diff --git a/crates/punktfunk-core/src/abr.rs b/crates/punktfunk-core/src/abr.rs
new file mode 100644
index 00000000..857fd39c
--- /dev/null
+++ b/crates/punktfunk-core/src/abr.rs
@@ -0,0 +1,300 @@
+//! Adaptive bitrate: the client-side AIMD controller behind the "Automatic" bitrate setting.
+//!
+//! Runs inside [`crate::client`]'s data-plane pump on the same 750 ms cadence as the adaptive-FEC
+//! [`crate::quic::LossReport`], deciding when to ask the host for a different encoder bitrate via
+//! [`crate::quic::SetBitrate`]. Division of labour with adaptive FEC: **FEC answers fast, random
+//! loss** (Wi-Fi bursts, RF noise — recoverable redundancy is the right tool); **bitrate answers
+//! persistent congestion** (the link simply can't carry the rate — more FEC only adds load). The
+//! controller therefore reacts to *sustained* signals only:
+//!
+//! - **unrecoverable frames** — loss exceeded the FEC budget (the stream visibly froze/recovered);
+//! - **heavy loss** — a window whose shard loss is beyond what FEC should be left to absorb alone;
+//! - **one-way-delay rise** — capture→received latency (host-clock skew corrected) climbing above
+//!   its rolling baseline: standing queue growth, the *pre-loss* signature of a saturated link
+//!   (bufferbloat) — this is the early-warning signal loss-based control lacks;
+//! - **a jump-to-live flush** — the pump discarded its backlog, the strongest "we were behind"
+//!   evidence there is.
+//!
+//! AIMD shape: two consecutive bad windows ⇒ multiplicative decrease (×0.7, floored); ~10 s of
+//! clean windows ⇒ additive-ish increase (+~6 %, ceilinged at the session's starting rate — the
+//! controller recovers *back to* what was negotiated, never beyond it). Changes are rate-limited
+//! (each one costs the IDR the host's rebuilt encoder opens with) and the whole controller
+//! disables itself against a host that never answers [`crate::quic::BitrateChanged`] (an older
+//! build that ignores unknown control messages).
+
+use std::collections::VecDeque;
+use std::time::{Duration, Instant};
+
+/// Never ask for less than this — below it the stream is unusable anyway and the floor keeps a
+/// mis-measured window from cratering the session.
+const FLOOR_KBPS: u32 = 5_000;
+/// Consecutive bad windows before a decrease — one window can be a scheduler blip or a single
+/// Wi-Fi scan; two in a row (1.5 s) is a condition.
+const BAD_WINDOWS_TO_DECREASE: u32 = 2;
+/// Consecutive clean windows before probing back up (~10 s at the 750 ms cadence): recovery is
+/// deliberately much slower than backoff, classic AIMD.
+const CLEAN_WINDOWS_TO_INCREASE: u32 = 13;
+/// Minimum gap between requested changes — every accepted change costs an encoder rebuild + IDR
+/// on the host, and back-to-back steps would outrun the ack/effect round trip.
+const CHANGE_COOLDOWN: Duration = Duration::from_secs(3);
+/// Window shard loss beyond which the window counts bad even without an unrecoverable frame:
+/// 2 % sustained is congestion territory, not the random tail FEC exists for.
+const HEAVY_LOSS_PPM: u32 = 20_000;
+/// How far the window's mean one-way delay may sit above the rolling baseline before it counts
+/// as queue growth. 25 ms is far beyond jitter at any streamable frame rate.
+const OWD_RISE_US: i64 = 25_000;
+/// Rolling window (in 750 ms report windows, ~30 s) whose minimum mean is the OWD baseline.
+/// Long enough to remember the uncongested floor, short enough to follow genuine path changes.
+const BASELINE_WINDOWS: usize = 40;
+/// Requests sent without a single [`crate::quic::BitrateChanged`] ack before concluding the host
+/// predates bitrate renegotiation and going quiet for the rest of the session.
+const MAX_UNACKED: u32 = 3;
+
+/// One decision per report window; `Some(kbps)` = send a [`crate::quic::SetBitrate`].
+pub(crate) struct BitrateController {
+    /// `false` = permanently off (explicit user bitrate, an old host, or ack silence).
+    enabled: bool,
+    /// The rate we believe the host encodes at (updated by acks; requests are not assumed).
+    current_kbps: u32,
+    /// The session's starting (negotiated) rate — the recovery ceiling.
+    ceiling_kbps: u32,
+    floor_kbps: u32,
+    /// Recent window mean OWDs (µs); the rolling min is the uncongested baseline.
+    owd_means: VecDeque<i64>,
+    bad_windows: u32,
+    clean_windows: u32,
+    last_change: Option<Instant>,
+    /// Requests since the last ack — reaching [`MAX_UNACKED`] disables the controller.
+    unacked: u32,
+}
+
+impl BitrateController {
+    /// `start_kbps` is the Welcome-resolved session bitrate when the user chose Automatic, or `0`
+    /// to build a permanently-disabled controller (explicit bitrate / an old host that didn't
+    /// echo one — no known ceiling to work against).
+    pub(crate) fn new(start_kbps: u32) -> Self {
+        BitrateController {
+            enabled: start_kbps > 0,
+            current_kbps: start_kbps,
+            ceiling_kbps: start_kbps,
+            floor_kbps: FLOOR_KBPS.min(start_kbps.max(1)),
+            owd_means: VecDeque::with_capacity(BASELINE_WINDOWS),
+            bad_windows: 0,
+            clean_windows: 0,
+            last_change: None,
+            unacked: 0,
+        }
+    }
+
+    /// The host's [`crate::quic::BitrateChanged`] ack: its clamp is authoritative for what the
+    /// encoder now targets, and any ack proves the host renegotiates (resets the silence counter).
+    pub(crate) fn on_ack(&mut self, kbps: u32) {
+        if kbps > 0 {
+            self.current_kbps = kbps;
+        }
+        self.unacked = 0;
+    }
+
+    /// Feed one report window; returns the rate to request now, if any. `dropped` = frames that
+    /// went FEC-unrecoverable in the window, `loss_ppm` the window's [`crate::quic::LossReport`]
+    /// figure, `owd_mean_us` the window's mean skew-corrected capture→received latency (`None`
+    /// without a clock handshake), `flushed` = the pump's jump-to-live fired in the window.
+    pub(crate) fn on_window(
+        &mut self,
+        now: Instant,
+        dropped: u64,
+        loss_ppm: u32,
+        owd_mean_us: Option<i64>,
+        flushed: bool,
+    ) -> Option<u32> {
+        if !self.enabled {
+            return None;
+        }
+        if self.unacked >= MAX_UNACKED {
+            // The host never answered: an older build. Go quiet instead of spamming a message it
+            // logs as unknown every few seconds.
+            self.enabled = false;
+            tracing::info!("adaptive bitrate off — host never acked a SetBitrate (older host)");
+            return None;
+        }
+        // OWD: compare against the rolling-min baseline of PRIOR windows (so a rising window
+        // doesn't drag its own baseline up), then record it.
+        let owd_bad = match owd_mean_us {
+            Some(mean) => {
+                let bad = self
+                    .owd_means
+                    .iter()
+                    .min()
+                    .is_some_and(|&base| mean > base + OWD_RISE_US);
+                if self.owd_means.len() == BASELINE_WINDOWS {
+                    self.owd_means.pop_front();
+                }
+                self.owd_means.push_back(mean);
+                bad
+            }
+            None => false,
+        };
+        let bad = dropped > 0 || loss_ppm >= HEAVY_LOSS_PPM || owd_bad || flushed;
+        if bad {
+            self.bad_windows += 1;
+            self.clean_windows = 0;
+        } else {
+            self.clean_windows += 1;
+            self.bad_windows = 0;
+        }
+        let cooled = self
+            .last_change
+            .is_none_or(|t| now.duration_since(t) >= CHANGE_COOLDOWN);
+        if !cooled {
+            return None;
+        }
+        if self.bad_windows >= BAD_WINDOWS_TO_DECREASE && self.current_kbps > self.floor_kbps {
+            let next = ((self.current_kbps as u64 * 7 / 10) as u32).max(self.floor_kbps);
+            self.bad_windows = 0;
+            return self.request(next, now);
+        }
+        if self.clean_windows >= CLEAN_WINDOWS_TO_INCREASE && self.current_kbps < self.ceiling_kbps
+        {
+            let next = (self.current_kbps + self.current_kbps / 16 + 1).min(self.ceiling_kbps);
+            self.clean_windows = 0;
+            return self.request(next, now);
+        }
+        None
+    }
+
+    fn request(&mut self, kbps: u32, now: Instant) -> Option<u32> {
+        self.last_change = Some(now);
+        self.unacked += 1;
+        // `current_kbps` is NOT updated here — the host's ack is authoritative. A lost/ignored
+        // request just recomputes from the same base next time (and counts toward MAX_UNACKED).
+        Some(kbps)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// A window cadence matching the pump's 750 ms tick, safely past the change cooldown when
+    /// stepped 5× between decisions.
+    const TICK: Duration = Duration::from_millis(750);
+
+    fn ticks(start: Instant, n: u32) -> Instant {
+        start + TICK * n
+    }
+
+    /// Drive `n` clean windows, asserting no decision fires before the clean threshold.
+    fn run_clean(c: &mut BitrateController, start: Instant, from: u32, n: u32) -> Option<u32> {
+        let mut out = None;
+        for i in from..from + n {
+            out = c.on_window(ticks(start, i), 0, 0, Some(10_000), false);
+            if out.is_some() {
+                return out;
+            }
+        }
+        out
+    }
+
+    #[test]
+    fn disabled_when_not_automatic_or_old_host() {
+        // start 0 = explicit user bitrate or a host that didn't echo one → permanently off.
+        let mut c = BitrateController::new(0);
+        let now = Instant::now();
+        assert_eq!(c.on_window(now, 5, 900_000, Some(500_000), true), None);
+    }
+
+    #[test]
+    fn two_bad_windows_step_down_multiplicatively() {
+        let mut c = BitrateController::new(20_000);
+        let start = Instant::now();
+        // One bad window is a blip — no reaction.
+        assert_eq!(c.on_window(ticks(start, 0), 1, 0, None, false), None);
+        // The second consecutive bad window backs off ×0.7.
+        assert_eq!(c.on_window(ticks(start, 1), 1, 0, None, false), Some(14_000));
+        c.on_ack(14_000);
+        // Still bad after the cooldown → another ×0.7 step from the ACKED rate.
+        assert_eq!(c.on_window(ticks(start, 6), 1, 0, None, false), None); // bad #1 again
+        assert_eq!(c.on_window(ticks(start, 7), 1, 0, None, false), Some(9_800));
+    }
+
+    #[test]
+    fn cooldown_blocks_back_to_back_steps() {
+        let mut c = BitrateController::new(20_000);
+        let start = Instant::now();
+        assert_eq!(c.on_window(ticks(start, 0), 1, 0, None, false), None);
+        assert_eq!(c.on_window(ticks(start, 1), 1, 0, None, false), Some(14_000));
+        c.on_ack(14_000);
+        // Two more bad windows land INSIDE the 3 s cooldown (ticks 2,3 = 1.5/2.25 s) → held.
+        assert_eq!(c.on_window(ticks(start, 2), 1, 0, None, false), None);
+        assert_eq!(c.on_window(ticks(start, 3), 1, 0, None, false), None);
+    }
+
+    #[test]
+    fn floor_is_never_crossed() {
+        let mut c = BitrateController::new(6_000);
+        let start = Instant::now();
+        assert_eq!(c.on_window(ticks(start, 0), 1, 0, None, false), None);
+        // ×0.7 of 6000 = 4200 < floor → clamped to 5000.
+        assert_eq!(c.on_window(ticks(start, 1), 1, 0, None, false), Some(5_000));
+        c.on_ack(5_000);
+        // At the floor, further bad windows request nothing.
+        assert_eq!(c.on_window(ticks(start, 6), 1, 0, None, false), None);
+        assert_eq!(c.on_window(ticks(start, 7), 1, 0, None, false), None);
+    }
+
+    #[test]
+    fn sustained_clean_recovers_toward_ceiling_only() {
+        let mut c = BitrateController::new(20_000);
+        let start = Instant::now();
+        assert_eq!(c.on_window(ticks(start, 0), 1, 0, None, false), None);
+        assert_eq!(c.on_window(ticks(start, 1), 1, 0, None, false), Some(14_000));
+        c.on_ack(14_000);
+        // 13 clean windows → one additive step up (14000 + 14000/16 + 1 = 14876).
+        let up = run_clean(&mut c, start, 2, 13);
+        assert_eq!(up, Some(14_876));
+        c.on_ack(14_876);
+        // Fully recovered → clean windows at the ceiling stay quiet (never probe past start).
+        c.on_ack(20_000);
+        assert_eq!(run_clean(&mut c, start, 40, 20), None);
+    }
+
+    #[test]
+    fn owd_rise_alone_is_a_congestion_signal() {
+        let mut c = BitrateController::new(20_000);
+        let start = Instant::now();
+        // Establish a ~10 ms baseline over a few clean windows.
+        for i in 0..4 {
+            assert_eq!(c.on_window(ticks(start, i), 0, 0, Some(10_000), false), None);
+        }
+        // Delay climbs 40 ms above baseline with ZERO loss — bufferbloat. Two windows → back off.
+        assert_eq!(c.on_window(ticks(start, 4), 0, 0, Some(50_000), false), None);
+        assert_eq!(
+            c.on_window(ticks(start, 5), 0, 0, Some(52_000), false),
+            Some(14_000)
+        );
+    }
+
+    #[test]
+    fn ack_silence_disables_the_controller() {
+        let mut c = BitrateController::new(20_000);
+        let start = Instant::now();
+        let mut sent = 0;
+        let mut i = 0;
+        // Keep every window bad and never ack: exactly MAX_UNACKED requests, then silence.
+        while i < 60 {
+            if c.on_window(ticks(start, i), 1, 0, None, false).is_some() {
+                sent += 1;
+            }
+            i += 1;
+        }
+        assert_eq!(sent, MAX_UNACKED);
+    }
+
+    #[test]
+    fn flush_counts_as_a_bad_window() {
+        let mut c = BitrateController::new(20_000);
+        let start = Instant::now();
+        assert_eq!(c.on_window(ticks(start, 0), 0, 0, None, true), None);
+        assert_eq!(c.on_window(ticks(start, 1), 0, 0, None, true), Some(14_000));
+    }
+}
diff --git a/crates/punktfunk-core/src/client.rs b/crates/punktfunk-core/src/client.rs
index c75c5501..b022e1c6 100644
--- a/crates/punktfunk-core/src/client.rs
+++ b/crates/punktfunk-core/src/client.rs
@@ -15,9 +15,11 @@ use crate::config::{CompositorPref, GamepadPref, Mode, Role};
 use crate::error::{PunktfunkError, Result};
 use crate::input::InputEvent;
 use crate::packet::FLAG_PROBE;
+use crate::abr::BitrateController;
 use crate::quic::{
-    endpoint, io, window_loss_ppm, ColorInfo, HdrMeta, Hello, HidOutput, LossReport, ProbeRequest,
-    ProbeResult, Reconfigure, Reconfigured, RequestKeyframe, RichInput, Start, Welcome,
+    endpoint, io, window_loss_ppm, BitrateChanged, ColorInfo, HdrMeta, Hello, HidOutput,
+    LossReport, ProbeRequest, ProbeResult, Reconfigure, Reconfigured, RequestKeyframe, RichInput,
+    SetBitrate, Start, Welcome,
 };
 use crate::session::{Frame, Session};
 use crate::transport::UdpTransport;
@@ -27,6 +29,19 @@ use std::sync::mpsc::{Receiver, RecvTimeoutError, SyncSender};
 use std::sync::{Arc, Condvar, Mutex};
 use std::time::{Duration, Instant};
 
+/// Join `host` and `port` for `SocketAddr` parsing, bracketing a bare IPv6 literal
+/// (`fd00::1` → `[fd00::1]:4770`) — without the brackets the joined string can never parse and
+/// the error blames the caller's input. The control/data sockets are still IPv4-bound today, so
+/// a v6 dial fails at connect (with an honest IO error); this is the parse-side groundwork for
+/// IPv6 support. V4 literals, hostnames, and already-bracketed input pass through unchanged.
+fn join_host_port(host: &str, port: u16) -> String {
+    if host.contains(':') && !host.starts_with('[') {
+        format!("[{host}]:{port}")
+    } else {
+        format!("{host}:{port}")
+    }
+}
+
 /// A control-stream request the embedder makes on the open handshake stream: a mode switch or a
 /// speed test. One outbound channel carries both so the worker's `select!` has a single writer
 /// (two `&mut ctrl_send` borrows across select branches don't compile).
@@ -35,6 +50,9 @@ enum CtrlRequest {
     Probe(ProbeRequest),
     Keyframe,
     Loss(LossReport),
+    /// Adaptive bitrate: ask the host to re-target its encoder (kbps). Sent by the pump's
+    /// [`BitrateController`] when the user's bitrate setting is Automatic.
+    SetBitrate(u32),
 }
 
 /// What the worker reports to [`NativeClient::connect`] once the handshake lands: the negotiated
@@ -642,7 +660,7 @@ impl NativeClient {
             .map_err(PunktfunkError::Io)?;
         let pin = pin.to_string();
         let name = name.to_string();
-        let remote: std::net::SocketAddr = format!("{host}:{port}")
+        let remote: std::net::SocketAddr = join_host_port(host, port)
             .parse()
             .map_err(|_| PunktfunkError::InvalidArg("host:port"))?;
 
@@ -1096,7 +1114,7 @@ async fn worker_main(args: WorkerArgs) {
         hot_tids,
     } = args;
     let setup = async {
-        let remote: std::net::SocketAddr = format!("{host}:{port}")
+        let remote: std::net::SocketAddr = join_host_port(&host, port)
             .parse()
             .map_err(|_| PunktfunkError::InvalidArg("host:port"))?;
         let (ep, observed) = endpoint::client_pinned_with_identity(
@@ -1289,6 +1307,10 @@ async fn worker_main(args: WorkerArgs) {
         }
     });
 
+    // Adaptive bitrate ack slot: the control task parks the latest BitrateChanged here; the
+    // pump's controller drains it on its report tick (`take()` — an ack is consumed once).
+    let bitrate_ack: Arc<Mutex<Option<u32>>> = Arc::new(Mutex::new(None));
+
     // Control task: the handshake stream stays open for mid-stream renegotiation + speed tests.
     // Outbound requests (mode switch, probe) and inbound replies (Reconfigured, ProbeResult) are
     // multiplexed with `select!`; a single outbound channel (`ctrl_rx`) keeps one writer so the
@@ -1296,6 +1318,7 @@ async fn worker_main(args: WorkerArgs) {
     {
         let mode_slot = mode_slot.clone();
         let probe = probe.clone();
+        let bitrate_ack = bitrate_ack.clone();
         tokio::spawn(async move {
             loop {
                 tokio::select! {
@@ -1306,6 +1329,7 @@ async fn worker_main(args: WorkerArgs) {
                             CtrlRequest::Probe(p) => p.encode(),
                             CtrlRequest::Keyframe => RequestKeyframe.encode(),
                             CtrlRequest::Loss(r) => r.encode(),
+                            CtrlRequest::SetBitrate(k) => SetBitrate { bitrate_kbps: k }.encode(),
                         };
                         if io::write_msg(&mut ctrl_send, &bytes).await.is_err() {
                             break;
@@ -1342,6 +1366,15 @@ async fn worker_main(args: WorkerArgs) {
                                 delivered_packets = p.delivered_packets,
                                 "speed-test probe result"
                             );
+                        } else if let Ok(ack) = BitrateChanged::decode(&msg) {
+                            // Adaptive bitrate: the host's clamp is authoritative — park it for
+                            // the pump's controller (which also reads any ack as "this host
+                            // renegotiates", arming further steps).
+                            tracing::info!(
+                                kbps = ack.bitrate_kbps,
+                                "host re-targeted encoder bitrate"
+                            );
+                            *bitrate_ack.lock().unwrap() = Some(ack.bitrate_kbps);
                         } else {
                             tracing::warn!("unknown control message — ignoring");
                         }
@@ -1417,6 +1450,18 @@ async fn worker_main(args: WorkerArgs) {
         const ADAPT_REPORT_INTERVAL: Duration = Duration::from_millis(750);
         let mut last_report = Instant::now();
         let (mut last_recovered, mut last_received, mut last_dropped) = (0u64, 0u64, 0u64);
+        // Adaptive bitrate (see `crate::abr`): armed only when the embedder asked for Automatic
+        // (`bitrate_kbps == 0`) and the host echoed the rate it actually configured (an old host
+        // echoes 0 → controller stays permanently off). Fed once per report window with the same
+        // deltas the LossReport uses, plus the window's mean skew-corrected one-way delay and
+        // whether a jump-to-live flush fired.
+        let mut abr = BitrateController::new(if bitrate_kbps == 0 {
+            resolved_bitrate_kbps
+        } else {
+            0
+        });
+        let (mut owd_sum_ns, mut owd_frames) = (0i128, 0u32);
+        let mut flush_in_window = false;
         // Jump-to-live state (see the guard in the loop below): the clock-based over-bound run
         // (`stale_frames`, armed only when the skew handshake succeeded so the clocks are comparable),
         // the clock-free non-draining-queue run (`standing_frames`), and the last-jump instant for the
@@ -1443,12 +1488,33 @@ async fn worker_main(args: WorkerArgs) {
                 p.active && !p.done
             };
             if !probe_active && last_report.elapsed() >= ADAPT_REPORT_INTERVAL {
+                let window_dropped = st.frames_dropped.wrapping_sub(last_dropped);
                 let loss_ppm = window_loss_ppm(
                     st.fec_recovered_shards.wrapping_sub(last_recovered),
                     st.packets_received.wrapping_sub(last_received),
-                    st.frames_dropped.wrapping_sub(last_dropped),
+                    window_dropped,
                 );
                 let _ = ctrl_tx.send(CtrlRequest::Loss(LossReport { loss_ppm }));
+                // Adaptive bitrate: drain any host ack first (its clamp is authoritative), then
+                // feed the controller this window's congestion signals; a decision becomes a
+                // SetBitrate on the control stream.
+                if let Some(acked) = bitrate_ack.lock().unwrap().take() {
+                    abr.on_ack(acked);
+                }
+                let owd_mean_us =
+                    (owd_frames > 0).then(|| (owd_sum_ns / owd_frames as i128 / 1000) as i64);
+                (owd_sum_ns, owd_frames) = (0, 0);
+                if let Some(kbps) = abr.on_window(
+                    Instant::now(),
+                    window_dropped,
+                    loss_ppm,
+                    owd_mean_us,
+                    flush_in_window,
+                ) {
+                    tracing::info!(kbps, "adaptive bitrate: requesting encoder re-target");
+                    let _ = ctrl_tx.send(CtrlRequest::SetBitrate(kbps));
+                }
+                flush_in_window = false;
                 last_report = Instant::now();
                 last_recovered = st.fec_recovered_shards;
                 last_received = st.packets_received;
@@ -1486,6 +1552,13 @@ async fn worker_main(args: WorkerArgs) {
                         } else {
                             0
                         };
+                        // Feed the adaptive-bitrate controller's OWD window (mean capture→received
+                        // delay): rising delay under zero loss is queue growth — the pre-loss
+                        // congestion signal. Only meaningful with a clock handshake.
+                        if clock_offset_ns != 0 && lat_ns > 0 {
+                            owd_sum_ns += lat_ns;
+                            owd_frames += 1;
+                        }
                         if clock_offset_ns != 0 && lat_ns > FLUSH_LATENCY.as_nanos() as i128 {
                             stale_frames += 1;
                         } else {
@@ -1505,6 +1578,7 @@ async fn worker_main(args: WorkerArgs) {
                             stale_frames = 0;
                             standing_frames = 0;
                             last_flush = Some(Instant::now());
+                            flush_in_window = true; // strongest "link can't hold the rate" signal
                             let flushed = session.flush_backlog().unwrap_or(0);
                             let dropped = frames.clear();
                             let _ = ctrl_tx.send(CtrlRequest::Keyframe);
@@ -1543,6 +1617,23 @@ async fn worker_main(args: WorkerArgs) {
     conn.close(close_code.into(), b"client closed");
 }
 
+#[cfg(test)]
+mod host_port_tests {
+    use super::join_host_port;
+
+    #[test]
+    fn brackets_bare_ipv6_only() {
+        assert_eq!(join_host_port("192.168.1.9", 4770), "192.168.1.9:4770");
+        assert_eq!(join_host_port("myhost", 4770), "myhost:4770");
+        assert_eq!(join_host_port("fd00::1", 4770), "[fd00::1]:4770");
+        assert_eq!(join_host_port("[fd00::1]", 4770), "[fd00::1]:4770");
+        // The bracketed form is what SocketAddr's parser actually accepts.
+        assert!(join_host_port("fd00::1", 4770)
+            .parse::<std::net::SocketAddr>()
+            .is_ok());
+    }
+}
+
 #[cfg(test)]
 mod frame_channel_tests {
     use super::{FrameChannel, FramePop, FRAME_QUEUE_HARD_CAP};
diff --git a/crates/punktfunk-core/src/lib.rs b/crates/punktfunk-core/src/lib.rs
index 60c201ee..f4c77936 100644
--- a/crates/punktfunk-core/src/lib.rs
+++ b/crates/punktfunk-core/src/lib.rs
@@ -25,6 +25,8 @@
 #![forbid(unsafe_op_in_unsafe_fn)]
 
 pub mod abi;
+#[cfg(feature = "quic")]
+mod abr;
 pub mod audio;
 #[cfg(feature = "quic")]
 pub mod client;
diff --git a/crates/punktfunk-core/src/quic.rs b/crates/punktfunk-core/src/quic.rs
index ff739047..f2508e54 100644
--- a/crates/punktfunk-core/src/quic.rs
+++ b/crates/punktfunk-core/src/quic.rs
@@ -369,6 +369,32 @@ pub struct LossReport {
     pub loss_ppm: u32,
 }
 
+/// `client → host`, any time after [`Start`]: reconfigure the encoder to a new target bitrate
+/// without reconnecting — the mid-stream lever of adaptive bitrate. The host clamps the request
+/// exactly like [`Hello::bitrate_kbps`] (its `[MIN, MAX]` band; `0` → host default), answers with
+/// [`BitrateChanged`] carrying the value it actually configured, and rebuilds the encoder in
+/// place at the same mode — the first new-rate frame is an IDR with in-band parameter sets, which
+/// every client decoder already follows (same discipline as a [`Reconfigure`] mode switch).
+///
+/// Sent by the client's automatic-bitrate controller (active when the user's bitrate setting is
+/// "Automatic", i.e. `Hello::bitrate_kbps == 0`) when the link can't sustain the current rate —
+/// or can sustain more again. A host that predates this ignores it (unknown control message) and
+/// never answers; the client's controller detects the silence and disables itself.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct SetBitrate {
+    /// Requested encoder bitrate in kilobits per second (`0` = host default, like Hello's field).
+    pub bitrate_kbps: u32,
+}
+
+/// `host → client`: answer to [`SetBitrate`] — the bitrate the host actually configured (the
+/// request clamped to its supported band). The encoder switches on the next frame (an IDR); the
+/// stream never pauses. Also the controller's liveness signal: no answer ⇒ an old host that
+/// doesn't renegotiate bitrate.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct BitrateChanged {
+    pub bitrate_kbps: u32,
+}
+
 /// `client → host`, any time after [`Start`]: run a bandwidth speed test. The host bursts
 /// filler access units (flagged [`crate::packet::FLAG_PROBE`]) over the data plane at
 /// `target_kbps` of application goodput for `duration_ms`, *pausing video for the duration*, then
@@ -455,6 +481,10 @@ pub const MSG_RECONFIGURED: u8 = 0x02;
 pub const MSG_REQUEST_KEYFRAME: u8 = 0x03;
 /// Type byte of [`LossReport`].
 pub const MSG_LOSS_REPORT: u8 = 0x04;
+/// Type byte of [`SetBitrate`].
+pub const MSG_SET_BITRATE: u8 = 0x05;
+/// Type byte of [`BitrateChanged`].
+pub const MSG_BITRATE_CHANGED: u8 = 0x06;
 /// Type byte of [`ProbeRequest`].
 pub const MSG_PROBE_REQUEST: u8 = 0x20;
 /// Type byte of [`ProbeResult`].
@@ -1128,6 +1158,46 @@ impl LossReport {
     }
 }
 
+impl SetBitrate {
+    pub fn encode(&self) -> Vec<u8> {
+        // magic[0..4] type[4] bitrate_kbps[5..9]
+        let mut b = Vec::with_capacity(9);
+        b.extend_from_slice(CTL_MAGIC);
+        b.push(MSG_SET_BITRATE);
+        b.extend_from_slice(&self.bitrate_kbps.to_le_bytes());
+        b
+    }
+
+    pub fn decode(b: &[u8]) -> Result<SetBitrate> {
+        if b.len() != 9 || &b[0..4] != CTL_MAGIC || b[4] != MSG_SET_BITRATE {
+            return Err(PunktfunkError::InvalidArg("bad SetBitrate"));
+        }
+        Ok(SetBitrate {
+            bitrate_kbps: u32::from_le_bytes(b[5..9].try_into().unwrap()),
+        })
+    }
+}
+
+impl BitrateChanged {
+    pub fn encode(&self) -> Vec<u8> {
+        // magic[0..4] type[4] bitrate_kbps[5..9]
+        let mut b = Vec::with_capacity(9);
+        b.extend_from_slice(CTL_MAGIC);
+        b.push(MSG_BITRATE_CHANGED);
+        b.extend_from_slice(&self.bitrate_kbps.to_le_bytes());
+        b
+    }
+
+    pub fn decode(b: &[u8]) -> Result<BitrateChanged> {
+        if b.len() != 9 || &b[0..4] != CTL_MAGIC || b[4] != MSG_BITRATE_CHANGED {
+            return Err(PunktfunkError::InvalidArg("bad BitrateChanged"));
+        }
+        Ok(BitrateChanged {
+            bitrate_kbps: u32::from_le_bytes(b[5..9].try_into().unwrap()),
+        })
+    }
+}
+
 /// Compute a [`LossReport`] `loss_ppm` from one window's session-stat deltas: shards FEC recovered
 /// (the loss it absorbed), shards received, and frames that went unrecoverable. Loss ≈ recovered /
 /// (received + recovered) — the fraction of shards that arrived missing. A frame drop means loss
@@ -2684,6 +2754,23 @@ mod tests {
         assert!(window_loss_ppm(u64::MAX, 1, 9) <= 1_000_000);
     }
 
+    #[test]
+    fn bitrate_messages_roundtrip() {
+        let req = SetBitrate {
+            bitrate_kbps: 14_000,
+        };
+        assert_eq!(SetBitrate::decode(&req.encode()).unwrap(), req);
+        let ack = BitrateChanged {
+            bitrate_kbps: 14_000,
+        };
+        assert_eq!(BitrateChanged::decode(&ack.encode()).unwrap(), ack);
+        // Same payload shape as LossReport — the type byte alone must keep them disjoint.
+        assert!(LossReport::decode(&req.encode()).is_err());
+        assert!(SetBitrate::decode(&ack.encode()).is_err());
+        assert!(BitrateChanged::decode(&req.encode()).is_err());
+        assert!(SetBitrate::decode(&LossReport { loss_ppm: 7 }.encode()).is_err());
+    }
+
     #[test]
     fn probe_messages_roundtrip() {
         let req = ProbeRequest {
diff --git a/crates/punktfunk-host/src/punktfunk1.rs b/crates/punktfunk-host/src/punktfunk1.rs
index 2cd6ec6a..0e747e36 100644
--- a/crates/punktfunk-host/src/punktfunk1.rs
+++ b/crates/punktfunk-host/src/punktfunk1.rs
@@ -32,14 +32,14 @@ use punktfunk_core::config::{
 use punktfunk_core::input::{InputEvent, InputKind};
 use punktfunk_core::packet::{FLAG_PIC, FLAG_PROBE, FLAG_SOF};
 use punktfunk_core::quic::{
-    endpoint, io, ClockEcho, ClockProbe, ColorInfo, Hello, LossReport, PairChallenge, PairProof,
-    PairRequest, PairResult, ProbeRequest, ProbeResult, Reconfigure, Reconfigured, RequestKeyframe,
-    Start, Welcome,
+    endpoint, io, BitrateChanged, ClockEcho, ClockProbe, ColorInfo, Hello, LossReport,
+    PairChallenge, PairProof, PairRequest, PairResult, ProbeRequest, ProbeResult, Reconfigure,
+    Reconfigured, RequestKeyframe, SetBitrate, Start, Welcome,
 };
 use punktfunk_core::transport::UdpTransport;
 use punktfunk_core::Session;
 use rand::RngCore;
-use std::sync::atomic::{AtomicBool, AtomicU8, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicU32, AtomicU8, Ordering};
 use std::sync::Arc;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -1054,6 +1054,7 @@ async fn serve_session(
     // (inbound requests, outbound probe results) are multiplexed with `select!`.
     let (reconfig_tx, reconfig_rx) = std::sync::mpsc::channel::<punktfunk_core::Mode>();
     let (keyframe_tx, keyframe_rx) = std::sync::mpsc::channel::<()>();
+    let (bitrate_tx, bitrate_rx) = std::sync::mpsc::channel::<u32>();
     let (probe_tx, probe_rx) = std::sync::mpsc::channel::<ProbeRequest>();
     let (probe_result_tx, mut probe_result_rx) =
         tokio::sync::mpsc::unbounded_channel::<ProbeResult>();
@@ -1124,6 +1125,27 @@ async fn serve_session(
                                 );
                             }
                         }
+                    } else if let Ok(req) = SetBitrate::decode(&msg) {
+                        // Mid-stream bitrate renegotiation (adaptive bitrate): clamp exactly like
+                        // the Hello request, ack the resolved value, then hand it to the data-plane
+                        // thread, which rebuilds the encoder in place at the same mode — the fresh
+                        // encoder's first frame is an IDR with in-band parameter sets, so the
+                        // client's decoder follows without a reconnect.
+                        let resolved = resolve_bitrate_kbps(req.bitrate_kbps);
+                        tracing::info!(
+                            requested_kbps = req.bitrate_kbps,
+                            resolved_kbps = resolved,
+                            "mid-stream bitrate change requested"
+                        );
+                        let ack = BitrateChanged {
+                            bitrate_kbps: resolved,
+                        };
+                        if io::write_msg(&mut ctrl_send, &ack.encode()).await.is_err() {
+                            break;
+                        }
+                        if bitrate_tx.send(resolved).is_err() {
+                            break; // data plane gone
+                        }
                     } else if let Ok(req) = ProbeRequest::decode(&msg) {
                         tracing::info!(
                             target_kbps = req.target_kbps,
@@ -1445,6 +1467,7 @@ async fn serve_session(
                         quit: quit_stream,
                         reconfig: reconfig_rx,
                         keyframe: keyframe_rx,
+                        bitrate_rx,
                         compositor,
                         bitrate_kbps,
                         bit_depth,
@@ -2738,7 +2761,9 @@ struct SendStats {
     fps: u32,
     codec: &'static str,
     client: String,
-    bitrate_kbps: u32,
+    /// Live encoder bitrate (kbps) — the capture thread updates it on a mid-stream adaptive
+    /// bitrate change, so the web-console sample reports what the encoder is ACTUALLY targeting.
+    bitrate_kbps: Arc<AtomicU32>,
 }
 
 #[allow(clippy::too_many_arguments)]
@@ -2921,7 +2946,7 @@ fn send_loop(
                     fps: (new_frames as f64 / secs) as f32,
                     repeat_fps: (repeat_frames as f64 / secs) as f32,
                     mbps: tx_mbps as f32,
-                    bitrate_kbps: stats.bitrate_kbps,
+                    bitrate_kbps: stats.bitrate_kbps.load(Ordering::Relaxed),
                     frames_dropped: s.frames_dropped.saturating_sub(last_frames_dropped) as u32,
                     packets_dropped: s.packets_dropped.saturating_sub(last_packets_dropped) as u32,
                     send_dropped: s.packets_send_dropped.saturating_sub(last_send_dropped) as u32,
@@ -3078,6 +3103,9 @@ struct SessionContext {
     reconfig: std::sync::mpsc::Receiver<punktfunk_core::Mode>,
     /// Client decode-recovery keyframe requests.
     keyframe: std::sync::mpsc::Receiver<()>,
+    /// Accepted mid-stream bitrate changes (adaptive bitrate, already clamped) — the encoder
+    /// alone is rebuilt in place at the new rate; capture + virtual output are untouched.
+    bitrate_rx: std::sync::mpsc::Receiver<u32>,
     /// The resolved compositor backend (moot on Windows — `vdisplay::open` ignores it there).
     compositor: crate::vdisplay::Compositor,
     /// Negotiated encoder bitrate (kbps).
@@ -3137,8 +3165,9 @@ fn virtual_stream(ctx: SessionContext) -> Result<()> {
         quit,
         reconfig,
         keyframe,
+        bitrate_rx,
         compositor,
-        bitrate_kbps,
+        mut bitrate_kbps,
         bit_depth,
         // The resolved chroma is already captured in `plan` (above); ignore the duplicate here.
         chroma: _,
@@ -3236,6 +3265,9 @@ fn virtual_stream(ctx: SessionContext) -> Result<()> {
     // The bounded channel applies backpressure (the encode thread blocks if the send falls behind,
     // so frames slow down rather than a dropped frame freezing the infinite-GOP stream).
     let (frame_tx, frame_rx) = std::sync::mpsc::sync_channel::<FrameMsg>(3);
+    // Live encoder bitrate, shared with the send thread's stats sample: a mid-stream adaptive
+    // bitrate change (bitrate_rx below) updates it so the console shows the actual target.
+    let live_bitrate = Arc::new(AtomicU32::new(bitrate_kbps));
     // The send thread emits the web-console stats sample (it owns `session.stats()`); clone the
     // recorder so the capture loop keeps its own handle for the per-frame `is_armed()` gate.
     let send_stats = SendStats {
@@ -3245,7 +3277,7 @@ fn virtual_stream(ctx: SessionContext) -> Result<()> {
         fps: mode.refresh_hz,
         codec: plan.codec.label(),
         client: client_label,
-        bitrate_kbps,
+        bitrate_kbps: live_bitrate.clone(),
     };
     let send_thread = std::thread::Builder::new()
         .name("punktfunk-send".into())
@@ -3447,6 +3479,51 @@ fn virtual_stream(ctx: SessionContext) -> Result<()> {
                 }
             }
         }
+        // Adaptive bitrate: drain to the NEWEST requested rate (the client's controller may step
+        // several times while we stream) and rebuild the ENCODER ONLY in place — the mode didn't
+        // change, so capture and the virtual output are untouched and the switch costs exactly the
+        // IDR the fresh encoder opens with (the same resync discipline as a mode switch, minus the
+        // pipeline churn). Rates arrive pre-clamped by the control task (`resolve_bitrate_kbps`).
+        let mut want_kbps = None;
+        while let Ok(k) = bitrate_rx.try_recv() {
+            want_kbps = Some(k);
+        }
+        if let Some(new_kbps) = want_kbps.filter(|&k| k != bitrate_kbps) {
+            // `interval` was built as 1/effective_hz, so the round-trip recovers the integer rate.
+            let hz = (1.0 / interval.as_secs_f64()).round() as u32;
+            match crate::encode::open_video(
+                plan.codec,
+                frame.format,
+                frame.width,
+                frame.height,
+                hz,
+                new_kbps as u64 * 1000,
+                frame.is_cuda(),
+                bit_depth,
+                plan.chroma,
+            ) {
+                Ok(new_enc) => {
+                    tracing::info!(
+                        from_kbps = bitrate_kbps,
+                        to_kbps = new_kbps,
+                        "encoder rebuilt at new bitrate (adaptive bitrate)"
+                    );
+                    enc = new_enc;
+                    bitrate_kbps = new_kbps;
+                    live_bitrate.store(new_kbps, Ordering::Relaxed);
+                    // The owed AUs died with the old encoder — same bookkeeping as a mode-switch
+                    // rebuild; the fresh encoder opens on an IDR, so anchor the IDR cooldown too.
+                    inflight.clear();
+                    last_au_at = std::time::Instant::now();
+                    encoder_resets = 0;
+                    last_forced_idr = Some(std::time::Instant::now());
+                }
+                Err(e) => {
+                    tracing::error!(error = %format!("{e:#}"), to_kbps = new_kbps,
+                        "bitrate-change encoder rebuild failed — keeping the current rate");
+                }
+            }
+        }
         // Client recovery: it asked for a fresh IDR (its decoder wedged on the cold opening
         // GOP). Coalesce the backlog — several requests fire before the IDR lands — and force
         // the next encoded frame to be a keyframe. (A reconfig rebuild above already opens with
diff --git a/include/punktfunk_core.h b/include/punktfunk_core.h
index fffeaee4..c757de4f 100644
--- a/include/punktfunk_core.h
+++ b/include/punktfunk_core.h
@@ -356,6 +356,16 @@
 #define MSG_LOSS_REPORT 4
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Type byte of [`SetBitrate`].
+#define MSG_SET_BITRATE 5
+#endif
+
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Type byte of [`BitrateChanged`].
+#define MSG_BITRATE_CHANGED 6
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Type byte of [`ProbeRequest`].
 #define MSG_PROBE_REQUEST 32