diff --git a/README.md b/README.md
index 1c69349..a3fe85a 100644
--- a/README.md
+++ b/README.md
@@ -71,21 +71,20 @@ A union always encodes the full form, in JSON, SQL and Spanner too, because the
 
 ### Read the prefix from a proto annotation
 
-Package `objectidproto` reads `object_type` and `separator` from the `trogon.object_id.v1alpha1.enum_value` annotation on an enum value, so every language that uses the proto shares one prefix:
+Package `objectidproto` reads `object_type` and `separator` from the `trogon.object_id.v1alpha1.enum_value` annotation on an enum value, so every language that uses the proto shares one prefix. The other fields come from the base `Definition` you pass:
 
 ```go
-var ticketDefinition = func() trogonobjectid.Definition {
-	def := objectidproto.MustDefinition(acmev1.ObjectType_OBJECT_TYPE_TICKET)
-	def.Validate = trogonobjectid.ValidateUUID
-	return def
-}()
+var ticketDefinition = objectidproto.MustDefinition(acmev1.ObjectType_OBJECT_TYPE_TICKET, trogonobjectid.Definition{
+	Validate: trogonobjectid.ValidateUUID,
+	Generate: trogonobjectid.GenerateUUIDv7,
+})
 
 type ticketType struct{}
 
 func (ticketType) Definition() trogonobjectid.Definition { return ticketDefinition }
 ```
 
-Keep the result in a package-level variable, because `Definition` methods run on every call.
+The package caches each type's definition, so `Definition` must always return the same value. `ObjectTypeOf[T]()` and `PrefixOf[T]()` read it without an ID.
 
 ## Options
 
diff --git a/cache.go b/cache.go
new file mode 100644
index 0000000..af0d74f
--- /dev/null
+++ b/cache.go
@@ -0,0 +1,24 @@
+package trogonobjectid
+
+import (
+	"reflect"
+	"sync"
+)
+
+type cached[V any] struct {
+	value V
+	err   error
+}
+
+// loadOrBuild returns the value cached for key, building it on first use. The cache
+// holds pointers, so a lookup on the encode and parse paths copies no struct.
+func loadOrBuild[V any](cache *sync.Map, key reflect.Type, build func() (V, error)) (*V, error) {
+	if c, ok := cache.Load(key); ok {
+		entry := c.(*cached[V])
+		return &entry.value, entry.err
+	}
+	value, err := build()
+	c, _ := cache.LoadOrStore(key, &cached[V]{value: value, err: err})
+	entry := c.(*cached[V])
+	return &entry.value, entry.err
+}
diff --git a/objectid.go b/objectid.go
index 0a3663b..e81f664 100644
--- a/objectid.go
+++ b/objectid.go
@@ -5,7 +5,9 @@ import (
 	"encoding/json"
 	"fmt"
 	"log/slog"
+	"reflect"
 	"strings"
+	"sync"
 )
 
 // DefaultSeparator is the separator used when Definition.Separator is empty.
@@ -38,7 +40,8 @@ type Definition struct {
 }
 
 // Definer is implemented by the marker type that names an object ID type.
-// Implement it on an empty struct with a value receiver:
+// The result of Definition is cached per type, so it must always return the same
+// value. Implement it on an empty struct with a value receiver:
 //
 //	type userType struct{}
 //
@@ -66,7 +69,15 @@ type spec struct {
 	prefix string
 }
 
-func specFor[T Definer]() (spec, error) {
+// specs caches each marker type's spec, keyed by reflect.Type, so encoding an ID does
+// not rebuild the prefix on every call.
+var specs sync.Map
+
+func specFor[T Definer]() (*spec, error) {
+	return loadOrBuild(&specs, reflect.TypeFor[T](), newSpec[T])
+}
+
+func newSpec[T Definer]() (spec, error) {
 	var marker T
 	def := marker.Definition()
 	if def.ObjectType == "" {
@@ -78,7 +89,7 @@ func specFor[T Definer]() (spec, error) {
 	return spec{Definition: def, prefix: def.ObjectType + def.Separator}, nil
 }
 
-func (s spec) check(id string) error {
+func (s *spec) check(id string) error {
 	if id == "" {
 		return &ValidationError{ObjectType: s.ObjectType, Value: id, Err: ErrEmptyValue}
 	}
@@ -90,7 +101,7 @@ func (s spec) check(id string) error {
 	return nil
 }
 
-func (s spec) parse(full string) (string, error) {
+func (s *spec) parse(full string) (string, error) {
 	bare, ok := strings.CutPrefix(full, s.prefix)
 	if !ok || bare == "" {
 		return "", &ValidationError{ObjectType: s.ObjectType, Value: full, Err: ErrInvalidFormat}
@@ -101,7 +112,7 @@ func (s spec) parse(full string) (string, error) {
 	return bare, nil
 }
 
-func (s spec) decode(format Format, value string) (string, error) {
+func (s *spec) decode(format Format, value string) (string, error) {
 	if format == FormatDropPrefix {
 		if err := s.check(value); err != nil {
 			return "", err
@@ -111,7 +122,7 @@ func (s spec) decode(format Format, value string) (string, error) {
 	return s.parse(value)
 }
 
-func (s spec) encode(format Format, id string) string {
+func (s *spec) encode(format Format, id string) string {
 	if format == FormatDropPrefix {
 		return id
 	}
@@ -195,7 +206,7 @@ func FromStorage[T Definer](value string) (ID[T], error) {
 }
 
 // mustSpec is for methods on an ID that already exists, which proves the definition is valid.
-func mustSpec[T Definer]() spec {
+func mustSpec[T Definer]() *spec {
 	s, err := specFor[T]()
 	if err != nil {
 		panic(err)
@@ -203,6 +214,14 @@ func mustSpec[T Definer]() spec {
 	return s
 }
 
+// ObjectTypeOf returns T's object type, for example "user". It panics if T's
+// Definition is invalid.
+func ObjectTypeOf[T Definer]() string { return mustSpec[T]().ObjectType }
+
+// PrefixOf returns T's object type and separator, for example "user_". It panics if
+// T's Definition is invalid.
+func PrefixOf[T Definer]() string { return mustSpec[T]().prefix }
+
 // IsZero reports whether id is the zero (absent) ID.
 func (id ID[T]) IsZero() bool { return id.id == "" }
 
@@ -210,10 +229,10 @@ func (id ID[T]) IsZero() bool { return id.id == "" }
 func (id ID[T]) Bare() string { return id.id }
 
 // ObjectType returns the type's object type, for example "user".
-func (id ID[T]) ObjectType() string { return mustSpec[T]().ObjectType }
+func (id ID[T]) ObjectType() string { return ObjectTypeOf[T]() }
 
 // Prefix returns the object type and separator, for example "user_".
-func (id ID[T]) Prefix() string { return mustSpec[T]().prefix }
+func (id ID[T]) Prefix() string { return PrefixOf[T]() }
 
 // String returns the prefixed form, for example "user_abc-123", or "" for the zero ID.
 func (id ID[T]) String() string {
diff --git a/objectid_test.go b/objectid_test.go
index d62c357..0412196 100644
--- a/objectid_test.go
+++ b/objectid_test.go
@@ -397,6 +397,47 @@ func TestSpanner(t *testing.T) {
 	})
 }
 
+func TestTypeLevelAccessors(t *testing.T) {
+	assert.Equal(t, "inv", trogonobjectid.ObjectTypeOf[invoiceType]())
+	assert.Equal(t, "inv_", trogonobjectid.PrefixOf[invoiceType]())
+	assert.Equal(t, "order#", trogonobjectid.PrefixOf[orderType]())
+	assert.Panics(t, func() { trogonobjectid.PrefixOf[badType]() })
+}
+
+var definitionCalls int
+
+type countedType struct{}
+
+func (countedType) Definition() trogonobjectid.Definition {
+	definitionCalls++
+	return trogonobjectid.Definition{ObjectType: "cnt"}
+}
+
+func TestDefinitionRunsOnce(t *testing.T) {
+	id := trogonobjectid.MustNew[countedType]("a")
+	for range 3 {
+		_ = id.String()
+		_, _ = trogonobjectid.Parse[countedType]("cnt_a")
+	}
+	assert.Equal(t, 1, definitionCalls)
+}
+
+func BenchmarkString(b *testing.B) {
+	id := trogonobjectid.MustNew[invoiceType](invoiceUUID)
+	b.ReportAllocs()
+	for b.Loop() {
+		_ = id.String()
+	}
+}
+
+func BenchmarkParse(b *testing.B) {
+	full := "inv_" + invoiceUUID
+	b.ReportAllocs()
+	for b.Loop() {
+		_, _ = trogonobjectid.Parse[invoiceType](full)
+	}
+}
+
 func BenchmarkValidateUUID(b *testing.B) {
 	b.ReportAllocs()
 	for b.Loop() {
diff --git a/objectidproto/objectidproto.go b/objectidproto/objectidproto.go
index 57038cc..ee78369 100644
--- a/objectidproto/objectidproto.go
+++ b/objectidproto/objectidproto.go
@@ -7,15 +7,12 @@
 //	}
 //
 // The object type and separator then live in one place for every language that uses
-// the proto. Keep the result in a package-level variable, because Definition methods
-// run on every call:
+// the proto, and the other fields come from a base Definition:
 //
-//	var ticketDefinition = func() trogonobjectid.Definition {
-//		def := objectidproto.MustDefinition(acmev1.ObjectType_OBJECT_TYPE_TICKET)
-//		def.StorageFormat = trogonobjectid.FormatDropPrefix
-//		def.Validate = trogonobjectid.ValidateUUID
-//		return def
-//	}()
+//	var ticketDefinition = objectidproto.MustDefinition(acmev1.ObjectType_OBJECT_TYPE_TICKET, trogonobjectid.Definition{
+//		StorageFormat: trogonobjectid.FormatDropPrefix,
+//		Validate:      trogonobjectid.ValidateUUID,
+//	})
 //
 //	type ticketType struct{}
 //
@@ -35,22 +32,23 @@ import (
 )
 
 // Definition returns a Definition with ObjectType and Separator read from the enum
-// value's annotation. An absent separator gives trogonobjectid.DefaultSeparator.
-// Set the other fields on the result.
-func Definition(value protoreflect.Enum) (trogonobjectid.Definition, error) {
+// value's annotation, over base. An absent separator gives
+// trogonobjectid.DefaultSeparator. base carries the fields the annotation does not, so
+// a new Definition field needs no change here.
+func Definition(value protoreflect.Enum, base trogonobjectid.Definition) (trogonobjectid.Definition, error) {
 	desc := value.Descriptor().Values().ByNumber(value.Number())
 	if desc == nil {
 		return trogonobjectid.Definition{}, fmt.Errorf("%w: %s has no value %d",
 			trogonobjectid.ErrInvalidDefinition, value.Descriptor().FullName(), value.Number())
 	}
 
-	opts, _ := desc.Options().(*descriptorpb.EnumValueOptions)
-	if opts == nil || !proto.HasExtension(opts, objectidv1alpha1.E_EnumValue) {
+	valueOpts, _ := desc.Options().(*descriptorpb.EnumValueOptions)
+	if valueOpts == nil || !proto.HasExtension(valueOpts, objectidv1alpha1.E_EnumValue) {
 		return trogonobjectid.Definition{}, fmt.Errorf("%w: %s has no %s annotation",
 			trogonobjectid.ErrInvalidDefinition, desc.FullName(), objectidv1alpha1.E_EnumValue.TypeDescriptor().FullName())
 	}
 
-	ext, _ := proto.GetExtension(opts, objectidv1alpha1.E_EnumValue).(*objectidv1alpha1.EnumValueOptions)
+	ext, _ := proto.GetExtension(valueOpts, objectidv1alpha1.E_EnumValue).(*objectidv1alpha1.EnumValueOptions)
 	if ext.GetObjectType() == "" {
 		return trogonobjectid.Definition{}, fmt.Errorf("%w: %s has an empty object_type",
 			trogonobjectid.ErrInvalidDefinition, desc.FullName())
@@ -60,12 +58,14 @@ func Definition(value protoreflect.Enum) (trogonobjectid.Definition, error) {
 	if ext.Separator != nil {
 		separator = ext.GetSeparator()
 	}
-	return trogonobjectid.Definition{ObjectType: ext.GetObjectType(), Separator: separator}, nil
+	base.ObjectType = ext.GetObjectType()
+	base.Separator = separator
+	return base, nil
 }
 
 // MustDefinition is like Definition but panics on error. Use it in package-level variables.
-func MustDefinition(value protoreflect.Enum) trogonobjectid.Definition {
-	def, err := Definition(value)
+func MustDefinition(value protoreflect.Enum, base trogonobjectid.Definition) trogonobjectid.Definition {
+	def, err := Definition(value, base)
 	if err != nil {
 		panic(err)
 	}
diff --git a/objectidproto/objectidproto_test.go b/objectidproto/objectidproto_test.go
index 9877ddd..9e41519 100644
--- a/objectidproto/objectidproto_test.go
+++ b/objectidproto/objectidproto_test.go
@@ -71,13 +71,13 @@ func TestDefinition(t *testing.T) {
 	enum := objectTypeEnum(t)
 
 	t.Run("reads the object type and uses the default separator", func(t *testing.T) {
-		def, err := objectidproto.Definition(enum.New(valueTicket))
+		def, err := objectidproto.Definition(enum.New(valueTicket), trogonobjectid.Definition{})
 		require.NoError(t, err)
 		assert.Equal(t, trogonobjectid.Definition{ObjectType: "tkt", Separator: "_"}, def)
 	})
 
 	t.Run("reads the separator", func(t *testing.T) {
-		def, err := objectidproto.Definition(enum.New(valueOrder))
+		def, err := objectidproto.Definition(enum.New(valueOrder), trogonobjectid.Definition{})
 		require.NoError(t, err)
 		assert.Equal(t, trogonobjectid.Definition{ObjectType: "order", Separator: "#"}, def)
 	})
@@ -92,13 +92,13 @@ func TestDefinition(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			_, err := objectidproto.Definition(enum.New(tt.number))
+			_, err := objectidproto.Definition(enum.New(tt.number), trogonobjectid.Definition{})
 			require.ErrorIs(t, err, trogonobjectid.ErrInvalidDefinition)
 		})
 	}
 
 	t.Run("MustDefinition panics on error", func(t *testing.T) {
-		assert.Panics(t, func() { objectidproto.MustDefinition(enum.New(valueUnspecified)) })
+		assert.Panics(t, func() { objectidproto.MustDefinition(enum.New(valueUnspecified), trogonobjectid.Definition{}) })
 	})
 }
 
@@ -107,7 +107,7 @@ var ticketEnum protoreflect.EnumType
 type ticketType struct{}
 
 func (ticketType) Definition() trogonobjectid.Definition {
-	def := objectidproto.MustDefinition(ticketEnum.New(valueTicket))
+	def := objectidproto.MustDefinition(ticketEnum.New(valueTicket), trogonobjectid.Definition{})
 	def.Validate = trogonobjectid.ValidateInteger
 	return def
 }
@@ -122,3 +122,21 @@ func TestDefinitionWithID(t *testing.T) {
 	_, err = trogonobjectid.Parse[ticketType]("tkt_abc")
 	require.ErrorIs(t, err, trogonobjectid.ErrInvalidInteger)
 }
+
+func TestDefinitionBase(t *testing.T) {
+	enum := objectTypeEnum(t)
+	base := trogonobjectid.Definition{
+		ObjectType:    "ignored",
+		Separator:     "ignored",
+		StorageFormat: trogonobjectid.FormatDropPrefix,
+		JSONFormat:    trogonobjectid.FormatDropPrefix,
+	}
+
+	def, err := objectidproto.Definition(enum.New(valueOrder), base)
+	require.NoError(t, err)
+
+	want := base
+	want.ObjectType = "order"
+	want.Separator = "#"
+	assert.Equal(t, want, def)
+}
diff --git a/union.go b/union.go
index 5a61853..c2f9c25 100644
--- a/union.go
+++ b/union.go
@@ -5,8 +5,10 @@ import (
 	"encoding/json"
 	"fmt"
 	"log/slog"
+	"reflect"
 	"sort"
 	"strings"
+	"sync"
 )
 
 // Member is an ID that a Union can hold. Every ID[T] implements it.
@@ -67,7 +69,13 @@ type unionSpec struct {
 	members []MemberType
 }
 
-func unionSpecFor[U UnionDefiner]() (unionSpec, error) {
+var unionSpecs sync.Map
+
+func unionSpecFor[U UnionDefiner]() (*unionSpec, error) {
+	return loadOrBuild(&unionSpecs, reflect.TypeFor[U](), newUnionSpec[U])
+}
+
+func newUnionSpec[U UnionDefiner]() (unionSpec, error) {
 	var marker U
 	members := marker.Members()
 	if len(members) == 0 {