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 58f8adabaea..0ffada86128 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
@@ -161,14 +161,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts
index c8c0c413a2b..b92abe55617 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts
index c8c0c413a2b..b92abe55617 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
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 e46e834fc01..c95b2ac453e 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
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
diff --git a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
index a1fe7aa98dd..2a90ca5b632 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
index 8ec41ab00fa..d4d2025e1e3 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
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 a1fe7aa98dd..2a90ca5b632 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
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
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 a1fe7aa98dd..2a90ca5b632 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
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 a1fe7aa98dd..2a90ca5b632 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
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
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 a1fe7aa98dd..2a90ca5b632 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
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
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 a1fe7aa98dd..2a90ca5b632 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
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 a1fe7aa98dd..2a90ca5b632 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
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
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 8ec41ab00fa..d4d2025e1e3 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
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };
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 4d355ad4aa2..8b52e158ff0 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
@@ -172,14 +172,20 @@ export class BaseAPI {
             }))
         };
 
+        let body: any;
+        if (isFormData(overriddenInit.body)
+            || (overriddenInit.body instanceof URLSearchParams)
+            || isBlob(overriddenInit.body)) {
+          body = overriddenInit.body;
+        } else if (this.isJsonMime(headers['Content-Type'])) {
+          body = JSON.stringify(overriddenInit.body);
+        } else {
+          body = overriddenInit.body;
+        }
+
         const init: RequestInit = {
             ...overriddenInit,
-            body:
-                isFormData(overriddenInit.body) ||
-                overriddenInit.body instanceof URLSearchParams ||
-                isBlob(overriddenInit.body)
-                    ? overriddenInit.body
-                    : JSON.stringify(overriddenInit.body),
+            body
         };
 
         return { url, init };