diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6013765..0b7175b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,20 @@ All notable changes to this module will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this module adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.10.0] - 2026-03-25
+
+### Changed
+
+- `Error(w http.ResponseWriter, err error)` — now includes `"platformCode"` in the JSON error body when the `*xerrors.Err` carries one; omitted otherwise
+- Upgraded `code.nochebuena.dev/go/xerrors` dependency to `v0.10.0`
+
+### Design Notes
+
+- `platformCode` is the domain-layer error identifier introduced in `xerrors` v0.10.0. It travels through `Error` transparently — no transport-layer logic depends on it.
+- Backwards-compatible: error responses without a platform code are identical to v0.9.0.
+
+[0.10.0]: https://code.nochebuena.dev/go/httputil/compare/v0.9.0...v0.10.0
+
 ## [0.9.0] - 2026-03-18
 
 ### Added
diff --git a/go.mod b/go.mod
index f35356a..b31572b 100644
--- a/go.mod
+++ b/go.mod
@@ -4,7 +4,7 @@ go 1.25
 
 require (
 	code.nochebuena.dev/go/valid v0.9.0
-	code.nochebuena.dev/go/xerrors v0.9.0
+	code.nochebuena.dev/go/xerrors v0.10.0
 )
 
 require (
diff --git a/go.sum b/go.sum
index acdeb32..21ffa88 100644
--- a/go.sum
+++ b/go.sum
@@ -2,6 +2,8 @@ code.nochebuena.dev/go/valid v0.9.0 h1:o8/tICIoed2+uwBp+TxXa3FE6KmyirU266O4jEUgF
 code.nochebuena.dev/go/valid v0.9.0/go.mod h1:SKpLcqpEsLMaEk7K3Y0kFF7Y3W5PHAQF6+U6wleFAhg=
 code.nochebuena.dev/go/xerrors v0.9.0 h1:8wrDto7e44ZW1YPOnT6JrxYXTqnvNuKpAO1/5bcT4TE=
 code.nochebuena.dev/go/xerrors v0.9.0/go.mod h1:mtXo7xscBreCB7w7smlBP5Onv8H1HVohCvF0I/VXbAY=
+code.nochebuena.dev/go/xerrors v0.10.0 h1:/4BCGZ7yZ384IC7t8SWGuAqEH/FmI9WtxiB5RKIoKWc=
+code.nochebuena.dev/go/xerrors v0.10.0/go.mod h1:mtXo7xscBreCB7w7smlBP5Onv8H1HVohCvF0I/VXbAY=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/gabriel-vasile/mimetype v1.4.12 h1:e9hWvmLYvtp846tLHam2o++qitpguFiYCKbn0w9jyqw=
diff --git a/httputil_test.go b/httputil_test.go
index 6c980b5..33faf8d 100644
--- a/httputil_test.go
+++ b/httputil_test.go
@@ -240,6 +240,28 @@ func TestError_NilError(t *testing.T) {
 	}
 }
 
+func TestError_PlatformCode_IncludedWhenSet(t *testing.T) {
+	rec := httptest.NewRecorder()
+	Error(rec, xerrors.New(xerrors.ErrNotFound, "employee not found").
+		WithPlatformCode("EMPLOYEE_NOT_FOUND"))
+	if rec.Code != http.StatusNotFound {
+		t.Errorf("want 404, got %d", rec.Code)
+	}
+	m := decodeMap(t, rec)
+	if m["platformCode"] != "EMPLOYEE_NOT_FOUND" {
+		t.Errorf("platformCode: want EMPLOYEE_NOT_FOUND, got %v", m["platformCode"])
+	}
+}
+
+func TestError_PlatformCode_OmittedWhenNotSet(t *testing.T) {
+	rec := httptest.NewRecorder()
+	Error(rec, xerrors.New(xerrors.ErrInternal, "unexpected error"))
+	m := decodeMap(t, rec)
+	if _, ok := m["platformCode"]; ok {
+		t.Errorf("platformCode should be absent for errors without a platform code, got %v", m["platformCode"])
+	}
+}
+
 func TestErrorCodeToStatus_AllCodes(t *testing.T) {
 	cases := []struct {
 		code   xerrors.Code
diff --git a/response.go b/response.go
index 789aedc..41a4be8 100644
--- a/response.go
+++ b/response.go
@@ -22,28 +22,31 @@ func NoContent(w http.ResponseWriter) {
 }
 
 // Error maps err to the appropriate HTTP status code and writes a JSON error body.
-// Understands *xerrors.Err — extracts Code and Message; fields are included if present.
+// Understands *xerrors.Err — extracts Code, PlatformCode, and Message.
 // Falls back to 500 for unknown errors.
 func Error(w http.ResponseWriter, err error) {
 	if err == nil {
-		JSON(w, http.StatusInternalServerError, errorBody("INTERNAL", "internal server error", nil))
+		JSON(w, http.StatusInternalServerError, errorBody("INTERNAL", "", "internal server error", nil))
 		return
 	}
 	var xe *xerrors.Err
 	if errors.As(err, &xe) {
 		status := errorCodeToStatus(xe.Code())
-		body := errorBody(string(xe.Code()), xe.Message(), xe.Fields())
+		body := errorBody(string(xe.Code()), xe.PlatformCode(), xe.Message(), xe.Fields())
 		JSON(w, status, body)
 		return
 	}
-	JSON(w, http.StatusInternalServerError, errorBody("INTERNAL", "internal server error", nil))
+	JSON(w, http.StatusInternalServerError, errorBody("INTERNAL", "", "internal server error", nil))
 }
 
-func errorBody(code, message string, fields map[string]any) map[string]any {
+func errorBody(code, platformCode, message string, fields map[string]any) map[string]any {
 	m := map[string]any{
 		"code":    code,
 		"message": message,
 	}
+	if platformCode != "" {
+		m["platformCode"] = platformCode
+	}
 	for k, v := range fields {
 		m[k] = v
 	}