diff --git a/src/storage/baseStorage.ts b/src/storage/baseStorage.ts
index e83b6841..53dcd901 100644
--- a/src/storage/baseStorage.ts
+++ b/src/storage/baseStorage.ts
@@ -25,7 +25,7 @@ import {
   NOUN_TYPE_COUNT,
   VERB_TYPE_COUNT
 } from '../types/graphTypes.js'
-import { getShardIdFromUuid } from './sharding.js'
+import { getShardId } from './sharding.js'
 import { BlobStorage, type BlobStoreAdapter } from './blobStorage.js'
 import { unwrapBinaryData } from './binaryDataCodec.js'
 import { prodLog } from '../utils/logger.js'
@@ -161,7 +161,7 @@ function isSingletonSystemKey(key: string): boolean {
  * No type parameter needed - direct O(1) lookup by ID
  */
 function getNounVectorPath(id: string): string {
-  const shard = getShardIdFromUuid(id)
+  const shard = getShardId(id)
   return `entities/nouns/${shard}/${id}/vectors.json`
 }
 
@@ -170,7 +170,7 @@ function getNounVectorPath(id: string): string {
  * No type parameter needed - direct O(1) lookup by ID
  */
 function getNounMetadataPath(id: string): string {
-  const shard = getShardIdFromUuid(id)
+  const shard = getShardId(id)
   return `entities/nouns/${shard}/${id}/metadata.json`
 }
 
@@ -179,7 +179,7 @@ function getNounMetadataPath(id: string): string {
  * No type parameter needed - direct O(1) lookup by ID
  */
 function getVerbVectorPath(id: string): string {
-  const shard = getShardIdFromUuid(id)
+  const shard = getShardId(id)
   return `entities/verbs/${shard}/${id}/vectors.json`
 }
 
@@ -188,7 +188,7 @@ function getVerbVectorPath(id: string): string {
  * No type parameter needed - direct O(1) lookup by ID
  */
 function getVerbMetadataPath(id: string): string {
-  const shard = getShardIdFromUuid(id)
+  const shard = getShardId(id)
   return `entities/verbs/${shard}/${id}/metadata.json`
 }
 
@@ -392,7 +392,7 @@ export abstract class BaseStorage extends BaseStorageAdapter {
     }
 
     // Valid entity UUID - apply sharding
-    const shardId = getShardIdFromUuid(id)
+    const shardId = getShardId(id)
 
     if (context === 'noun-metadata') {
       return {
diff --git a/src/storage/sharding.ts b/src/storage/sharding.ts
index 96ff2c30..f13791dc 100644
--- a/src/storage/sharding.ts
+++ b/src/storage/sharding.ts
@@ -1,55 +1,77 @@
 /**
- * Unified UUID-based sharding for all storage adapters
+ * Unified id-based sharding for all storage adapters.
  *
- * Uses first 2 hex characters of UUID for consistent, predictable sharding
- * that scales from hundreds to millions of entities without configuration.
+ * Maps any entity id to one of 256 buckets (00-ff) for consistent, predictable
+ * on-disk layout that scales from hundreds to millions of entities with no
+ * configuration.
  *
  * Sharding characteristics:
  * - 256 buckets (00-ff)
- * - Deterministic (same UUID always maps to same shard)
+ * - Deterministic (the same id always maps to the same shard)
  * - No configuration required
- * - Works across all storage types (filesystem, S3, GCS, memory)
+ * - Works across the filesystem and memory adapters
  * - Efficient for list operations and pagination
+ *
+ * Two id shapes are supported, by design:
+ * - **UUID-format ids** (32 hex chars) shard by their first byte. This is the
+ *   original scheme, kept byte-for-byte so existing on-disk layouts never move.
+ * - **Application ids** (`'user-123'`, slugs, emails — anything else) shard by a
+ *   stable FNV-1a hash. Requiring callers to mint UUIDs would break the
+ *   documented `add({ id: '…' })` contract; a shard is only a bucket, so any
+ *   deterministic, well-distributed mapping is correct.
+ *
+ * The hash is part of the on-disk contract: **never change it** — doing so would
+ * relocate every application-id record.
  */
 
 /**
- * Extract shard ID from UUID
+ * 32-bit FNV-1a hash of a string, returned as a 2-hex-char shard bucket (00-ff).
+ * Deterministic and uniformly distributed across the 256 buckets.
  *
- * Uses first 2 hex characters of the UUID as the shard ID.
- * This provides 256 evenly-distributed buckets (00-ff).
+ * @param value - The string to hash (an entity id that is not UUID-format).
+ * @returns A 2-character hex shard id.
+ */
+function hashToShardId(value: string): string {
+  let hash = 0x811c9dc5 // FNV offset basis
+  for (let i = 0; i < value.length; i++) {
+    hash ^= value.charCodeAt(i)
+    hash = Math.imul(hash, 0x01000193) // FNV prime
+  }
+  return ((hash >>> 0) & 0xff).toString(16).padStart(2, '0')
+}
+
+/**
+ * Resolve the shard bucket (00-ff) for an entity id.
  *
- * @param uuid - UUID string (with or without hyphens)
- * @returns 2-character hex shard ID (00-ff)
+ * UUID-format ids (32 hex chars, with or without hyphens) shard by their first
+ * byte — preserving the original on-disk layout. Any other id is hashed (FNV-1a)
+ * into the same 256-bucket space, so application-supplied ids like `'user-123'`
+ * work without forcing callers to mint UUIDs.
+ *
+ * @param id - The entity id (UUID-format or any application string).
+ * @returns A 2-character hex shard id (00-ff).
+ * @throws If `id` is empty.
  *
  * @example
  * ```typescript
- * getShardIdFromUuid('ab123456-1234-5678-9abc-def012345678') // returns 'ab'
- * getShardIdFromUuid('cd987654-4321-8765-cba9-fed543210987') // returns 'cd'
- * getShardIdFromUuid('00000000-0000-0000-0000-000000000000') // returns '00'
+ * getShardId('ab123456-1234-5678-9abc-def012345678') // 'ab' (UUID first byte)
+ * getShardId('user-12345')                            // FNV-1a hash bucket
  * ```
  */
-export function getShardIdFromUuid(uuid: string): string {
-  if (!uuid) {
-    throw new Error('UUID is required for sharding')
+export function getShardId(id: string): string {
+  if (!id) {
+    throw new Error('id is required for sharding')
   }
 
-  // Remove hyphens and convert to lowercase
-  const normalized = uuid.toLowerCase().replace(/-/g, '')
-
-  // Validate UUID format (32 hex characters)
-  if (normalized.length !== 32) {
-    throw new Error(`Invalid UUID format: ${uuid} (expected 32 hex chars, got ${normalized.length})`)
+  // UUID-format ids (32 hex chars) keep the original first-byte bucketing so
+  // existing on-disk data is never relocated.
+  const normalized = id.toLowerCase().replace(/-/g, '')
+  if (/^[0-9a-f]{32}$/.test(normalized)) {
+    return normalized.substring(0, 2)
   }
 
-  // Extract first 2 characters
-  const shardId = normalized.substring(0, 2)
-
-  // Validate hex format
-  if (!/^[0-9a-f]{2}$/.test(shardId)) {
-    throw new Error(`Invalid UUID prefix: ${shardId} (expected 2 hex chars)`)
-  }
-
-  return shardId
+  // Any other id (application keys, slugs, emails) is hashed into a bucket.
+  return hashToShardId(id)
 }
 
 /**
diff --git a/tests/unit/storage/sharding.test.ts b/tests/unit/storage/sharding.test.ts
new file mode 100644
index 00000000..3b85a695
--- /dev/null
+++ b/tests/unit/storage/sharding.test.ts
@@ -0,0 +1,52 @@
+/**
+ * Shard-bucket resolution for entity ids.
+ *
+ * `getShardId` buckets every entity id into one of 256 dirs (00-ff). It must
+ * accept BOTH UUID-format ids (bucketed by first byte — the original on-disk
+ * layout) AND arbitrary application ids like `'user-123'` (bucketed by a stable
+ * hash). A refactor once required UUID format and threw on everything else,
+ * which broke the documented `add({ id: '…' })` contract. These tests pin the
+ * dual scheme, determinism, and distribution.
+ */
+import { describe, it, expect } from 'vitest'
+import { getShardId, getAllShardIds } from '../../../src/storage/sharding.js'
+
+describe('getShardId', () => {
+  it('buckets a UUID by its first byte (with or without hyphens)', () => {
+    expect(getShardId('ab123456-1234-5678-9abc-def012345678')).toBe('ab')
+    expect(getShardId('ab123456123456789abcdef012345678')).toBe('ab')
+    expect(getShardId('CD987654-4321-8765-cba9-fed543210987')).toBe('cd') // case-insensitive
+    expect(getShardId('00000000-0000-0000-0000-000000000000')).toBe('00')
+  })
+
+  it('buckets an arbitrary application id without throwing (the regression)', () => {
+    const shard = getShardId('user-12345')
+    expect(shard).toMatch(/^[0-9a-f]{2}$/)
+  })
+
+  it('is deterministic for application ids', () => {
+    expect(getShardId('order-42')).toBe(getShardId('order-42'))
+    expect(getShardId('alice@example.com')).toBe(getShardId('alice@example.com'))
+  })
+
+  it('distributes application ids across many buckets (not one hot bucket)', () => {
+    const buckets = new Set<string>()
+    for (let i = 0; i < 1000; i++) {
+      buckets.add(getShardId(`entity-${i}`))
+    }
+    // FNV-1a over 1000 sequential keys should touch a large fraction of the
+    // 256-bucket space; a degenerate hash would collapse to a handful.
+    expect(buckets.size).toBeGreaterThan(200)
+  })
+
+  it('every produced shard is a valid bucket id', () => {
+    const valid = new Set(getAllShardIds())
+    for (const sample of ['user-1', 'café', '🚀', 'a', 'A_VERY_LONG_APPLICATION_KEY_0001']) {
+      expect(valid.has(getShardId(sample))).toBe(true)
+    }
+  })
+
+  it('throws on an empty id', () => {
+    expect(() => getShardId('')).toThrow(/id is required/)
+  })
+})