diff --git a/.changeset/preserve-mixed-case-enum-keys.md b/.changeset/preserve-mixed-case-enum-keys.md
new file mode 100644
index 000000000..44b65b2df
--- /dev/null
+++ b/.changeset/preserve-mixed-case-enum-keys.md
@@ -0,0 +1,5 @@
+---
+"swagger-typescript-api": patch
+---
+
+Fix enum key formatting to preserve mixed case names with underscores (e.g., `_123ValCamelCase`, `Val_Snake_Case`, `_Val_12_CamelCase`) while still transforming pure lowercase snake_case to PascalCase (e.g., `local_only` → `LocalOnly`)
diff --git a/src/type-name-formatter.ts b/src/type-name-formatter.ts
index 58743a863..4030c6e66 100644
--- a/src/type-name-formatter.ts
+++ b/src/type-name-formatter.ts
@@ -36,6 +36,17 @@ export class TypeNameFormatter {
       return lodash.compact([typePrefix, name, typeSuffix]).join("_");
     }
 
+    // for enum keys, preserve mixed case names with underscores like Val_Snake_Case, _Val_12_CamelCase
+    // but transform pure lowercase snake_case like local_only to PascalCase
+    if (
+      schemaType === "enum-key" &&
+      /^(?!\d)([A-Za-z0-9_]{1,})$/g.test(name) &&
+      name.includes("_") &&
+      /[A-Z]/.test(name)
+    ) {
+      return lodash.compact([typePrefix, name, typeSuffix]).join("_");
+    }
+
     if (this.formattedModelNamesMap.has(hashKey)) {
       return this.formattedModelNamesMap.get(hashKey);
     }
diff --git a/tests/spec/enumIncludesNumber/__snapshots__/basic.test.ts.snap b/tests/spec/enumIncludesNumber/__snapshots__/basic.test.ts.snap
index 571ebdb11..cda046887 100644
--- a/tests/spec/enumIncludesNumber/__snapshots__/basic.test.ts.snap
+++ b/tests/spec/enumIncludesNumber/__snapshots__/basic.test.ts.snap
@@ -1,5 +1,89 @@
 // Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
 
+exports[`basic > edge case with prefix and suffix 1`] = `
+"/* eslint-disable */
+/* tslint:disable */
+// @ts-nocheck
+/*
+ * ---------------------------------------------------------------
+ * ## THIS FILE WAS GENERATED VIA SWAGGER-TYPESCRIPT-API        ##
+ * ##                                                           ##
+ * ## AUTHOR: acacode                                           ##
+ * ## SOURCE: https://github.com/acacode/swagger-typescript-api ##
+ * ---------------------------------------------------------------
+ */
+
+/** Pure lowercase snake_case values that should be transformed to PascalCase */
+export enum LowercaseSnakeCaseEnum {
+  VALLocalOnlyCaSeCamelCase = "local_only",
+  VALPhoneNumberCaSeCamelCase = "phone_number",
+  VALWaitDependencyCaSeCamelCase = "wait_dependency",
+  VALAuthFailedCaSeCamelCase = "auth_failed",
+  VALWaitingToPushCaSeCamelCase = "waiting_to_push",
+}
+
+export enum StringEnumIncludesNumbersAndUnderscore {
+  /** Foo */
+  V_aL_VAL_1_Ca_Se_CamelCase = "VAL_1",
+  /** Bar */
+  V_aL_VAL_2_Ca_Se_CamelCase = "VAL_2",
+  /** Baz */
+  V_aL_VAL_3_Ca_Se_CamelCase = "VAL_3",
+  V_aL__1_VAL_Ca_Se_CamelCase = "_1_VAL",
+  V_aL_A_1_B_2_C_3_Ca_Se_CamelCase = "A_1_B_2_C_3",
+  V_aL__A_1_B_2_C_3_Ca_Se_CamelCase = "_A_1_B_2_C_3",
+  V_aL__1_A_2_B_3_C_Ca_Se_CamelCase = "_1_A_2_B_3_C",
+  V_aL__123ValCamelCase_Ca_Se_CamelCase = "_123ValCamelCase",
+  V_aL_Val_Snake_Case_Ca_Se_CamelCase = "Val_Snake_Case",
+  V_aL_Val_12_CamelCase_Ca_Se_CamelCase = "Val_12_CamelCase",
+  V_aL__Val_12_Snake_Case_Ca_Se_CamelCase = "_Val_12_Snake_Case",
+  V_aL__Val_12_CamelCase_Ca_Se_CamelCase = "_Val_12_CamelCase",
+}
+"
+`;
+
+exports[`basic > transform pure lowercase snake_case to PascalCase 1`] = `
+"/* eslint-disable */
+/* tslint:disable */
+// @ts-nocheck
+/*
+ * ---------------------------------------------------------------
+ * ## THIS FILE WAS GENERATED VIA SWAGGER-TYPESCRIPT-API        ##
+ * ##                                                           ##
+ * ## AUTHOR: acacode                                           ##
+ * ## SOURCE: https://github.com/acacode/swagger-typescript-api ##
+ * ---------------------------------------------------------------
+ */
+
+/** Pure lowercase snake_case values that should be transformed to PascalCase */
+export enum LowercaseSnakeCaseEnum {
+  LocalOnly = "local_only",
+  PhoneNumber = "phone_number",
+  WaitDependency = "wait_dependency",
+  AuthFailed = "auth_failed",
+  WaitingToPush = "waiting_to_push",
+}
+
+export enum StringEnumIncludesNumbersAndUnderscore {
+  /** Foo */
+  VAL_1 = "VAL_1",
+  /** Bar */
+  VAL_2 = "VAL_2",
+  /** Baz */
+  VAL_3 = "VAL_3",
+  _1_VAL = "_1_VAL",
+  A_1_B_2_C_3 = "A_1_B_2_C_3",
+  _A_1_B_2_C_3 = "_A_1_B_2_C_3",
+  _1_A_2_B_3_C = "_1_A_2_B_3_C",
+  _123ValCamelCase = "_123ValCamelCase",
+  Val_Snake_Case = "Val_Snake_Case",
+  Val_12_CamelCase = "Val_12_CamelCase",
+  _Val_12_Snake_Case = "_Val_12_Snake_Case",
+  _Val_12_CamelCase = "_Val_12_CamelCase",
+}
+"
+`;
+
 exports[`basic > use x-enumNames as the key for enum 1`] = `
 "/* eslint-disable */
 /* tslint:disable */
@@ -13,6 +97,15 @@ exports[`basic > use x-enumNames as the key for enum 1`] = `
  * ---------------------------------------------------------------
  */
 
+/** Pure lowercase snake_case values that should be transformed to PascalCase */
+export enum LowercaseSnakeCaseEnum {
+  LocalOnly = "local_only",
+  PhoneNumber = "phone_number",
+  WaitDependency = "wait_dependency",
+  AuthFailed = "auth_failed",
+  WaitingToPush = "waiting_to_push",
+}
+
 export enum StringEnumIncludesNumbersAndUnderscore {
   /** Foo */
   VAL_1 = "VAL_1",
@@ -24,6 +117,11 @@ export enum StringEnumIncludesNumbersAndUnderscore {
   A_1_B_2_C_3 = "A_1_B_2_C_3",
   _A_1_B_2_C_3 = "_A_1_B_2_C_3",
   _1_A_2_B_3_C = "_1_A_2_B_3_C",
+  _123ValCamelCase = "_123ValCamelCase",
+  Val_Snake_Case = "Val_Snake_Case",
+  Val_12_CamelCase = "Val_12_CamelCase",
+  _Val_12_Snake_Case = "_Val_12_Snake_Case",
+  _Val_12_CamelCase = "_Val_12_CamelCase",
 }
 "
 `;
diff --git a/tests/spec/enumIncludesNumber/basic.test.ts b/tests/spec/enumIncludesNumber/basic.test.ts
index 404e7688a..de44025fe 100644
--- a/tests/spec/enumIncludesNumber/basic.test.ts
+++ b/tests/spec/enumIncludesNumber/basic.test.ts
@@ -31,4 +31,41 @@ describe("basic", async () => {
 
     expect(content).toMatchSnapshot();
   });
+
+  test("edge case with prefix and suffix", async () => {
+    await generateApi({
+      fileName: "schema",
+      input: path.resolve(import.meta.dirname, "schema.json"),
+      output: tmpdir,
+      silent: true,
+      enumNamesAsValues: false,
+      generateClient: false,
+      enumKeyPrefix: "V_aL",
+      enumKeySuffix: "Ca_Se_CamelCase",
+    });
+
+    const content = await fs.readFile(path.join(tmpdir, "schema.ts"), {
+      encoding: "utf8",
+    });
+
+    expect(content).toMatchSnapshot();
+  });
+
+  test("transform pure lowercase snake_case to PascalCase", async () => {
+    await generateApi({
+      fileName: "schema",
+      input: path.resolve(import.meta.dirname, "schema.json"),
+      output: tmpdir,
+      silent: true,
+      enumNamesAsValues: false,
+      generateClient: false,
+    });
+
+    const content = await fs.readFile(path.join(tmpdir, "schema.ts"), {
+      encoding: "utf8",
+    });
+
+    // Check that pure lowercase snake_case values are transformed to PascalCase
+    expect(content).toMatchSnapshot();
+  });
 });
diff --git a/tests/spec/enumIncludesNumber/schema.json b/tests/spec/enumIncludesNumber/schema.json
index 324298f10..623fffbbe 100644
--- a/tests/spec/enumIncludesNumber/schema.json
+++ b/tests/spec/enumIncludesNumber/schema.json
@@ -13,9 +13,25 @@
           "_1_VAL",
           "A_1_B_2_C_3",
           "_A_1_B_2_C_3",
-          "_1_A_2_B_3_C"
+          "_1_A_2_B_3_C",
+          "_123ValCamelCase",
+          "Val_Snake_Case",
+          "Val_12_CamelCase",
+          "_Val_12_Snake_Case",
+          "_Val_12_CamelCase"
         ],
         "x-enum-descriptions": ["Foo", "Bar", "Baz"]
+      },
+      "LowercaseSnakeCaseEnum": {
+        "type": "string",
+        "enum": [
+          "local_only",
+          "phone_number",
+          "wait_dependency",
+          "auth_failed",
+          "waiting_to_push"
+        ],
+        "description": "Pure lowercase snake_case values that should be transformed to PascalCase"
       }
     }
   }