rhino-mcp

name	rhino-mcp
description	Drive a live Rhino 8 / Grasshopper from the session via the Rhino MCP tools — build or regenerate a .ghx fixture, place and wire components, read live param data, and debug Selva.gha plugin behaviour. Use whenever the task involves building/rebuilding a Grasshopper fixture, inspecting what a component actually outputs at solve time, reproducing a plugin bug in Rhino, or any mcp__rhino__* tool.

Rhino MCP

Verified against Rhino 8.33, Grasshopper 1.0.0008, Rhino MCP Platform 0.1.5 (2026-07-31).

Check first

Tools are mcp__rhino__*. If absent, they cannot be added mid-session: MCP servers are fixed at session start. Tell the user to run MCPConnect in Rhino and start a new session — don't try to work around it.

If the plugin itself isn't loading (MCPStart/MCPConnect don't exist as Rhino commands): Rhino 8 blocks unsigned .NET plugins and RhinoMcpPlatform.rhp isn't Authenticode-signed. The block must be lifted in Rhino; a clean yak install hits the same wall.

Install (once)

Rhino MCP Platform (McNeel, MIT). Rhino 8 packages are per-user and yak install only resolves names from a package server, so a local .yak is extracted by hand:

# from a plugin-vX.Y.Z release, asset *-rh8_29-win.yak
Expand-Archive rhino-mcp-platform-0.1.5-rh8_29-win.yak -DestinationPath `
  "$env:APPDATA\McNeel\Rhinoceros\packages\8.0\Rhino-MCP-Platform\0.1.5"

Restart Rhino. It loads as RhinoMcpPlatform.rhp, independent of Selva.gha in Libraries-8/, and its transport is a router process over a filesystem slot directory — no contention with Selva's WebSocket on 8765.

The plugin is pre-1.0 (0.1.5 stable, 0.2.1-wip at tip); tool names are not yet stable.

Two tools are broken — don't use them

g1_apply_graph and g1_connect error and change nothing, even for one stock component with zero wires. They fail before touching the document, so a failed call leaves the canvas clean.

Build graphs with run_csharp instead. Ready-to-paste C# for every case below — place-and-wire, save, graft a schema, enumerate components, reset, API discovery — is in reference.csx next to this file. Read it when you need the code; the traps that decide whether the code works are here.

Working tools: g1_start, g1_place_component (resolves third-party Guids fine), g1_describe_component, g1_search_components, g1_get_canvas_graph, g1_clear_canvas (needs confirm=true), run_csharp.

Method

Verify against g1_get_canvas_graph, never against a tool's return value. It returns per-object Messages (balloon warnings/errors) and per-param DataSummary {Branches, Items, Sample[]} — that is ground truth and what makes a debugging walk assertable instead of eyeballed. Read back after a solve; placement runs on Grasshopper's UI thread and an immediate read can miss it.

run_csharp has no implicit using System — fully qualify or declare usings. Its System.Windows.Forms CS1701 warning is harmless. When a call won't compile, reflect over the type rather than guessing at method names ("Discover an API" in reference.csx).

Selva components

All visible — 61 proxies across Display, Drawing, Elements, IO, UI, Utilities, with full param metadata. Two caveats from the OBSOLETE + upgrader procedure:

• 17 are obsolete and share their live component's name (Display has 6 under one name, Create File and Geometry To File 3 each, UI Bridge 2). g1_describe_component and g1_search_components match by name, so both can hand back a retired variant. Pin by Guid.
• Searching "Selva" does not list Selva components — g1_search_components matches Description too, so it returns whatever mentions "the Selva UI" in prose. Filter ObjectProxies by Desc.Category in C# instead.

Component traps:

• UI Bridge self-scaffolds. Placing 593bc967-… alone creates a pre-wired Boolean Toggle and Context Bake — 3 objects, 2 wires, from one placement. Also placing those two yields 5 objects and duplicate wires into Enable. For a bare placement wrap AddObject in GH_UIBuilderComponent.SuppressAutoWire() (public since 0.16.1; the flag is [ThreadStatic]).
• Toggling Enable starts the WebSocket server on 8765 and opens the web UI. Set it back to false and re-solve when done, or the server outlives the graph.
• Context Bake's input is named Content, nicknamed Schema. Wire it by index 0.
• Params only exist for the schema if a real component carries them. SchemaSynchronizer.ValidateSchema resolves every id/paramId against GH_Document.FindObject and silently purges what it can't find — a schema of invented Guids validates down to inputs=0, outputs=0, keeping only its name. Inputs need a real upstream object (a slider, a getter component); outputs are only discovered through Context Bake / Context Print — a bare output Guid is never picked up. Place the components first, then build the schema from their live InstanceGuids.

Fixtures

GH_Document.SaveAs does not exist in this build and save_doc is the Rhino document. Use GH_Archive: set doc.FilePath first (or Name writes empty), AppendObject(doc, "Definition"), then add a CreateTopLevelNode("Thumbnail") chunk — Grasshopper's own save writes two root chunks, and omitting the thumbnail leaves one.

A rebuild silently drops authored state unless you put it back. Fresh InstanceGuids and timestamps are expected. What is not: a freshly-placed component has no persisted data. A rebuilt UI Bridge reads "Offline • No Schema" and its Container chunk is missing the Schema item — the graph is right, the content is empty. Nicknames go too (UI Bridge reverts to UIBridge).

Before overwriting any committed fixture, diff for dropped items:

git show HEAD:path/to/fixture.ghx > /tmp/old.ghx
diff <(grep -oE '<item name="[^"]+"' /tmp/old.ghx | sort -u) \
     <(grep -oE '<item name="[^"]+"' path/to/fixture.ghx | sort -u)

A missing <item name="Schema" means the fixture is degraded. Don't commit it — graft the schema back (next section), or restore from git.

fixture_dynamic_value_list.ghx additionally needs 8 C# Script bodies and a locked group, which have no equivalent graft path — hand-edit that one.

Authoring the embedded UI schema

GH_UIBuilderComponent.Schema is a public property (Selva >= 0.16.1) over the _embeddedSchema field that Read() populates; the setter expires the solution for you. Assigning it persists through Write() and survives a reload — including correct polymorphic layout-item types. On older builds, write the private _embeddedSchema field instead; both persist identically.

You still reach the property by reflection from run_csharp — the script sandbox can't reference Selva.Schema, so UISchema isn't nameable there even though the property is public.

This makes fixtures with arbitrary UI schemas buildable: typed inputs with defaults, outputs, tabs, groups, widget config. Verified round-tripping InputNumberLayoutItem + InputTextLayoutItem through save→reload. Snippet: "Graft an embedded UI schema" in reference.csx.

Validate before grafting — do not hand-check the shape.

node .claude/skills/rhino-mcp/validate-ui-schema.mjs <file.json>

packages/schemas/ui-schema.json is a real draft-07 JSON Schema rooted at UISchema, authoritative for both stacks. Every trap below is caught by it and by nothing else at authoring time — Newtonsoft ignores unknown keys and binds enums case-insensitively, so a wrong payload deserializes without complaint and quietly loses content:

• Nesting is layout.tabs[] → groups[] → items[]. A tab has no items — putting them there yields items=0 with no error.
• Layout items key on paramId, not inputId, and the discriminator is two fields: type: "input" plus widgetType: "number" — not a single type: "input-number".
• paramType is lowercase: number, integer, boolean, text, valueList, dynamicValueList, file, color, generic. "Number" binds fine and is still wrong.

Required: TabConfig needs id/label/groups, GroupConfig needs id/label/items, LayoutItemBase needs id/paramId, SchemaInput needs id/nickname/paramType.

For semantic checks beyond shape — orphaned params, missing widget config, version drift — the plugin's own Selva.Schema.Services.Validation.SchemaValidator runs six rule classes over a live UISchema and is reachable from run_csharp. Use the JSON Schema on a payload before it goes in; use SchemaValidator on a schema already grafted onto a component.

Set documentId to the live doc.DocumentID, not the value copied from another fixture.

Verify by reloading, never by assuming the save worked:

back.ExtractObject(fresh, "Definition");   // then walk tabs → groups → items

The [Selva Warning] Could not write schema history: Method not found: JToken.ToString on save is a Newtonsoft mismatch in the MCP script sandbox. It affects only the optional history backup — the Schema item itself is written correctly. Confirm with the diff above rather than trusting it.