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world-building-3d is a skill from the Summer skill library: a guide your AI agent reads when the task matches, so it builds the good version on the first try. Compose, ground and space 3D scenes with Summer’s four spatial tools, one geometric decision at a time.

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When your agent uses it

  • Placing or grounding props on floors, shelves, ramps, or tables without hand-tuning transforms
  • Aligning or evenly spacing a group of 3D objects along one world axis
  • Verifying an NPC, pickup, or portal position is reachable on the navigation map

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Install

summer setup installs every skill for your agent. To add only this one:
Swap claude-code for your agent, such as codex or cursor. See Skills for every option.

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The skill

Use exact scene and node paths, make one geometric decision at a time, and verify the saved result. Treat spatial tools as bounded evidence: they reduce guesswork but do not replace a final rendered or gameplay check. Read references/spatial-tools.md before the first spatial-tool call in a task. It defines each tool’s evidence boundary and the result fields that must not be overinterpreted.

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Choose the narrowest tool

Use ordinary summer_get_scene_tree and summer_inspect_node first when paths, hierarchy, authored transforms, collision layers, or camera settings are unknown. For modular kit pieces (facade modules, storeys, pipes, gutters, wall lamps, clamp rows) use the spatial-placement skill: it adds summer_inspect_asset (measure a piece before placing it), placed summer_instantiate_scene, summer_place_adjacent, summer_attach_to_surface, summer_repeat_along, summer_connect_ports, summer_raycast, summer_measure and summer_starcast to the four tools here.

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Follow the composition loop

  1. Open the exact target scene and resolve exact ./-relative paths.
  2. Inspect the subject, its visual/collider descendants, intended support or neighbors, and the camera.
  3. State the intended invariant: contact gap, clearance, spacing axis, or reachable destination. If the support, ordering, or destination is genuinely ambiguous, ask Proceed? with the concrete choice before making a user-visible mutation.
  4. Query before mutation when a read-only tool exists.
  5. Apply the smallest mutation. Prefer the dedicated solver over hand-tuned transform loops.
  6. Re-query the saved pose. Do not treat a successful mutation receipt as proof of visual quality.
  7. Run a rendered or gameplay verification when renderer visibility, transparent materials, concave silhouettes, navigation behavior, or player perception matters.
Keep scene A and scene B independent. Never copy a solved world transform between scenes unless their relevant parents, bounds, and obstacles are proven identical. There is no raw .tscn fallback for these evidence queries or solver mutations. They depend on the running Summer Engine’s live scene, physics, camera, and navigation state. If the required tool is unavailable, inspect and explain the limitation instead of hand-editing a scene around it.

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Place and ground props

For a known candidate pose, call summer_test_placement before setting the transform. Preserve the current global scale unless the user asked to resize the asset.
  • overlapCount > 0 or known overlap evidence means reject the pose.
  • fits: null means the physics backend could not prove completeness; it is not true.
  • Require grounded: true and an acceptable absolute floorGap for seated props.
  • Treat visual_aabb as broad-phase evidence. Irregular meshes can have empty-looking AABB corners.
For a direct seat operation, call summer_snap_to_surface with a world-space direction. Use alignUp: true only when the object should inherit an exact physics support normal. After slope alignment, verify both up alignment and preserved heading. Do not infer a usable normal from visual_aabb fallback. SnapToSurface does not return the raw support normal or a heading residual. Require evidence: physics, evidenceDetails.contactResolved: true, the expected supportPath, alignApplied: true, a plausible slopeDeg, and inspect the saved after.basis plus a rendered view. Keep maxDistance tight because the current solver cannot take an expected-support path and will stop at the first surface. On mixed-height or sloped supports, arrange the subjects laterally first, then snap each subject. A later alignment can lift a grounded prop or bury it in a slope. For a final placement re-query, use the exact saved global pose; if the post-alignment basis cannot be expressed confidently as global Euler degrees, do not invent angles - use the snap receipt and an independent rendered or physics check.

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Arrange groups

Pass subjects to summer_align_distribute_3d in intentional order. Choose a world axis explicitly.
  • Use align_min, align_center, or align_max to share one projected anchor.
  • Use distribute_centers when equal center spacing matters.
  • Use distribute_gaps when visible edge clearance matters; endpoints remain fixed.
Inspect the result’s residuals and changedCount; infer unchanged subjects only as subjectCount - changedCount. Rerun placement checks for dense groups because alignment solves one axis and does not prove full 3D clearance.

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Validate navigation placement

Use summer_navigation_probe before moving an NPC, pickup, portal, or encounter anchor whose route matters.
  • ready: false means no trustworthy navigation evidence; wait or fix the navigation map.
  • reachable: false is meaningful only when ready: true.
  • Check requested-to-snapped endpoint distance. A route to a distant snapped point does not validate the requested placement.
  • After moving the subject, probe the final authored position again.
Probe each destination separately from the intended player/start point. Use the final inspected global origin and the scene’s actual navigation-layer mask. Set the acceptable snap distance from the navigation agent radius or the authored placement tolerance; if neither is known, report the distance instead of declaring it small. For iteration 0, allow one bounded navigation-sync wait and retry once - never loop indefinitely.

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Fail closed

Do not work around world-mismatch failures such as subject_world_mismatch or subject_geometry_world_mismatch, non-finite transforms, hierarchy limits, unavailable physics, or missing bounds by guessing. Fix the scene structure or choose a clearly labeled manual fallback, then verify it. Do not retry a mutation after an oversized or ambiguous result unless the receipt explicitly says it is retry-safe; the first mutation may already have landed.

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From a shell

No Summer MCP tools in this session? Run each one as a command; when summer --version is missing or below 4.0, use npx -y summer-engine@latest in place of summer. Fields are flags in kebab-case (--scene-path res://main.tscn) or one --input '<json>', and summer ops describe <op> prints the exact input. The summer-cli skill has the rest.

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Files in this skill

  • scene-composition: Scene structure: node hierarchy, when to extract sub-scenes, reusable prefabs, instance vs add node.
  • verifying-scenes: Prove scene work landed: snapshot, diff, screenshots, runtime reads, 3D audits, shot sheets and zooms.
  • scene-scripting: Build scenes, levels, HUDs and wiring with one editor GDScript (summer_run_script) instead of call chains.
  • spatial-placement: Place 3D objects and modular kit pieces from measured geometry: floors, walls, facades, pipes, alcoves.
Source: library/skills/world-building-3d/SKILL.md, rendered from summer-engine@4.3.0 on npm.