diff --git a/.changeset/fix-empty-error-body-truthy.md b/.changeset/fix-empty-error-body-truthy.md
new file mode 100644
index 000000000..f31b797c1
--- /dev/null
+++ b/.changeset/fix-empty-error-body-truthy.md
@@ -0,0 +1,5 @@
+---
+"openapi-fetch": patch
+---
+
+Fix `error` being `undefined` when a server returns a 4xx/5xx status with no response body. Previously `if (error)` checks would silently pass even on error responses. The `error` field now contains the HTTP status text (e.g. `"Not Found"`) or status code string (e.g. `"500"`) when the body is empty, making `if (error)` reliable for all error responses.
diff --git a/packages/openapi-fetch/biome.json b/packages/openapi-fetch/biome.json
index 99724ae16..f32a6de1e 100644
--- a/packages/openapi-fetch/biome.json
+++ b/packages/openapi-fetch/biome.json
@@ -3,7 +3,7 @@
   "$schema": "https://biomejs.dev/schemas/2.3.14/schema.json",
   "extends": "//",
   "files": {
-    "includes": ["src/**", "test/**", "!**/examples/**", "!test/**/schemas/**", "!test/bench/**/*.min.js"]
+    "includes": ["src/**", "test/**", "!**/examples", "!test/**/schemas", "!test/bench/**/*.min.js"]
   },
   "linter": {
     "rules": {
diff --git a/packages/openapi-fetch/src/index.js b/packages/openapi-fetch/src/index.js
index 4c7250fb0..5df9636cf 100644
--- a/packages/openapi-fetch/src/index.js
+++ b/packages/openapi-fetch/src/index.js
@@ -4,7 +4,7 @@ const PATH_PARAM_RE = /\{[^{}]+\}/g;
 const supportsRequestInitExt = () => {
   return (
     typeof process === "object" &&
-    Number.parseInt(process?.versions?.node?.substring(0, 2)) >= 18 &&
+    Number.parseInt(process?.versions?.node?.substring(0, 2), 10) >= 18 &&
     process.versions.undici
   );
 };
@@ -242,7 +242,7 @@ export default function createClient(clientOptions) {
       request.method === "HEAD" ||
       (contentLength === "0" && !response.headers.get("Transfer-Encoding")?.includes("chunked"))
     ) {
-      return response.ok ? { data: undefined, response } : { error: undefined, response };
+      return response.ok ? { data: undefined, response } : { error: response.statusText || String(response.status), response };
     }
 
     // parse response (falling back to .text() when necessary)
@@ -267,8 +267,7 @@ export default function createClient(clientOptions) {
     // handle errors (use text() when no content-length to safely handle empty bodies from proxies)
     const raw = await response.text();
     if (!raw) {
-      // empty error body - return undefined to be consistent with status 204 handling
-      return { error: undefined, response };
+      return { error: response.statusText || String(response.status), response };
     }
     let error = raw;
     try {
@@ -625,6 +624,9 @@ export function defaultBodySerializer(body, headers) {
   if (body instanceof FormData) {
     return body;
   }
+  if (typeof body === "string") {
+    return body;
+  }
   if (headers) {
     const contentType =
       headers.get instanceof Function
diff --git a/packages/openapi-fetch/test/common/create-client-e2e.test.js b/packages/openapi-fetch/test/common/create-client-e2e.test.js
index 88536e083..51fe78f6d 100644
--- a/packages/openapi-fetch/test/common/create-client-e2e.test.js
+++ b/packages/openapi-fetch/test/common/create-client-e2e.test.js
@@ -67,7 +67,7 @@ const caToBuffer = (ca) => {
 const API_PORT = process.env.API_PORT || 4578;
 
 const app = express();
-app.get("/v1/foo", (req, res) => {
+app.get("/v1/foo", (_req, res) => {
   res.send("bar");
 });
 
diff --git a/packages/openapi-fetch/test/common/request.test.ts b/packages/openapi-fetch/test/common/request.test.ts
index 6a836520e..be926327a 100644
--- a/packages/openapi-fetch/test/common/request.test.ts
+++ b/packages/openapi-fetch/test/common/request.test.ts
@@ -236,7 +236,19 @@ describe("request", () => {
       });
 
       expect(bodyUsed).toBe(true);
-      expect(bodyText).toBe('""');
+      expect(bodyText).toBe("");
+    });
+
+    test.each(BODY_ACCEPTING_METHODS)("string body is passed through without JSON serialization - %s", async (method) => {
+      const { bodyUsed, bodyText } = await fireRequestAndGetBodyInformation({
+        method,
+        fetchOptions: {
+          body: "pre-serialized string",
+        },
+      });
+
+      expect(bodyUsed).toBe(true);
+      expect(bodyText).toBe("pre-serialized string");
     });
 
     test.each(BODY_ACCEPTING_METHODS)("`0` body (with body serializer) - %s", async (method) => {
diff --git a/packages/openapi-fetch/test/common/response.test.ts b/packages/openapi-fetch/test/common/response.test.ts
index 72f4f1f6e..0144cda10 100644
--- a/packages/openapi-fetch/test/common/response.test.ts
+++ b/packages/openapi-fetch/test/common/response.test.ts
@@ -127,7 +127,7 @@ describe("response", () => {
 
   describe("response object", () => {
     test.each([200, 404, 500] as const)("%s", async (status) => {
-      const client = createObservedClient<paths>({}, async (req) =>
+      const client = createObservedClient<paths>({}, async (_req) =>
         Response.json({ status, message: "OK" }, { status }),
       );
       const result = await client.GET(status === 200 ? "/resources" : `/error-${status}`);
diff --git a/packages/openapi-fetch/test/e2e/index.e2e.ts b/packages/openapi-fetch/test/e2e/index.e2e.ts
index ecd6cfaa5..38a371704 100644
--- a/packages/openapi-fetch/test/e2e/index.e2e.ts
+++ b/packages/openapi-fetch/test/e2e/index.e2e.ts
@@ -1,4 +1,4 @@
-import { expect, type Page, test } from "@playwright/test";
+import { type Page, test } from "@playwright/test";
 
 // note: these tests load Chrome, Firefox, and Safari in Playwright to test a browser-realistic runtime.
 // the frontend is prepared via Vite to create a production-accurate app (and throw add’l type errors)
diff --git a/packages/openapi-fetch/test/http-methods/delete.test.ts b/packages/openapi-fetch/test/http-methods/delete.test.ts
index 16979ca7a..24ed10bb1 100644
--- a/packages/openapi-fetch/test/http-methods/delete.test.ts
+++ b/packages/openapi-fetch/test/http-methods/delete.test.ts
@@ -62,14 +62,31 @@ describe("DELETE", () => {
     // assert data is undefined for error response
     expect(data).toBeUndefined();
 
-    // assert error is undefined for empty body (consistent with 204 and Content-Length: 0 handling)
-    expect(error).toBeUndefined();
+    // assert error is truthy for error response with empty body
+    expect(error).toBe("500");
 
     // assert response status is preserved
     expect(response.status).toBe(500);
     expect(response.ok).toBe(false);
   });
 
+  test("error response with Content-Length: 0 returns truthy error", async () => {
+    const client = createObservedClient<paths>(
+      {},
+      async () => new Response(null, { status: 404, statusText: "Not Found", headers: { "Content-Length": "0" } }),
+    );
+    const { data, error, response } = await client.DELETE("/tags/{name}", {
+      params: {
+        path: { name: "Tag" },
+      },
+    });
+
+    expect(data).toBeUndefined();
+    expect(error).toBe("Not Found");
+    expect(response.status).toBe(404);
+    expect(response.ok).toBe(false);
+  });
+
   test("handles success response with empty body when Content-Length header is stripped by proxy", async () => {
     // Simulate proxy stripping Content-Length header from an empty success response
     const client = createObservedClient<paths>(
diff --git a/packages/openapi-fetch/test/http-methods/get.test.ts b/packages/openapi-fetch/test/http-methods/get.test.ts
index 991f72453..36ad107a4 100644
--- a/packages/openapi-fetch/test/http-methods/get.test.ts
+++ b/packages/openapi-fetch/test/http-methods/get.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, test } from "vitest";
 import { createObservedClient } from "../helpers.js";
-import type { components, paths } from "./schemas/get.js";
+import type { paths } from "./schemas/get.js";
 
 describe("GET", () => {
   test("sends correct method", async () => {
diff --git a/packages/openapi-fetch/test/middleware/middleware.test.ts b/packages/openapi-fetch/test/middleware/middleware.test.ts
index 0a1fecdea..4ad25ad9c 100644
--- a/packages/openapi-fetch/test/middleware/middleware.test.ts
+++ b/packages/openapi-fetch/test/middleware/middleware.test.ts
@@ -98,7 +98,7 @@ test("can modify response", async () => {
 
 test("returns original errors if nothing is returned", async () => {
   const actualError = new Error();
-  const client = createObservedClient<paths>({}, async (req) => {
+  const client = createObservedClient<paths>({}, async (_req) => {
     throw actualError;
   });
   client.use({