diff --git a/packages/core/src/registry/index.ts b/packages/core/src/registry/index.ts
index ee5fc49b..6648f7cf 100644
--- a/packages/core/src/registry/index.ts
+++ b/packages/core/src/registry/index.ts
@@ -1,4 +1,11 @@
 export { loadPlugin, nodeRegistry, registerNode } from './registry'
+export {
+  type CascadeContext,
+  type ChildQuery,
+  cascadeDirty,
+  collectDescendants,
+  type SpatialQuery,
+} from './relations-resolver'
 export { createSceneApi, type SceneStoreLike } from './scene-api'
 export type {
   Affordance,
diff --git a/packages/core/src/registry/relations-resolver.test.ts b/packages/core/src/registry/relations-resolver.test.ts
new file mode 100644
index 00000000..2784c1d1
--- /dev/null
+++ b/packages/core/src/registry/relations-resolver.test.ts
@@ -0,0 +1,207 @@
+import { beforeEach, describe, expect, test } from 'bun:test'
+import { z } from 'zod'
+import type { AnyNode, AnyNodeId } from '../schema/types'
+import { nodeRegistry, registerNode } from './registry'
+import { cascadeDirty, collectDescendants, type SpatialQuery } from './relations-resolver'
+import type { AnyNodeDefinition, Relations, SceneApi } from './types'
+
+const id = (s: string) => s as AnyNodeId
+
+function makeDef(
+  kind: string,
+  relations?: Relations,
+  overrides: Partial<AnyNodeDefinition> = {},
+): AnyNodeDefinition {
+  return {
+    kind,
+    schemaVersion: 1,
+    schema: z.object({ type: z.literal(kind) }) as any,
+    category: 'utility',
+    defaults: () => ({}) as any,
+    capabilities: {},
+    relations,
+    renderer: { kind: 'parametric', module: async () => ({ default: () => null }) },
+    ...overrides,
+  }
+}
+
+function makeNode(kind: string, idStr: string, extra: Partial<AnyNode> = {}): AnyNode {
+  return {
+    id: id(idStr),
+    type: kind,
+    parentId: null,
+    visible: true,
+    ...extra,
+  } 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('cascadeDirty', () => {
+  beforeEach(() => {
+    nodeRegistry._reset()
+  })
+
+  test('starting node alone when registry is empty', () => {
+    const scene = makeFakeScene({ a: makeNode('unknown', 'a') })
+    const dirty = cascadeDirty(id('a'), { scene })
+    expect(Array.from(dirty)).toEqual([id('a')])
+  })
+
+  test('starting node alone when no relations declared', () => {
+    registerNode(makeDef('thing'))
+    const scene = makeFakeScene({ a: makeNode('thing', 'a') })
+    const dirty = cascadeDirty(id('a'), { scene })
+    expect(Array.from(dirty)).toEqual([id('a')])
+  })
+
+  test('hosts cascade marks matching children dirty', () => {
+    registerNode(makeDef('wall', { hosts: ['door', 'window'] }))
+    registerNode(makeDef('door'))
+    registerNode(makeDef('window'))
+    registerNode(makeDef('lamp'))
+
+    const wall = makeNode('wall', 'w1', {
+      children: [id('d1'), id('w2'), id('l1')],
+    } as Partial<AnyNode>)
+    const scene = makeFakeScene({
+      w1: wall,
+      d1: makeNode('door', 'd1', { parentId: id('w1') }),
+      w2: makeNode('window', 'w2', { parentId: id('w1') }),
+      l1: makeNode('lamp', 'l1', { parentId: id('w1') }), // not in hosts list
+    })
+
+    const dirty = cascadeDirty(id('w1'), { scene })
+    const ids = Array.from(dirty).sort()
+    expect(ids).toEqual([id('d1'), id('w1'), id('w2')]) // l1 excluded
+  })
+
+  test('hosts cascade is recursive but bounded by maxDepth', () => {
+    registerNode(makeDef('a', { hosts: ['a'] })) // a hosts more a
+    const nodes: Record<string, AnyNode> = {}
+    for (let i = 0; i < 25; i++) {
+      const childId = i < 24 ? id(`a${i + 1}`) : undefined
+      nodes[`a${i}`] = makeNode('a', `a${i}`, {
+        children: childId ? [childId] : [],
+      } as Partial<AnyNode>)
+    }
+    const scene = makeFakeScene(nodes)
+    const dirty = cascadeDirty(id('a0'), { scene, maxDepth: 5 })
+    expect(dirty.size).toBe(6) // a0 + 5 descendants
+  })
+
+  test('affectsSpatial cascade uses spatialQuery to find neighbors', () => {
+    registerNode(makeDef('wall', { affectsSpatial: ['slab', 'zone'] }))
+    registerNode(makeDef('slab'))
+    registerNode(makeDef('zone'))
+
+    const scene = makeFakeScene({
+      w1: makeNode('wall', 'w1'),
+      s1: makeNode('slab', 's1'),
+      z1: makeNode('zone', 'z1'),
+      unrelated: makeNode('door', 'unrelated'),
+    })
+
+    const spatialQuery: SpatialQuery = (node, kinds) => {
+      if (node.id !== id('w1')) return []
+      const matches: AnyNodeId[] = []
+      if (kinds.includes('slab')) matches.push(id('s1'))
+      if (kinds.includes('zone')) matches.push(id('z1'))
+      return matches
+    }
+
+    const dirty = cascadeDirty(id('w1'), { scene, spatialQuery })
+    expect(Array.from(dirty).sort()).toEqual([id('s1'), id('w1'), id('z1')])
+  })
+
+  test('affectsSpatial is a no-op when no spatialQuery is provided', () => {
+    registerNode(makeDef('wall', { affectsSpatial: ['slab'] }))
+    const scene = makeFakeScene({ w1: makeNode('wall', 'w1') })
+    const dirty = cascadeDirty(id('w1'), { scene })
+    expect(Array.from(dirty)).toEqual([id('w1')]) // spatial branch silently skipped
+  })
+
+  test('cycle in hosts cascade does not loop forever', () => {
+    registerNode(makeDef('a', { hosts: ['a'] }))
+    const nodes: Record<string, AnyNode> = {
+      a1: makeNode('a', 'a1', { children: [id('a2')] } as Partial<AnyNode>),
+      a2: makeNode('a', 'a2', { children: [id('a1')] } as Partial<AnyNode>), // cycle
+    }
+    const scene = makeFakeScene(nodes)
+    const dirty = cascadeDirty(id('a1'), { scene })
+    expect(dirty.size).toBe(2)
+    expect(dirty.has(id('a1'))).toBe(true)
+    expect(dirty.has(id('a2'))).toBe(true)
+  })
+
+  test('custom childQuery overrides the default node.children lookup', () => {
+    registerNode(makeDef('wall', { hosts: ['door'] }))
+    registerNode(makeDef('door'))
+    const scene = makeFakeScene({
+      w1: makeNode('wall', 'w1'), // no children field
+      d1: makeNode('door', 'd1', { parentId: id('w1') }),
+    })
+
+    // childQuery iterates the scene to find parentId matches — what you would
+    // do for kinds that don't carry an explicit children array.
+    const childQuery = (node: AnyNode) => {
+      const result: AnyNodeId[] = []
+      for (const candidate of [id('d1')]) {
+        const c = scene.get(candidate)
+        if (c && c.parentId === node.id) result.push(c.id)
+      }
+      return result
+    }
+
+    const dirty = cascadeDirty(id('w1'), { scene, childQuery })
+    expect(Array.from(dirty).sort()).toEqual([id('d1'), id('w1')])
+  })
+})
+
+describe('collectDescendants', () => {
+  beforeEach(() => {
+    nodeRegistry._reset()
+  })
+
+  test('returns just the start when no children', () => {
+    const scene = makeFakeScene({ a: makeNode('thing', 'a') })
+    const result = collectDescendants(id('a'), { scene })
+    expect(Array.from(result)).toEqual([id('a')])
+  })
+
+  test('returns full subtree regardless of relations declarations', () => {
+    // No def registered — descendants still found via the children array.
+    const scene = makeFakeScene({
+      root: makeNode('thing', 'root', { children: [id('c1'), id('c2')] } as Partial<AnyNode>),
+      c1: makeNode('thing', 'c1', { children: [id('g1')] } as Partial<AnyNode>),
+      c2: makeNode('thing', 'c2'),
+      g1: makeNode('thing', 'g1'),
+    })
+
+    const result = collectDescendants(id('root'), { scene })
+    expect(Array.from(result).sort()).toEqual([id('c1'), id('c2'), id('g1'), id('root')])
+  })
+
+  test('respects maxDepth', () => {
+    const scene = makeFakeScene({
+      a: makeNode('thing', 'a', { children: [id('b')] } as Partial<AnyNode>),
+      b: makeNode('thing', 'b', { children: [id('c')] } as Partial<AnyNode>),
+      c: makeNode('thing', 'c', { children: [id('d')] } as Partial<AnyNode>),
+      d: makeNode('thing', 'd'),
+    })
+    const result = collectDescendants(id('a'), { scene, maxDepth: 2 })
+    expect(Array.from(result).sort()).toEqual([id('a'), id('b'), id('c')]) // d truncated
+  })
+})
diff --git a/packages/core/src/registry/relations-resolver.ts b/packages/core/src/registry/relations-resolver.ts
new file mode 100644
index 00000000..e406928f
--- /dev/null
+++ b/packages/core/src/registry/relations-resolver.ts
@@ -0,0 +1,124 @@
+import type { AnyNode, AnyNodeId } from '../schema/types'
+import { nodeRegistry } from './registry'
+import type { SceneApi } from './types'
+
+/**
+ * Spatial neighbor query — given a node and a set of kinds, returns IDs of
+ * neighboring nodes of those kinds. The runtime provides this from
+ * `spatialGridManager`; tests can pass a stub.
+ */
+export type SpatialQuery = (node: AnyNode, kinds: readonly string[]) => Iterable<AnyNodeId>
+
+/**
+ * Returns the IDs of nodes that share `node` as a parent. The runtime can
+ * pass an optimized index; the default fallback iterates the scene.
+ */
+export type ChildQuery = (node: AnyNode) => Iterable<AnyNodeId>
+
+export type CascadeContext = {
+  scene: SceneApi
+  /** Optional: bounded spatial neighbor lookup. Required for `affectsSpatial`. */
+  spatialQuery?: SpatialQuery
+  /** Optional: children-by-parent lookup. Defaults to iterating the scene. */
+  childQuery?: ChildQuery
+  /** Safety cap on cascade depth — guards against bad data and pathological
+   * registry configurations. Default 16 (deeper than the maxHostDepth of 6). */
+  maxDepth?: number
+}
+
+const DEFAULT_MAX_DEPTH = 16
+
+/**
+ * Walks the relations graph from one dirty node and returns the full set of
+ * IDs (including the starting one) that should be marked dirty. Pure — does
+ * NOT call `scene.markDirty`; callers iterate the result.
+ *
+ * Phase 1 implements:
+ * - `hosts`: marks children whose `type` matches the kind list
+ * - `affectsSpatial`: marks neighbors found via `spatialQuery`
+ *
+ * Phase 3 will add `linkedBy: 'endpoint-match'` for wall corner propagation.
+ */
+export function cascadeDirty(startId: AnyNodeId, ctx: CascadeContext): Set<AnyNodeId> {
+  const result = new Set<AnyNodeId>()
+  const maxDepth = ctx.maxDepth ?? DEFAULT_MAX_DEPTH
+  walk(startId, ctx, result, 0, maxDepth)
+  return result
+}
+
+function walk(
+  id: AnyNodeId,
+  ctx: CascadeContext,
+  result: Set<AnyNodeId>,
+  depth: number,
+  maxDepth: number,
+): void {
+  if (result.has(id) || depth > maxDepth) return
+  result.add(id)
+
+  const node = ctx.scene.get(id)
+  if (!node) return
+
+  const def = nodeRegistry.get(node.type)
+  if (!def?.relations) return
+
+  const { hosts, affectsSpatial } = def.relations
+
+  if (hosts && hosts.length > 0) {
+    const childIds = ctx.childQuery ? ctx.childQuery(node) : defaultChildIds(node, ctx.scene)
+    for (const childId of childIds) {
+      const child = ctx.scene.get(childId)
+      if (child && (hosts as readonly string[]).includes(child.type)) {
+        walk(childId, ctx, result, depth + 1, maxDepth)
+      }
+    }
+  }
+
+  if (affectsSpatial && affectsSpatial.length > 0 && ctx.spatialQuery) {
+    for (const neighborId of ctx.spatialQuery(node, affectsSpatial)) {
+      walk(neighborId, ctx, result, depth + 1, maxDepth)
+    }
+  }
+}
+
+/**
+ * Fallback children lookup that reads the node's `children: AnyNodeId[]`
+ * field if present. Most parametric nodes carry one; nodes that don't will
+ * need a `childQuery` override on the context.
+ */
+function defaultChildIds(node: AnyNode, _scene: SceneApi): AnyNodeId[] {
+  const maybeChildren = (node as unknown as { children?: AnyNodeId[] }).children
+  return Array.isArray(maybeChildren) ? maybeChildren : []
+}
+
+/**
+ * Recursively collects every descendant of a node, plus the node itself.
+ * Used by `cascadeDelete: 'descendants'` and by tools that need to delete a
+ * subtree atomically. Independent of dirty-marking — pure traversal.
+ */
+export function collectDescendants(
+  startId: AnyNodeId,
+  ctx: Pick<CascadeContext, 'scene' | 'childQuery' | 'maxDepth'>,
+): Set<AnyNodeId> {
+  const result = new Set<AnyNodeId>()
+  const maxDepth = ctx.maxDepth ?? DEFAULT_MAX_DEPTH
+  walkDescendants(startId, ctx, result, 0, maxDepth)
+  return result
+}
+
+function walkDescendants(
+  id: AnyNodeId,
+  ctx: Pick<CascadeContext, 'scene' | 'childQuery' | 'maxDepth'>,
+  result: Set<AnyNodeId>,
+  depth: number,
+  maxDepth: number,
+): void {
+  if (result.has(id) || depth > maxDepth) return
+  result.add(id)
+  const node = ctx.scene.get(id)
+  if (!node) return
+  const childIds = ctx.childQuery ? ctx.childQuery(node) : defaultChildIds(node, ctx.scene)
+  for (const childId of childIds) {
+    walkDescendants(childId, ctx, result, depth + 1, maxDepth)
+  }
+}