diff --git a/.changeset/supabase-encryption-error.md b/.changeset/supabase-encryption-error.md
new file mode 100644
index 00000000..d83143e0
--- /dev/null
+++ b/.changeset/supabase-encryption-error.md
@@ -0,0 +1,11 @@
+---
+'@cipherstash/stack-supabase': patch
+---
+
+Populate `EncryptedSupabaseError.encryptionError` on encryption failures (#626).
+The query builder's catch block previously hardcoded `encryptionError: undefined`,
+so the typed field was always empty and callers had to detect encryption failures
+indirectly (via `status`/`statusText` or `.throwOnError()`). It now threads the
+underlying `EncryptionError` through — for both the v2 and v3 dialects — when the
+failure originates in an encrypt/decrypt step, and leaves it unset for plain
+PostgREST/API errors.
diff --git a/packages/stack-supabase/__tests__/supabase-encryption-error.test.ts b/packages/stack-supabase/__tests__/supabase-encryption-error.test.ts
new file mode 100644
index 00000000..5154b8c1
--- /dev/null
+++ b/packages/stack-supabase/__tests__/supabase-encryption-error.test.ts
@@ -0,0 +1,165 @@
+import type { EncryptionClient } from '@cipherstash/stack/encryption'
+import { encryptedTable, types } from '@cipherstash/stack/eql/v3'
+import { EncryptionErrorTypes } from '@cipherstash/stack/errors'
+import {
+  encryptedColumn,
+  encryptedTable as encryptedTableV2,
+} from '@cipherstash/stack/schema'
+import { describe, expect, it } from 'vitest'
+import { EncryptedQueryBuilderImpl } from '../src/query-builder'
+import { EncryptedQueryBuilderV3Impl } from '../src/query-builder-v3'
+import {
+  createMockEncryptionClient,
+  createMockSupabase,
+  fakeEnvelope,
+  operation,
+} from './helpers/supabase-mock'
+
+/**
+ * Regression coverage for #626: the query builder's catch block used to hardcode
+ * `encryptionError: undefined`, so the typed `EncryptedSupabaseError.encryptionError`
+ * field was dead. The v2 tests pin that a genuine encryption failure now threads its
+ * `EncryptionError` through the shared base `execute()` catch, while a plain
+ * (non-encryption) throw leaves it unset. The v3 tests cover the dialect's own
+ * `encryptionFailure` path, which synthesizes an `EncryptionError` for its two
+ * query-term contract-violation cases (length mismatch, null envelope) that have
+ * no operation failure to wrap.
+ */
+
+const usersV2 = encryptedTableV2('users', {
+  email: encryptedColumn('email').freeTextSearch().equality(),
+})
+
+const usersV3 = encryptedTable('users', {
+  email: types.TextEq('email'),
+})
+
+/** A chainable op that resolves to `{ failure }`, like a real failed operation. */
+function failingOperation(failure: { type: string; message: string }) {
+  const op = {
+    withLockContext: () => op,
+    audit: () => op,
+    then: (
+      onfulfilled?: ((value: { failure: typeof failure }) => unknown) | null,
+      onrejected?: ((reason: unknown) => unknown) | null,
+    ) => Promise.resolve({ failure }).then(onfulfilled, onrejected),
+  }
+  return op
+}
+
+describe('EncryptedSupabaseError.encryptionError (#626)', () => {
+  it('v2: threads the EncryptionError through on an encryption failure', async () => {
+    const failure = {
+      type: EncryptionErrorTypes.EncryptionError,
+      message: 'zerokms unreachable',
+    }
+    const encryptionClient = createMockEncryptionClient() as unknown as Record<
+      string,
+      unknown
+    >
+    encryptionClient['encryptModel'] = () => failingOperation(failure)
+
+    const { client: supabase } = createMockSupabase()
+    const builder = new EncryptedQueryBuilderImpl(
+      'users',
+      usersV2,
+      encryptionClient as unknown as EncryptionClient,
+      supabase,
+    )
+
+    const { data, error } = await builder.insert({ email: 'ada@example.com' })
+
+    expect(data).toBeNull()
+    expect(error).not.toBeNull()
+    expect(error?.encryptionError).toEqual(failure)
+    expect(error?.encryptionError?.type).toBe(
+      EncryptionErrorTypes.EncryptionError,
+    )
+  })
+
+  it('v2: leaves encryptionError unset on a plain (non-encryption) error', async () => {
+    const encryptionClient = createMockEncryptionClient()
+    const { client: supabase } = createMockSupabase()
+    // Make the underlying supabase call throw a non-encryption error: an insert
+    // with no encrypted columns skips encryption and goes straight to the wire.
+    supabase.from = () => {
+      throw new Error('PostgREST is down')
+    }
+
+    const builder = new EncryptedQueryBuilderImpl(
+      'users',
+      usersV2,
+      encryptionClient,
+      supabase,
+    )
+
+    const { data, error } = await builder.insert({ id: 1 } as never)
+
+    expect(data).toBeNull()
+    expect(error?.message).toContain('PostgREST is down')
+    expect(error?.encryptionError).toBeUndefined()
+  })
+
+  it('v3: synthesizes an EncryptionError for a query-term contract violation', async () => {
+    // The v3 dialect's own `encryptionFailure` path has no operation failure to
+    // wrap for its contract-violation cases, so it synthesizes an EncryptionError.
+    // Drive it directly: a two-element `in` list whose bulkEncrypt returns one
+    // term trips the length-mismatch check. (The base `execute()` threading is
+    // already covered by the v2 test above; overriding `encryptModel` here would
+    // only re-run that shared path, not this v3-specific branch.)
+    const encryptionClient = createMockEncryptionClient() as unknown as {
+      bulkEncrypt: (...args: unknown[]) => unknown
+    }
+    encryptionClient.bulkEncrypt = () =>
+      operation([{ data: fakeEnvelope('ada', 'email') }])
+
+    const { client: supabase } = createMockSupabase()
+    const { error, status } = await new EncryptedQueryBuilderV3Impl(
+      'users',
+      usersV3,
+      encryptionClient as unknown as EncryptionClient,
+      supabase,
+      ['id', 'email'],
+    )
+      .select('id')
+      .in('email', ['ada@example.com', 'grace@example.com'])
+
+    expect(status).toBe(500)
+    expect(error?.encryptionError?.type).toBe(
+      EncryptionErrorTypes.EncryptionError,
+    )
+    expect(error?.encryptionError?.message).toMatch(
+      /1 term(s)? for 2 value(s)?/,
+    )
+  })
+
+  it('v3: synthesizes an EncryptionError for a null-envelope contract violation', async () => {
+    // The other no-cause `encryptionFailure` path: a length-matched bulk response
+    // whose position 0 is a null envelope. Same synthesized-EncryptionError branch
+    // as the length-mismatch case above, reached from a different call site.
+    const encryptionClient = createMockEncryptionClient() as unknown as {
+      bulkEncrypt: (...args: unknown[]) => unknown
+    }
+    encryptionClient.bulkEncrypt = () =>
+      operation([{ data: null }, { data: fakeEnvelope('grace', 'email') }])
+
+    const { client: supabase } = createMockSupabase()
+    const { error, status } = await new EncryptedQueryBuilderV3Impl(
+      'users',
+      usersV3,
+      encryptionClient as unknown as EncryptionClient,
+      supabase,
+      ['id', 'email'],
+    )
+      .select('id')
+      .in('email', ['ada@example.com', 'grace@example.com'])
+
+    expect(status).toBe(500)
+    expect(error?.encryptionError?.type).toBe(
+      EncryptionErrorTypes.EncryptionError,
+    )
+    expect(error?.encryptionError?.message).toMatch(
+      /null envelope at position 0/,
+    )
+  })
+})
diff --git a/packages/stack-supabase/src/query-builder-v3.ts b/packages/stack-supabase/src/query-builder-v3.ts
index b1bb131d..a84de692 100644
--- a/packages/stack-supabase/src/query-builder-v3.ts
+++ b/packages/stack-supabase/src/query-builder-v3.ts
@@ -5,6 +5,10 @@ import {
 } from '@cipherstash/stack/adapter-kit'
 import type { EncryptionClient } from '@cipherstash/stack/encryption'
 import type { AnyV3Table } from '@cipherstash/stack/eql/v3'
+import {
+  type EncryptionError,
+  EncryptionErrorTypes,
+} from '@cipherstash/stack/errors'
 import type {
   ColumnSchema,
   EncryptedTable,
@@ -417,13 +421,17 @@ export class EncryptedQueryBuilderV3Impl<
     return column
   }
 
-  private encryptionFailure(message: string, cause?: unknown): never {
+  private encryptionFailure(message: string, cause?: EncryptionError): never {
     logger.error(
       `Supabase: failed to encrypt query terms for table "${this.tableName}"`,
     )
+    // Most callers pass the operation's own `EncryptionError`; the contract-
+    // violation cases (bulk length mismatch, null envelope) have none, so
+    // synthesize one — a broken query encryption is still an encryption failure,
+    // and callers branch on `error.encryptionError` regardless.
     throw new EncryptionFailedError(
       `Failed to encrypt query terms: ${message}`,
-      cause,
+      cause ?? { type: EncryptionErrorTypes.EncryptionError, message },
     )
   }
 
diff --git a/packages/stack-supabase/src/query-builder.ts b/packages/stack-supabase/src/query-builder.ts
index 29b80903..57d0fc03 100644
--- a/packages/stack-supabase/src/query-builder.ts
+++ b/packages/stack-supabase/src/query-builder.ts
@@ -6,6 +6,7 @@ import {
   modelToEncryptedPgComposites,
 } from '@cipherstash/stack/adapter-kit'
 import type { EncryptionClient } from '@cipherstash/stack/encryption'
+import type { EncryptionError } from '@cipherstash/stack/errors'
 import type { LockContext } from '@cipherstash/stack/identity'
 import type {
   EncryptedTable,
@@ -419,9 +420,17 @@ export class EncryptedQueryBuilderImpl<
         `Supabase encrypted query failed on table "${this.tableName}": ${message}`,
       )
 
+      // A failure inside any of the encrypt/decrypt steps above is thrown as an
+      // `EncryptionFailedError` wrapping the operation's `EncryptionError` (or, in
+      // the v3 dialect, a synthesized one for its contract-violation cases).
+      // Thread it through so callers can branch on `error.encryptionError`; a plain
+      // PostgREST/API error is not an `EncryptionFailedError` and leaves it unset.
       const error: EncryptedSupabaseError = {
         message,
-        encryptionError: undefined,
+        encryptionError:
+          err instanceof EncryptionFailedError
+            ? err.encryptionError
+            : undefined,
       }
 
       if (this.shouldThrowOnError) {
@@ -1599,9 +1608,9 @@ type RawSupabaseResult = {
 }
 
 export class EncryptionFailedError extends Error {
-  public encryptionError: unknown
+  public encryptionError: EncryptionError
 
-  constructor(message: string, encryptionError: unknown) {
+  constructor(message: string, encryptionError: EncryptionError) {
     super(message)
     this.name = 'EncryptionFailedError'
     this.encryptionError = encryptionError