diff --git a/.gitignore b/.gitignore
index 221da82..ae6d551 100644
--- a/.gitignore
+++ b/.gitignore
@@ -36,3 +36,4 @@ Thumbs.db
 # VCS files
 COMMIT.md
 RELEASE.md
+PR.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7e86d6c..c619738 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,15 @@ 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).
 
+## [1.0.2] - 2026-05-22
+
+### Fixed
+
+- `MarshalJSON` JSON tag corrected from `"platformCode"` to `"platform_code"` to
+  match the OpenAPI spec snake_case convention.
+
+[1.0.2]: https://code.nochebuena.dev/go/xerrors/compare/v1.0.1...v1.0.2
+
 ## [1.0.0] — 2026-05-08
 
 ### Added
diff --git a/xerrors.go b/xerrors.go
index fab664e..9669b2c 100644
--- a/xerrors.go
+++ b/xerrors.go
@@ -188,12 +188,12 @@ func (e *Err) ErrorContext() map[string]any {
 }
 
 // MarshalJSON implements [json.Marshaler].
-// Output: {"code":"NOT_FOUND","platformCode":"EMPLOYEE_NOT_FOUND","message":"...","fields":{...}}
-// platformCode and fields are omitted when empty.
+// Output: {"code":"NOT_FOUND","platform_code":"IDG-USR-0004","message":"...","fields":{...}}
+// platform_code and fields are omitted when empty.
 func (e *Err) MarshalJSON() ([]byte, error) {
 	return json.Marshal(struct {
 		Code         string         `json:"code"`
-		PlatformCode string         `json:"platformCode,omitempty"`
+		PlatformCode string         `json:"platform_code,omitempty"`
 		Message      string         `json:"message"`
 		Fields       map[string]any `json:"fields,omitempty"`
 	}{
diff --git a/xerrors_test.go b/xerrors_test.go
index d3164c4..8597f7d 100644
--- a/xerrors_test.go
+++ b/xerrors_test.go
@@ -312,8 +312,8 @@ func TestErr_MarshalJSON_PlatformCode(t *testing.T) {
 		if jsonErr = json.Unmarshal(b, &out); jsonErr != nil {
 			t.Fatalf("unmarshal error: %v", jsonErr)
 		}
-		if out["platformCode"] != "EMPLOYEE_NOT_FOUND" {
-			t.Errorf("json platformCode = %v, want EMPLOYEE_NOT_FOUND", out["platformCode"])
+		if out["platform_code"] != "EMPLOYEE_NOT_FOUND" {
+			t.Errorf("json platform_code = %v, want EMPLOYEE_NOT_FOUND", out["platform_code"])
 		}
 	})
 
@@ -323,8 +323,8 @@ func TestErr_MarshalJSON_PlatformCode(t *testing.T) {
 		if jsonErr != nil {
 			t.Fatalf("MarshalJSON error: %v", jsonErr)
 		}
-		if strings.Contains(string(b), "platformCode") {
-			t.Errorf("json should omit platformCode when empty, got: %s", b)
+		if strings.Contains(string(b), "platform_code") {
+			t.Errorf("json should omit platform_code when empty, got: %s", b)
 		}
 	})
 }