fix(openapi-fetch): handle empty error responses when Content-Length header is stripped (#2605)

* fix(openapi-fetch): handle empty error responses when Content-Length header is stripped

When a proxy (like Cloudflare) strips the Content-Length header from an
error response with no body, the client would return an empty string
as the error value, which is falsy and causes issues when checking
`if (error)`.

This fix applies the same safe handling to error responses that already
exists for success responses - when Content-Length is absent and the
body is empty, return undefined for the error value to be consistent
with 204 and Content-Length: 0 handling.

Fixes #2574

* Add changeset

---------

Co-authored-by: Kai Gritun <kai@kaigritun.com>

Author: kaigritun

.changeset/fix-empty-error-response.md

@@ -0,0 +1,5 @@
+---
+"openapi-fetch": patch
+---
+
+fix(openapi-fetch): handle empty error responses when Content-Length header is stripped

packages/openapi-fetch/src/index.js

@@ -264,12 +264,17 @@ export default function createClient(clientOptions) {
       return { data: await getResponseData(), response };
     }
 
-    // handle errors
-    let error = await response.text();
+    // 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 };
+    }
+    let error = raw;
     try {
-      error = JSON.parse(error); // attempt to parse as JSON
+      error = JSON.parse(raw); // attempt to parse as JSON
     } catch {
-      // noop
+      // noop - keep as raw text
     }
     return { error, response };
   }

packages/openapi-fetch/test/http-methods/delete.test.ts

@@ -46,4 +46,50 @@ describe("DELETE", () => {
     // assert error is empty
     expect(error).toBeUndefined();
   });
+
+  test("handles error response with empty body when Content-Length header is stripped by proxy", async () => {
+    // Simulate proxy stripping Content-Length header from an empty error response
+    const client = createObservedClient<paths>(
+      {},
+      async () => new Response(null, { status: 500 }), // No Content-Length header
+    );
+    const { data, error, response } = await client.DELETE("/tags/{name}", {
+      params: {
+        path: { name: "Tag" },
+      },
+    });
+
+    // 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 response status is preserved
+    expect(response.status).toBe(500);
+    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>(
+      {},
+      async () => new Response(null, { status: 200 }), // No Content-Length header
+    );
+    const { data, error, response } = await client.DELETE("/tags/{name}", {
+      params: {
+        path: { name: "Tag" },
+      },
+    });
+
+    // assert data is undefined for empty body
+    expect(data).toBeUndefined();
+
+    // assert error is undefined for success response
+    expect(error).toBeUndefined();
+
+    // assert response status is preserved
+    expect(response.status).toBe(200);
+    expect(response.ok).toBe(true);
+  });
 });