diff --git a/docs/extensions.md b/docs/extensions.md
index b8a2b23..2577961 100644
--- a/docs/extensions.md
+++ b/docs/extensions.md
@@ -96,7 +96,10 @@ Two extensions set a name:
 
 - Both take part in clash resolution with the highest rank ([naming](naming.md#clashes)).
 - A name that still has to change gets a warning.
-- The name is exported unless `x-go-name-exact: true` is set.
+- The name is used as written, with its first letter upper case: `ThreeDSACSURL` stays
+  `ThreeDSACSURL`. `x-go-name-exact: true` keeps the first letter too, so the name can be
+  unexported. A name that is still not exported, such as `_note`, goes through the
+  [naming rules](naming.md).
 
 ## x-sensitive-data
 
diff --git a/internal/gomodel/extensions.go b/internal/gomodel/extensions.go
index e600102..83a9398 100644
--- a/internal/gomodel/extensions.go
+++ b/internal/gomodel/extensions.go
@@ -9,9 +9,12 @@ package gomodel
 
 import (
 	"fmt"
+	"go/token"
 	"slices"
 	"strconv"
 	"strings"
+	"unicode"
+	"unicode/utf8"
 
 	"github.com/mockzilla/mockzilla-codegen/internal/diag"
 	"github.com/mockzilla/mockzilla-codegen/internal/extension"
@@ -100,11 +103,16 @@ func (r *extReader) clash(p *spec.Parameter, name, outer, inner string) {
 	})
 }
 
-// goName is the Go name x-go-name asks for: as written with x-go-name-exact, else exported.
+// goName is the Go name x-go-name asks for, as written with its first letter upper unless exact.
 func (r *extReader) goName(set extension.Set, name string) string {
 	if name == "" || set.IsExactName {
 		return name
 	}
+
+	first, size := utf8.DecodeRuneInString(name)
+	if upper := string(unicode.ToUpper(first)) + name[size:]; token.IsExported(upper) {
+		return upper
+	}
 	return r.namer.Exported(name)
 }
 
diff --git a/internal/gomodel/testdata/extensions.golden b/internal/gomodel/testdata/extensions.golden
index bd075b8..96f346f 100644
--- a/internal/gomodel/testdata/extensions.golden
+++ b/internal/gomodel/testdata/extensions.golden
@@ -1,6 +1,8 @@
 Customer struct models.types
   Identifier string json=id required yaml=id
   accountSecret *string json=secret omitempty jsonIgnored yaml=secret,omitempty
+  ThreeDSACSURL *string json=acs_url omitempty yaml=acs_url,omitempty
+  Note *string json=_note omitempty yaml=_note,omitempty
   Nickname string json=nickname omitempty yaml=nickname,omitempty
   Count *int json=count yaml=count
   Code *string json=code omitempty yaml=code,omitempty
@@ -40,6 +42,6 @@ op GetAccount GET /accounts/{id}
 warning extension-value 11:11 /paths/~1accounts~1{id}/get/parameters/3: x-go-extra-tags db of query parameter "sort" is "sort", and "order" in its schema; the parameter's is used
 warning extension-value 11:11 /paths/~1accounts~1{id}/get/parameters/3: x-go-name of query parameter "sort" is "SortBy", and "Order" in its schema; the parameter's is used
 warning extension-value 21:13 /components/parameters/Cursor: x-go-name of query parameter "cursor" is "After", and "Before" in its schema; the parameter's is used
-warning extension-unknown 69:11 /components/schemas/Account/properties/typo/x-go-nam: unknown extension x-go-nam; it is left out
-warning enum-name-unexported 76:7 /components/schemas/Shade/x-enum-names/0: x-enum-names "light" is unexported; it is used as written
-warning name-clash 93:16 /components/schemas/Renamed/properties/clash: "Validate" is reserved, renamed to "Validate2"
+warning extension-unknown 75:11 /components/schemas/Account/properties/typo/x-go-nam: unknown extension x-go-nam; it is left out
+warning enum-name-unexported 82:7 /components/schemas/Shade/x-enum-names/0: x-enum-names "light" is unexported; it is used as written
+warning name-clash 99:16 /components/schemas/Renamed/properties/clash: "Validate" is reserved, renamed to "Validate2"
diff --git a/internal/gomodel/testdata/extensions.yaml b/internal/gomodel/testdata/extensions.yaml
index bd59653..57f3f94 100644
--- a/internal/gomodel/testdata/extensions.yaml
+++ b/internal/gomodel/testdata/extensions.yaml
@@ -33,6 +33,12 @@ components:
           x-go-name: accountSecret
           x-go-name-exact: true
           x-go-json-ignore: true
+        acs_url:
+          type: string
+          x-go-name: ThreeDSACSURL
+        _note:
+          type: string
+          x-go-name: _note
         nickname:
           type: string
           x-go-type-skip-optional-pointer: true