diff --git a/src/document/scene-document-validation.ts b/src/document/scene-document-validation.ts
new file mode 100644
index 00000000..428e44aa
--- /dev/null
+++ b/src/document/scene-document-validation.ts
@@ -0,0 +1,171 @@
+import type { EntityInstance } from "../entities/entity-instances";
+import { BOX_FACE_IDS, hasPositiveBoxSize } from "./brushes";
+import type { SceneDocument } from "./scene-document";
+
+export type SceneDiagnosticSeverity = "error" | "warning";
+export type SceneDiagnosticScope = "document" | "build";
+
+export interface SceneDiagnostic {
+  code: string;
+  severity: SceneDiagnosticSeverity;
+  scope: SceneDiagnosticScope;
+  message: string;
+  path?: string;
+}
+
+export interface SceneDocumentValidationResult {
+  diagnostics: SceneDiagnostic[];
+  errors: SceneDiagnostic[];
+  warnings: SceneDiagnostic[];
+}
+
+function createDiagnostic(
+  severity: SceneDiagnosticSeverity,
+  code: string,
+  message: string,
+  path?: string,
+  scope: SceneDiagnosticScope = "document"
+): SceneDiagnostic {
+  return {
+    code,
+    severity,
+    scope,
+    message,
+    path
+  };
+}
+
+function isFiniteNumber(value: unknown): value is number {
+  return typeof value === "number" && Number.isFinite(value);
+}
+
+function isFiniteVec3(vector: { x: unknown; y: unknown; z: unknown }): boolean {
+  return isFiniteNumber(vector.x) && isFiniteNumber(vector.y) && isFiniteNumber(vector.z);
+}
+
+function validatePlayerStartEntity(entity: EntityInstance, path: string, diagnostics: SceneDiagnostic[]) {
+  if (!isFiniteVec3(entity.position)) {
+    diagnostics.push(
+      createDiagnostic("error", "invalid-player-start-position", "Player Start position must remain finite on every axis.", `${path}.position`)
+    );
+  }
+
+  if (!isFiniteNumber(entity.yawDegrees)) {
+    diagnostics.push(createDiagnostic("error", "invalid-player-start-yaw", "Player Start yaw must remain a finite number.", `${path}.yawDegrees`));
+  }
+}
+
+function registerAuthoredId(id: string, path: string, seenIds: Map<string, string>, diagnostics: SceneDiagnostic[]) {
+  const previousPath = seenIds.get(id);
+
+  if (previousPath !== undefined) {
+    diagnostics.push(
+      createDiagnostic("error", "duplicate-authored-id", `Duplicate authored id ${id} is already used at ${previousPath}.`, path)
+    );
+    return;
+  }
+
+  seenIds.set(id, path);
+}
+
+export function formatSceneDiagnostic(diagnostic: SceneDiagnostic): string {
+  return diagnostic.path === undefined ? diagnostic.message : `${diagnostic.path}: ${diagnostic.message}`;
+}
+
+export function formatSceneDiagnosticSummary(diagnostics: SceneDiagnostic[], limit = 3): string {
+  if (diagnostics.length === 0) {
+    return "No diagnostics.";
+  }
+
+  const visibleDiagnostics = diagnostics.slice(0, Math.max(1, limit));
+  const summary = visibleDiagnostics.map((diagnostic) => formatSceneDiagnostic(diagnostic)).join("; ");
+  const remainingCount = diagnostics.length - visibleDiagnostics.length;
+
+  return remainingCount > 0 ? `${summary}; +${remainingCount} more` : summary;
+}
+
+export function validateSceneDocument(document: SceneDocument): SceneDocumentValidationResult {
+  const diagnostics: SceneDiagnostic[] = [];
+  const seenIds = new Map<string, string>();
+
+  for (const [materialKey, material] of Object.entries(document.materials)) {
+    const path = `materials.${materialKey}`;
+
+    if (material.id !== materialKey) {
+      diagnostics.push(
+        createDiagnostic("error", "material-id-mismatch", "Material ids must match their registry key.", `${path}.id`)
+      );
+    }
+
+    registerAuthoredId(material.id, path, seenIds, diagnostics);
+  }
+
+  for (const [brushKey, brush] of Object.entries(document.brushes)) {
+    const path = `brushes.${brushKey}`;
+
+    if (brush.id !== brushKey) {
+      diagnostics.push(createDiagnostic("error", "brush-id-mismatch", "Brush ids must match their registry key.", `${path}.id`));
+    }
+
+    registerAuthoredId(brush.id, path, seenIds, diagnostics);
+
+    if (!isFiniteVec3(brush.size) || !hasPositiveBoxSize(brush.size)) {
+      diagnostics.push(
+        createDiagnostic("error", "invalid-box-size", "Box brush sizes must remain finite and positive on every axis.", `${path}.size`)
+      );
+    }
+
+    for (const faceId of BOX_FACE_IDS) {
+      const materialId = brush.faces[faceId].materialId;
+
+      if (materialId !== null && document.materials[materialId] === undefined) {
+        diagnostics.push(
+          createDiagnostic(
+            "error",
+            "missing-material-ref",
+            `Face material reference ${materialId} does not exist in the document material registry.`,
+            `${path}.faces.${faceId}.materialId`
+          )
+        );
+      }
+    }
+  }
+
+  for (const [entityKey, entity] of Object.entries(document.entities)) {
+    const path = `entities.${entityKey}`;
+
+    if (entity.id !== entityKey) {
+      diagnostics.push(createDiagnostic("error", "entity-id-mismatch", "Entity ids must match their registry key.", `${path}.id`));
+    }
+
+    registerAuthoredId(entity.id, path, seenIds, diagnostics);
+
+    if (entity.kind === "playerStart") {
+      validatePlayerStartEntity(entity, path, diagnostics);
+      continue;
+    }
+
+    diagnostics.push(
+      createDiagnostic(
+        "error",
+        "unsupported-entity-kind",
+        `Unsupported entity kind ${(entity as { kind: string }).kind}.`,
+        `${path}.kind`
+      )
+    );
+  }
+
+  return {
+    diagnostics,
+    errors: diagnostics.filter((diagnostic) => diagnostic.severity === "error"),
+    warnings: diagnostics.filter((diagnostic) => diagnostic.severity === "warning")
+  };
+}
+
+export function assertSceneDocumentIsValid(document: SceneDocument) {
+  const validation = validateSceneDocument(document);
+
+  if (validation.errors.length > 0) {
+    throw new Error(`Scene document has ${validation.errors.length} validation error(s): ${formatSceneDiagnosticSummary(validation.errors)}`);
+  }
+}