diff --git a/src/transport/socks5/pool.rs b/src/transport/socks5/pool.rs
index 969f8268..4a57f4e4 100644
--- a/src/transport/socks5/pool.rs
+++ b/src/transport/socks5/pool.rs
@@ -370,7 +370,7 @@ pub(crate) async fn proxied_receive_loop<S: ProxiedStats, M>(
 mod tests {
     use super::*;
     use crate::transport::packet_channel;
-    use crate::transport::stream::next_conn_id;
+    use crate::transport::stream::{next_conn_id, park_writer};
     use portable_atomic::{AtomicU64, Ordering};
     use tokio::io::AsyncReadExt;
     use tokio::net::TcpListener;
@@ -555,33 +555,19 @@ mod tests {
             },
         );
 
-        // Fill without stopping at the first refusal, yielding so the writer
-        // runs, and never keep a sender past the fill.
+        // Never keep a sender past the fill: the teardown below relies on the
+        // pool entry holding the only one.
         let frame = vec![0xAB; 1400];
-        let mut queued = 0usize;
-        let mut refused = 0usize;
-        for _ in 0..8000 {
-            let sent = {
-                let guard = pool.lock().await;
-                guard
+        let queued = park_writer(
+            async || {
+                pool.lock()
+                    .await
                     .get(&remote)
-                    .map(|c| c.send_tx.try_send(frame.clone()).is_ok())
-            };
-            if sent == Some(true) {
-                queued += 1;
-            } else {
-                refused += 1;
-            }
-            tokio::task::yield_now().await;
-        }
-        tokio::time::sleep(Duration::from_millis(300)).await;
-        let capacity = pool.lock().await.get(&remote).map(|c| c.send_tx.capacity());
-        assert_eq!(
-            capacity,
-            Some(0),
-            "setup did not park the writer: queued={queued} refused={refused}"
-        );
-        assert!(refused > 0, "setup never filled the queue: queued={queued}");
+                    .is_some_and(|c| c.send_tx.try_send(frame.clone()).is_ok())
+            },
+            async || pool.lock().await.get(&remote).map(|c| c.send_tx.capacity()),
+        )
+        .await;
 
         peer.shutdown().await.unwrap();
         assert!(
diff --git a/src/transport/stream.rs b/src/transport/stream.rs
index 103ed68d..64a99bb1 100644
--- a/src/transport/stream.rs
+++ b/src/transport/stream.rs
@@ -75,6 +75,72 @@ pub(crate) fn drain_writer(send_task: JoinHandle<()>, bound: Duration) -> JoinHa
     })
 }
 
+/// How long a filled send queue must stay full, with nothing sending, before a
+/// test treats its writer as parked.
+#[cfg(test)]
+pub(crate) const PARK_SETTLE: Duration = Duration::from_millis(300);
+
+/// How many settle intervals a test waits for its writer to stay parked.
+#[cfg(test)]
+pub(crate) const PARK_ROUNDS: usize = 10;
+
+/// Fill a connection's send queue behind a peer that does not read, until the
+/// writer is parked in `write_all`, and return how many frames were queued.
+///
+/// `offer` tries to queue one frame and says whether it was accepted;
+/// `capacity` reads the queue's free slots, `None` if the connection is gone.
+/// Every one of 8000 offers is made whatever the previous one returned, with a
+/// yield between them so the writer runs. Then the queue must read full after
+/// a settle interval with nothing sending. Free capacity only grows while
+/// nothing sends, so a full reading means the writer took no frame for the
+/// whole interval, which it can do only while blocked in `write_all`.
+///
+/// A queue that is not full is topped up and the interval repeated, because a
+/// late ACK can free send-buffer space after the fill ends and let the writer
+/// take a few frames before it parks again; FreeBSD delays that ACK on
+/// loopback. A writer that never stays parked for a whole interval panics.
+#[cfg(test)]
+pub(crate) async fn park_writer(
+    mut offer: impl AsyncFnMut() -> bool,
+    mut capacity: impl AsyncFnMut() -> Option<usize>,
+) -> usize {
+    let mut queued = 0usize;
+    let mut refused = 0usize;
+    for _ in 0..8000 {
+        if offer().await {
+            queued += 1;
+        } else {
+            refused += 1;
+        }
+        tokio::task::yield_now().await;
+    }
+    let mut seen = Vec::with_capacity(PARK_ROUNDS);
+    for _ in 0..PARK_ROUNDS {
+        tokio::time::sleep(PARK_SETTLE).await;
+        let free = capacity().await;
+        seen.push(free);
+        match free {
+            Some(0) => {
+                assert!(refused > 0, "setup never filled the queue: queued={queued}");
+                return queued;
+            }
+            Some(n) => {
+                for _ in 0..=n {
+                    if !offer().await {
+                        break;
+                    }
+                    queued += 1;
+                }
+            }
+            None => break,
+        }
+    }
+    panic!(
+        "setup did not park the writer: queued={queued} refused={refused} \
+         capacity after each settle={seen:?}"
+    );
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -116,4 +182,82 @@ mod tests {
             .expect("the drain timer kept running after the writer exited")
             .unwrap();
     }
+
+    /// Depth of the modelled send queue in the `park_writer` tests.
+    const MODEL_DEPTH: usize = 4;
+
+    /// A queue a late ACK drained after the fill is topped up, and the helper
+    /// returns at the first settle that finds it still full.
+    ///
+    /// The first capacity read drains 2 frames and later reads drain none, so
+    /// the fill queues 4, the top-up queues 2 more, and the second read ends
+    /// the wait.
+    #[tokio::test(start_paused = true)]
+    async fn park_writer_tops_up_a_queue_a_late_ack_drained_and_returns_once_it_stays_full() {
+        let len = std::cell::Cell::new(0usize);
+        let drain = std::cell::Cell::new(2usize);
+        let calls = std::cell::Cell::new(0usize);
+        let queued = park_writer(
+            async || {
+                let accept = len.get() < MODEL_DEPTH;
+                if accept {
+                    len.set(len.get() + 1);
+                }
+                accept
+            },
+            async || {
+                calls.set(calls.get() + 1);
+                len.set(len.get().saturating_sub(drain.take()));
+                Some(MODEL_DEPTH - len.get())
+            },
+        )
+        .await;
+        assert_eq!(queued, 6, "fill of 4 plus a top-up of 2");
+        assert_eq!(
+            calls.get(),
+            2,
+            "the second settle should find the queue full"
+        );
+    }
+
+    /// A writer that takes a frame in every settle interval never counts as
+    /// parked.
+    #[tokio::test(start_paused = true)]
+    #[should_panic(expected = "setup did not park the writer")]
+    async fn park_writer_panics_when_the_writer_takes_a_frame_in_every_settle() {
+        let len = std::cell::Cell::new(0usize);
+        let drain = std::cell::Cell::new(1usize);
+        park_writer(
+            async || {
+                let accept = len.get() < MODEL_DEPTH;
+                if accept {
+                    len.set(len.get() + 1);
+                }
+                accept
+            },
+            async || {
+                len.set(len.get().saturating_sub(drain.get()));
+                Some(MODEL_DEPTH - len.get())
+            },
+        )
+        .await;
+    }
+
+    /// A connection that is gone by the settle check never counts as parked.
+    #[tokio::test(start_paused = true)]
+    #[should_panic(expected = "setup did not park the writer")]
+    async fn park_writer_panics_when_the_connection_is_gone() {
+        let len = std::cell::Cell::new(0usize);
+        park_writer(
+            async || {
+                let accept = len.get() < MODEL_DEPTH;
+                if accept {
+                    len.set(len.get() + 1);
+                }
+                accept
+            },
+            async || None,
+        )
+        .await;
+    }
 }
diff --git a/src/transport/tcp/mod.rs b/src/transport/tcp/mod.rs
index 97b74e62..e4643fed 100644
--- a/src/transport/tcp/mod.rs
+++ b/src/transport/tcp/mod.rs
@@ -1356,6 +1356,7 @@ mod tests {
     use super::*;
     use crate::transport::framing::build_msg1_frame;
     use crate::transport::packet_channel;
+    use crate::transport::stream::park_writer;
     use tokio::time::{Duration, timeout};
 
     /// Poll `f` every 10ms until it holds or `limit` elapses.
@@ -2386,40 +2387,25 @@ mod tests {
         socket.listen(8).unwrap()
     }
 
-    /// Fill `remote`'s send queue behind a peer that does not read, until the
+    /// Fill `remote`'s send queue behind a peer that does not read until the
     /// writer is parked in `write_all`, and return how many frames were
-    /// queued.
-    ///
-    /// Every send is attempted whatever the previous one returned, with a
-    /// yield between sends so the writer runs. A queue still full 300 ms
-    /// after the last send means the writer could not drain it, which is the
-    /// state the caller's teardown needs; anything else panics rather than
-    /// letting the caller pass without it.
+    /// queued. The fill and the parked-writer check are `park_writer`'s.
     async fn park_tcp_writer(t: &TcpTransport, remote: &TransportAddr, frame: &[u8]) -> usize {
-        let mut queued = 0usize;
-        let mut refused = 0usize;
-        for _ in 0..8000 {
-            match timeout(Duration::from_secs(2), t.send_async(remote, frame)).await {
-                Ok(Ok(_)) => queued += 1,
-                Ok(Err(_)) => refused += 1,
+        park_writer(
+            async || match timeout(Duration::from_secs(2), t.send_async(remote, frame)).await {
+                Ok(Ok(_)) => true,
+                Ok(Err(_)) => false,
                 Err(_) => panic!("send blocked on a peer that stopped reading"),
-            }
-            tokio::task::yield_now().await;
-        }
-        tokio::time::sleep(Duration::from_millis(300)).await;
-        let capacity = t
-            .pool
-            .lock()
-            .await
-            .get(remote)
-            .map(|c| c.send_tx.capacity());
-        assert_eq!(
-            capacity,
-            Some(0),
-            "setup did not park the writer: queued={queued} refused={refused}"
-        );
-        assert!(refused > 0, "setup never filled the queue: queued={queued}");
-        queued
+            },
+            async || {
+                t.pool
+                    .lock()
+                    .await
+                    .get(remote)
+                    .map(|c| c.send_tx.capacity())
+            },
+        )
+        .await
     }
 
     /// Read `stream` to EOF within `limit`, returning the byte count, or