diff --git a/src/generateValidRootSchema.ts b/src/generateValidRootSchema.ts
new file mode 100644
index 00000000..45b27bc3
--- /dev/null
+++ b/src/generateValidRootSchema.ts
@@ -0,0 +1,33 @@
+import type * as Types from "./types";
+
+export const generateValidRootSchema = (input: Types.OpenApi.Document): Types.OpenApi.Document => {
+  if (!input.paths) {
+    return input;
+  }
+  /** update undefined operation id */
+  for (const [path, methods] of Object.entries(input.paths || {})) {
+    const targets = {
+      get: methods.get,
+      put: methods.put,
+      post: methods.post,
+      delete: methods.delete,
+      options: methods.options,
+      head: methods.head,
+      patch: methods.patch,
+      trace: methods.trace,
+    } satisfies Record<string, Types.OpenApi.Operation | undefined>;
+    for (const [method, operation] of Object.entries(targets)) {
+      if (!operation) {
+        continue;
+      }
+      // skip reference object
+      if ("$ref" in operation) {
+        continue;
+      }
+      if (!operation.operationId) {
+        operation.operationId = `${method.toLowerCase()}${path.charAt(0).toUpperCase() + path.slice(1)}`;
+      }
+    }
+  }
+  return input;
+};
diff --git a/src/index.ts b/src/index.ts
index 26d83eb0..fa6d0c45 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,6 +2,7 @@ import { EOL } from "os";
 
 import * as Api from "./api";
 import type * as Types from "./types";
+import { generateValidRootSchema } from "./generateValidRootSchema";
 
 export interface Option {
   allowOperationIds?: string[];
@@ -17,7 +18,7 @@ export class CodeGenerator {
     private option?: Option,
   ) {
     if (typeof entryPointOrDocument === "string") {
-      this.rootSchema = Api.FileSystem.loadJsonOrYaml(entryPointOrDocument);
+      this.rootSchema = generateValidRootSchema(Api.FileSystem.loadJsonOrYaml(entryPointOrDocument));
       this.resolvedReferenceDocument = Api.ResolveReference.resolve(
         entryPointOrDocument,
         entryPointOrDocument,
diff --git a/test/__tests__/class/__snapshots__/typedef-with-template-test.ts.snap b/test/__tests__/class/__snapshots__/typedef-with-template-test.ts.snap
index 62a9e4a7..78ad58f7 100644
--- a/test/__tests__/class/__snapshots__/typedef-with-template-test.ts.snap
+++ b/test/__tests__/class/__snapshots__/typedef-with-template-test.ts.snap
@@ -1,4 +1,4 @@
-// Jest Snapshot v1, https://goo.gl/fbAQLP
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
 
 exports[`Typedef with template api.test.domain 1`] = `
 "// 
@@ -572,6 +572,12 @@ export interface Response$postHelloWorldReadonly$Status$200 {
         message?: Schemas.Message;
     };
 }
+export interface Response$get$undefined$operation$$id$Status$200 {
+    "application/json": {
+        /** Undefined operation response */
+        message?: string;
+    };
+}
 export type ResponseContentType$getHelloWorld = keyof Response$getHelloWorld$Status$200;
 export interface Params$getHelloWorld {
     parameter: Parameter$getHelloWorld;
@@ -581,6 +587,7 @@ export type ResponseContentType$postHelloWorldReadonly = keyof Response$postHell
 export interface Params$postHelloWorldReadonly {
     requestBody: RequestBody$postHelloWorldReadonly["application/json"];
 }
+export type ResponseContentType$get$undefined$operation$$id = keyof Response$get$undefined$operation$$id$Status$200;
 export type HttpMethod = "GET" | "PUT" | "POST" | "DELETE" | "OPTIONS" | "HEAD" | "PATCH" | "TRACE";
 export interface ObjectLike {
     [key: string]: any;
@@ -593,10 +600,11 @@ export interface QueryParameter {
 export interface QueryParameters {
     [key: string]: QueryParameter;
 }
-export type SuccessResponses = Response$getHelloWorld$Status$200 | Response$postHelloWorldReadonly$Status$200;
+export type SuccessResponses = Response$getHelloWorld$Status$200 | Response$postHelloWorldReadonly$Status$200 | Response$get$undefined$operation$$id$Status$200;
 export namespace ErrorResponse {
     export type getHelloWorld = void;
     export type postHelloWorldReadonly = void;
+    export type get$undefined$operation$$id = void;
 }
 export interface Encoding {
     readonly contentType?: string;
@@ -647,6 +655,17 @@ export class Client<RequestOption> {
             requestBody: params.requestBody
         }, option);
     }
+    public async get$undefined$operation$$id(option?: RequestOption): Promise<Response$get$undefined$operation$$id$Status$200["application/json"]> {
+        const url = this.baseUrl + \`/undefined/operation/:id\`;
+        const headers = {
+            Accept: "application/json"
+        };
+        return this.apiClient.request({
+            httpMethod: "GET",
+            url,
+            headers
+        }, option);
+    }
 }
 "
 `;
diff --git a/test/__tests__/functional/__snapshots__/typedef-with-template-test.ts.snap b/test/__tests__/functional/__snapshots__/typedef-with-template-test.ts.snap
index a3b74f17..0e75991c 100644
--- a/test/__tests__/functional/__snapshots__/typedef-with-template-test.ts.snap
+++ b/test/__tests__/functional/__snapshots__/typedef-with-template-test.ts.snap
@@ -1,4 +1,4 @@
-// Jest Snapshot v1, https://goo.gl/fbAQLP
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
 
 exports[`Typedef with template api.test.domain 1`] = `
 "// 
@@ -575,6 +575,12 @@ export interface Response$postHelloWorldReadonly$Status$200 {
         message?: Schemas.Message;
     };
 }
+export interface Response$get$undefined$operation$$id$Status$200 {
+    "application/json": {
+        /** Undefined operation response */
+        message?: string;
+    };
+}
 export type ResponseContentType$getHelloWorld = keyof Response$getHelloWorld$Status$200;
 export interface Params$getHelloWorld {
     parameter: Parameter$getHelloWorld;
@@ -584,6 +590,7 @@ export type ResponseContentType$postHelloWorldReadonly = keyof Response$postHell
 export interface Params$postHelloWorldReadonly {
     requestBody: RequestBody$postHelloWorldReadonly["application/json"];
 }
+export type ResponseContentType$get$undefined$operation$$id = keyof Response$get$undefined$operation$$id$Status$200;
 export type HttpMethod = "GET" | "PUT" | "POST" | "DELETE" | "OPTIONS" | "HEAD" | "PATCH" | "TRACE";
 export interface ObjectLike {
     [key: string]: any;
@@ -596,10 +603,11 @@ export interface QueryParameter {
 export interface QueryParameters {
     [key: string]: QueryParameter;
 }
-export type SuccessResponses = Response$getHelloWorld$Status$200 | Response$postHelloWorldReadonly$Status$200;
+export type SuccessResponses = Response$getHelloWorld$Status$200 | Response$postHelloWorldReadonly$Status$200 | Response$get$undefined$operation$$id$Status$200;
 export namespace ErrorResponse {
     export type getHelloWorld = void;
     export type postHelloWorldReadonly = void;
+    export type get$undefined$operation$$id = void;
 }
 export interface Encoding {
     readonly contentType?: string;
@@ -649,6 +657,17 @@ export const createClient = <RequestOption>(apiClient: ApiClient<RequestOption>,
                 headers,
                 requestBody: params.requestBody
             }, option);
+        },
+        get$undefined$operation$$id: (option?: RequestOption): Promise<Response$get$undefined$operation$$id$Status$200["application/json"]> => {
+            const url = _baseUrl + \`/undefined/operation/:id\`;
+            const headers = {
+                Accept: "application/json"
+            };
+            return apiClient.request({
+                httpMethod: "GET",
+                url,
+                headers
+            }, option);
         }
     };
 };
diff --git a/test/api.v2.domain/index.yml b/test/api.v2.domain/index.yml
index 69a60210..32987c5d 100644
--- a/test/api.v2.domain/index.yml
+++ b/test/api.v2.domain/index.yml
@@ -66,3 +66,16 @@ paths:
                 properties:
                   message:
                     $ref: "#/components/schemas/Message"
+  /undefined/operation/:id:
+    get:
+      responses:
+        200:
+          description: 成功
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  message:
+                    type: string
+                    description: Undefined operation response