Geometry Nodes

The execution model is in blender-core. Build trees with scripts/gn_builder.py, a compact JSON spec that's far cheaper than raw bpy. Look up exact socket names first, because they change between versions:

scripts/gn_builder.py -- --describe GeometryNodeDistributePointsOnFaces GeometryNodeInstanceOnPoints
scripts/gn_builder.py -- --spec scatter.json --apply Ground --set Density=20 Seed=3 Mask=@paint_mask

--set Name=@attr feeds a modifier input from a named attribute. The result reports evaluated vert/face counts and node warnings. The spec format is in the script docstring; full examples are in references/recipes.md.

Core concepts (get these right or nothing works)

• Geometry flows left→right through sockets with circle shapes. Fields (diamond sockets) are functions evaluated per element on the node that consumes them, in that node's domain/context.
• Domains: Point, Edge, Face, Face Corner, Spline, Instance, Layer. Capture Attribute freezes a field at a point in the graph (before topology changes), and Evaluate on Domain converts between domains.
• Attributes: built-ins (position, normal, sharp_face, material_index, UVMap...) plus named ones via Store Named Attribute. Named attributes persist after the modifier. That's how GN data reaches shaders (Attribute node) and exporters (glTF export_attributes, which exports names starting with _).
• Instances are cheap (one mesh, many transforms). Realize Instances only when you must edit per-instance geometry or export a single mesh. Realize as late as possible.
• Inputs exposed on the group interface become modifier inputs (5.2: mod.properties.inputs.<identifier>.value). Panels group them, menu sockets give enums.

Zones and advanced types

Feature	Nodes	Use
Simulation zone	GeometryNodeSimulationInput/Output (paired)	per-frame state: growth, particles, erosion; bake to keep
Repeat zone	GeometryNodeRepeatInput/Output	iterate N times (L-systems, relaxation, recursive subdivision)
For Each Element	GeometryNodeForeachGeometryElementInput/Output	run a sub-graph per element (per building lot, per island)
Closures / Bundles (5.0+)	NodeClosureInput/Output, NodeCombineBundle/NodeSeparateBundle, Evaluate Closure	pass functions/structured data around; reusable "strategy" node groups
Lists (5.2)	GeometryNodeFieldToList, ListGetItem, ListLength, SortList, FilterList	real arrays, not fields
Grids/SDF (4.x–5.x)	MeshToSDFGrid, SDFGridBoolean, SDFGridFillet, SDFGridOffset, GridToMesh	smooth booleans, remeshing, fillets, voxel effects
Physics (5.2, experimental)	GeometryNodeXPBDSolver + bundles	node-based cloth/hair, see blender-simulation
Gizmos	GizmoLinear/Dial/Transform	viewport handles for inputs
Zones need input.pair_with_output(output) (spec key pairs), plus items on the output node (repeat_items, state_items, ...; spec 4th node field).

Procedure

1. Sketch the data flow: source geometry → selection/masks (fields) → generate/instance → transform/randomize → realize/merge → set materials/shading → output.
2. --describe every node type you'll use, then write the spec and build. Check node_warnings and the evaluated counts.
3. Expose only meaningful inputs (density, seed, scale range, materials), with min/max set.
4. Look at it (look), check blender-asset-qa if the result will be exported (realized geometry), and check blender-game-optimization for instance counts and tris.
5. Ship: either keep it procedural (the .blend/asset library), or apply the modifier / visual_geometry_to_objects for exporters.

Rules

• Randomness: always expose a Seed, and use Random Value with the ID/Index field so results stay stable when counts change.
• Scale-independent setups use Object Info (Relative) and real units. Apply the object scale or read it explicitly.
• Materials: Set Material inside the tree. The material must exist in the file, and instances keep their own materials.
• Shading: GN output loses auto-smooth unless you add Set Shade Smooth + (5.x) the Smooth by Angle essentials group, or store sharp_edge.
• UVs: primitives output UVMap via their UV output. Store it with Store Named Attribute (Face Corner, 2D Vector, name UVMap) or it's dropped.
• Performance: avoid realizing millions of instances, prefer Points + instances for scatter, use Merge by Distance sparingly, and simulation zones need baking for playback/export (object.geometry_node_bake_single or modifier bakes).
• Node tools (ng.is_tool = True, is_mode_edit etc.) turn a group into an operator in the 3D View menus, for editing workflows.

Gotchas

• 5.2: mod["Socket_2"] no longer works. Use getattr(mod.properties.inputs, ident).value, and .type = "ATTRIBUTE" + .attribute_name for attribute inputs. Compare and Random Value sockets changed: Random Value now has single dynamic Min/Max/Value sockets typed by data_type. Link by name, and --describe when unsure.
• Sockets that are disabled for the current mode/data type are still in node.inputs with the same name. The builder picks the enabled one. Set data_type/mode props first (the builder sets props before links).
• nt.interface.new_socket(...) is the 4.0+ API (ng.inputs.new is gone).
• Exporters see the evaluated mesh. Instances become glTF instancing (export_gpu_instances) or are realized on export.