diff --git a/apps/editor/app/api/scenes/[id]/route.ts b/apps/editor/app/api/scenes/[id]/route.ts
index 1712ad4ad4..10fc523b9b 100644
--- a/apps/editor/app/api/scenes/[id]/route.ts
+++ b/apps/editor/app/api/scenes/[id]/route.ts
@@ -7,6 +7,7 @@ import {
   sceneApiPreflight,
   withSceneApiHeaders,
 } from '@/lib/scene-api-security'
+import { countContentNodes, wouldClearSceneContent } from '@/lib/scene-content-guard'
 import { getSceneOperations } from '@/lib/scene-store-server'
 
 export const dynamic = 'force-dynamic'
@@ -18,6 +19,12 @@ const putSceneSchema = z.object({
   graph: apiGraphSchema,
   thumbnailUrl: z.string().url().nullable().optional(),
   expectedVersion: z.number().int().nonnegative().optional(),
+  /**
+   * Opt out of the content-clear guard. Required for a deliberate
+   * "delete everything" save, which is otherwise indistinguishable from the
+   * client autosaving before it has loaded the scene.
+   */
+  allowContentClear: z.boolean().optional(),
 })
 
 const patchSceneSchema = z.object({
@@ -83,6 +90,30 @@ export async function PUT(request: NextRequest, { params }: RouteParams) {
     if (!existing) {
       return sceneApiJson(request, { error: 'not_found' }, { status: 404 })
     }
+
+    // A client that autosaves before its initial load resolves sends an empty
+    // graph — or the bare site/building/level scaffold — and the version still
+    // matches, so `expectedVersion` waves it straight through and the stored
+    // scene is destroyed. Refuse to be the last writer in that sequence: a save
+    // may shrink a scene freely, but it may not take the last content node with
+    // it unless the caller says so explicitly.
+    if (
+      !parsed.data.allowContentClear &&
+      wouldClearSceneContent(existing.graph, parsed.data.graph)
+    ) {
+      return sceneApiJson(
+        request,
+        {
+          error: 'content_clear_rejected',
+          details:
+            'Refusing to replace a non-empty scene with an empty one. Reload the scene, or resend with allowContentClear: true if the clear is intentional.',
+          existingContentNodes: countContentNodes(existing.graph),
+          version: existing.version,
+        },
+        { status: 409, headers: { ETag: `"${existing.version}"` } },
+      )
+    }
+
     const meta = await operations.saveScene({
       id,
       name: parsed.data.name ?? existing.name,
diff --git a/apps/editor/lib/scene-content-guard.test.ts b/apps/editor/lib/scene-content-guard.test.ts
new file mode 100644
index 0000000000..b6f5717a9d
--- /dev/null
+++ b/apps/editor/lib/scene-content-guard.test.ts
@@ -0,0 +1,67 @@
+import { describe, expect, test } from 'bun:test'
+import { countContentNodes, wouldClearSceneContent } from './scene-content-guard'
+
+const scaffold = {
+  nodes: {
+    site_a: { type: 'site' },
+    building_a: { type: 'building' },
+    level_a: { type: 'level' },
+  },
+}
+
+const house = {
+  nodes: {
+    site_a: { type: 'site' },
+    building_a: { type: 'building' },
+    level_a: { type: 'level' },
+    slab_a: { type: 'slab' },
+    wall_a: { type: 'wall' },
+    wall_b: { type: 'wall' },
+    zone_a: { type: 'zone' },
+  },
+}
+
+describe('countContentNodes', () => {
+  test('ignores the site/building/level scaffold', () => {
+    expect(countContentNodes(scaffold)).toBe(0)
+  })
+
+  test('counts authored nodes only', () => {
+    expect(countContentNodes(house)).toBe(4)
+  })
+
+  test('treats an empty or malformed graph as no content', () => {
+    expect(countContentNodes({ nodes: {} })).toBe(0)
+    expect(countContentNodes({})).toBe(0)
+    expect(countContentNodes(null)).toBe(0)
+    expect(countContentNodes(undefined)).toBe(0)
+    expect(countContentNodes({ nodes: { a: null } })).toBe(0)
+  })
+})
+
+describe('wouldClearSceneContent', () => {
+  test('rejects the empty-graph autosave that wiped the stored scene', () => {
+    expect(wouldClearSceneContent(house, { nodes: {} })).toBe(true)
+  })
+
+  test('rejects a scaffold-only autosave, not just a zero-node one', () => {
+    // The regression this guards: versions 51/53/55 wrote 3-4 scaffold nodes,
+    // which a naive "is the graph empty" check would have let through.
+    expect(wouldClearSceneContent(house, scaffold)).toBe(true)
+  })
+
+  test('allows ordinary edits that shrink the scene', () => {
+    const trimmed = {
+      nodes: { site_a: { type: 'site' }, level_a: { type: 'level' }, wall_a: { type: 'wall' } },
+    }
+    expect(wouldClearSceneContent(house, trimmed)).toBe(false)
+  })
+
+  test('allows growing a scene', () => {
+    expect(wouldClearSceneContent(scaffold, house)).toBe(false)
+  })
+
+  test('allows writing a scaffold over a scene that had no content anyway', () => {
+    expect(wouldClearSceneContent(scaffold, { nodes: {} })).toBe(false)
+  })
+})
diff --git a/apps/editor/lib/scene-content-guard.ts b/apps/editor/lib/scene-content-guard.ts
new file mode 100644
index 0000000000..a725fb81e5
--- /dev/null
+++ b/apps/editor/lib/scene-content-guard.ts
@@ -0,0 +1,42 @@
+/**
+ * Guard against a save that silently destroys an authored scene.
+ *
+ * The editor autosaves on a debounce. If that timer fires before the initial
+ * scene load resolves, the client sends the empty graph it started from — or
+ * the bare site/building/level scaffold it falls back to. The stored version is
+ * still the one the client read, so `expectedVersion` matches and optimistic
+ * concurrency waves the write through. The authored scene is then gone.
+ *
+ * Shrinking a scene is legitimate; removing its last authored node as part of
+ * an unattended write is not distinguishable from the failure above, so callers
+ * that mean it have to say so explicitly.
+ */
+
+/**
+ * The scaffold a fresh editor session starts from. These carry no authored
+ * content on their own, so a graph containing only these is "empty" for the
+ * purposes of this guard.
+ */
+export const SCAFFOLD_NODE_TYPES: ReadonlySet<string> = new Set(['site', 'building', 'level'])
+
+/** Counts nodes a user or agent actually authored (walls, slabs, zones, openings, roofs, items). */
+export function countContentNodes(graph: unknown): number {
+  const nodes = (graph as { nodes?: Record<string, { type?: string } | null> } | null | undefined)
+    ?.nodes
+  if (!nodes || typeof nodes !== 'object') return 0
+
+  let count = 0
+  for (const node of Object.values(nodes)) {
+    const type = node?.type
+    if (typeof type === 'string' && !SCAFFOLD_NODE_TYPES.has(type)) count += 1
+  }
+  return count
+}
+
+/**
+ * True when writing `incoming` over `existing` would strip the scene of every
+ * authored node. Callers should reject such a write unless it is explicit.
+ */
+export function wouldClearSceneContent(existing: unknown, incoming: unknown): boolean {
+  return countContentNodes(existing) > 0 && countContentNodes(incoming) === 0
+}
diff --git a/apps/editor/next.config.ts b/apps/editor/next.config.ts
index 18fb182064..d01630cb91 100644
--- a/apps/editor/next.config.ts
+++ b/apps/editor/next.config.ts
@@ -4,6 +4,18 @@ const nextConfig: NextConfig = {
   logging: {
     browserToTerminal: true,
   },
+  // The scene store hands out `/editor/<id>` as the canonical project URL
+  // (`packages/mcp/src/storage/sqlite-scene-store.ts` `editorUrlForScene`), and
+  // the hosted product serves that path. This app names its route `/scene/[id]`,
+  // so every MCP-reported `editorUrl` 404s here.
+  //
+  // Rewrite rather than redirect: the browser path has to keep the `/editor/`
+  // prefix because client code parses the project id back out of it (see
+  // `packages/editor/src/components/ui/action-menu/view-toggles.tsx`, scan
+  // upload). `/scene/<id>` keeps working for the app's own links.
+  async rewrites() {
+    return [{ source: '/editor/:id', destination: '/scene/:id' }]
+  },
   typescript: {
     ignoreBuildErrors: true,
   },
diff --git a/apps/editor/package.json b/apps/editor/package.json
index 862df6503f..6818c59d14 100644
--- a/apps/editor/package.json
+++ b/apps/editor/package.json
@@ -4,7 +4,7 @@
   "type": "module",
   "private": true,
   "scripts": {
-    "dev": "dotenv -e ../../.env.local -- next dev --port ${PORT:-3002}",
+    "dev": "dotenv -e ../../.env.local -- next dev --hostname 0.0.0.0 --port 3002",
     "build": "dotenv -e ../../.env.local -- next build",
     "start": "next start",
     "lint": "biome lint",