--- a/templates/typescript-fetch/apis.mustache
+++ b/templates/typescript-fetch/apis.mustache
@@ -287,7 +287,9 @@ export class {{classname}} extends runtime.BaseAPI {
         return new runtime.JSONApiResponse<any>(response);
         {{/isListContainer}}
         {{#returnSimpleType}}
-        return new runtime.TextApiResponse(response) as any;
+        const contentType = response.headers.get('content-type');
+        const isJson = contentType && contentType.includes('application/json');
+        return new runtime[isJson ? 'JSONApiResponse' : 'TextApiResponse'](response) as any;
         {{/returnSimpleType}}
         {{/returnTypeIsPrimitive}}
         {{^returnTypeIsPrimitive}}

--- a/templates/typescript-fetch/runtime.mustache
+++ b/templates/typescript-fetch/runtime.mustache
@@ -50,7 +50,7 @@ export class BaseAPI {
             // do not handle correctly sometimes.
             url += '?' + this.configuration.queryParamsStringify(context.query);
         }
-        const body = ((typeof FormData !== "undefined" && context.body instanceof FormData) || context.body instanceof URLSearchParams || isBlob(context.body))
+        const body = ((typeof FormData !== "undefined" && context.body instanceof FormData) || context.body instanceof URLSearchParams || isBlob(context.body)) || context.headers['Content-Type'] !== 'application/json'
            ? context.body
            : JSON.stringify(context.body);