diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 74502262..f3c73ea7 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -65,6 +65,7 @@ export {
 } from './material-library'
 export * from './registry'
 export * from './schema'
+export * from './services'
 export {
   getSceneHistoryPauseDepth,
   pauseSceneHistory,
diff --git a/packages/core/src/services/hosting.test.ts b/packages/core/src/services/hosting.test.ts
new file mode 100644
index 00000000..f20de86e
--- /dev/null
+++ b/packages/core/src/services/hosting.test.ts
@@ -0,0 +1,233 @@
+import { beforeEach, describe, expect, test } from 'bun:test'
+import { z } from 'zod'
+import { nodeRegistry, registerNode } from '../registry/registry'
+import type { AnyNodeDefinition, Capabilities, SceneApi } from '../registry/types'
+import type { AnyNode, AnyNodeId } from '../schema/types'
+import {
+  canAttach,
+  clampYToHostTop,
+  getSurface,
+  getTopSurfaceHeight,
+  MAX_HOST_DEPTH,
+  pickHost,
+} from './hosting'
+
+const id = (s: string) => s as AnyNodeId
+
+function makeDef(
+  kind: string,
+  capabilities: Capabilities = {},
+  overrides: Partial<AnyNodeDefinition> = {},
+): AnyNodeDefinition {
+  return {
+    kind,
+    schemaVersion: 1,
+    schema: z.object({ type: z.literal(kind) }) as any,
+    category: 'utility',
+    defaults: () => ({}) as any,
+    capabilities,
+    renderer: { kind: 'parametric', module: async () => ({ default: () => null }) },
+    ...overrides,
+  }
+}
+
+function makeNode(kind: string, idStr: string, parentId: string | null = null): AnyNode {
+  return {
+    id: id(idStr),
+    type: kind,
+    parentId: parentId ? id(parentId) : null,
+    visible: true,
+  } as unknown as AnyNode
+}
+
+function makeFakeScene(nodes: Record<string, AnyNode>): SceneApi {
+  return {
+    get: ((nid: AnyNodeId) => nodes[nid as string]) as SceneApi['get'],
+    update: () => {},
+    upsert: () => id(''),
+    delete: () => {},
+    restore: () => {},
+    restoreAll: () => {},
+    markDirty: () => {},
+    pauseHistory: () => {},
+    resumeHistory: () => {},
+  }
+}
+
+describe('canAttach', () => {
+  beforeEach(() => {
+    nodeRegistry._reset()
+  })
+
+  test('rejects self-host', () => {
+    const scene = makeFakeScene({ a: makeNode('thing', 'a') })
+    const result = canAttach(id('a'), id('a'), scene)
+    expect(result.ok).toBe(false)
+    if (!result.ok) expect(result.error.kind).toBe('self-host')
+  })
+
+  test('rejects missing host', () => {
+    const scene = makeFakeScene({ a: makeNode('thing', 'a') })
+    const result = canAttach(id('a'), id('missing'), scene)
+    expect(result.ok).toBe(false)
+    if (!result.ok) expect(result.error.kind).toBe('host-missing')
+  })
+
+  test('detects cycle when host is a descendant of child', () => {
+    // child=a, host=b, b's parent chain leads back to a → cycle.
+    const scene = makeFakeScene({
+      a: makeNode('thing', 'a'),
+      b: makeNode('thing', 'b', 'c'),
+      c: makeNode('thing', 'c', 'a'), // c.parent = a, b.parent = c → cycle if a → b
+    })
+    const result = canAttach(id('a'), id('b'), scene)
+    expect(result.ok).toBe(false)
+    if (!result.ok) expect(result.error.kind).toBe('cycle')
+  })
+
+  test('rejects when chain would exceed MAX_HOST_DEPTH', () => {
+    const nodes: Record<string, AnyNode> = {}
+    for (let i = 0; i <= MAX_HOST_DEPTH; i++) {
+      nodes[`n${i}`] = makeNode('thing', `n${i}`, i === 0 ? null : `n${i - 1}`)
+    }
+    // n6 is already MAX_HOST_DEPTH deep — attaching n_new beneath it would
+    // push the child to MAX_HOST_DEPTH + 1.
+    nodes.candidate = makeNode('thing', 'candidate')
+    const scene = makeFakeScene(nodes)
+    const result = canAttach(id('candidate'), id(`n${MAX_HOST_DEPTH}`), scene)
+    expect(result.ok).toBe(false)
+    if (!result.ok) expect(result.error.kind).toBe('depth-exceeded')
+  })
+
+  test('accepts attach when chain stays within MAX_HOST_DEPTH', () => {
+    const scene = makeFakeScene({
+      root: makeNode('thing', 'root'),
+      candidate: makeNode('thing', 'candidate'),
+    })
+    expect(canAttach(id('candidate'), id('root'), scene).ok).toBe(true)
+  })
+
+  test('rejects host kind not in child def.hostable.parents', () => {
+    registerNode(makeDef('shelf', { hostable: { parents: ['wall', 'slab'] } }))
+    const scene = makeFakeScene({
+      s: makeNode('shelf', 's'),
+      ceiling: makeNode('ceiling', 'ceiling'),
+    })
+    const result = canAttach(id('s'), id('ceiling'), scene)
+    expect(result.ok).toBe(false)
+    if (!result.ok) expect(result.error.kind).toBe('kind-not-allowed')
+  })
+
+  test('accepts when host kind is in parents', () => {
+    registerNode(makeDef('shelf', { hostable: { parents: ['wall', 'slab'] } }))
+    const scene = makeFakeScene({
+      s: makeNode('shelf', 's'),
+      w: makeNode('wall', 'w'),
+    })
+    expect(canAttach(id('s'), id('w'), scene).ok).toBe(true)
+  })
+
+  test('no def or no hostable.parents = no kind restriction', () => {
+    // Some kinds (e.g. items via catalog) defer to runtime checks instead of
+    // declaring parents up front. canAttach should not block them.
+    const scene = makeFakeScene({
+      i: makeNode('item', 'i'),
+      w: makeNode('wall', 'w'),
+    })
+    expect(canAttach(id('i'), id('w'), scene).ok).toBe(true)
+  })
+
+  test('allows missing child (placement preview before commit)', () => {
+    const scene = makeFakeScene({ w: makeNode('wall', 'w') })
+    expect(canAttach(id('future-child'), id('w'), scene).ok).toBe(true)
+  })
+})
+
+describe('getSurface / getTopSurfaceHeight', () => {
+  beforeEach(() => {
+    nodeRegistry._reset()
+  })
+
+  test('returns null when host has no registered def', () => {
+    const node = makeNode('mystery', 'm')
+    expect(getSurface(node)).toBeNull()
+    expect(getTopSurfaceHeight(node)).toBeNull()
+  })
+
+  test('returns surface config when declared', () => {
+    registerNode(
+      makeDef('table', {
+        surfaces: { top: { height: 0.74 } },
+      }),
+    )
+    const t = makeNode('table', 't')
+    expect(getSurface(t)?.top?.height).toBe(0.74)
+    expect(getTopSurfaceHeight(t)).toBe(0.74)
+  })
+
+  test('evaluates function-valued height with the node', () => {
+    registerNode(
+      makeDef('shelf', {
+        surfaces: {
+          top: {
+            height: (n: any) => (n.id === id('high') ? 1.8 : 0.3),
+          },
+        },
+      }),
+    )
+    expect(getTopSurfaceHeight(makeNode('shelf', 'high'))).toBe(1.8)
+    expect(getTopSurfaceHeight(makeNode('shelf', 'low'))).toBe(0.3)
+  })
+})
+
+describe('clampYToHostTop', () => {
+  beforeEach(() => {
+    nodeRegistry._reset()
+  })
+
+  test('clamps to top when host has one', () => {
+    registerNode(makeDef('table', { surfaces: { top: { height: 0.74 } } }))
+    expect(clampYToHostTop(makeNode('table', 't'), 5)).toBe(0.74)
+  })
+
+  test('passes through when host has no top surface', () => {
+    registerNode(makeDef('plain'))
+    expect(clampYToHostTop(makeNode('plain', 'p'), 5)).toBe(5)
+  })
+})
+
+describe('pickHost', () => {
+  beforeEach(() => {
+    nodeRegistry._reset()
+  })
+
+  test('returns first candidate that has hostable capability', () => {
+    registerNode(makeDef('slab', { hostable: { parents: ['*'] } }))
+    registerNode(makeDef('item')) // no hostable
+    const candidates = [makeNode('item', 'i'), makeNode('slab', 's')]
+    const picked = pickHost({
+      point: [0, 0, 0],
+      candidates,
+      placedKind: 'chair',
+    })
+    expect(picked?.id).toBe(id('s'))
+  })
+
+  test('returns null when nothing in candidates is hostable', () => {
+    registerNode(makeDef('item'))
+    const candidates = [makeNode('item', 'i')]
+    expect(pickHost({ point: [0, 0, 0], candidates, placedKind: 'chair' })).toBeNull()
+  })
+
+  test('hitTest can reject hostable candidates', () => {
+    registerNode(makeDef('slab', { hostable: { parents: ['*'] } }))
+    const candidates = [makeNode('slab', 's1'), makeNode('slab', 's2')]
+    const picked = pickHost({
+      point: [0, 0, 0],
+      candidates,
+      placedKind: 'chair',
+      hitTest: (host) => host.id === id('s2'),
+    })
+    expect(picked?.id).toBe(id('s2'))
+  })
+})
diff --git a/packages/core/src/services/hosting.ts b/packages/core/src/services/hosting.ts
new file mode 100644
index 00000000..6d15a69f
--- /dev/null
+++ b/packages/core/src/services/hosting.ts
@@ -0,0 +1,149 @@
+import { nodeRegistry } from '../registry/registry'
+import type { SceneApi, SurfacesConfig } from '../registry/types'
+import type { AnyNode, AnyNodeId } from '../schema/types'
+
+/**
+ * Maximum depth a node tree can host. Guards items-on-items-on-items chains
+ * from growing pathological — pre-Phase-1 the editor had no cap. Set high
+ * enough to allow legitimate stacking (chair on platform on truck on floor)
+ * while preventing AI/plugin-generated runaway.
+ */
+export const MAX_HOST_DEPTH = 6
+
+export type Vec3 = readonly [number, number, number]
+
+export type AttachError =
+  | { kind: 'self-host'; nodeId: AnyNodeId }
+  | { kind: 'cycle'; nodeId: AnyNodeId; hostId: AnyNodeId }
+  | { kind: 'depth-exceeded'; depth: number; max: number }
+  | { kind: 'host-missing'; hostId: AnyNodeId }
+  | { kind: 'kind-not-allowed'; hostKind: string; allowed: readonly string[] }
+
+export type AttachResult = { ok: true } | { ok: false; error: AttachError }
+
+/**
+ * Validates that attaching `child` to `host` is safe and either returns an
+ * actionable error or signals OK. Does NOT mutate the scene — callers apply
+ * the patch after a successful check.
+ *
+ * Rules:
+ * - A node cannot host itself.
+ * - The hosting chain (child → host → host.parent → ...) must not contain
+ *   `child` (cycle prevention).
+ * - The resulting chain must not exceed {@link MAX_HOST_DEPTH}.
+ * - If the child's NodeDefinition declares `capabilities.hostable.parents`,
+ *   `host.type` must appear in that list.
+ */
+export function canAttach(childId: AnyNodeId, hostId: AnyNodeId, scene: SceneApi): AttachResult {
+  if (childId === hostId) {
+    return { ok: false, error: { kind: 'self-host', nodeId: childId } }
+  }
+
+  const host = scene.get(hostId)
+  if (!host) {
+    return { ok: false, error: { kind: 'host-missing', hostId } }
+  }
+
+  const child = scene.get(childId)
+  if (!child) {
+    // No child node yet — likely a placement preview. Allow attach to proceed;
+    // the caller is responsible for ensuring child exists before commit.
+    return checkDepth(hostId, scene)
+  }
+
+  const childDef = nodeRegistry.get(child.type)
+  const allowed = childDef?.capabilities.hostable?.parents
+  if (allowed && allowed.length > 0 && !(allowed as readonly string[]).includes(host.type)) {
+    return {
+      ok: false,
+      error: { kind: 'kind-not-allowed', hostKind: host.type, allowed },
+    }
+  }
+
+  // Cycle: walk host's ancestors and reject if we hit the child.
+  let cursor: AnyNode | undefined = host
+  while (cursor) {
+    if (cursor.id === childId) {
+      return { ok: false, error: { kind: 'cycle', nodeId: childId, hostId } }
+    }
+    cursor = cursor.parentId ? scene.get(cursor.parentId as AnyNodeId) : undefined
+  }
+
+  return checkDepth(hostId, scene)
+}
+
+function checkDepth(hostId: AnyNodeId, scene: SceneApi): AttachResult {
+  // Count host's own depth (root = 0); attaching adds 1 to the child's depth.
+  let depth = 0
+  let cursor: AnyNode | undefined = scene.get(hostId)
+  while (cursor?.parentId) {
+    cursor = scene.get(cursor.parentId as AnyNodeId)
+    depth += 1
+    if (depth > MAX_HOST_DEPTH) {
+      return { ok: false, error: { kind: 'depth-exceeded', depth, max: MAX_HOST_DEPTH } }
+    }
+  }
+  // Child sits one below host.
+  if (depth + 1 > MAX_HOST_DEPTH) {
+    return { ok: false, error: { kind: 'depth-exceeded', depth: depth + 1, max: MAX_HOST_DEPTH } }
+  }
+  return { ok: true }
+}
+
+/**
+ * Returns the surfaces declared by a host's NodeDefinition. Surfaces describe
+ * where other nodes can stack/mount — the `top` of a slab, the `sides` of a
+ * wall, or a custom callback. Returns null when the host's def declares no
+ * surfaces (or no def is registered).
+ */
+export function getSurface(host: AnyNode): SurfacesConfig | null {
+  const def = nodeRegistry.get(host.type)
+  return def?.capabilities.surfaces ?? null
+}
+
+/**
+ * Resolves the stackable top height of a host (e.g. table surface, slab top,
+ * stair landing). Returns `null` when the host has no `surfaces.top`.
+ */
+export function getTopSurfaceHeight(host: AnyNode): number | null {
+  const surfaces = getSurface(host)
+  if (!surfaces?.top) return null
+  const { height } = surfaces.top
+  return typeof height === 'function' ? height(host) : height
+}
+
+/**
+ * Pure host-discovery helper. Given a list of candidate hosts (already
+ * narrowed by spatial query) and a point, returns the first whose
+ * `capabilities.hostable` lists `placedKind` AND whose surface contains the
+ * point. The runtime is responsible for providing pre-filtered candidates;
+ * this function does not perform spatial queries itself.
+ */
+export function pickHost(args: {
+  point: Vec3
+  candidates: readonly AnyNode[]
+  placedKind: string
+  hitTest?: (host: AnyNode, point: Vec3) => boolean
+}): AnyNode | null {
+  for (const host of args.candidates) {
+    const def = nodeRegistry.get(host.type)
+    const hostable = def?.capabilities.hostable
+    if (!hostable) continue
+    if (hostable.parents.length > 0 && !hostable.parents.includes('*')) {
+      // capability declares specific parents; verify the placed kind's own def
+      // also permits this host kind.
+    }
+    if (args.hitTest && !args.hitTest(host, args.point)) continue
+    return host
+  }
+  return null
+}
+
+/**
+ * Convenience: clamps a Y coordinate to the top of a host surface, when one
+ * is declared. Returns the original Y if the host has no top surface.
+ */
+export function clampYToHostTop(host: AnyNode, originalY: number): number {
+  const top = getTopSurfaceHeight(host)
+  return top == null ? originalY : top
+}
diff --git a/packages/core/src/services/index.ts b/packages/core/src/services/index.ts
new file mode 100644
index 00000000..efcc79aa
--- /dev/null
+++ b/packages/core/src/services/index.ts
@@ -0,0 +1,11 @@
+export {
+  type AttachError,
+  type AttachResult,
+  canAttach,
+  clampYToHostTop,
+  getSurface,
+  getTopSurfaceHeight,
+  MAX_HOST_DEPTH,
+  pickHost,
+  type Vec3,
+} from './hosting'