Build scenes with Crucible over MCP

You drive Crucible through the scene_* tools of the crucible-mcp server. No checkout is needed. Start the server with npx --package @codepawlhq/crucible crucible-mcp --backend gpu --assets <abs dir> --jobs <abs dir> (--assets for GLB, HDR and cube files, --jobs for render jobs, --backend cpu without a GPU).

Loop

1. scene_capabilities: with no arguments it returns a short index of features, limits and topic names. Pass topic for the field list of one feature (environment, materialFields, geometries, meshEdit.bevel, motion) or examples.scene_apply for a complete call of a hard tool. Tool schemas list only common fields, so read the topic before a feature you have not used.
2. scene_create with a recipe: { "format": "crucible.scene", "schemaVersion": 1, "geometries": [], "materials": [], "nodes": [] } plus optional camera, environment, effects, shadow, motion, bake, timeline.
3. scene_inspect: node IDs, bounds, diagnostics, current revision.
4. scene_look: the PNG. Add labels: true to match node names to the image. Judge the image, not the lint.
5. scene_apply with the current expectedRevision and a small operations batch. A stale revision fails whole.
6. scene_checkpoint before a risky batch, scene_restore to undo, scene_release_checkpoint after. Without a checkpoint, scene_restore with revision goes back to one of the last 4 earlier revisions.
7. scene_export_recipe keeps the result. scene_dispose a scene you finished with.

Beside the loop: mesh_inspect, mesh_edit and mesh_stack edit polygon meshes (see the modeling row below), and scene_walkmap returns a grid of walkable and blocked cells for placing characters and paths. scene_ask_user, where the server lists it, opens the view for the person to pin notes on: use it when a change is hard to put in words.

sessionId may be left out: the last used scene is meant. With two scenes open, pass it on every call. The server keeps 4 scenes and closes the least recently used (evictedSessions in the create result), and 16 checkpoints per scene, oldest first (evictedCheckpointIds). Variants: scene_apply with a branch name, compare in one scene_review (a branch per shot), keep one with scene_restore.

Pick the recipe

Defaults that save work

• Start from the default look: environment.setup daylight, AO off. studio suits products, overcast soft light, sunset warm low light. Do not hand-tune key, fill and ambient first.
• The preview draws a floor with a contact shadow under the lowest geometry. environment.ground: false removes it, so never add a floor node only for the shadow.
• Sizes: box is a unit cube; cylinder and sphere have radius 1, so node scale x/z (cylinder) or scale (sphere) is the radius, not the diameter. A 6-segment cylinder has flats on Z (1.732 x scale apart) and corners on X.
• Stack parts from numbers: write each part's bottom and top Y first, adding the Y of the surface it stands on (a height "above the slab" is measured from the slab's top, not from Y = 0). After building, compare scene_inspect bounds with the task's numbers.
• Reuse one geometry ID for repeated parts and move them with node transforms.
• Put parts against each other with node relations (attach, gap, flush, align, centered) so one set-node resizes the body and the handle follows.
• Surfaces without UVs: material pattern (wood, bricks, marble, metal, fabric). Glass: transmission with a nonzero thickness. Details are in scene_capabilities.
• Hide a node with visible: false; castShadow, receiveShadow, holdout and shadowCatcher are node flags too (the last two need a transparent canvas, so they matter for exported composites, not scene_look).
• Characters and creatures: recipe armatures plus skin per mesh node, motion.ik, imported GLBs through the asset node field and scene_asset_catalog. Physics, cloth, particles and hair are recipe sections. scene_inspect reports their results; read those instead of guessing from a picture.
• Third-party controller or camera kinds exist only when the server owner started it with --addons and --allow-addon: read scene_capabilities topic addons for the names, then set game.player.controller or game.camera to one. You cannot load an addon from a tool.
• A GLB for Blender: scene_export_glb (needs --jobs), it returns a path.

Finish

Deliver the recipe, one scene_look image and any limit you hit. Say what you measured (counts, bounds, seconds) apart from what you judged by eye. Do not claim Blender parity or general speed over three.js. Changes, including unreleased and breaking ones: https://crucible.codepawl.com/changelog.json. A Crucible bug: gh issue create -R codepawl/crucible with the steps, the recipe and a still; no local paths.