fix(openapi-fetch): return truthy error for empty error response bodies

.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.

packages/openapi-fetch/src/index.js

@@ -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

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) => {

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>(