diff --git a/README.md b/README.md
index 910176c..a1342f5 100644
--- a/README.md
+++ b/README.md
@@ -178,8 +178,9 @@ Swedish personnummer): `address`, `color`, `company`, `date`, `email`, `ip`,
 `data/misc` carries locale-neutral categories — universal data that isn't tied to
 a language or region:
 
-- ids & networking: `uuid` (a proper random v4), `mac`, `creditcard` (per-network
-  numbers ending in a valid `{luhn()}` digit)
+- ids & networking: `uuid` (a proper random v4), `mac`, `objectid` (MongoDB
+  ObjectID, 24 hex chars), `creditcard` (per-network numbers ending in a valid
+  `{luhn()}` digit)
 - reference codes: `currency` (ISO 4217), `country` (ISO 3166), `language` (ISO
   639), `timezone` (IANA)
 - web & systems: `mimetype`, `httpstatus`, `useragent`
@@ -276,7 +277,6 @@ fat-fingered `hex(2000000000)` can't try to allocate gigabytes at render.
 | `{ean()}` | derivation | EAN-13 / UPC-A / ISBN-13 / GTIN check digit |
 | `{uuid()}` | generator | UUID v7 (v4 ships as data — see [Data](#data)) |
 | `{ulid()}` | generator | ULID, 26-char Crockford base32 |
-| `{objectid()}` | generator | MongoDB ObjectID, 24 hex chars |
 | `{nanoid(n)}` | generator | URL-safe Nano ID, `n` chars |
 | `{hex(n)}` | generator | `n` lowercase hex digits |
 | `{base64(n)}` | generator | `n` random bytes, base64 |
diff --git a/builtins.go b/builtins.go
index 357869c..c0d3aa5 100644
--- a/builtins.go
+++ b/builtins.go
@@ -20,18 +20,17 @@ const (
 // the digits emitted so far in the current expansion (luhn, mod11, ean — place
 // them after their payload); generators read only the rng (uuid, ulid, ...). All
 // must stay pure over (rng, emitted, args) so a seeded faker is reproducible — a
-// time-based id (uuid v7, ulid, objectid) draws its timestamp from the rng, not
+// time-based id (uuid v7, ulid) draws its timestamp from the rng, not
 // the wall clock. Add a builtin only for what data can't express: a random v4
 // UUID already ships as data (data/misc/uuid.json), so the builtin is v7.
 var builtins = map[string]builtin{
 	"luhn": {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string {
 		return string(rune('0' + luhnCheck(e)))
 	}},
-	"mod11":    {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string { return mod11Check(e) }},
-	"ean":      {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string { return eanCheck(e) }},
-	"uuid":     {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return uuidV7(s) }},
-	"ulid":     {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return ulid(s) }},
-	"objectid": {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return randHex(s, 24) }},
+	"mod11": {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string { return mod11Check(e) }},
+	"ean":   {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string { return eanCheck(e) }},
+	"uuid":  {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return uuidV7(s) }},
+	"ulid":  {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return ulid(s) }},
 	"nanoid": {arity: 1, check: posIntArg, prep: func(a []string) callFn {
 		n := atoi(a[0])
 		return func(s *session, _ string, _ map[string]node) string { return nanoid(s, n) }
diff --git a/data/misc/objectid.json b/data/misc/objectid.json
new file mode 100644
index 0000000..9681734
--- /dev/null
+++ b/data/misc/objectid.json
@@ -0,0 +1,3 @@
+[
+  { "format": "{hex(24)}" }
+]