gets stuck

If a component doesn’t have a clean GH2 equivalent (or relies on a GH1 API that’s gone), have the assistant skip it and surface the list of skipped components at the end, rather than trying to fake it. A short “these need human eyes” list is more useful than a stubbed component that silently solves to nothing.

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