Blender Geometry Nodes (procedural systems and simulations)

Expert Geometry Nodes work means thinking in fields evaluated on domains, keeping systems declarative (data and instructions come in from outside), and shipping tools that artists drive from modifier inputs without opening the tree. For an agent it also means proving every tree with numbers read from the evaluated geometry and a render, because the node editor's red links and warnings are invisible from Python unless you ask for them. If a sibling skill named here is missing from your available skills, ask the user to install it (npx skills add scenario-labs/skills --skill <name>); unattended, proceed from tool schemas and flag the gap.

REQUIRED BACKGROUND: scenario-blender-expert (execution channel, review loop, 5.2 API changes). Hair node groups (Interpolate, Clump, Curl, Hair Dynamics grooms): scenario-blender-hair.

Stance (the expert delta)

• A field is a deferred question, not data. Thommes: it is evaluated by the node that consumes it, on that node's domain and geometry; values exist only once named or consumed. Consequence [added, verified]: two coupled updates (position and velocity) must be captured together in one Capture Attribute, or the second reads the first's new value.
• Domain changes interpolate; everything becomes a Boolean by a rule. Erindale: float > 0 is true (vectors: any non-zero), so a subdivided vertex-group selection bleeds; decide with Float to Integer or Compare with epsilon. Prefer trait selections (position, normal, proximity, raycast) over index ones, unless you generated the geometry and its order is the design (the Blender Studio matrix video drives a stack from Index == 0).
• Simulation only when history matters. Thommes: a simulation is rules applied to state; an animation is a function of time. If nothing depends on the previous frame, drive it from Scene Time and get scrubbing and sub-frames free.
• Loops are a last resort. Thommes: built-in nodes working on all elements use acceleration structures, loops cannot. For-each is for black-box generators run once per element: field outside, single value inside.
• Everything is object-local. Thommes: a (0, 0, -1) vector rotates with the object. World directions go through Self Object > Object Info > Invert Matrix > Transform Direction.
• Tools, not trees. Thommes: expose every scene dependency and tweakable value as a group input, never hard-reference an object inside a reusable tree, and drive gizmos only from Group Input sockets, through invertible math, into the first input of each math node.
• 5.2 physics is declarative. Thommes (BCON26): forces, colliders and constraints travel as bundles and closures into a black-box solver. Customize from outside first (inputs, bundles, Custom Effector closures); make the asset local only to edit a built-in constraint. Any per-step custom attribute must be listed in Extra Sim Attributes, and topology-changing sims must append new elements at the end.

Establish first