diff --git a/packages/generator/src/client.ts b/packages/generator/src/client.ts
index fa95d7f..3ccea19 100644
--- a/packages/generator/src/client.ts
+++ b/packages/generator/src/client.ts
@@ -54,17 +54,34 @@ export class HTTPError extends Error {
   readonly status: number;
   readonly statusText: string;
   readonly response: Response;
-  constructor(response: Response) {
+  readonly responseText: string | null;
+  readonly responseJson: unknown | null;
+  constructor(
+    response: Response,
+    responseText: string | null,
+    responseJson: unknown | null
+  ) {
     super(\`HTTP Error: \${response.status} \${response.statusText}\`);
     this.name = 'HTTPError';
     this.status = response.status;
     this.statusText = response.statusText;
     this.response = response;
+    this.responseText = responseText;
+    this.responseJson = responseJson;
   }
 }
 
-function _checkOk(response: Response): Response {
-  if (!response.ok) throw new HTTPError(response);
+async function _checkOk(response: Response): Promise<Response> {
+  if (!response.ok) {
+    const responseText = await response.text().catch(() => null);
+    let responseJson: unknown | null = null;
+    if (responseText) {
+      try {
+        responseJson = JSON.parse(responseText);
+      } catch {}
+    }
+    throw new HTTPError(response, responseText, responseJson);
+  }
   return response;
 }
 `;
diff --git a/packages/generator/tests/client.test.ts b/packages/generator/tests/client.test.ts
index 13b8929..af8fe3d 100644
--- a/packages/generator/tests/client.test.ts
+++ b/packages/generator/tests/client.test.ts
@@ -99,17 +99,34 @@ describe('generateClient', () => {
     	  readonly status: number;
     	  readonly statusText: string;
     	  readonly response: Response;
-    	  constructor(response: Response) {
+    	  readonly responseText: string | null;
+    	  readonly responseJson: unknown | null;
+    	  constructor(
+    	    response: Response,
+    	    responseText: string | null,
+    	    responseJson: unknown | null
+    	  ) {
     	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
     	    this.name = 'HTTPError';
     	    this.status = response.status;
     	    this.statusText = response.statusText;
     	    this.response = response;
+    	    this.responseText = responseText;
+    	    this.responseJson = responseJson;
     	  }
     	}
 
-    	function _checkOk(response: Response): Response {
-    	  if (!response.ok) throw new HTTPError(response);
+    	async function _checkOk(response: Response): Promise<Response> {
+    	  if (!response.ok) {
+    	    const responseText = await response.text().catch(() => null);
+    	    let responseJson: unknown | null = null;
+    	    if (responseText) {
+    	      try {
+    	        responseJson = JSON.parse(responseText);
+    	      } catch {}
+    	    }
+    	    throw new HTTPError(response, responseText, responseJson);
+    	  }
     	  return response;
     	}
 
@@ -129,17 +146,34 @@ describe('generateClient', () => {
         	  readonly status: number;
         	  readonly statusText: string;
         	  readonly response: Response;
-        	  constructor(response: Response) {
+        	  readonly responseText: string | null;
+        	  readonly responseJson: unknown | null;
+        	  constructor(
+        	    response: Response,
+        	    responseText: string | null,
+        	    responseJson: unknown | null
+        	  ) {
         	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
         	    this.name = 'HTTPError';
         	    this.status = response.status;
         	    this.statusText = response.statusText;
         	    this.response = response;
+        	    this.responseText = responseText;
+        	    this.responseJson = responseJson;
         	  }
         	}
 
-        	function _checkOk(response: Response): Response {
-        	  if (!response.ok) throw new HTTPError(response);
+        	async function _checkOk(response: Response): Promise<Response> {
+        	  if (!response.ok) {
+        	    const responseText = await response.text().catch(() => null);
+        	    let responseJson: unknown | null = null;
+        	    if (responseText) {
+        	      try {
+        	        responseJson = JSON.parse(responseText);
+        	      } catch {}
+        	    }
+        	    throw new HTTPError(response, responseText, responseJson);
+        	  }
         	  return response;
         	}
 
@@ -176,17 +210,34 @@ describe('generateClient', () => {
       	  readonly status: number;
       	  readonly statusText: string;
       	  readonly response: Response;
-      	  constructor(response: Response) {
+      	  readonly responseText: string | null;
+      	  readonly responseJson: unknown | null;
+      	  constructor(
+      	    response: Response,
+      	    responseText: string | null,
+      	    responseJson: unknown | null
+      	  ) {
       	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
       	    this.name = 'HTTPError';
       	    this.status = response.status;
       	    this.statusText = response.statusText;
       	    this.response = response;
+      	    this.responseText = responseText;
+      	    this.responseJson = responseJson;
       	  }
       	}
 
-      	function _checkOk(response: Response): Response {
-      	  if (!response.ok) throw new HTTPError(response);
+      	async function _checkOk(response: Response): Promise<Response> {
+      	  if (!response.ok) {
+      	    const responseText = await response.text().catch(() => null);
+      	    let responseJson: unknown | null = null;
+      	    if (responseText) {
+      	      try {
+      	        responseJson = JSON.parse(responseText);
+      	      } catch {}
+      	    }
+      	    throw new HTTPError(response, responseText, responseJson);
+      	  }
       	  return response;
       	}
 
@@ -224,17 +275,34 @@ describe('generateClient', () => {
         	  readonly status: number;
         	  readonly statusText: string;
         	  readonly response: Response;
-        	  constructor(response: Response) {
+        	  readonly responseText: string | null;
+        	  readonly responseJson: unknown | null;
+        	  constructor(
+        	    response: Response,
+        	    responseText: string | null,
+        	    responseJson: unknown | null
+        	  ) {
         	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
         	    this.name = 'HTTPError';
         	    this.status = response.status;
         	    this.statusText = response.statusText;
         	    this.response = response;
+        	    this.responseText = responseText;
+        	    this.responseJson = responseJson;
         	  }
         	}
 
-        	function _checkOk(response: Response): Response {
-        	  if (!response.ok) throw new HTTPError(response);
+        	async function _checkOk(response: Response): Promise<Response> {
+        	  if (!response.ok) {
+        	    const responseText = await response.text().catch(() => null);
+        	    let responseJson: unknown | null = null;
+        	    if (responseText) {
+        	      try {
+        	        responseJson = JSON.parse(responseText);
+        	      } catch {}
+        	    }
+        	    throw new HTTPError(response, responseText, responseJson);
+        	  }
         	  return response;
         	}
 
@@ -284,17 +352,34 @@ describe('generateClient', () => {
       	  readonly status: number;
       	  readonly statusText: string;
       	  readonly response: Response;
-      	  constructor(response: Response) {
+      	  readonly responseText: string | null;
+      	  readonly responseJson: unknown | null;
+      	  constructor(
+      	    response: Response,
+      	    responseText: string | null,
+      	    responseJson: unknown | null
+      	  ) {
       	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
       	    this.name = 'HTTPError';
       	    this.status = response.status;
       	    this.statusText = response.statusText;
       	    this.response = response;
+      	    this.responseText = responseText;
+      	    this.responseJson = responseJson;
       	  }
       	}
 
-      	function _checkOk(response: Response): Response {
-      	  if (!response.ok) throw new HTTPError(response);
+      	async function _checkOk(response: Response): Promise<Response> {
+      	  if (!response.ok) {
+      	    const responseText = await response.text().catch(() => null);
+      	    let responseJson: unknown | null = null;
+      	    if (responseText) {
+      	      try {
+      	        responseJson = JSON.parse(responseText);
+      	      } catch {}
+      	    }
+      	    throw new HTTPError(response, responseText, responseJson);
+      	  }
       	  return response;
       	}
 
@@ -342,17 +427,34 @@ describe('generateClient', () => {
       	  readonly status: number;
       	  readonly statusText: string;
       	  readonly response: Response;
-      	  constructor(response: Response) {
+      	  readonly responseText: string | null;
+      	  readonly responseJson: unknown | null;
+      	  constructor(
+      	    response: Response,
+      	    responseText: string | null,
+      	    responseJson: unknown | null
+      	  ) {
       	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
       	    this.name = 'HTTPError';
       	    this.status = response.status;
       	    this.statusText = response.statusText;
       	    this.response = response;
+      	    this.responseText = responseText;
+      	    this.responseJson = responseJson;
       	  }
       	}
 
-      	function _checkOk(response: Response): Response {
-      	  if (!response.ok) throw new HTTPError(response);
+      	async function _checkOk(response: Response): Promise<Response> {
+      	  if (!response.ok) {
+      	    const responseText = await response.text().catch(() => null);
+      	    let responseJson: unknown | null = null;
+      	    if (responseText) {
+      	      try {
+      	        responseJson = JSON.parse(responseText);
+      	      } catch {}
+      	    }
+      	    throw new HTTPError(response, responseText, responseJson);
+      	  }
       	  return response;
       	}
 
@@ -390,17 +492,34 @@ describe('generateClient', () => {
         	  readonly status: number;
         	  readonly statusText: string;
         	  readonly response: Response;
-        	  constructor(response: Response) {
+        	  readonly responseText: string | null;
+        	  readonly responseJson: unknown | null;
+        	  constructor(
+        	    response: Response,
+        	    responseText: string | null,
+        	    responseJson: unknown | null
+        	  ) {
         	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
         	    this.name = 'HTTPError';
         	    this.status = response.status;
         	    this.statusText = response.statusText;
         	    this.response = response;
+        	    this.responseText = responseText;
+        	    this.responseJson = responseJson;
         	  }
         	}
 
-        	function _checkOk(response: Response): Response {
-        	  if (!response.ok) throw new HTTPError(response);
+        	async function _checkOk(response: Response): Promise<Response> {
+        	  if (!response.ok) {
+        	    const responseText = await response.text().catch(() => null);
+        	    let responseJson: unknown | null = null;
+        	    if (responseText) {
+        	      try {
+        	        responseJson = JSON.parse(responseText);
+        	      } catch {}
+        	    }
+        	    throw new HTTPError(response, responseText, responseJson);
+        	  }
         	  return response;
         	}
 
@@ -460,17 +579,34 @@ describe('generateClient', () => {
       	  readonly status: number;
       	  readonly statusText: string;
       	  readonly response: Response;
-      	  constructor(response: Response) {
+      	  readonly responseText: string | null;
+      	  readonly responseJson: unknown | null;
+      	  constructor(
+      	    response: Response,
+      	    responseText: string | null,
+      	    responseJson: unknown | null
+      	  ) {
       	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
       	    this.name = 'HTTPError';
       	    this.status = response.status;
       	    this.statusText = response.statusText;
       	    this.response = response;
+      	    this.responseText = responseText;
+      	    this.responseJson = responseJson;
       	  }
       	}
 
-      	function _checkOk(response: Response): Response {
-      	  if (!response.ok) throw new HTTPError(response);
+      	async function _checkOk(response: Response): Promise<Response> {
+      	  if (!response.ok) {
+      	    const responseText = await response.text().catch(() => null);
+      	    let responseJson: unknown | null = null;
+      	    if (responseText) {
+      	      try {
+      	        responseJson = JSON.parse(responseText);
+      	      } catch {}
+      	    }
+      	    throw new HTTPError(response, responseText, responseJson);
+      	  }
       	  return response;
       	}
 
@@ -508,17 +644,34 @@ describe('generateClient', () => {
         	  readonly status: number;
         	  readonly statusText: string;
         	  readonly response: Response;
-        	  constructor(response: Response) {
+        	  readonly responseText: string | null;
+        	  readonly responseJson: unknown | null;
+        	  constructor(
+        	    response: Response,
+        	    responseText: string | null,
+        	    responseJson: unknown | null
+        	  ) {
         	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
         	    this.name = 'HTTPError';
         	    this.status = response.status;
         	    this.statusText = response.statusText;
         	    this.response = response;
+        	    this.responseText = responseText;
+        	    this.responseJson = responseJson;
         	  }
         	}
 
-        	function _checkOk(response: Response): Response {
-        	  if (!response.ok) throw new HTTPError(response);
+        	async function _checkOk(response: Response): Promise<Response> {
+        	  if (!response.ok) {
+        	    const responseText = await response.text().catch(() => null);
+        	    let responseJson: unknown | null = null;
+        	    if (responseText) {
+        	      try {
+        	        responseJson = JSON.parse(responseText);
+        	      } catch {}
+        	    }
+        	    throw new HTTPError(response, responseText, responseJson);
+        	  }
         	  return response;
         	}
 
@@ -572,17 +725,34 @@ describe('generateClient', () => {
       	  readonly status: number;
       	  readonly statusText: string;
       	  readonly response: Response;
-      	  constructor(response: Response) {
+      	  readonly responseText: string | null;
+      	  readonly responseJson: unknown | null;
+      	  constructor(
+      	    response: Response,
+      	    responseText: string | null,
+      	    responseJson: unknown | null
+      	  ) {
       	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
       	    this.name = 'HTTPError';
       	    this.status = response.status;
       	    this.statusText = response.statusText;
       	    this.response = response;
+      	    this.responseText = responseText;
+      	    this.responseJson = responseJson;
       	  }
       	}
 
-      	function _checkOk(response: Response): Response {
-      	  if (!response.ok) throw new HTTPError(response);
+      	async function _checkOk(response: Response): Promise<Response> {
+      	  if (!response.ok) {
+      	    const responseText = await response.text().catch(() => null);
+      	    let responseJson: unknown | null = null;
+      	    if (responseText) {
+      	      try {
+      	        responseJson = JSON.parse(responseText);
+      	      } catch {}
+      	    }
+      	    throw new HTTPError(response, responseText, responseJson);
+      	  }
       	  return response;
       	}
 
@@ -636,17 +806,34 @@ describe('generateClient', () => {
       	  readonly status: number;
       	  readonly statusText: string;
       	  readonly response: Response;
-      	  constructor(response: Response) {
+      	  readonly responseText: string | null;
+      	  readonly responseJson: unknown | null;
+      	  constructor(
+      	    response: Response,
+      	    responseText: string | null,
+      	    responseJson: unknown | null
+      	  ) {
       	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
       	    this.name = 'HTTPError';
       	    this.status = response.status;
       	    this.statusText = response.statusText;
       	    this.response = response;
+      	    this.responseText = responseText;
+      	    this.responseJson = responseJson;
       	  }
       	}
 
-      	function _checkOk(response: Response): Response {
-      	  if (!response.ok) throw new HTTPError(response);
+      	async function _checkOk(response: Response): Promise<Response> {
+      	  if (!response.ok) {
+      	    const responseText = await response.text().catch(() => null);
+      	    let responseJson: unknown | null = null;
+      	    if (responseText) {
+      	      try {
+      	        responseJson = JSON.parse(responseText);
+      	      } catch {}
+      	    }
+      	    throw new HTTPError(response, responseText, responseJson);
+      	  }
       	  return response;
       	}
 
@@ -701,17 +888,34 @@ describe('generateClient', () => {
       	  readonly status: number;
       	  readonly statusText: string;
       	  readonly response: Response;
-      	  constructor(response: Response) {
+      	  readonly responseText: string | null;
+      	  readonly responseJson: unknown | null;
+      	  constructor(
+      	    response: Response,
+      	    responseText: string | null,
+      	    responseJson: unknown | null
+      	  ) {
       	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
       	    this.name = 'HTTPError';
       	    this.status = response.status;
       	    this.statusText = response.statusText;
       	    this.response = response;
+      	    this.responseText = responseText;
+      	    this.responseJson = responseJson;
       	  }
       	}
 
-      	function _checkOk(response: Response): Response {
-      	  if (!response.ok) throw new HTTPError(response);
+      	async function _checkOk(response: Response): Promise<Response> {
+      	  if (!response.ok) {
+      	    const responseText = await response.text().catch(() => null);
+      	    let responseJson: unknown | null = null;
+      	    if (responseText) {
+      	      try {
+      	        responseJson = JSON.parse(responseText);
+      	      } catch {}
+      	    }
+      	    throw new HTTPError(response, responseText, responseJson);
+      	  }
       	  return response;
       	}
 
@@ -764,17 +968,34 @@ describe('generateClient', () => {
       	  readonly status: number;
       	  readonly statusText: string;
       	  readonly response: Response;
-      	  constructor(response: Response) {
+      	  readonly responseText: string | null;
+      	  readonly responseJson: unknown | null;
+      	  constructor(
+      	    response: Response,
+      	    responseText: string | null,
+      	    responseJson: unknown | null
+      	  ) {
       	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
       	    this.name = 'HTTPError';
       	    this.status = response.status;
       	    this.statusText = response.statusText;
       	    this.response = response;
+      	    this.responseText = responseText;
+      	    this.responseJson = responseJson;
       	  }
       	}
 
-      	function _checkOk(response: Response): Response {
-      	  if (!response.ok) throw new HTTPError(response);
+      	async function _checkOk(response: Response): Promise<Response> {
+      	  if (!response.ok) {
+      	    const responseText = await response.text().catch(() => null);
+      	    let responseJson: unknown | null = null;
+      	    if (responseText) {
+      	      try {
+      	        responseJson = JSON.parse(responseText);
+      	      } catch {}
+      	    }
+      	    throw new HTTPError(response, responseText, responseJson);
+      	  }
       	  return response;
       	}
 
@@ -837,17 +1058,34 @@ describe('generateClient', () => {
       	  readonly status: number;
       	  readonly statusText: string;
       	  readonly response: Response;
-      	  constructor(response: Response) {
+      	  readonly responseText: string | null;
+      	  readonly responseJson: unknown | null;
+      	  constructor(
+      	    response: Response,
+      	    responseText: string | null,
+      	    responseJson: unknown | null
+      	  ) {
       	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
       	    this.name = 'HTTPError';
       	    this.status = response.status;
       	    this.statusText = response.statusText;
       	    this.response = response;
+      	    this.responseText = responseText;
+      	    this.responseJson = responseJson;
       	  }
       	}
 
-      	function _checkOk(response: Response): Response {
-      	  if (!response.ok) throw new HTTPError(response);
+      	async function _checkOk(response: Response): Promise<Response> {
+      	  if (!response.ok) {
+      	    const responseText = await response.text().catch(() => null);
+      	    let responseJson: unknown | null = null;
+      	    if (responseText) {
+      	      try {
+      	        responseJson = JSON.parse(responseText);
+      	      } catch {}
+      	    }
+      	    throw new HTTPError(response, responseText, responseJson);
+      	  }
       	  return response;
       	}
 
diff --git a/packages/generator/tests/generate.test.ts b/packages/generator/tests/generate.test.ts
index 133920e..ac427db 100644
--- a/packages/generator/tests/generate.test.ts
+++ b/packages/generator/tests/generate.test.ts
@@ -146,17 +146,34 @@ describe('generateFromObject', () => {
     	  readonly status: number;
     	  readonly statusText: string;
     	  readonly response: Response;
-    	  constructor(response: Response) {
+    	  readonly responseText: string | null;
+    	  readonly responseJson: unknown | null;
+    	  constructor(
+    	    response: Response,
+    	    responseText: string | null,
+    	    responseJson: unknown | null
+    	  ) {
     	    super(\`HTTP Error: \${response.status} \${response.statusText}\`);
     	    this.name = 'HTTPError';
     	    this.status = response.status;
     	    this.statusText = response.statusText;
     	    this.response = response;
+    	    this.responseText = responseText;
+    	    this.responseJson = responseJson;
     	  }
     	}
 
-    	function _checkOk(response: Response): Response {
-    	  if (!response.ok) throw new HTTPError(response);
+    	async function _checkOk(response: Response): Promise<Response> {
+    	  if (!response.ok) {
+    	    const responseText = await response.text().catch(() => null);
+    	    let responseJson: unknown | null = null;
+    	    if (responseText) {
+    	      try {
+    	        responseJson = JSON.parse(responseText);
+    	      } catch {}
+    	    }
+    	    throw new HTTPError(response, responseText, responseJson);
+    	  }
     	  return response;
     	}