diff --git a/README.md b/README.md
index ec96304..da8471a 100644
--- a/README.md
+++ b/README.md
@@ -256,6 +256,49 @@ process registry: `urus.server` (the listener-pool supervisor) and
 `urus.listener.{i}` (each accept loop). `smarm::whereis(name)` resolves
 them from any actor inside the runtime.
 
+### WebSocket (v0.4)
+
+A route handler accepts the upgrade by handing `Conn::upgrade` a
+`WsHandler`; after the `101` the connection actor leaves HTTP and drives
+the handler from a duplex select loop (smarm RFC 008 fd arms: first-of
+outbound-channel / fd-readable, one actor, no fd dup):
+
+```rust
+struct Echo;
+
+impl urus::WsHandler for Echo {
+    fn on_message(&mut self, msg: urus::Message, sender: &urus::WsSender) {
+        let _ = sender.send(msg); // or .text(..) / .binary(..) / .ping() / .close(code, reason)
+    }
+    fn on_close(&mut self, code: Option<u16>, reason: &str) {
+        // what the PEER said: Some(code) iff its close frame arrived;
+        // None for EOF / write failure / handshake timeout.
+        let _ = (code, reason);
+    }
+}
+
+Router::new().get("/echo", |c: Conn, _n: Next| c.upgrade(Echo))
+```
+
+Callbacks run **inside the connection actor** — a slow `on_message`
+stops reads, which is backpressure, not a bug. A handler that wants
+concurrency spawns its own actor and hands it a `WsSender` clone (the
+SSE-producer pattern); every `WsSender` method returns `Err(WsClosed)`
+once the connection ends. While a large outbound frame is mid-write the
+actor isn't reading — accepted v1 trade-off of the one-actor topology.
+
+Control frames are invisible to the handler in v1: pings are
+auto-ponged, a peer close is auto-echoed and surfaces as `on_close`.
+Protocol violations close with the mapped RFC 6455 code (1002/1009/1007;
+handler panic → 1011). Caps: `Config.max_frame_payload` (1 MiB default,
+enforced from the frame header before payload buffers) and
+`Config.max_message_bytes` (4 MiB default, spans fragments).
+
+Like SSE, an open WebSocket has no request clock: idle is fine, a dead
+client is caught when a write stalls past `write_timeout`, and graceful
+shutdown force-stops the connection at the drain deadline. See
+`examples/ws_echo.rs`.
+
 ## Examples
 
 ### CRUD with Actor Ownership
@@ -326,9 +369,10 @@ This avoids the complexity of async ecosystems while maintaining full concurrenc
 ## Roadmap
 
 See [`ROADMAP.md`](ROADMAP.md). Summary: v0.2 (supervised listener pool,
-graceful shutdown, enforced timeouts, registry names) and v0.3 (streaming
-bodies, chunked requests, SSE) are done; next up are WebSocket (v0.4),
-PubSub (v0.5), and Phoenix-style channels (v0.6).
+graceful shutdown, enforced timeouts, registry names), v0.3 (streaming
+bodies, chunked requests, SSE) and v0.4 (WebSocket: handshake, frame
+codec, duplex handler API) are done; next up are PubSub (v0.5) and
+Phoenix-style channels (v0.6).
 
 Refer to `urus-spec.md` and `urus-v1-build-notes.md` in the artifact persistence for the original design and implementation notes.
 
diff --git a/ROADMAP.md b/ROADMAP.md
index 177ba4e..c82262e 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -159,7 +159,7 @@ force-stopped at the drain deadline on shutdown; full suite green.
 
 ---
 
-## v0.4 — WebSocket upgrade path
+## v0.4 — WebSocket upgrade path ✅ (landed 2026-06, chunks 1-3)
 
 1. **Handshake** ✅ (49538cc) — `Conn::upgrade()` (handler arg comes
    with chunk 3): §4.2.1 validation, 101 + accept key, marker field on
@@ -175,22 +175,46 @@ force-stopped at the drain deadline on shutdown; full suite green.
    close-code wire validation, `Assembler` for fragmentation with
    whole-message UTF-8 check, `FrameError`→close-code map
    (1002/1009/1007). §5.7 examples pinned verbatim.
-3. **Duplex topology** (the design decision from the assessment):
-   - **(a) Two actors, dup'd fd**: reader actor on the dup, writer actor
-     on the original, writer owns a `Receiver<Frame>`; the user handler
-     talks to both via channels. Proven trick (listener pool), works today.
-   - **(b) One actor, smarm first-of(fd-readable, channel)**: fd arms in
-     select landed with RFC 008 (`393cdd0`) — no longer blocked. Simpler
-     topology, no dup; use the fallible `try_select` twins (registration
-     errors, incl. `AlreadyExists`, surface as `Err` and the wait fully
-     retires).
-   Was "default (a), revisit if (b) lands" — (b) landed, so this is a
-   live choice for the v0.4 design pass. Note RFC 008 is phase 1:
-   single-waiter-per-fd stands, `dup` remains the documented answer for
-   true simultaneous read+write waits.
-4. Handler API sketch (settle in design pass): trait with
-   `on_message/on_close` + a `WsSender` handle — gen_server-shaped, so a
-   ws connection *is* an actor you can monitor, link, and register.
+3. **Duplex wiring** ✅ — topology decision: **(b) one actor, RFC 008
+   fd arms** (urus exists to put smarm through its paces; first fd-arm
+   consumer is the point). As landed:
+   - `ws::duplex::run_duplex` replaces the HTTP loop after the 101:
+     `try_select(&[&outbound_rx, &FdArm::readable(fd)])` per iteration —
+     outbound at index 0 (select is priority-ordered: owed writes drain
+     before reading more). Accepted gap documented: mid-write of a large
+     outbound frame the actor isn't reading.
+   - Handler API (the deferred questions, as answered): in-actor
+     callbacks — `WsHandler { on_message, on_close }` runs inside the
+     select loop; concurrency = spawn your own actor with a `WsSender`
+     clone (the SSE-producer pattern). `Conn::upgrade(self, handler)`
+     flat signature (builder deferred until per-upgrade options exist,
+     e.g. subprotocols). `WsSender`: Clone, `send/text/binary/ping/
+     close → Err(WsClosed)`, unbounded channel like SSE (slow client
+     bounded by write_timeout). Control frames invisible v1: auto-pong
+     inline (§5.5.2), peer close auto-echoed (§5.5.1, server closes TCP
+     first per §7.1.1) surfacing only as `on_close(Some(code), reason)`;
+     wordless endings (EOF, write failure, handshake timeout) =
+     `on_close(None, "")`. `on_ping`/`on_pong` can land later as default
+     trait methods, non-breaking.
+   - Server-initiated close (WsSender::close or FrameError→1002/1009/
+     1007, handler panic→1011): close frame out, then a bounded drain
+     (one write_timeout budget) for the peer's echo, discarding data
+     (§1.4). Handler panics re-raise smarm's stop sentinel before being
+     treated as panics — the pipeline's catch_unwind dance, replicated.
+   - Caps: `Config.max_frame_payload` (1 MiB) / `max_message_bytes`
+     (4 MiB) — Config fields per the max_body_bytes precedent; header
+     cap checked before payload buffers.
+   - Buf handover: bytes pipelined past the upgrade head carry into the
+     duplex loop (tested).
+   - Registry: conn stays **Busy** for the ws lifetime (an open ws is
+     in-flight work); graceful shutdown force-stops it out of the select
+     park at the drain deadline (tested — the in-runtime request_stop
+     wake covers select parks, not just channel recv).
+   - Validated against a real client (python websocket-client): echo,
+     ping/pong payload, close handshake. `examples/ws_echo.rs` shows
+     both handler shapes (/echo in-actor, /clock producer-spawning).
+4. **Handler API** ✅ — folded into chunk 3 above (was: settle in design
+   pass; settled: gen_server-shaped trait + WsSender handle).
 
 ## v0.5 — PubSub
 
diff --git a/examples/ws_echo.rs b/examples/ws_echo.rs
new file mode 100644
index 0000000..0a96e44
--- /dev/null
+++ b/examples/ws_echo.rs
@@ -0,0 +1,103 @@
+//! WebSocket echo server (v0.4).
+//!
+//!   cargo run --example ws_echo
+//!   websocat ws://127.0.0.1:8080/echo
+//!
+//! Demonstrates the chunk-3 handler model:
+//! - `EchoHandler` runs INSIDE the connection actor's select loop — echo
+//!   is exactly the workload that wants zero extra moving parts.
+//! - `/clock` shows the other shape: the handler spawns a producer actor
+//!   at `on_message("start")` and hands it a `WsSender` clone — the
+//!   SSE-producer pattern, verbatim. The producer exits when its send
+//!   fails (`WsClosed`: client gone or server shutting down).
+//!
+//! Routing happens before the upgrade, so one server can host both
+//! endpoints — `Conn::upgrade(handler)` is just another thing a route
+//! handler returns.
+
+use std::time::Duration;
+
+use urus::{
+    serve_with_shutdown, shutdown_handle, Config, Conn, Message, Next, Pipeline, Router,
+    WsHandler, WsSender,
+};
+
+// ---------------------------------------------------------------------------
+// /echo — everything comes straight back
+// ---------------------------------------------------------------------------
+
+struct EchoHandler;
+
+impl WsHandler for EchoHandler {
+    fn on_message(&mut self, msg: Message, sender: &WsSender) {
+        if matches!(&msg, Message::Text(t) if t == "bye") {
+            let _ = sender.close(1000, "you said bye");
+            return;
+        }
+        let _ = sender.send(msg);
+    }
+
+    fn on_close(&mut self, code: Option<u16>, reason: &str) {
+        println!("ws_echo: /echo closed (code {code:?}, reason {reason:?})");
+    }
+}
+
+// ---------------------------------------------------------------------------
+// /clock — "start" spawns a producer actor ticking once a second
+// ---------------------------------------------------------------------------
+
+struct ClockHandler {
+    started: bool,
+}
+
+impl WsHandler for ClockHandler {
+    fn on_message(&mut self, msg: Message, sender: &WsSender) {
+        match msg {
+            Message::Text(t) if t == "start" && !self.started => {
+                self.started = true;
+                let out = sender.clone();
+                smarm::spawn(move || {
+                    let mut n = 0u64;
+                    loop {
+                        if out.text(format!("tick {n}")).is_err() {
+                            return; // WsClosed: connection over, we follow
+                        }
+                        n += 1;
+                        smarm::sleep(Duration::from_secs(1));
+                    }
+                });
+            }
+            _ => {
+                let _ = sender.text("send \"start\" to begin");
+            }
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// main
+// ---------------------------------------------------------------------------
+
+fn main() {
+    let pipeline = Pipeline::new().plug(
+        Router::new()
+            .get("/echo", |c: Conn, _n: Next| c.upgrade(EchoHandler))
+            .get("/clock", |c: Conn, _n: Next| {
+                c.upgrade(ClockHandler { started: false })
+            }),
+    );
+
+    let cfg = Config::new("127.0.0.1:8080".parse().unwrap());
+    println!("ws_echo: ws://127.0.0.1:8080/echo and /clock — press Enter to shut down");
+
+    let (handle, signal) = shutdown_handle();
+    std::thread::spawn(move || {
+        let mut line = String::new();
+        let _ = std::io::stdin().read_line(&mut line);
+        println!("ws_echo: shutting down…");
+        handle.shutdown();
+    });
+
+    serve_with_shutdown(cfg, pipeline, signal).unwrap();
+    println!("ws_echo: bye");
+}
diff --git a/src/conn.rs b/src/conn.rs
index 4a91e65..46460c8 100644
--- a/src/conn.rs
+++ b/src/conn.rs
@@ -366,25 +366,31 @@ impl Conn {
         self
     }
 
-    /// Accept a WebSocket upgrade on this request (RFC 6455 §4.2).
+    /// Accept a WebSocket upgrade on this request (RFC 6455 §4.2),
+    /// handing `handler` to the duplex loop that takes the socket over.
     ///
     /// On a valid handshake: status `101`, the `upgrade`/`connection`/
-    /// `sec-websocket-accept` response headers, the [`WsUpgrade`] marker
-    /// the connection actor acts on, and the pipeline is halted.
+    /// `sec-websocket-accept` response headers, the [`WsUpgrade`] payload
+    /// the connection actor acts on, and the pipeline is halted. After
+    /// the 101 the connection actor leaves HTTP and drives `handler` —
+    /// see [`WsHandler`](crate::ws::WsHandler) for the callback contract.
     ///
-    /// On an invalid handshake: the appropriate rejection (`400`, or
-    /// `426` + `sec-websocket-version: 13` for a version mismatch),
-    /// empty body, pipeline halted; the connection stays plain HTTP with
-    /// normal keep-alive semantics. Pre-check with
+    /// On an invalid handshake: `handler` is dropped untouched and the
+    /// appropriate rejection goes out (`400`, or `426` +
+    /// `sec-websocket-version: 13` for a version mismatch), empty body,
+    /// pipeline halted; the connection stays plain HTTP with normal
+    /// keep-alive semantics. Pre-check with
     /// [`ws::handshake::is_upgrade_request`](crate::ws::handshake::is_upgrade_request)
     /// to branch before committing.
     ///
     /// [`WsUpgrade`]: crate::ws::WsUpgrade
-    pub fn upgrade(mut self) -> Self {
+    pub fn upgrade(mut self, handler: impl crate::ws::WsHandler) -> Self {
         use crate::ws::{handshake, Rejection};
         match handshake::validate(&self) {
             Ok(accept) => {
-                self.upgrade = Some(crate::ws::WsUpgrade { _priv: () });
+                self.upgrade = Some(crate::ws::WsUpgrade {
+                    handler: Box::new(handler),
+                });
                 self.put_status(101)
                     .put_header("upgrade", "websocket")
                     .put_header("connection", "Upgrade")
diff --git a/src/conn_actor.rs b/src/conn_actor.rs
index 52adb7e..40fa3a3 100644
--- a/src/conn_actor.rs
+++ b/src/conn_actor.rs
@@ -59,6 +59,13 @@ pub struct ConnLimits {
     /// A client that stops reading mid-response is dropped when its
     /// socket buffer fills and a write stalls past the budget.
     pub write_timeout:      Duration,
+    /// WebSocket: hard cap on a single frame's payload, enforced from
+    /// the frame header BEFORE the payload is buffered. Violation closes
+    /// with 1009.
+    pub max_frame_payload:  usize,
+    /// WebSocket: hard cap on a complete (reassembled) message; spans
+    /// fragments. Violation closes with 1009.
+    pub max_message_bytes:  usize,
 }
 
 impl Default for ConnLimits {
@@ -71,6 +78,8 @@ impl Default for ConnLimits {
             keep_alive_timeout: Duration::from_secs(60),
             request_timeout:    Duration::from_secs(30),
             write_timeout:      Duration::from_secs(30),
+            max_frame_payload:  1024 * 1024,
+            max_message_bytes:  4 * 1024 * 1024,
         }
     }
 }
@@ -231,17 +240,25 @@ pub fn run_connection(
                 .put_body(RespBody::Empty);
         }
 
-        // ----- 3.5 WebSocket upgrade short-circuit. -----
-        // An accepted handshake (marker + 101) ends HTTP on this socket:
-        // write the 101 head and leave the request loop. Chunk 3 (duplex
-        // wiring) takes the socket over right here; until then leaving
-        // the loop closes it via OwnedFd::drop — the 101 is still the
-        // honest, testable wire artefact. The status check is defensive:
-        // a post-handler plug that clobbered the 101 forfeits the
-        // upgrade and falls through to plain HTTP.
+        // ----- 3.5 WebSocket upgrade. -----
+        // An accepted handshake (payload + 101) ends HTTP on this socket:
+        // write the 101 head, hand the fd to the duplex loop with the
+        // boxed handler and any bytes already read past this request (a
+        // client may pipeline its first frame behind the handshake — those
+        // bytes are ws bytes now). The registry entry stays Busy for the
+        // whole ws lifetime: an open WebSocket is in-flight work, not
+        // reapable idle HTTP; graceful shutdown force-stops it out of the
+        // select park at the drain deadline. The status check is
+        // defensive: a post-handler plug that clobbered the 101 forfeits
+        // the upgrade and falls through to plain HTTP.
         if response_conn.upgrade.is_some() && response_conn.status == Some(101) {
             let head = parser::serialise_response(&response_conn, true);
-            let _ = write_all(raw, &head, Instant::now() + limits.write_timeout);
+            if write_all(raw, &head, Instant::now() + limits.write_timeout).is_err() {
+                return;
+            }
+            let upgrade = response_conn.upgrade.take().expect("checked above");
+            buf.drain(..head_len + consumed_past_head);
+            crate::ws::duplex::run_duplex(raw, buf, upgrade.handler, &limits);
             return;
         }
 
@@ -550,7 +567,7 @@ fn read_chunked_body(
 // `ErrorKind::TimedOut` when `deadline` passes before the fd turns
 // readable, or the last io error.
 
-fn read_some(
+pub(crate) fn read_some(
     fd: RawFd,
     buf: &mut Vec<u8>,
     chunk: usize,
@@ -616,7 +633,7 @@ fn try_write_once(fd: RawFd, buf: &[u8]) {
 // are down — a client that stops reading must not pin this actor in
 // wait_writable forever (the write-side twin of slowloris).
 
-fn write_all(fd: RawFd, mut buf: &[u8], deadline: Instant) -> io::Result<()> {
+pub(crate) fn write_all(fd: RawFd, mut buf: &[u8], deadline: Instant) -> io::Result<()> {
     while !buf.is_empty() {
         // Park on writability before each syscall, bounded by the budget.
         let remaining = deadline.saturating_duration_since(Instant::now());
diff --git a/src/lib.rs b/src/lib.rs
index 551a2a7..cb910ab 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -34,6 +34,7 @@ pub use conn::{Assigns, Body, Conn, HeaderMap, HttpVersion, Method, Params, Resp
 pub use plug::{Next, Pipeline, Plug};
 pub use router::Router;
 pub use sse::{EventSender, SseClosed};
+pub use ws::{Message, WsClosed, WsHandler, WsSender};
 pub use serve::{
     serve, serve_with, serve_with_shutdown, shutdown_handle, Config, Handle, ShutdownSignal,
 };
diff --git a/src/serve.rs b/src/serve.rs
index 9135937..98ae7b5 100644
--- a/src/serve.rs
+++ b/src/serve.rs
@@ -47,6 +47,12 @@ pub struct Config {
     /// connections are closed immediately on shutdown and do not run the
     /// clock out.
     pub drain_timeout:      Duration,
+    /// WebSocket: cap on a single frame's payload (header-checked
+    /// before buffering; violation closes 1009).
+    pub max_frame_payload:  usize,
+    /// WebSocket: cap on a complete reassembled message (spans
+    /// fragments; violation closes 1009).
+    pub max_message_bytes:  usize,
     /// Number of smarm scheduler OS threads. `None` means smarm's default
     /// (one per CPU). Set this to a small fixed number in tests so multiple
     /// concurrent test servers don't oversubscribe the host.
@@ -69,6 +75,8 @@ impl Config {
             write_timeout:      Duration::from_secs(30),
             max_body_bytes:     16 * 1024 * 1024,
             drain_timeout:      Duration::from_secs(30),
+            max_frame_payload:  1024 * 1024,
+            max_message_bytes:  4 * 1024 * 1024,
             scheduler_threads:  None,
         }
     }
@@ -82,6 +90,8 @@ impl Config {
             keep_alive_timeout: self.keep_alive_timeout,
             request_timeout:    self.request_timeout,
             write_timeout:      self.write_timeout,
+            max_frame_payload:  self.max_frame_payload,
+            max_message_bytes:  self.max_message_bytes,
         }
     }
 }
diff --git a/src/ws/duplex.rs b/src/ws/duplex.rs
new file mode 100644
index 0000000..c1276dd
--- /dev/null
+++ b/src/ws/duplex.rs
@@ -0,0 +1,398 @@
+//! The WebSocket duplex loop — v0.4 chunk 3.
+//!
+//! After an accepted upgrade the connection actor calls [`run_duplex`],
+//! which replaces the HTTP request loop for the rest of the socket's
+//! life. ONE actor speaks both directions (the v0.4 topology decision):
+//! each iteration parks on a two-arm select —
+//!
+//!   try_select(&[&outbound_rx, &FdArm::readable(fd)])
+//!
+//! — smarm's RFC 008 fd arms composed with a channel arm on one wait
+//! epoch; urus is deliberately the first consumer of that machinery.
+//! The outbound arm sits at index 0 (select is priority-ordered): owed
+//! writes drain before we read more, which is honest backpressure.
+//! Accepted gap, by design: while a large outbound frame is mid-write
+//! the actor isn't reading, so a ping arriving during that write is
+//! answered after it completes.
+//!
+//! Handler model (the v0.4 chunk-3 decisions, mirroring SSE):
+//! - [`WsHandler`] callbacks run IN the connection actor, inside the
+//!   select loop. A slow `on_message` stops reads — backpressure, not a
+//!   bug. Handlers needing concurrency spawn their own actor and hand it
+//!   a [`WsSender`] clone, exactly like an SSE producer.
+//! - [`WsSender`] is the clonable outbound handle; every method returns
+//!   `Err(WsClosed)` once the connection is gone.
+//! - Control frames are invisible to the handler in v1: pings are
+//!   auto-ponged (RFC 6455 §5.5.2 MUST), pongs are absorbed, a peer
+//!   close is auto-echoed (§5.5.1) and surfaces only as `on_close`.
+//!   `on_ping`/`on_pong` hooks can land later as default trait methods
+//!   without breaking anyone.
+//!
+//! Liveness mirrors SSE: there is no request clock on an open
+//! WebSocket. An idle connection parks in the select (stoppable by a
+//! draining registry — graceful shutdown force-stops it at the drain
+//! deadline); a dead client is caught when a write stalls past
+//! `write_timeout`. Reads have no deadline of their own — the select
+//! only wakes the read path when bytes are pending.
+
+use crate::conn_actor::{read_some, write_all, ConnLimits};
+use crate::ws::frame::{self, Assembler, Frame, FrameError, Message, Opcode};
+
+use smarm::FdArm;
+
+use std::os::fd::RawFd;
+use std::time::Instant;
+
+// ---------------------------------------------------------------------------
+// Handler trait
+// ---------------------------------------------------------------------------
+
+/// Application callbacks for an upgraded WebSocket connection. Both run
+/// inside the connection actor's select loop — see the module docs for
+/// what that implies (backpressure, spawn-for-concurrency).
+///
+/// A panic in `on_message` closes the connection with a `1011 internal
+/// error` close frame and skips `on_close` (the handler is not re-entered
+/// after it panicked).
+pub trait WsHandler: Send + 'static {
+    /// A complete data message (fragments already reassembled, text
+    /// already UTF-8 validated) arrived from the client.
+    fn on_message(&mut self, msg: Message, sender: &WsSender);
+
+    /// The connection is over. `code`/`reason` are what the PEER said:
+    /// `Some` iff a close frame was received from the client (whether it
+    /// initiated or echoed ours); `None` for everything wordless — EOF,
+    /// write failure, or a close handshake that timed out. Called exactly
+    /// once, last, on every exit path except a handler panic or a
+    /// force-stop unwind.
+    fn on_close(&mut self, code: Option<u16>, reason: &str) {
+        let _ = (code, reason);
+    }
+}
+
+// ---------------------------------------------------------------------------
+// WsSender — the clonable outbound handle
+// ---------------------------------------------------------------------------
+
+/// The connection is gone (client disconnect, write timeout, close
+/// handshake completed, or server shutdown). Producers should exit when
+/// they see this — the exact contract of `SseClosed`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct WsClosed;
+
+impl std::fmt::Display for WsClosed {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("WebSocket connection closed")
+    }
+}
+impl std::error::Error for WsClosed {}
+
+pub(crate) enum Outbound {
+    Msg(Message),
+    Ping,
+    Close(u16, String),
+}
+
+/// Outbound handle for an upgraded connection. Clonable — hand copies to
+/// producer actors freely; the connection outlives none of them (sends
+/// after it ends return `Err(WsClosed)`).
+///
+/// v1 sends each message as a single unfragmented frame; outbound
+/// fragmentation is not implemented.
+#[derive(Clone)]
+pub struct WsSender {
+    pub(crate) tx: smarm::Sender<Outbound>,
+}
+
+impl WsSender {
+    /// Queue a data message for the client.
+    pub fn send(&self, msg: Message) -> Result<(), WsClosed> {
+        self.tx.send(Outbound::Msg(msg)).map_err(|_| WsClosed)
+    }
+
+    /// Sugar: queue a text message.
+    pub fn text(&self, s: impl Into<String>) -> Result<(), WsClosed> {
+        self.send(Message::Text(s.into()))
+    }
+
+    /// Sugar: queue a binary message.
+    pub fn binary(&self, b: impl Into<Vec<u8>>) -> Result<(), WsClosed> {
+        self.send(Message::Binary(b.into()))
+    }
+
+    /// Queue a ping (empty payload). Application-level liveness beyond
+    /// the built-in write-timeout detection; the pong is absorbed by the
+    /// loop in v1.
+    pub fn ping(&self) -> Result<(), WsClosed> {
+        self.tx.send(Outbound::Ping).map_err(|_| WsClosed)
+    }
+
+    /// Initiate the close handshake: a close frame with `code`/`reason`
+    /// goes out, the loop then waits (bounded by `write_timeout`) for the
+    /// peer's echo before dropping the socket. `reason` is truncated to
+    /// fit the 125-byte control payload on a char boundary.
+    pub fn close(&self, code: u16, reason: &str) -> Result<(), WsClosed> {
+        self.tx
+            .send(Outbound::Close(code, reason.to_string()))
+            .map_err(|_| WsClosed)
+    }
+}
+
+// ---------------------------------------------------------------------------
+// The loop
+// ---------------------------------------------------------------------------
+
+/// Run the duplex loop until the connection ends. `buf` carries any bytes
+/// already read past the upgrade request's head — a client may pipeline
+/// its first frame behind the handshake, and those bytes belong to us
+/// now.
+pub(crate) fn run_duplex(
+    raw: RawFd,
+    mut buf: Vec<u8>,
+    mut handler: Box<dyn WsHandler>,
+    limits: &ConnLimits,
+) {
+    let (tx, rx) = smarm::channel::<Outbound>();
+    // We hold this sender for handler callbacks, so the rx arm can never
+    // report closed while the loop runs — no closed-arm starvation case.
+    let sender = WsSender { tx };
+    let mut asm = Assembler::new(limits.max_message_bytes);
+
+    loop {
+        // ----- 1. Decode everything already buffered. -----
+        // Runs before the first select so a pipelined first frame is
+        // served without waiting for fresh readability.
+        loop {
+            let (frame, consumed) =
+                match frame::decode(&buf, true, limits.max_frame_payload) {
+                    Ok(Some(hit)) => hit,
+                    Ok(None) => break, // incomplete; need more bytes
+                    Err(e) => {
+                        let peer = close_and_drain(raw, e, &mut buf, limits);
+                        finish(&mut handler, peer);
+                        return;
+                    }
+                };
+            buf.drain(..consumed);
+
+            match frame.opcode {
+                Opcode::Ping => {
+                    // §5.5.2 MUST pong, payload echoed. Written inline —
+                    // "as soon as practical" beats channel ordering.
+                    let pong = Frame::new(Opcode::Pong, frame.payload);
+                    if write_frame(raw, &pong, limits).is_err() {
+                        finish(&mut handler, None);
+                        return;
+                    }
+                }
+                Opcode::Pong => {} // invisible in v1
+                Opcode::Close => {
+                    // Peer-initiated close: echo its status code (§5.5.1)
+                    // and end. The server side closes the TCP connection
+                    // first by design (§7.1.1 puts that burden on us).
+                    let echo_payload =
+                        frame.payload.get(..2).map(<[u8]>::to_vec).unwrap_or_default();
+                    let peer = match frame::parse_close_payload(&frame.payload) {
+                        Ok(cp) => {
+                            let _ = write_frame(
+                                raw,
+                                &Frame::new(Opcode::Close, echo_payload),
+                                limits,
+                            );
+                            cp
+                        }
+                        Err(e) => {
+                            // Malformed close payload: answer with the
+                            // mapped code instead of an echo.
+                            let _ = write_frame(
+                                raw,
+                                &Frame::new(
+                                    Opcode::Close,
+                                    frame::close_payload(e.close_code(), ""),
+                                ),
+                                limits,
+                            );
+                            None
+                        }
+                    };
+                    finish_with(&mut handler, peer);
+                    return;
+                }
+                _ => match asm.push(frame) {
+                    Ok(Some(msg)) => {
+                        // A panicking handler must not swallow smarm's stop
+                        // sentinel — same re-raise dance as the pipeline.
+                        let r = std::panic::catch_unwind(
+                            std::panic::AssertUnwindSafe(|| {
+                                handler.on_message(msg, &sender)
+                            }),
+                        );
+                        if r.is_err() {
+                            smarm::preempt::check_cancelled();
+                            // Genuine handler panic: 1011 out, no further
+                            // handler calls (no on_close).
+                            let _ = close_and_drain_code(
+                                raw, 1011, "", &mut buf, limits,
+                            );
+                            return;
+                        }
+                    }
+                    Ok(None) => {} // mid-fragmentation
+                    Err(e) => {
+                        let peer = close_and_drain(raw, e, &mut buf, limits);
+                        finish(&mut handler, peer);
+                        return;
+                    }
+                },
+            }
+        }
+
+        // ----- 2. Park on first-of outbound / fd-readable. -----
+        // Outbound at index 0: select is priority-ordered, so owed writes
+        // go out before we read more. try_select over select: an fd-arm
+        // registration failure (EBADF after a peer RST, EMFILE on the
+        // epoll set) is a connection event, not a crash. NOTE from the
+        // RFC 008 review: an `AlreadyExists` here would mean a stale
+        // waiter survived select's eager cleanup — that is a smarm bug
+        // candidate; capture it with --features smarm-trace and report,
+        // do not paper over.
+        let fd_arm = FdArm::readable(raw);
+        let idx = match smarm::try_select(&[&rx, &fd_arm]) {
+            Ok(i) => i,
+            Err(_) => {
+                finish(&mut handler, None);
+                return;
+            }
+        };
+
+        if idx == 0 {
+            // Outbound arm won. Single receiver + we hold a sender: a
+            // ready arm means a message is queued (Empty/closed are
+            // impossible here, but stay defensive).
+            let Ok(Some(out)) = rx.try_recv() else { continue };
+            match out {
+                Outbound::Msg(msg) => {
+                    let f = match msg {
+                        Message::Text(s) => Frame::new(Opcode::Text, s.into_bytes()),
+                        Message::Binary(b) => Frame::new(Opcode::Binary, b),
+                    };
+                    if write_frame(raw, &f, limits).is_err() {
+                        finish(&mut handler, None);
+                        return;
+                    }
+                }
+                Outbound::Ping => {
+                    let f = Frame::new(Opcode::Ping, Vec::new());
+                    if write_frame(raw, &f, limits).is_err() {
+                        finish(&mut handler, None);
+                        return;
+                    }
+                }
+                Outbound::Close(code, reason) => {
+                    // We initiate: close out, bounded wait for the echo.
+                    let peer = close_and_drain_code(raw, code, &reason, &mut buf, limits);
+                    finish(&mut handler, peer);
+                    return;
+                }
+            }
+        } else {
+            // Readable. The deadline is nominal — the fd already reported
+            // ready, this returns without a fresh park in practice.
+            match read_some(
+                raw,
+                &mut buf,
+                limits.initial_read_buf,
+                Instant::now() + limits.write_timeout,
+            ) {
+                Ok(0) => {
+                    // EOF without a close frame: abnormal but common.
+                    finish(&mut handler, None);
+                    return;
+                }
+                Ok(_) => {}
+                Err(_) => {
+                    finish(&mut handler, None);
+                    return;
+                }
+            }
+        }
+    }
+}
+
+/// `on_close(None, "")` — wordless endings (EOF, write failure, errors).
+fn finish(handler: &mut Box<dyn WsHandler>, peer: Option<(u16, String)>) {
+    finish_with(handler, peer);
+}
+
+/// `on_close` with whatever the peer said, panic-shielded like
+/// `on_message` (stop sentinel re-raised; a panic here just ends the
+/// already-ending connection).
+fn finish_with(handler: &mut Box<dyn WsHandler>, peer: Option<(u16, String)>) {
+    let r = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+        match &peer {
+            Some((code, reason)) => handler.on_close(Some(*code), reason),
+            None => handler.on_close(None, ""),
+        }
+    }));
+    if r.is_err() {
+        smarm::preempt::check_cancelled();
+    }
+}
+
+/// One frame onto the wire under a fresh `write_timeout` budget — the
+/// streaming-chunk discipline, verbatim.
+fn write_frame(raw: RawFd, f: &Frame, limits: &ConnLimits) -> std::io::Result<()> {
+    write_all(raw, &f.encode(), Instant::now() + limits.write_timeout)
+}
+
+/// Server-initiated close for a frame/assembly error: map to the §7.4
+/// code and run the handshake tail.
+fn close_and_drain(
+    raw: RawFd,
+    e: FrameError,
+    buf: &mut Vec<u8>,
+    limits: &ConnLimits,
+) -> Option<(u16, String)> {
+    close_and_drain_code(raw, e.close_code(), "", buf, limits)
+}
+
+/// Write our close frame, then wait — bounded by ONE `write_timeout`
+/// budget — for the peer's close frame, discarding data frames (§1.4: an
+/// endpoint that has sent close may discard incoming data). Returns what
+/// the peer's close said, `None` if it never arrived (EOF, timeout,
+/// garbage). The socket drops at the caller either way.
+fn close_and_drain_code(
+    raw: RawFd,
+    code: u16,
+    reason: &str,
+    buf: &mut Vec<u8>,
+    limits: &ConnLimits,
+) -> Option<(u16, String)> {
+    let ours = Frame::new(Opcode::Close, frame::close_payload(code, reason));
+    if write_frame(raw, &ours, limits).is_err() {
+        return None;
+    }
+
+    let deadline = Instant::now() + limits.write_timeout;
+    loop {
+        loop {
+            match frame::decode(buf, true, limits.max_frame_payload) {
+                Ok(Some((frame, consumed))) => {
+                    buf.drain(..consumed);
+                    if frame.opcode == Opcode::Close {
+                        return frame::parse_close_payload(&frame.payload)
+                            .ok()
+                            .flatten();
+                    }
+                    // Data/ping mid-handshake: discarded.
+                }
+                Ok(None) => break,
+                Err(_) => return None, // garbage during the tail: give up
+            }
+        }
+        match read_some(raw, buf, limits.initial_read_buf, deadline) {
+            Ok(0) | Err(_) => return None,
+            Ok(_) => {}
+        }
+    }
+}
diff --git a/src/ws/mod.rs b/src/ws/mod.rs
index 0f59f80..8ac494c 100644
--- a/src/ws/mod.rs
+++ b/src/ws/mod.rs
@@ -12,17 +12,24 @@
 //! vendoring. Base64 here is the 20-byte *encode* direction only and is
 //! not cryptographic, so that stays in-tree (`handshake::b64`).
 
+pub mod duplex;
 pub mod frame;
 pub mod handshake;
 
+pub use duplex::{WsClosed, WsHandler, WsSender};
 pub use frame::{Frame, FrameError, Message, Opcode};
 pub use handshake::Rejection;
 
-/// Marker carried on a [`Conn`](crate::Conn) whose handler accepted a
-/// WebSocket upgrade. Opaque: chunk 3 grows this into the handler/duplex
-/// handoff payload, so nothing outside the crate should depend on its
-/// shape.
-#[derive(Debug)]
+/// Payload carried on a [`Conn`](crate::Conn) whose handler accepted a
+/// WebSocket upgrade: the boxed [`WsHandler`] the duplex loop will drive
+/// (the chunk-3 growth this was reserved for). Still opaque outside the
+/// crate.
 pub struct WsUpgrade {
-    pub(crate) _priv: (),
+    pub(crate) handler: Box<dyn WsHandler>,
+}
+
+impl std::fmt::Debug for WsUpgrade {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("WsUpgrade { handler: .. }")
+    }
 }
diff --git a/tests/integration.rs b/tests/integration.rs
index e5cd91c..756d7ce 100644
--- a/tests/integration.rs
+++ b/tests/integration.rs
@@ -1019,88 +1019,264 @@ fn sse_heartbeats_on_silence() {
 // closes the socket; the tests assert the wire artefact, not ws traffic.
 // ---------------------------------------------------------------------------
 
+const WS_HANDSHAKE: &[u8] =
+    b"GET /ws HTTP/1.1\r\nHost: x\r\nUpgrade: websocket\r\nConnection: Upgrade\r\n\
+      Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\nSec-WebSocket-Version: 13\r\n\r\n";
+
+/// Echo handler: data messages come straight back; the text "bye" makes
+/// the SERVER initiate the close handshake (1000/"goodbye").
+struct Echo;
+
+impl urus::WsHandler for Echo {
+    fn on_message(&mut self, msg: urus::Message, sender: &urus::WsSender) {
+        if matches!(&msg, urus::Message::Text(t) if t == "bye") {
+            let _ = sender.close(1000, "goodbye");
+            return;
+        }
+        let _ = sender.send(msg);
+    }
+}
+
 fn ws_pipeline() -> Pipeline {
-    Pipeline::new().plug(Router::new().get("/ws", |c: Conn, _n: Next| c.upgrade()))
+    Pipeline::new().plug(Router::new().get("/ws", |c: Conn, _n: Next| c.upgrade(Echo)))
 }
 
-#[test]
-fn ws_handshake_101_with_accept_key() {
-    let port = spawn_server(ws_pipeline());
-    let resp = send_request(
-        port,
-        b"GET /ws HTTP/1.1\r\nHost: x\r\nUpgrade: websocket\r\nConnection: Upgrade\r\n\
-          Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\nSec-WebSocket-Version: 13\r\n\r\n",
-    );
-    assert_eq!(http_status(&resp), 101);
-    let head = String::from_utf8_lossy(&resp).to_lowercase();
-    assert!(head.contains("upgrade: websocket"), "{head}");
-    assert!(head.contains("connection: upgrade"), "{head}");
-    // RFC 6455 §1.3 worked example key -> known accept value.
-    assert!(
-        head.contains("sec-websocket-accept: s3pplmbitxaq9kygzzhzrbk+xoo="),
-        "{head}"
-    );
-    // 1xx carries no body framing.
-    assert!(!head.contains("content-length"), "{head}");
-    assert!(!head.contains("transfer-encoding"), "{head}");
-    assert_eq!(http_body(&resp), b"", "no body after a 101");
-}
+// ----- client-side helpers (the test IS the ws client) -----
 
-#[test]
-fn ws_handshake_wrong_version_426() {
-    let port = spawn_server(ws_pipeline());
-    let resp = send_request(
-        port,
-        b"GET /ws HTTP/1.1\r\nHost: x\r\nUpgrade: websocket\r\nConnection: Upgrade\r\n\
-          Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\nSec-WebSocket-Version: 8\r\n\r\n",
-    );
-    assert_eq!(http_status(&resp), 426);
-    let head = String::from_utf8_lossy(&resp).to_lowercase();
-    assert!(head.contains("sec-websocket-version: 13"), "{head}");
-}
+use urus::ws::frame::{self as wsframe, Frame, Opcode};
 
-#[test]
-fn ws_handshake_missing_key_400_keeps_http_alive() {
-    let port = spawn_server(ws_pipeline());
-    // A rejected upgrade is a normal HTTP response: same connection must
-    // serve a second request afterwards.
+/// Complete the handshake on a fresh socket; panics on anything but 101.
+fn ws_connect(port: u16) -> TcpStream {
     let mut s = TcpStream::connect(("127.0.0.1", port)).unwrap();
     s.set_read_timeout(Some(Duration::from_secs(5))).unwrap();
-    s.write_all(
-        b"GET /ws HTTP/1.1\r\nHost: x\r\nUpgrade: websocket\r\nConnection: Upgrade\r\n\
-          Sec-WebSocket-Version: 13\r\n\r\n",
-    )
-    .unwrap();
-    let mut buf = [0u8; 1024];
-    let n = s.read(&mut buf).unwrap();
-    let first = String::from_utf8_lossy(&buf[..n]);
-    assert!(first.starts_with("HTTP/1.1 400"), "{first}");
+    s.write_all(WS_HANDSHAKE).unwrap();
+    let head = String::from_utf8(read_response_head(&mut s)).unwrap();
+    assert!(head.starts_with("HTTP/1.1 101"), "handshake: {head}");
+    s
+}
 
-    s.write_all(
-        b"GET /ws HTTP/1.1\r\nHost: x\r\nUpgrade: websocket\r\nConnection: Upgrade\r\n\
-          Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\nSec-WebSocket-Version: 13\r\n\r\n",
-    )
-    .unwrap();
-    let n = s.read(&mut buf).unwrap();
-    let second = String::from_utf8_lossy(&buf[..n]);
-    assert!(second.starts_with("HTTP/1.1 101"), "{second}");
+fn ws_send(s: &mut TcpStream, frame: &Frame) {
+    s.write_all(&wsframe::encode_masked(frame, [0x11, 0x22, 0x33, 0x44]))
+        .unwrap();
+}
+
+/// Read one server frame (server frames are unmasked). `buf` carries
+/// partial bytes across calls.
+fn ws_read_frame(s: &mut TcpStream, buf: &mut Vec<u8>) -> Frame {
+    loop {
+        if let Some((f, consumed)) =
+            wsframe::decode(buf, false, 16 * 1024 * 1024).expect("bad server frame")
+        {
+            buf.drain(..consumed);
+            return f;
+        }
+        let mut tmp = [0u8; 4096];
+        let n = s.read(&mut tmp).expect("read mid-frame");
+        assert!(n > 0, "EOF mid-frame; buffered: {buf:?}");
+        buf.extend_from_slice(&tmp[..n]);
+    }
+}
+
+/// Assert the socket yields EOF (the server closed).
+fn ws_expect_eof(s: &mut TcpStream, buf: &[u8]) {
+    assert!(buf.is_empty(), "unconsumed server bytes at EOF check: {buf:?}");
+    let mut rest = Vec::new();
+    let _ = s.read_to_end(&mut rest); // EOF or reset; both are closed
+    assert!(rest.is_empty(), "unexpected bytes before EOF: {rest:?}");
 }
 
 #[test]
-fn ws_accepted_upgrade_leaves_http_loop() {
-    // Chunk-1 semantics: after the 101 the actor exits and the fd drops —
-    // the client sees EOF after the 101 head. (Chunk 3 replaces the EOF
-    // with the ws duplex loop; this test will be rewritten then.)
+fn ws_echo_roundtrip_text_and_binary() {
     let port = spawn_server(ws_pipeline());
-    let mut s = TcpStream::connect(("127.0.0.1", port)).unwrap();
-    s.set_read_timeout(Some(Duration::from_secs(5))).unwrap();
-    s.write_all(
-        b"GET /ws HTTP/1.1\r\nHost: x\r\nUpgrade: websocket\r\nConnection: Upgrade\r\n\
-          Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\nSec-WebSocket-Version: 13\r\n\r\n",
-    )
-    .unwrap();
+    let mut s = ws_connect(port);
     let mut buf = Vec::new();
-    s.read_to_end(&mut buf).unwrap(); // EOF must arrive, not a hang
-    let head = String::from_utf8_lossy(&buf);
+
+    ws_send(&mut s, &Frame::new(Opcode::Text, "hello"));
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.opcode, Opcode::Text);
+    assert_eq!(f.payload, b"hello");
+
+    ws_send(&mut s, &Frame::new(Opcode::Binary, vec![0u8, 159, 146, 150]));
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.opcode, Opcode::Binary);
+    assert_eq!(f.payload, vec![0u8, 159, 146, 150]);
+}
+
+/// A first frame pipelined in the SAME write as the handshake must be
+/// served: bytes read past the 101 request head carry into the duplex
+/// loop (the buf-handover requirement).
+#[test]
+fn ws_pipelined_first_frame_served() {
+    let port = spawn_server(ws_pipeline());
+    let mut s = TcpStream::connect(("127.0.0.1", port)).unwrap();
+    s.set_read_timeout(Some(Duration::from_secs(5))).unwrap();
+
+    let mut wire = WS_HANDSHAKE.to_vec();
+    wire.extend_from_slice(&wsframe::encode_masked(
+        &Frame::new(Opcode::Text, "early"),
+        [9, 9, 9, 9],
+    ));
+    s.write_all(&wire).unwrap();
+
+    let head = String::from_utf8(read_response_head(&mut s)).unwrap();
     assert!(head.starts_with("HTTP/1.1 101"), "{head}");
+    let mut buf = Vec::new();
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.payload, b"early");
+}
+
+/// §5.5.2: a ping is answered with a pong echoing the payload, with no
+/// handler involvement.
+#[test]
+fn ws_ping_gets_pong_with_payload() {
+    let port = spawn_server(ws_pipeline());
+    let mut s = ws_connect(port);
+    let mut buf = Vec::new();
+
+    ws_send(&mut s, &Frame::new(Opcode::Ping, "abc"));
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.opcode, Opcode::Pong);
+    assert_eq!(f.payload, b"abc");
+
+    // The connection is still a working websocket afterwards.
+    ws_send(&mut s, &Frame::new(Opcode::Text, "still here"));
+    assert_eq!(ws_read_frame(&mut s, &mut buf).payload, b"still here");
+}
+
+/// Client-initiated close: the server echoes the status code (§5.5.1)
+/// and closes the TCP connection (§7.1.1 — server closes first).
+#[test]
+fn ws_client_close_is_echoed_then_eof() {
+    let port = spawn_server(ws_pipeline());
+    let mut s = ws_connect(port);
+    let mut buf = Vec::new();
+
+    ws_send(
+        &mut s,
+        &Frame::new(Opcode::Close, wsframe::close_payload(1000, "done")),
+    );
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.opcode, Opcode::Close);
+    let (code, _) = wsframe::parse_close_payload(&f.payload).unwrap().unwrap();
+    assert_eq!(code, 1000);
+    ws_expect_eof(&mut s, &buf);
+}
+
+/// Server-initiated close via WsSender::close: the close frame carries
+/// our code/reason; after the client echoes, the socket closes.
+#[test]
+fn ws_server_initiated_close_handshake() {
+    let port = spawn_server(ws_pipeline());
+    let mut s = ws_connect(port);
+    let mut buf = Vec::new();
+
+    ws_send(&mut s, &Frame::new(Opcode::Text, "bye"));
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.opcode, Opcode::Close);
+    let (code, reason) = wsframe::parse_close_payload(&f.payload).unwrap().unwrap();
+    assert_eq!(code, 1000);
+    assert_eq!(reason, "goodbye");
+
+    // Echo the close; server completes the handshake and drops the fd.
+    ws_send(
+        &mut s,
+        &Frame::new(Opcode::Close, wsframe::close_payload(1000, "")),
+    );
+    ws_expect_eof(&mut s, &buf);
+}
+
+/// A protocol violation (unmasked client frame) starts the close
+/// handshake with 1002.
+#[test]
+fn ws_unmasked_client_frame_closes_1002() {
+    let port = spawn_server(ws_pipeline());
+    let mut s = ws_connect(port);
+    let mut buf = Vec::new();
+
+    // Server-style (unmasked) encoding from the client = violation.
+    s.write_all(&Frame::new(Opcode::Text, "naughty").encode()).unwrap();
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.opcode, Opcode::Close);
+    let (code, _) = wsframe::parse_close_payload(&f.payload).unwrap().unwrap();
+    assert_eq!(code, 1002);
+
+    // Complete the handshake so the server returns inside the test budget.
+    ws_send(
+        &mut s,
+        &Frame::new(Opcode::Close, wsframe::close_payload(1002, "")),
+    );
+    ws_expect_eof(&mut s, &buf);
+}
+
+/// A frame whose HEADER already exceeds max_frame_payload is rejected
+/// before any payload is buffered: 1009 with no payload bytes sent.
+#[test]
+fn ws_oversize_frame_header_closes_1009() {
+    let port = spawn_server(ws_pipeline());
+    let mut s = ws_connect(port);
+    let mut buf = Vec::new();
+
+    // fin+text, masked, 64-bit length claiming 2 MiB (cap is 1 MiB).
+    let header: &[u8] = &[
+        0x81, 0xFF, 0, 0, 0, 0, 0, 0x20, 0, 0, // len = 0x200000
+        9, 9, 9, 9, // mask key
+    ];
+    s.write_all(header).unwrap();
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.opcode, Opcode::Close);
+    let (code, _) = wsframe::parse_close_payload(&f.payload).unwrap().unwrap();
+    assert_eq!(code, 1009);
+
+    ws_send(
+        &mut s,
+        &Frame::new(Opcode::Close, wsframe::close_payload(1009, "")),
+    );
+    ws_expect_eof(&mut s, &buf);
+}
+
+/// Fragmented text reassembles into one message; control frames may
+/// interleave mid-fragmentation (§5.4) without disturbing assembly.
+#[test]
+fn ws_fragmented_message_with_interleaved_ping() {
+    let port = spawn_server(ws_pipeline());
+    let mut s = ws_connect(port);
+    let mut buf = Vec::new();
+
+    let first = Frame { fin: false, opcode: Opcode::Text, payload: b"hel".to_vec() };
+    ws_send(&mut s, &first);
+    ws_send(&mut s, &Frame::new(Opcode::Ping, "mid"));
+    let cont = Frame { fin: true, opcode: Opcode::Continuation, payload: b"lo".to_vec() };
+    ws_send(&mut s, &cont);
+
+    // Pong for the interleaved ping arrives first (written inline on
+    // receipt), then the reassembled echo.
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.opcode, Opcode::Pong);
+    assert_eq!(f.payload, b"mid");
+    let f = ws_read_frame(&mut s, &mut buf);
+    assert_eq!(f.opcode, Opcode::Text);
+    assert_eq!(f.payload, b"hello");
+}
+
+/// An open WebSocket is in-flight work: graceful shutdown waits for the
+/// drain deadline, then force-stops the conn actor parked in the duplex
+/// select. serve returns; the client sees the socket drop.
+#[test]
+fn shutdown_force_stops_open_ws() {
+    let (port, handle, done_rx) =
+        spawn_server_with_handle(ws_pipeline(), Duration::from_millis(300));
+    let mut s = ws_connect(port);
+    let mut buf = Vec::new();
+
+    // Prove the duplex loop is live before shutting down.
+    ws_send(&mut s, &Frame::new(Opcode::Text, "alive?"));
+    assert_eq!(ws_read_frame(&mut s, &mut buf).payload, b"alive?");
+
+    handle.shutdown();
+    done_rx
+        .recv_timeout(Duration::from_secs(5))
+        .expect("serve did not return: open ws not force-stopped at drain deadline");
+
+    let mut rest = Vec::new();
+    let _ = s.read_to_end(&mut rest); // EOF or reset; both fine
 }