diff --git a/test/unit/incremental-cache/generate-cache-key.test.ts b/test/unit/incremental-cache/generate-cache-key.test.ts
new file mode 100644
index 00000000..f320376b
--- /dev/null
+++ b/test/unit/incremental-cache/generate-cache-key.test.ts
@@ -0,0 +1,252 @@
+import { IncrementalCache } from '../../../packages/next/src/server/lib/incremental-cache'
+
+function createCache() {
+  return new IncrementalCache({
+    dev: false,
+    requestHeaders: {},
+    getPrerenderManifest: () =>
+      ({
+        version: 4,
+        routes: {},
+        dynamicRoutes: {},
+        notFoundRoutes: [],
+        preview: {
+          previewModeId: 'id',
+          previewModeSigningKey: 'key',
+          previewModeEncryptionKey: 'key',
+        },
+      }) as any,
+  })
+}
+
+describe('IncrementalCache.generateCacheKey', () => {
+  const cache = createCache()
+  const url = 'https://example.com/api'
+
+  it('distinguishes binary bodies that UTF-8 decoding would collapse', async () => {
+    // 0xff and 0xfe both decode to U+FFFD as UTF-8; the key must tell them
+    // apart by their raw bytes.
+    const a = await cache.generateCacheKey(url, {
+      body: new Uint8Array([0xff]),
+    })
+    const b = await cache.generateCacheKey(url, {
+      body: new Uint8Array([0xfe]),
+    })
+    expect(a).not.toBe(b)
+  })
+
+  it('distinguishes a string body from its UTF-8 encoded bytes', async () => {
+    // A string and the bytes it encodes to are different request shapes.
+    const a = await cache.generateCacheKey(url, { body: 'hello' })
+    const b = await cache.generateCacheKey(url, {
+      body: new TextEncoder().encode('hello'),
+    })
+    expect(a).not.toBe(b)
+  })
+
+  it('uses the selected bytes of arbitrary ArrayBuffer views', async () => {
+    const backing = new Uint8Array([9, 1, 2, 3, 9])
+    const dataView = new DataView(backing.buffer, 1, 3)
+    const typedView = new Uint8Array([1, 2, 3])
+
+    const a = await cache.generateCacheKey(url, { body: dataView })
+    const b = await cache.generateCacheKey(url, { body: typedView })
+    expect(a).toBe(b)
+  })
+
+  it('distinguishes URLSearchParams, FormData, and string bodies', async () => {
+    const form = new FormData()
+    form.append('x', 'a')
+
+    const formKey = await cache.generateCacheKey(url, { body: form })
+    const paramsKey = await cache.generateCacheKey(url, {
+      body: new URLSearchParams([['x', 'a']]),
+    })
+    const stringKey = await cache.generateCacheKey(url, { body: 'x=a' })
+
+    expect(new Set([formKey, paramsKey, stringKey]).size).toBe(3)
+  })
+
+  it('distinguishes a Blob from its raw content bytes', async () => {
+    const bytes = new Uint8Array([1, 2, 3])
+    const blobKey = await cache.generateCacheKey(url, {
+      body: new Blob([bytes]),
+    })
+    const bytesKey = await cache.generateCacheKey(url, { body: bytes })
+
+    expect(blobKey).not.toBe(bytesKey)
+  })
+
+  it('is deterministic for identical bodies', async () => {
+    const a = await cache.generateCacheKey(url, {
+      body: new Uint8Array([1, 2, 3]),
+    })
+    const b = await cache.generateCacheKey(url, {
+      body: new Uint8Array([1, 2, 3]),
+    })
+    expect(a).toBe(b)
+  })
+
+  it('does not collide FormData multi-values with a comma-joined string', async () => {
+    // The two values ['a', 'b'] must not hash the same as the single 'a,b'.
+    const multi = new FormData()
+    multi.append('x', 'a')
+    multi.append('x', 'b')
+
+    const joined = new FormData()
+    joined.append('x', 'a,b')
+
+    const a = await cache.generateCacheKey(url, { body: multi })
+    const b = await cache.generateCacheKey(url, { body: joined })
+    expect(a).not.toBe(b)
+  })
+
+  it('distinguishes blobs that differ only by content type', async () => {
+    const bytes = new Uint8Array([1, 2, 3])
+    const a = await cache.generateCacheKey(url, {
+      body: new Blob([bytes], { type: 'text/plain' }),
+    })
+    const b = await cache.generateCacheKey(url, {
+      body: new Blob([bytes], { type: 'application/json' }),
+    })
+    expect(a).not.toBe(b)
+  })
+
+  it('distinguishes ArrayBuffers', async () => {
+    const a = await cache.generateCacheKey(url, {
+      body: new Uint8Array([1, 2, 3, 4]).buffer,
+    })
+    const b = await cache.generateCacheKey(url, {
+      body: new Uint8Array([1, 2, 3]).buffer,
+    })
+    expect(a).not.toBe(b)
+  })
+
+  it('produces the same key for identical bytes in different typed arrays', async () => {
+    const uint8View = new Uint8Array([1, 2, 3, 4])
+    const uint16View = new Uint16Array(
+      uint8View.buffer,
+      uint8View.byteOffset,
+      uint8View.byteLength / 2
+    )
+    const a = await cache.generateCacheKey(url, {
+      body: uint8View,
+    })
+    const b = await cache.generateCacheKey(url, {
+      body: uint16View,
+    })
+    expect(a).toBe(b)
+  })
+
+  it('does not collide FormData with differently interleaved values', async () => {
+    const a = new FormData()
+    a.append('x', 'a')
+    a.append('x', 'b')
+    a.append('y', 'a')
+
+    const b = new FormData()
+    b.append('x', 'a')
+    b.append('y', 'a')
+    b.append('x', 'b')
+
+    const keyA = await cache.generateCacheKey(url, { body: a })
+    const keyB = await cache.generateCacheKey(url, { body: b })
+    expect(keyA).not.toBe(keyB)
+  })
+
+  it('does not collide FormData whose value forges quoted entry delimiters', async () => {
+    // User values may resemble framing text and must remain data.
+    const a = new FormData()
+    a.append('x', 'a"key:"y"str:"b')
+
+    const b = new FormData()
+    b.append('x', 'a')
+    b.append('y', 'b')
+
+    const keyA = await cache.generateCacheKey(url, { body: a })
+    const keyB = await cache.generateCacheKey(url, { body: b })
+    expect(keyA).not.toBe(keyB)
+  })
+
+  it('does not collide FormData whose value spans an entry boundary', async () => {
+    // One value may contain text resembling a complete following entry.
+    const a = new FormData()
+    a.append('x', 'akey:ystr:b')
+
+    const b = new FormData()
+    b.append('x', 'a')
+    b.append('y', 'b')
+
+    const keyA = await cache.generateCacheKey(url, { body: a })
+    const keyB = await cache.generateCacheKey(url, { body: b })
+    expect(keyA).not.toBe(keyB)
+  })
+
+  it('does not collide FormData files that differ only by name/type split', async () => {
+    // File name and content type are separate fetch-relevant metadata.
+    const bytes = new Uint8Array([1, 2, 3])
+
+    const a = new FormData()
+    a.append('f', new Blob([bytes], { type: 'c' }), 'ab')
+
+    const b = new FormData()
+    b.append('f', new Blob([bytes], { type: 'bc' }), 'a')
+
+    const keyA = await cache.generateCacheKey(url, { body: a })
+    const keyB = await cache.generateCacheKey(url, { body: b })
+    expect(keyA).not.toBe(keyB)
+  })
+
+  it('does not collide a file whose content forges a following entry', async () => {
+    // File bytes may resemble framing for a following field.
+    const enc = new TextEncoder()
+
+    const a = new FormData()
+    a.append('f', new Blob([enc.encode('AAAkey:1kstr:1v')], { type: '' }), 'n')
+
+    const b = new FormData()
+    b.append('f', new Blob([enc.encode('AAA')], { type: '' }), 'n')
+    b.append('k', 'v')
+
+    const keyA = await cache.generateCacheKey(url, { body: a })
+    const keyB = await cache.generateCacheKey(url, { body: b })
+    expect(keyA).not.toBe(keyB)
+  })
+
+  it('does not collide blobs that differ only by type/content split', async () => {
+    // Blob content types and bytes must be framed as separate values.
+    const enc = new TextEncoder()
+
+    const a = new Blob([enc.encode('y')], { type: 'bytes:x' })
+    const b = new Blob([enc.encode('xbytes:y')], { type: '' })
+
+    const keyA = await cache.generateCacheKey(url, { body: a })
+    const keyB = await cache.generateCacheKey(url, { body: b })
+    expect(keyA).not.toBe(keyB)
+  })
+
+  it('does not collide a FormData value whose length digits absorb a forged entry', async () => {
+    // Length digits adjacent to user data must not make framing ambiguous.
+    const a = new FormData()
+    a.append('x', 'key:1astr:0')
+
+    const b = new FormData()
+    b.append('x', '1')
+    b.append('a', '')
+
+    const keyA = await cache.generateCacheKey(url, { body: a })
+    const keyB = await cache.generateCacheKey(url, { body: b })
+    expect(keyA).not.toBe(keyB)
+  })
+
+  it('does not collide blobs whose type-length digits absorb the content', async () => {
+    // Type lengths and content beginning with digits remain distinct.
+    const enc = new TextEncoder()
+    const a = new Blob([new Uint8Array(0)], { type: 'bytes:aaaaa' })
+    const b = new Blob([enc.encode('aaaaabytes:')], { type: '1' })
+
+    const keyA = await cache.generateCacheKey(url, { body: a })
+    const keyB = await cache.generateCacheKey(url, { body: b })
+    expect(keyA).not.toBe(keyB)
+  })
+})