diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
index 1a89aee59a18..a1bd07719c16 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
@@ -226,7 +226,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -255,26 +255,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 {{^withoutRuntimeChecks}}
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
index 3af1b477731c..aa29f74f8a4d 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
index c44768d37ab0..2467db552714 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
index 22a74a4f7d18..fbcfb338deaf 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
index c44768d37ab0..2467db552714 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
index c44768d37ab0..2467db552714 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
index c44768d37ab0..2467db552714 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
index c44768d37ab0..2467db552714 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
index c44768d37ab0..2467db552714 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
index c44768d37ab0..2467db552714 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
index 22a74a4f7d18..fbcfb338deaf 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -264,26 +264,31 @@ export function exists(json: any, key: string) {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 export function mapValues(data: any, fn: (item: any) => any) {
   return Object.keys(data).reduce(
     (acc, key) => ({ ...acc, [key]: fn(data[key]) }),
diff --git a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts
index ac27ba301abe..6c79ad1a4380 100644
--- a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts
@@ -237,7 +237,7 @@ export type FetchAPI = WindowOrWorkerGlobalScope['fetch'];
 export type Json = any;
 export type HTTPMethod = 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE' | 'OPTIONS' | 'HEAD';
 export type HTTPHeaders = { [key: string]: string };
-export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | HTTPQuery };
+export type HTTPQuery = { [key: string]: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery };
 export type HTTPBody = Json | FormData | URLSearchParams;
 export type HTTPRequestInit = { headers?: HTTPHeaders; method: HTTPMethod; credentials?: RequestCredentials; body?: HTTPBody }
 export type ModelPropertyNaming = 'camelCase' | 'snake_case' | 'PascalCase' | 'original';
@@ -260,26 +260,31 @@ export interface RequestOpts {
 
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
-        .map((key) => {
-            const fullKey = prefix + (prefix.length ? `[${key}]` : key);
-            const value = params[key];
-            if (value instanceof Array) {
-                const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
-                    .join(`&${encodeURIComponent(fullKey)}=`);
-                return `${encodeURIComponent(fullKey)}=${multiValue}`;
-            }
-            if (value instanceof Date) {
-                return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
-            }
-            if (value instanceof Object) {
-                return querystring(value as HTTPQuery, fullKey);
-            }
-            return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
-        })
+        .map(key => querystringSingleKey(key, params[key], prefix))
         .filter(part => part.length > 0)
         .join('&');
 }
 
+function querystringSingleKey(key: string, value: string | number | null | boolean | Array<string | number | null | boolean> | Set<string | number | null | boolean> | HTTPQuery, keyPrefix: string = ''): string {
+    const fullKey = keyPrefix + (keyPrefix.length ? `[${key}]` : key);
+    if (value instanceof Array) {
+        const multiValue = value.map(singleValue => encodeURIComponent(String(singleValue)))
+            .join(`&${encodeURIComponent(fullKey)}=`);
+        return `${encodeURIComponent(fullKey)}=${multiValue}`;
+    }
+    if (value instanceof Set) {
+        const valueAsArray = Array.from(value);
+        return querystringSingleKey(key, valueAsArray, keyPrefix);
+    }
+    if (value instanceof Date) {
+        return `${encodeURIComponent(fullKey)}=${encodeURIComponent(value.toISOString())}`;
+    }
+    if (value instanceof Object) {
+        return querystring(value as HTTPQuery, fullKey);
+    }
+    return `${encodeURIComponent(fullKey)}=${encodeURIComponent(String(value))}`;
+}
+
 
 export function canConsumeForm(consumes: Consume[]): boolean {
     for (const consume of consumes) {