diff --git a/crates/irtt-client/src/event.rs b/crates/irtt-client/src/event.rs
index 7d2dd13..3f58cbf 100644
--- a/crates/irtt-client/src/event.rs
+++ b/crates/irtt-client/src/event.rs
@@ -361,7 +361,10 @@ pub struct OneWayDelaySample {
 pub struct ReceivedStatsSample {
     /// Server-reported count of received packets, when available.
     pub count: Option<u32>,
-    /// Server-reported receive window value, when available.
+    /// Raw server-reported receive window bitmap, when available.
+    ///
+    /// The bitmap is preserved exactly as it appeared on the wire; the client
+    /// does not interpret its bits.
     pub window: Option<u64>,
 }
 
diff --git a/crates/irtt-stats/src/lib.rs b/crates/irtt-stats/src/lib.rs
index e721dc8..b89809a 100644
--- a/crates/irtt-stats/src/lib.rs
+++ b/crates/irtt-stats/src/lib.rs
@@ -229,8 +229,20 @@ pub struct PacketCounts {
     /// exposed separately.
     pub bytes_received: u64,
     /// Highest cumulative server-reported packets received, when available.
+    ///
+    /// This is the only server-reported input to the directional loss estimates
+    /// in [`LossStats`].
     pub server_packets_received: Option<u64>,
-    /// Server-reported receive window, when available.
+    /// Raw server-reported receive window from the most recently processed
+    /// reply that carried one, when available.
+    ///
+    /// The value is stored exactly as the server reported it and is not
+    /// interpreted by this crate. It is a bounded bitmap of recent server
+    /// receive history rather than a fixed 64-packet loss mask, so a value of
+    /// `0x1` means no earlier history is represented and not that the previous
+    /// 63 packets were lost. Replies are aggregated in local arrival order, so
+    /// a late reply can leave an older window here than an already-processed
+    /// reply carried.
     pub server_received_window: Option<u64>,
 }
 
diff --git a/crates/irtt-stats/src/loss.rs b/crates/irtt-stats/src/loss.rs
index a705f24..579aeaa 100644
--- a/crates/irtt-stats/src/loss.rs
+++ b/crates/irtt-stats/src/loss.rs
@@ -6,6 +6,11 @@ use crate::PacketCounts;
 /// Percentage fields are percentages from `0.0` to `100.0` in normal cases.
 /// Directional upstream and downstream loss can be negative when
 /// server-reported counts exceed local expectations.
+///
+/// The directional fields are derived from
+/// [`PacketCounts::server_packets_received`] and are `None` when the server
+/// reported no receive count. [`PacketCounts::server_received_window`] is never
+/// used to derive loss.
 pub struct LossStats {
     /// Locally inferred total lost packets.
     pub lost_packets: u64,
diff --git a/crates/irtt-stats/tests/common/mod.rs b/crates/irtt-stats/tests/common/mod.rs
index bb60b2d..731ea71 100644
--- a/crates/irtt-stats/tests/common/mod.rs
+++ b/crates/irtt-stats/tests/common/mod.rs
@@ -84,6 +84,22 @@ fn reply_with_rtt(seq: u32, raw_ms: u64, rtt: RttSample) -> ClientEvent {
     }
 }
 
+/// Builds a normal reply carrying exactly the supplied server received stats.
+pub fn reply_with_received_stats(
+    seq: u32,
+    raw_ms: u64,
+    count: Option<u32>,
+    window: Option<u64>,
+) -> ClientEvent {
+    let mut event = unadjusted_reply(seq, raw_ms);
+    let ClientEvent::EchoReply { received_stats, .. } = &mut event else {
+        unreachable!();
+    };
+    *received_stats =
+        (count.is_some() || window.is_some()).then_some(ReceivedStatsSample { count, window });
+    event
+}
+
 pub fn unadjusted_late_reply(seq: u32, raw_ms: u64) -> ClientEvent {
     late_reply_from(unadjusted_reply(seq, raw_ms))
 }
diff --git a/crates/irtt-stats/tests/stats.rs b/crates/irtt-stats/tests/stats.rs
index dd487f5..a4a92ed 100644
--- a/crates/irtt-stats/tests/stats.rs
+++ b/crates/irtt-stats/tests/stats.rs
@@ -2,7 +2,9 @@ mod common;
 
 use std::time::{Duration, Instant, UNIX_EPOCH};
 
-use common::{adjusted_reply, sent, ts, unadjusted_late_reply, unadjusted_reply};
+use common::{
+    adjusted_reply, reply_with_received_stats, sent, ts, unadjusted_late_reply, unadjusted_reply,
+};
 use irtt_client::{
     ClientEvent, ClientTimestamp, OneWayDelaySample, PacketMeta, RttSample, SignedDuration,
 };
@@ -378,6 +380,32 @@ fn empty_and_all_lost_edges_are_defined() {
     assert_eq!(all_lost.loss.packet_loss_percent, 100.0);
 }
 
+#[test]
+fn receive_window_is_stored_raw_and_never_drives_directional_loss() {
+    // Window-only negotiation: the server reports a bitmap but no receive count.
+    let mut collector = StatsCollector::new(StatsConfig::finite());
+    collector.process(&sent(0, ts(0)));
+    collector.process(&reply_with_received_stats(0, 10, None, Some(0x1)));
+
+    // `0x1` represents no earlier history. It must be preserved verbatim and
+    // must not be read as 63 lost packets or stand in for a receive count.
+    let snapshot = collector.snapshot();
+    assert_eq!(snapshot.packets.server_received_window, Some(0x1));
+    assert_eq!(snapshot.packets.server_packets_received, None);
+    assert_eq!(snapshot.loss.upstream_loss_packets, None);
+    assert_eq!(snapshot.loss.downstream_loss_packets, None);
+    assert_eq!(snapshot.loss.upstream_loss_percent, 0.0);
+    assert_eq!(snapshot.loss.downstream_loss_percent, 0.0);
+
+    // An advancing bitmap is stored verbatim across the full 64-bit range.
+    collector.process(&sent(1, ts(10)));
+    collector.process(&reply_with_received_stats(1, 10, None, Some(u64::MAX)));
+    assert_eq!(
+        collector.snapshot().packets.server_received_window,
+        Some(u64::MAX)
+    );
+}
+
 #[test]
 fn directional_loss_uses_server_received_count_when_available() {
     let mut collector = StatsCollector::new(StatsConfig::finite());