diff --git a/common/src/util/__tests__/response-ad-positions.test.ts b/common/src/util/__tests__/response-ad-positions.test.ts
new file mode 100644
index 0000000000..6944e1ee4e
--- /dev/null
+++ b/common/src/util/__tests__/response-ad-positions.test.ts
@@ -0,0 +1,60 @@
+import { describe, expect, it } from 'bun:test'
+
+import {
+  responseAdSlotCount,
+  responseAdNodePositions,
+  RESPONSE_AD_FIRST_NODE_COUNT,
+  RESPONSE_AD_NODE_STEP,
+} from '../response-ad-positions'
+
+describe('responseAdSlotCount', () => {
+  it('returns 0 when nodeCount is less than firstAdAfterNodes', () => {
+    expect(responseAdSlotCount({ nodeCount: 1 })).toBe(0)
+    expect(responseAdSlotCount({ nodeCount: 0 })).toBe(0)
+  })
+
+  it('returns correct count for valid nodeCount', () => {
+    expect(responseAdSlotCount({ nodeCount: 10 })).toBe(3)
+    expect(responseAdSlotCount({ nodeCount: 20 })).toBe(6)
+  })
+
+  it('handles NaN nodeCount gracefully', () => {
+    expect(responseAdSlotCount({ nodeCount: NaN })).toBe(0)
+  })
+
+  it('handles Infinity nodeCount gracefully', () => {
+    expect(responseAdSlotCount({ nodeCount: Infinity })).toBe(0)
+  })
+
+  it('handles negative nodeCount gracefully', () => {
+    expect(responseAdSlotCount({ nodeCount: -5 })).toBe(0)
+  })
+
+  it('uses custom step when provided', () => {
+    expect(responseAdSlotCount({ nodeCount: 20, step: 5 })).toBe(4)
+  })
+
+  it('uses custom firstAdAfterNodes when provided', () => {
+    expect(responseAdSlotCount({ nodeCount: 20, firstAdAfterNodes: 5 })).toBe(5)
+  })
+})
+
+describe('responseAdNodePositions', () => {
+  it('returns empty array when adCount is 0', () => {
+    expect(responseAdNodePositions({ nodeCount: 10, adCount: 0 })).toEqual([])
+  })
+
+  it('returns correct positions for valid inputs', () => {
+    const result = responseAdNodePositions({ nodeCount: 20, adCount: 2 })
+    expect(result).toEqual([1, 4])
+  })
+
+  it('handles NaN nodeCount gracefully', () => {
+    expect(responseAdNodePositions({ nodeCount: NaN, adCount: 2 })).toEqual([])
+  })
+
+  it('handles Infinity nodeCount gracefully', () => {
+    expect(responseAdNodePositions({ nodeCount: Infinity, adCount: 2 })).toEqual([])
+  })
+})
+
diff --git a/common/src/util/response-ad-positions.ts b/common/src/util/response-ad-positions.ts
index bdaf6db288..9b06df9349 100644
--- a/common/src/util/response-ad-positions.ts
+++ b/common/src/util/response-ad-positions.ts
@@ -18,14 +18,14 @@ export function responseAdSlotCount(params: {
   step?: number
   firstAdAfterNodes?: number
 }): number {
-  const step = Math.max(1, params.step ?? RESPONSE_AD_NODE_STEP)
-  const firstAdAfterNodes = Math.max(
-    1,
-    params.firstAdAfterNodes ?? RESPONSE_AD_FIRST_NODE_COUNT,
-  )
+  const safeNodeCount = Number.isFinite(params.nodeCount) ? params.nodeCount : 0
+  const safeStep = params.step !== undefined && Number.isFinite(params.step) ? params.step : RESPONSE_AD_NODE_STEP
+  const safeFirstAdAfterNodes = params.firstAdAfterNodes !== undefined && Number.isFinite(params.firstAdAfterNodes) ? params.firstAdAfterNodes : RESPONSE_AD_FIRST_NODE_COUNT
+  const step = Math.max(1, safeStep)
+  const firstAdAfterNodes = Math.max(1, safeFirstAdAfterNodes)
   return Math.max(
     0,
-    Math.floor((params.nodeCount - firstAdAfterNodes - 1) / step) + 1,
+    Math.floor((safeNodeCount - firstAdAfterNodes - 1) / step) + 1,
   )
 }