diff --git a/engine/crates/layerfs-sandbox/src/backend/docker/listener.rs b/engine/crates/layerfs-sandbox/src/backend/docker/listener.rs
index f7b870920..137ec4375 100644
--- a/engine/crates/layerfs-sandbox/src/backend/docker/listener.rs
+++ b/engine/crates/layerfs-sandbox/src/backend/docker/listener.rs
@@ -4,6 +4,7 @@ use super::{body::exact, Sandbox};
 use super::{body::Body, socket::Socket};
 use crate::{RuntimeError, WireFailure};
 use std::{
+    collections::VecDeque,
     io::{self, BufReader, Read},
     net::Shutdown,
     time::{Duration, Instant},
@@ -161,7 +162,7 @@ impl Sandbox {
                 None,
             )));
         }
-        let mut kept = Vec::new();
+        let mut kept = VecDeque::new();
         let parsed = (|| {
             let mut payload = [0; 8192];
             loop {
@@ -182,13 +183,19 @@ impl Sandbox {
                         return Err(io::Error::from(io::ErrorKind::UnexpectedEof).into());
                     }
                     left -= n as u32;
-                    kept.extend_from_slice(&payload[..n]);
-                    kept.drain(..kept.len().saturating_sub(cap));
+                    if n >= cap {
+                        kept.clear();
+                        kept.extend(payload[n - cap..n].iter().copied());
+                    } else {
+                        let discard = kept.len().saturating_sub(cap - n);
+                        kept.drain(..discard);
+                        kept.extend(payload[..n].iter().copied());
+                    }
                 }
             }
         })();
         parsed.map_err(|e| self.selected(r.fail(e, None)))?;
-        Ok(kept)
+        Ok(kept.into_iter().collect())
     }
 }
 
diff --git a/engine/crates/layerfs-sandbox/tests/engine_log_tail.rs b/engine/crates/layerfs-sandbox/tests/engine_log_tail.rs
new file mode 100644
index 000000000..bbf89bfa1
--- /dev/null
+++ b/engine/crates/layerfs-sandbox/tests/engine_log_tail.rs
@@ -0,0 +1,113 @@
+//! Log suffix semantics through the public Sandbox API and a bounded Engine peer.
+#![cfg(unix)]
+#[allow(dead_code)]
+#[path = "support/engine.rs"]
+mod peer;
+use layerfs_sandbox::{
+    backend::docker::{Sandbox, SandboxRequest},
+    ContainerId, RuntimeError,
+};
+use peer::{frame, Peer};
+
+fn read_tail(wire: Vec<u8>, cap: usize, split: usize) -> Vec<u8> {
+    let (docker, server) = Peer::new(vec![wire], split);
+    let sandbox = Sandbox::stopped(
+        docker,
+        ContainerId::parse(&"b".repeat(64)).unwrap(),
+        SandboxRequest {
+            image: format!("sha256:{}", "a".repeat(64)),
+            store_volume: "log-tail-store".into(),
+            port: 30421,
+            labels: vec![],
+        },
+    );
+    let before = sandbox.disposition();
+    let tail = sandbox.log_tail(20, cap).unwrap();
+    assert_eq!(sandbox.disposition(), before);
+    let requests = server.finish();
+    assert_eq!(requests.len(), 1);
+    let request = String::from_utf8_lossy(&requests[0]);
+    assert!(request.contains("/logs?stdout=true&stderr=true&follow=false&tail=20"));
+    tail
+}
+
+fn logs(frames: &[u8]) -> Vec<u8> {
+    let mut wire = format!(
+        "HTTP/1.1 200 OK\r\nContent-Type: application/vnd.docker.multiplexed-stream\r\nContent-Length: {}\r\n\r\n",
+        frames.len()
+    ).into_bytes();
+    wire.extend_from_slice(frames);
+    wire
+}
+
+#[test]
+fn mixed_streams_keep_exact_suffix_across_windows_and_repeated_wraps() {
+    let mut frames = Vec::new();
+    let mut all = Vec::new();
+    for (i, length) in [0, 3, 8191, 8192, 8193, 65537, 137, 65536]
+        .into_iter()
+        .enumerate()
+    {
+        let bytes: Vec<u8> = (0..length).map(|n| ((n + i * 17) % 251) as u8).collect();
+        frames.extend(frame(if i % 2 == 0 { 1 } else { 2 }, &bytes));
+        all.extend(bytes);
+    }
+    let wire = logs(&frames);
+    for cap in [
+        0,
+        1,
+        7,
+        2048,
+        8191,
+        8192,
+        8193,
+        65536,
+        all.len(),
+        all.len() + 1,
+    ] {
+        let tail = read_tail(wire.clone(), cap, 8192);
+        assert_eq!(tail, all[all.len().saturating_sub(cap)..], "cap={cap}");
+    }
+}
+
+#[test]
+fn fragmented_frames_preserve_binary_and_utf8_bytes() {
+    let mut bytes = "中文日志\n".as_bytes().to_vec();
+    bytes.extend(0..=255);
+    let mut frames = frame(1, &bytes[..5]);
+    frames.extend(frame(2, &bytes[5..]));
+    for cap in [0, 1, 7, 17, bytes.len()] {
+        let tail = read_tail(logs(&frames), cap, 1);
+        assert_eq!(tail, bytes[bytes.len().saturating_sub(cap)..]);
+    }
+}
+
+#[test]
+fn zero_capacity_still_validates_the_entire_response() {
+    let container = ContainerId::parse(&"b".repeat(64)).unwrap();
+    let mut invalid_tag = frame(1, b"valid prefix");
+    invalid_tag.extend(frame(3, b"invalid stream"));
+    let mut incomplete = frame(1, b"truncated");
+    incomplete.pop();
+    for frames in [invalid_tag, incomplete] {
+        let (docker, server) = Peer::new(vec![logs(&frames)], 7);
+        let sandbox = Sandbox::stopped(
+            docker,
+            container,
+            SandboxRequest {
+                image: format!("sha256:{}", "a".repeat(64)),
+                store_volume: "log-tail-store".into(),
+                port: 30421,
+                labels: vec![],
+            },
+        );
+        let failure = sandbox.log_tail(20, 0).unwrap_err();
+        assert_eq!(failure.status, Some(200));
+        assert_eq!(failure.requested_container, Some(container));
+        assert!(matches!(
+            failure.cause,
+            RuntimeError::Protocol(_) | RuntimeError::Io(_)
+        ));
+        assert_eq!(server.finish().len(), 1);
+    }
+}