diff --git a/src/runtime-three/runtime-scene-validation.ts b/src/runtime-three/runtime-scene-validation.ts
new file mode 100644
index 00000000..c0ecaedb
--- /dev/null
+++ b/src/runtime-three/runtime-scene-validation.ts
@@ -0,0 +1,49 @@
+import type { SceneDocument } from "../document/scene-document";
+import {
+  assertSceneDocumentIsValid,
+  createDiagnostic,
+  formatSceneDiagnosticSummary,
+  type SceneDiagnostic
+} from "../document/scene-document-validation";
+import { getPrimaryPlayerStartEntity } from "../entities/entity-instances";
+
+export interface RuntimeSceneBuildValidationResult {
+  diagnostics: SceneDiagnostic[];
+  errors: SceneDiagnostic[];
+  warnings: SceneDiagnostic[];
+}
+
+export function validateRuntimeSceneBuild(
+  document: SceneDocument,
+  navigationMode: "firstPerson" | "orbitVisitor"
+): RuntimeSceneBuildValidationResult {
+  const diagnostics: SceneDiagnostic[] = [];
+
+  if (navigationMode === "firstPerson" && getPrimaryPlayerStartEntity(document.entities) === null) {
+    diagnostics.push(
+      createDiagnostic(
+        "error",
+        "missing-player-start",
+        "First-person run requires an authored Player Start. Place one or switch to Orbit Visitor.",
+        "entities",
+        "build"
+      )
+    );
+  }
+
+  return {
+    diagnostics,
+    errors: diagnostics.filter((diagnostic) => diagnostic.severity === "error"),
+    warnings: diagnostics.filter((diagnostic) => diagnostic.severity === "warning")
+  };
+}
+
+export function assertRuntimeSceneBuildable(document: SceneDocument, navigationMode: "firstPerson" | "orbitVisitor") {
+  assertSceneDocumentIsValid(document);
+
+  const validation = validateRuntimeSceneBuild(document, navigationMode);
+
+  if (validation.errors.length > 0) {
+    throw new Error(`Runtime build is blocked: ${formatSceneDiagnosticSummary(validation.errors)}`);
+  }
+}