fix: Remove content encoding header when body is actually not gzipped

Author: seedspirit

src/ai/backend/web/proxy.py

@@ -221,10 +221,29 @@ async def web_handler(
             async with api_request.fetch() as up_resp:
                 down_resp = web.StreamResponse()
                 down_resp.set_status(up_resp.status, up_resp.reason)
-                down_resp.headers.update(up_resp.headers)
+
+                final_headers = dict(up_resp.headers)
+
+                # figure out if the response body is gzipped by checking the first chunk
+                first_chunk = await up_resp.read(8192)
+                if first_chunk:
+                    body_is_actually_gzipped = first_chunk.startswith(b"\x1f\x8b")
+                    header_says_gzipped = (
+                        "gzip" in final_headers.get("Content-Encoding", "").lower()
+                    )
+                    # If reponse header says gzipped but the body is not actually gzipped,
+                    # remove the header to prevent confusion.
+                    if header_says_gzipped and not body_is_actually_gzipped:
+                        del final_headers["Content-Encoding"]
+
                 # We already have configured CORS handlers and the API server
                 # also provides those headers.  Just let them as-is.
+                down_resp.headers.update(final_headers)
                 await down_resp.prepare(request)
+
+                if first_chunk:
+                    await down_resp.write(first_chunk)
+
                 while True:
                     chunk = await up_resp.read(8192)
                     if not chunk: