diff --git a/crates/script/src/socket.rs b/crates/script/src/socket.rs
--- a/crates/script/src/socket.rs
+++ b/crates/script/src/socket.rs
@@ -394,19 +394,33 @@ mod tests {
 
     /// Fill bounded socket buffers nonblocking, rather than assuming a 64 MiB write stalls.
     /// In particular, Windows may accept that entire write with its default send buffering.
+    /// Writes must stay blocked for [`SETTLED`] before the buffers count as full: on macOS the
+    /// accepted stream does not keep the listener's 4 KiB receive buffer (it gets about 114 KiB),
+    /// so the first `WouldBlock` only means the 4 KiB send buffer is full, and the send buffer then
+    /// drains into the peer's larger receive buffer.
     fn backpressure(id: u32) {
+        const SETTLED: Duration = Duration::from_millis(200);
         let mut stream = super::imp::test_stream(id);
         socket2::SockRef::from(&stream).set_send_buffer_size(4096).unwrap();
         stream.set_nonblocking(true).unwrap();
         let start = Instant::now();
         let chunk = [b'x'; 4096];
         let mut sent = 0;
+        let mut blocked_since = None;
         loop {
             assert!(sent < 16 * 1024 * 1024 && start.elapsed() < Duration::from_secs(5), "could not establish backpressure");
             match stream.write(&chunk) {
                 Ok(0) => panic!("peer closed while establishing backpressure"),
-                Ok(n) => sent += n,
-                Err(e) if e.kind() == ErrorKind::WouldBlock => break,
+                Ok(n) => {
+                    sent += n;
+                    blocked_since = None;
+                }
+                Err(e) if e.kind() == ErrorKind::WouldBlock => {
+                    if blocked_since.get_or_insert_with(Instant::now).elapsed() >= SETTLED {
+                        break;
+                    }
+                    std::thread::sleep(Duration::from_millis(5));
+                }
                 Err(e) if e.kind() == ErrorKind::Interrupted => continue,
                 Err(e) => panic!("establishing backpressure: {e}"),
             }