Do not treat Content-Length=0 as empty when Transfer-Encoding is chunked (#2572)

* fix(response): handle zero Content-Length correctly for chunked and non-chunked responses

Some HTTP servers incorrectly send `Content-Length: 0` together with
`Transfer-Encoding: chunked`. According to RFC 7230, chunked transfer
encoding overrides Content-Length and the body may still contain data.

Previously, the client treated any response with Content-Length=0 as
having an empty body, which caused valid chunked responses to be
discarded.

This change only treats Content-Length=0 as empty when the response is
not chunked, preserving correct behavior for misconfigured servers.

* chore: fix linting errors

* chore: revert accidental line changes

* chore: added changeset

* Resolve merge conflicts

* Properly check if content length is zero

Author: luis-guideti

.changeset/empty-ghosts-sip.md

@@ -0,0 +1,5 @@
+---
+"openapi-fetch": minor
+---
+
+Do not treat Content-Length=0 as empty when Transfer-Encoding is chunked

packages/openapi-fetch/src/index.js

@@ -237,7 +237,11 @@ export default function createClient(clientOptions) {
 
     const contentLength = response.headers.get("Content-Length");
     // handle empty content
-    if (response.status === 204 || request.method === "HEAD" || contentLength === "0") {
+    if (
+      response.status === 204 ||
+      request.method === "HEAD" ||
+      (contentLength === "0" && !response.headers.get("Transfer-Encoding")?.includes("chunked"))
+    ) {
       return response.ok ? { data: undefined, response } : { error: undefined, response };
     }
 

packages/openapi-fetch/test/common/response.test.ts

@@ -204,4 +204,36 @@ describe("response", () => {
       }
     });
   });
+
+  describe("chunked transfer with zero Content-Length", () => {
+    test("does not treat chunked body with Content-Length: 0 as empty", async () => {
+      const mock = [{ id: 1 }];
+      const client = createObservedClient<paths>({}, async () =>
+        Response.json(mock, {
+          status: 200,
+          headers: { "Content-Length": "0", "Transfer-Encoding": "chunked" },
+        }),
+      );
+
+      const { data, error, response } = await client.GET("/resources");
+      expect(response.status).toBe(200);
+      expect(error).toBeUndefined();
+      expect(data).toEqual(mock);
+    });
+  });
+  describe("Content-Length: 0 without chunked", () => {
+    test("treats as empty when not chunked", async () => {
+      const client = createObservedClient<paths>({}, async () =>
+        Response.json([{ id: 1 }], {
+          status: 200,
+          headers: { "Content-Length": "0" },
+        }),
+      );
+
+      const { data, error, response } = await client.GET("/resources");
+      expect(response.status).toBe(200);
+      expect(error).toBeUndefined();
+      expect(data).toBeUndefined();
+    });
+  });
 });