diff --git a/stdlib/socket.rail b/stdlib/socket.rail
index 6ae20f5..3acf5fc 100644
--- a/stdlib/socket.rail
+++ b/stdlib/socket.rail
@@ -193,12 +193,16 @@ accept_tcp listen_fd =
   cfd
 
 -- Copy `n` bytes from buf[off..off+n) into a Rail string.
--- Uses char_from_int + join; O(N²) alloc, fine for small reads.
+-- Builds a list of single-char strings then joins once: O(N) allocation
+-- instead of O(N²) from repeated prefix-copying concat.
 tcp_bytes_to_str buf off n acc =
-  if n <= 0 then acc
+  join "" (tcp_bytes_to_list buf off n)
+
+tcp_bytes_to_list buf off n =
+  if n <= 0 then []
   else
     let c = byte_at buf off
-    tcp_bytes_to_str buf (off + 1) (n - 1) (join "" [acc, char_from_int c])
+    (char_from_int c) : (tcp_bytes_to_list buf (off + 1) (n - 1))
 
 -- Receive bytes until the HTTP header terminator \r\n\r\n appears, then
 -- continue reading Content-Length bytes of body if present. Returns the