diff --git a/frontend/src/lib/utils.test.ts b/frontend/src/lib/utils.test.ts
new file mode 100644
index 0000000..6cabae1
--- /dev/null
+++ b/frontend/src/lib/utils.test.ts
@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest'
+
+import { formatBytes } from './utils'
+
+describe('formatBytes', () => {
+  it('formats zero and whole-byte values', () => {
+    expect(formatBytes(0)).toBe('0 B')
+    expect(formatBytes(512)).toBe('512.0 B')
+  })
+
+  it('keeps fractional bytes in the byte unit', () => {
+    expect(formatBytes(0.5)).toBe('0.5 B')
+  })
+
+  it('handles invalid sizes without rendering undefined units', () => {
+    expect(formatBytes(-1)).toBe('0 B')
+    expect(formatBytes(Number.NaN)).toBe('0 B')
+    expect(formatBytes(Number.POSITIVE_INFINITY)).toBe('0 B')
+  })
+})
diff --git a/frontend/src/lib/utils.ts b/frontend/src/lib/utils.ts
index 502c33a..8c2ca65 100644
--- a/frontend/src/lib/utils.ts
+++ b/frontend/src/lib/utils.ts
@@ -20,10 +20,10 @@ export function formatDuration(seconds: number): string {
 }
 
 export function formatBytes(bytes: number): string {
-  if (bytes === 0) return '0 B'
+  if (!Number.isFinite(bytes) || bytes <= 0) return '0 B'
   const k = 1024
   const sizes = ['B', 'KB', 'MB', 'GB', 'TB']
-  const i = Math.floor(Math.log(bytes) / Math.log(k))
+  const i = Math.min(Math.max(Math.floor(Math.log(bytes) / Math.log(k)), 0), sizes.length - 1)
   return `${(bytes / Math.pow(k, i)).toFixed(1)} ${sizes[i]}`
 }
 
diff --git a/frontend/src/test/setup.ts b/frontend/src/test/setup.ts
new file mode 100644
index 0000000..c44951a
--- /dev/null
+++ b/frontend/src/test/setup.ts
@@ -0,0 +1 @@
+import '@testing-library/jest-dom'