diff --git a/frontend/src/lib/components/graph/graphBuilder.test.ts b/frontend/src/lib/components/graph/graphBuilder.test.ts
new file mode 100644
index 0000000000..1203ab57cd
--- /dev/null
+++ b/frontend/src/lib/components/graph/graphBuilder.test.ts
@@ -0,0 +1,23 @@
+import { describe, it, expect, vi } from 'vitest'
+
+// Mock modules that transitively import CSS/Monaco
+vi.mock('monaco-editor', () => ({}))
+vi.mock('@xyflow/svelte', () => ({}))
+vi.mock('./renderers/nodes/AssetNode.svelte', () => ({
+	assetDisplaysAsOutputInFlowGraph: () => false
+}))
+vi.mock('../modulesTest.svelte', () => ({}))
+
+import { topologicalSort } from './graphBuilder.svelte'
+
+describe('topologicalSort', () => {
+	it('skips parent ids with no node instead of throwing', () => {
+		const sorted = topologicalSort([
+			{ id: 'a' },
+			{ id: 'b', parentIds: ['a'] },
+			{ id: 'orphan', parentIds: ['deleted'] }
+		])
+
+		expect(sorted.map((n) => n.id)).toEqual(['orphan', 'b', 'a'])
+	})
+})