diff --git a/crates/irtt-client/src/client.rs b/crates/irtt-client/src/client.rs
index cdb8a6e..14fc88a 100644
--- a/crates/irtt-client/src/client.rs
+++ b/crates/irtt-client/src/client.rs
@@ -12,7 +12,7 @@ use crate::{
     error::ClientError,
     event::{ClientEvent, OpenOutcome},
     receive::recv_datagram,
-    session::machine::{recv_buffer_size, SendProbeResult, SessionMachine, MAX_OPEN_PACKET_SIZE},
+    session::machine::{recv_buffer_size, ProbeSent, SessionMachine, MAX_OPEN_PACKET_SIZE},
     socket::{connect_udp_socket, resolve_remote, validate_open_timeouts},
     socket_options::{apply_dscp_to_socket, clear_dscp_on_socket},
     timing::ClientTimestamp,
@@ -22,6 +22,12 @@ pub(crate) mod schedule;
 
 use schedule::ProbeSchedule;
 
+#[derive(Debug, Clone, Copy)]
+enum ProbeScheduleMode {
+    CallerPaced,
+    Managed { scheduled_at: Instant },
+}
+
 #[cfg(test)]
 use crate::{
     session::machine::{
@@ -45,6 +51,10 @@ pub struct Client {
     socket: UdpSocket,
     remote: SocketAddr,
     recv_buffer: Vec<u8>,
+    #[cfg(test)]
+    probe_reported_len: Option<usize>,
+    #[cfg(test)]
+    probe_send_error: bool,
 }
 
 impl Client {
@@ -65,6 +75,10 @@ impl Client {
             socket,
             remote,
             recv_buffer: vec![0_u8; recv_buffer_size(false, None)?],
+            #[cfg(test)]
+            probe_reported_len: None,
+            #[cfg(test)]
+            probe_send_error: false,
         })
     }
 
@@ -302,24 +316,7 @@ impl Client {
         &mut self,
         override_ts: Option<ClientTimestamp>,
     ) -> Result<Vec<ClientEvent>, ClientError> {
-        let socket = &self.socket;
-        self.runtime.ensure_open()?;
-        let schedule = self
-            .schedule
-            .as_mut()
-            .expect("open sessions always have a probe schedule");
-        self.runtime
-            .send_probe_with(schedule, override_ts, |packet| {
-                let sent_at = override_ts.unwrap_or_else(ClientTimestamp::now);
-                let send_call_start = Instant::now();
-                let bytes = socket.send(packet)?;
-                let send_call = send_call_start.elapsed();
-                Ok(SendProbeResult {
-                    sent_at,
-                    bytes,
-                    send_call,
-                })
-            })
+        self.send_probe_transaction(override_ts, ProbeScheduleMode::CallerPaced)
     }
 
     fn send_managed_probe_inner(
@@ -327,24 +324,102 @@ impl Client {
         scheduled_at: Instant,
         override_ts: Option<ClientTimestamp>,
     ) -> Result<Vec<ClientEvent>, ClientError> {
-        let socket = &self.socket;
+        self.send_probe_transaction(override_ts, ProbeScheduleMode::Managed { scheduled_at })
+    }
+
+    fn send_probe_transaction(
+        &mut self,
+        override_ts: Option<ClientTimestamp>,
+        mode: ProbeScheduleMode,
+    ) -> Result<Vec<ClientEvent>, ClientError> {
         self.runtime.ensure_open()?;
-        let schedule = self
-            .schedule
+        let sent_at = override_ts.unwrap_or_else(ClientTimestamp::now);
+        #[cfg(test)]
+        let fail_send = std::mem::take(&mut self.probe_send_error);
+        let remote = self.remote;
+        let (runtime, schedule, socket) = (&mut self.runtime, &mut self.schedule, &self.socket);
+        let schedule = schedule
             .as_mut()
             .expect("open sessions always have a probe schedule");
-        self.runtime
-            .send_probe_for_deadline(schedule, override_ts, scheduled_at, |packet| {
-                let sent_at = override_ts.unwrap_or_else(ClientTimestamp::now);
-                let send_call_start = Instant::now();
-                let bytes = socket.send(packet)?;
-                let send_call = send_call_start.elapsed();
-                Ok(SendProbeResult {
-                    sent_at,
-                    bytes,
-                    send_call,
-                })
-            })
+        if !schedule.permit_probe_at(sent_at.mono) {
+            return Ok(Vec::new());
+        }
+
+        let Some(prepared) = runtime.prepare_probe()? else {
+            return Ok(Vec::new());
+        };
+        let machine_commit = runtime.preflight_probe_commit(&prepared, sent_at)?;
+        let schedule_commit = match mode {
+            ProbeScheduleMode::CallerPaced => {
+                schedule.preflight_caller_commit(sent_at.mono, machine_commit.next_packets_sent)?
+            }
+            ProbeScheduleMode::Managed { scheduled_at } => {
+                schedule.preflight_managed_commit(scheduled_at, sent_at.mono)?
+            }
+        };
+        let mut events = Vec::new();
+        events
+            .try_reserve(1)
+            .map_err(|source| ClientError::AllocationFailed {
+                operation: "probe event result",
+                source,
+            })?;
+
+        let expected_bytes = prepared.bytes.len();
+        let scheduled_at = schedule_commit.scheduled_at;
+        let timer_error = schedule_commit.timer_error;
+        #[cfg(test)]
+        let reported_bytes = self.probe_reported_len.take();
+        #[cfg(test)]
+        if fail_send {
+            return Err(ClientError::Socket(io::Error::other(
+                "injected probe send failure",
+            )));
+        }
+        let send_call_start = Instant::now();
+        let bytes = socket.send(&prepared.bytes)?;
+        let sent = runtime.commit_probe_sent(machine_commit, bytes);
+        schedule.commit(schedule_commit);
+
+        let send_call = send_call_start.elapsed();
+        #[cfg(test)]
+        let bytes = reported_bytes.unwrap_or(bytes);
+        validate_datagram_length(expected_bytes, bytes)?;
+
+        events.push(echo_sent_event(
+            remote,
+            sent,
+            send_call,
+            scheduled_at,
+            timer_error,
+        ));
+        Ok(events)
+    }
+}
+
+pub(crate) fn echo_sent_event(
+    remote: SocketAddr,
+    sent: ProbeSent,
+    send_call: Duration,
+    scheduled_at: Instant,
+    timer_error: Duration,
+) -> ClientEvent {
+    ClientEvent::EchoSent {
+        seq: sent.seq,
+        remote,
+        scheduled_at,
+        sent_at: sent.sent_at,
+        bytes: sent.bytes,
+        send_call,
+        timer_error,
+    }
+}
+
+pub(crate) fn validate_datagram_length(expected: usize, actual: usize) -> Result<(), ClientError> {
+    if actual == expected {
+        Ok(())
+    } else {
+        Err(ClientError::DatagramLengthMismatch { expected, actual })
     }
 }
 
diff --git a/crates/irtt-client/src/client/tests/probes_replies/probe_schedule.rs b/crates/irtt-client/src/client/tests/probes_replies/probe_schedule.rs
index 4b6d7cc..b1d6815 100644
--- a/crates/irtt-client/src/client/tests/probes_replies/probe_schedule.rs
+++ b/crates/irtt-client/src/client/tests/probes_replies/probe_schedule.rs
@@ -51,6 +51,113 @@ fn send_probe_sends_valid_echo_request() {
     server.join();
 }
 
+#[test]
+fn blocking_short_probe_commits_before_presentation_length_error() {
+    let params = default_params();
+    let server = silent_open_server(params.clone());
+    let mut client = Client::connect(default_test_config(server.addr)).unwrap();
+    assert_open_started(client.open().unwrap());
+
+    let expected = echo_packet_len(false, &params);
+    let initial_deadline = client.next_send_deadline().unwrap();
+    client.probe_reported_len = Some(expected - 1);
+
+    assert!(matches!(
+        client.send_probe(),
+        Err(ClientError::DatagramLengthMismatch {
+            expected: error_expected,
+            actual,
+        }) if error_expected == expected && actual + 1 == expected
+    ));
+    assert_eq!(client.runtime.packets_sent(), 1);
+    assert_eq!(
+        client.next_send_deadline(),
+        Some(initial_deadline + client.runtime.config().interval)
+    );
+
+    client.close().unwrap();
+    server.join();
+}
+
+#[test]
+fn blocking_failed_probe_send_preserves_machine_and_schedule() {
+    let params = default_params();
+    let server = silent_open_server(params);
+    let mut client = Client::connect(default_test_config(server.addr)).unwrap();
+    assert_open_started(client.open().unwrap());
+
+    let initial_deadline = client.next_send_deadline().unwrap();
+    client.probe_send_error = true;
+
+    assert!(matches!(client.send_probe(), Err(ClientError::Socket(_))));
+    assert_eq!(client.runtime.packets_sent(), 0);
+    assert_eq!(client.next_send_deadline(), Some(initial_deadline));
+
+    assert!(matches!(
+        client.send_probe().unwrap().as_slice(),
+        [ClientEvent::EchoSent { seq: 0, .. }]
+    ));
+
+    client.close().unwrap();
+    server.join();
+}
+
+#[test]
+fn blocking_probe_uses_one_timestamp_for_permission_and_schedule_commit() {
+    let duration = Duration::from_secs(1);
+    let interval = Duration::from_millis(100);
+    let params = Params {
+        duration_ns: 1_000_000_000,
+        interval_ns: 100_000_000,
+        ..default_params()
+    };
+    let server = silent_open_server(params.clone());
+    let config = ClientConfig {
+        duration: Some(duration),
+        interval,
+        ..default_test_config(server.addr)
+    };
+    let mut client = Client::connect(config).unwrap();
+    assert_open_started(client.open().unwrap());
+
+    let opened_mono = Instant::now()
+        .checked_sub(Duration::from_secs(2))
+        .expect("test host monotonic clock has at least two seconds of history");
+    client.schedule = Some(
+        ProbeSchedule::new(
+            opened_mono,
+            &NegotiatedParams {
+                params,
+                restrictions: Vec::new(),
+            },
+        )
+        .unwrap(),
+    );
+    let sent_at = ClientTimestamp {
+        wall: SystemTime::now(),
+        mono: opened_mono + Duration::from_millis(500),
+    };
+
+    let events = client.send_probe_at(sent_at).unwrap();
+
+    assert!(matches!(
+        events.as_slice(),
+        [ClientEvent::EchoSent {
+            scheduled_at,
+            sent_at: event_sent_at,
+            timer_error,
+            ..
+        }] if *scheduled_at == opened_mono
+            && *event_sent_at == sent_at
+            && *timer_error == Duration::from_millis(500)
+    ));
+    assert_eq!(client.next_send_deadline(), Some(opened_mono + interval));
+    assert_eq!(client.runtime.packets_sent(), 1);
+
+    client.close().unwrap();
+    server.join();
+}
+
 #[test]
 fn send_probe_respects_finite_duration_exclusive_end() {
     let params = Params {
diff --git a/crates/irtt-client/src/error.rs b/crates/irtt-client/src/error.rs
index 9aa8fe3..325946c 100644
--- a/crates/irtt-client/src/error.rs
+++ b/crates/irtt-client/src/error.rs
@@ -152,6 +152,16 @@ pub enum ClientError {
     /// can be reused.
     #[error("wire sequence {seq} is still pending")]
     PendingSequenceCollision { seq: u32 },
+    /// A prepared probe no longer matches the session's next wire sequence.
+    ///
+    /// Prepare the probe again after any intervening successful send.
+    #[error(
+        "prepared probe sequence {prepared_seq} is stale; next wire sequence is {next_wire_seq}"
+    )]
+    StalePreparedProbe {
+        prepared_seq: u32,
+        next_wire_seq: u32,
+    },
     /// Reserving memory required by an operation failed before any datagram was
     /// sent or logical protocol state was changed.
     #[error("failed to reserve memory for {operation}: {source}")]
@@ -160,6 +170,13 @@ pub enum ClientError {
         #[source]
         source: TryReserveError,
     },
+    /// UDP reported successful acceptance of a datagram with an unexpected
+    /// byte count.
+    ///
+    /// The send is committed locally before this transport invariant failure
+    /// is returned because the kernel reported success.
+    #[error("UDP accepted {actual} bytes for a {expected}-byte datagram")]
+    DatagramLengthMismatch { expected: usize, actual: usize },
     /// Client configuration failed validation before the requested operation.
     ///
     /// The reported reason names a value that must be changed before retrying.
diff --git a/crates/irtt-client/src/managed/group.rs b/crates/irtt-client/src/managed/group.rs
index 4335272..7a24a7c 100644
--- a/crates/irtt-client/src/managed/group.rs
+++ b/crates/irtt-client/src/managed/group.rs
@@ -12,13 +12,17 @@ use std::{
 };
 
 use crate::{
-    client::schedule::{advance_cadence, ProbeSchedule},
+    client::{
+        echo_sent_event,
+        schedule::{advance_cadence, ProbeSchedule},
+        validate_datagram_length,
+    },
     config::{ClientAuthConfig, ClientConfig},
     error::ClientError,
     event::{ClientEvent, OpenOutcome},
     metadata::ReceiveMeta,
     receive::recv_datagram_from,
-    session::machine::{params_from_config, SendProbeResult, SessionMachine},
+    session::machine::{params_from_config, SessionMachine},
     socket::{bind_unconnected_udp_socket, validate_open_timeouts},
     socket_options::apply_dscp_to_socket,
     timing::ClientTimestamp,
@@ -633,6 +637,10 @@ struct TargetState {
     counters: TargetCounters,
     order: u64,
     final_reason: Option<ManagedTargetEndReason>,
+    #[cfg(test)]
+    probe_reported_len: Option<usize>,
+    #[cfg(test)]
+    probe_send_error: bool,
 }
 
 type ScheduledTarget = (Arc<Mutex<TargetState>>, Instant);
@@ -727,6 +735,10 @@ impl TargetState {
             counters: TargetCounters::default(),
             order,
             final_reason: None,
+            #[cfg(test)]
+            probe_reported_len: None,
+            #[cfg(test)]
+            probe_send_error: false,
         })
     }
 
@@ -1311,28 +1323,7 @@ fn send_echo_to_target(
     if !matches!(target.status, TargetStatus::Active) {
         return;
     }
-    let remote = target.remote;
-    target
-        .runtime
-        .ensure_open()
-        .expect("active targets are open");
-    let TargetState {
-        runtime, schedule, ..
-    } = &mut *target;
-    let schedule = schedule
-        .as_mut()
-        .expect("active targets always have a probe schedule");
-    let result = runtime.send_probe_for_deadline(schedule, None, scheduled_at, |packet| {
-        let sent_at = ClientTimestamp::now();
-        let send_call_start = Instant::now();
-        let bytes = socket.send_to(packet, remote)?;
-        let send_call = send_call_start.elapsed();
-        Ok(SendProbeResult {
-            sent_at,
-            bytes,
-            send_call,
-        })
-    });
+    let result = send_echo_to_locked_target(socket, &mut target, scheduled_at);
 
     match result {
         Ok(events) => {
@@ -1348,6 +1339,77 @@ fn send_echo_to_target(
     }
 }
 
+fn send_echo_to_locked_target(
+    socket: &UdpSocket,
+    target: &mut TargetState,
+    scheduled_at: Instant,
+) -> Result<Vec<ClientEvent>, ClientError> {
+    send_echo_to_locked_target_at(socket, target, scheduled_at, ClientTimestamp::now())
+}
+
+fn send_echo_to_locked_target_at(
+    socket: &UdpSocket,
+    target: &mut TargetState,
+    scheduled_at: Instant,
+    sent_at: ClientTimestamp,
+) -> Result<Vec<ClientEvent>, ClientError> {
+    target.runtime.ensure_open()?;
+    let remote = target.remote;
+    #[cfg(test)]
+    let reported_bytes = target.probe_reported_len.take();
+    #[cfg(test)]
+    let fail_send = std::mem::take(&mut target.probe_send_error);
+    let TargetState {
+        runtime, schedule, ..
+    } = target;
+    let schedule = schedule
+        .as_mut()
+        .expect("active targets always have a probe schedule");
+    if !schedule.permit_probe_at(sent_at.mono) {
+        return Ok(Vec::new());
+    }
+
+    let Some(prepared) = runtime.prepare_probe()? else {
+        return Ok(Vec::new());
+    };
+    let machine_commit = runtime.preflight_probe_commit(&prepared, sent_at)?;
+    let schedule_commit = schedule.preflight_managed_commit(scheduled_at, sent_at.mono)?;
+    let mut events = Vec::new();
+    events
+        .try_reserve(1)
+        .map_err(|source| ClientError::AllocationFailed {
+            operation: "managed probe event result",
+            source,
+        })?;
+
+    let expected_bytes = prepared.bytes.len();
+    let event_scheduled_at = schedule_commit.scheduled_at;
+    let timer_error = schedule_commit.timer_error;
+    #[cfg(test)]
+    if fail_send {
+        return Err(ClientError::Socket(std::io::Error::other(
+            "injected managed probe send failure",
+        )));
+    }
+    let send_call_start = Instant::now();
+    let bytes = socket.send_to(&prepared.bytes, remote)?;
+    let sent = runtime.commit_probe_sent(machine_commit, bytes);
+    schedule.commit(schedule_commit);
+
+    let send_call = send_call_start.elapsed();
+    #[cfg(test)]
+    let bytes = reported_bytes.unwrap_or(bytes);
+    validate_datagram_length(expected_bytes, bytes)?;
+    events.push(echo_sent_event(
+        remote,
+        sent,
+        send_call,
+        event_scheduled_at,
+        timer_error,
+    ));
+    Ok(events)
+}
+
 fn collect_finished_targets(shared: &GroupShared, records: &mut TargetOutcomeHistory) {
     let finished = {
         let mut registry = shared
@@ -1853,23 +1915,29 @@ mod tests {
         let scheduled_at = Instant::now();
         let sent_at = ClientTimestamp::now();
         let sent_events = {
+            let remote = target.remote;
             let TargetState {
                 runtime, schedule, ..
             } = &mut target;
-            runtime.send_probe_for_deadline(
-                schedule.as_mut().unwrap(),
-                Some(sent_at),
-                scheduled_at,
-                |packet| {
-                    Ok(SendProbeResult {
-                        sent_at,
-                        bytes: packet.len(),
-                        send_call: Duration::ZERO,
-                    })
-                },
-            )
-        }
-        .unwrap();
+            let schedule = schedule.as_mut().unwrap();
+            assert!(schedule.permit_probe_at(sent_at.mono));
+            let prepared = runtime.prepare_probe().unwrap().unwrap();
+            let machine_commit = runtime.preflight_probe_commit(&prepared, sent_at).unwrap();
+            let schedule_commit = schedule
+                .preflight_managed_commit(scheduled_at, sent_at.mono)
+                .unwrap();
+            let event_scheduled_at = schedule_commit.scheduled_at;
+            let timer_error = schedule_commit.timer_error;
+            let sent = runtime.commit_probe_sent(machine_commit, prepared.bytes.len());
+            schedule.commit(schedule_commit);
+            vec![echo_sent_event(
+                remote,
+                sent,
+                Duration::ZERO,
+                event_scheduled_at,
+                timer_error,
+            )]
+        };
         publish_events(&hub, &mut target, sent_events);
 
         ActiveTargetFixture {
@@ -1934,6 +2002,29 @@ mod tests {
         }
     }
 
+    fn active_probe_target(
+        config: &ClientConfig,
+        params: &Params,
+        remote: SocketAddr,
+        opened_at: ClientTimestamp,
+    ) -> TargetState {
+        let mut target =
+            TargetState::new(config, target("peer", remote), 0, opened_at.mono).unwrap();
+        let packet = open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, params);
+        let reply = target.runtime.decode_open_reply(&packet).unwrap();
+        let mut schedule = None;
+        target
+            .runtime
+            .accept_open_reply(reply, opened_at, |negotiated| {
+                schedule = Some(ProbeSchedule::new(opened_at.mono, negotiated)?);
+                Ok(())
+            })
+            .unwrap();
+        target.schedule = schedule;
+        target.status = TargetStatus::Active;
+        target
+    }
+
     fn target_outcome(
         id: &str,
         remote: SocketAddr,
@@ -2883,6 +2974,137 @@ mod tests {
             .all(|target| target.end_reason.failure().is_some()));
     }
 
+    #[test]
+    fn group_probe_uses_one_timestamp_for_permission_and_schedule_commit() {
+        let duration = Duration::from_secs(1);
+        let interval = Duration::from_millis(100);
+        let params = test_params(Some(duration), interval);
+        let config = group_config(Some(duration), interval, ManagedGroupPacing::Staggered).client;
+        let peer = UdpSocket::bind("127.0.0.1:0").unwrap();
+        let remote = peer.local_addr().unwrap();
+        let opened_mono = Instant::now()
+            .checked_sub(Duration::from_secs(2))
+            .expect("test host monotonic clock has at least two seconds of history");
+        let opened_at = ClientTimestamp {
+            wall: std::time::SystemTime::now(),
+            mono: opened_mono,
+        };
+        let mut target = active_probe_target(&config, &params, remote, opened_at);
+        let sent_at = ClientTimestamp {
+            wall: std::time::SystemTime::now(),
+            mono: opened_mono + Duration::from_millis(500),
+        };
+        let socket = UdpSocket::bind("127.0.0.1:0").unwrap();
+
+        let events =
+            send_echo_to_locked_target_at(&socket, &mut target, opened_mono, sent_at).unwrap();
+
+        assert!(matches!(
+            events.as_slice(),
+            [ClientEvent::EchoSent {
+                scheduled_at,
+                sent_at: event_sent_at,
+                timer_error,
+                ..
+            }] if *scheduled_at == sent_at.mono
+                && *event_sent_at == sent_at
+                && *timer_error == Duration::ZERO
+        ));
+        assert_eq!(
+            target
+                .schedule
+                .as_ref()
+                .and_then(ProbeSchedule::next_send_deadline),
+            Some(opened_mono + Duration::from_millis(600))
+        );
+        assert_eq!(target.runtime.packets_sent(), 1);
+    }
+
+    #[test]
+    fn group_failed_probe_send_preserves_machine_and_schedule() {
+        let duration = Duration::from_secs(1);
+        let interval = Duration::from_millis(100);
+        let params = test_params(Some(duration), interval);
+        let config = group_config(Some(duration), interval, ManagedGroupPacing::Staggered).client;
+        let peer = UdpSocket::bind("127.0.0.1:0").unwrap();
+        let remote = peer.local_addr().unwrap();
+        let opened_mono = Instant::now()
+            .checked_sub(Duration::from_secs(2))
+            .expect("test host monotonic clock has at least two seconds of history");
+        let opened_at = ClientTimestamp {
+            wall: std::time::SystemTime::now(),
+            mono: opened_mono,
+        };
+        let mut target = active_probe_target(&config, &params, remote, opened_at);
+        let sent_at = ClientTimestamp {
+            wall: std::time::SystemTime::now(),
+            mono: opened_mono + Duration::from_millis(500),
+        };
+        let socket = UdpSocket::bind("127.0.0.1:0").unwrap();
+        target.probe_send_error = true;
+
+        assert!(matches!(
+            send_echo_to_locked_target_at(&socket, &mut target, opened_mono, sent_at),
+            Err(ClientError::Socket(_))
+        ));
+        assert_eq!(target.runtime.packets_sent(), 0);
+        assert_eq!(
+            target
+                .schedule
+                .as_ref()
+                .and_then(ProbeSchedule::next_send_deadline),
+            Some(opened_mono)
+        );
+
+        assert!(matches!(
+            send_echo_to_locked_target_at(&socket, &mut target, opened_mono, sent_at)
+                .unwrap()
+                .as_slice(),
+            [ClientEvent::EchoSent { seq: 0, .. }]
+        ));
+    }
+
+    #[test]
+    fn group_short_probe_commits_before_presentation_length_error() {
+        let duration = Duration::from_secs(1);
+        let interval = Duration::from_millis(100);
+        let params = test_params(Some(duration), interval);
+        let config = group_config(Some(duration), interval, ManagedGroupPacing::Staggered).client;
+        let peer = UdpSocket::bind("127.0.0.1:0").unwrap();
+        let remote = peer.local_addr().unwrap();
+        let opened_mono = Instant::now()
+            .checked_sub(Duration::from_secs(2))
+            .expect("test host monotonic clock has at least two seconds of history");
+        let opened_at = ClientTimestamp {
+            wall: std::time::SystemTime::now(),
+            mono: opened_mono,
+        };
+        let mut target = active_probe_target(&config, &params, remote, opened_at);
+        let sent_at = ClientTimestamp {
+            wall: std::time::SystemTime::now(),
+            mono: opened_mono + Duration::from_millis(500),
+        };
+        let expected = echo_packet_len(false, &params).unwrap();
+        target.probe_reported_len = Some(expected - 1);
+        let socket = UdpSocket::bind("127.0.0.1:0").unwrap();
+
+        assert!(matches!(
+            send_echo_to_locked_target_at(&socket, &mut target, opened_mono, sent_at),
+            Err(ClientError::DatagramLengthMismatch {
+                expected: error_expected,
+                actual,
+            }) if error_expected == expected && actual + 1 == expected
+        ));
+        assert_eq!(target.runtime.packets_sent(), 1);
+        assert_eq!(
+            target
+                .schedule
+                .as_ref()
+                .and_then(ProbeSchedule::next_send_deadline),
+            Some(opened_mono + Duration::from_millis(600))
+        );
+    }
+
     #[test]
     fn silent_opening_peer_times_out_all_attempts_and_publishes_once() {
         let silent_peer = UdpSocket::bind("127.0.0.1:0").unwrap();
diff --git a/crates/irtt-client/src/probe.rs b/crates/irtt-client/src/probe.rs
index 26ae1b2..c14916b 100644
--- a/crates/irtt-client/src/probe.rs
+++ b/crates/irtt-client/src/probe.rs
@@ -72,13 +72,13 @@ impl PendingMap {
     }
 
     #[cfg(test)]
-    fn capacity(&self) -> usize {
-        self.map.capacity()
+    pub fn contains(&self, wire_seq: u32) -> bool {
+        self.map.contains_key(&wire_seq)
     }
 
     #[cfg(test)]
-    fn contains(&self, wire_seq: u32) -> bool {
-        self.map.contains_key(&wire_seq)
+    pub fn capacity(&self) -> usize {
+        self.map.capacity()
     }
 }
 
@@ -139,6 +139,11 @@ impl TimedOutMap {
         self.map.len()
     }
 
+    #[cfg(test)]
+    pub fn contains(&self, wire_seq: u32) -> bool {
+        self.map.contains_key(&wire_seq)
+    }
+
     #[cfg(test)]
     fn insertion_order_len(&self) -> usize {
         self.insertion_order.len()
@@ -186,6 +191,14 @@ impl CompletedSet {
         self.set.contains(&seq)
     }
 
+    pub fn remove(&mut self, seq: u32) -> bool {
+        let removed = self.set.remove(&seq);
+        if removed {
+            self.insertion_order.retain(|entry| *entry != seq);
+        }
+        removed
+    }
+
     fn evict_oldest(&mut self) {
         while let Some(oldest_seq) = self.insertion_order.pop_front() {
             if self.set.remove(&oldest_seq) {
diff --git a/crates/irtt-client/src/session/machine.rs b/crates/irtt-client/src/session/machine.rs
index ba56f41..e60fa46 100644
--- a/crates/irtt-client/src/session/machine.rs
+++ b/crates/irtt-client/src/session/machine.rs
@@ -7,7 +7,6 @@ use irtt_proto::{
 };
 
 use crate::{
-    client::schedule::ProbeSchedule,
     config::{
         ClientConfig, RunMode, MAX_DSCP_CODEPOINT, MAX_SERVER_FILL_BYTES, MAX_UDP_PAYLOAD_LENGTH,
     },
@@ -62,11 +61,24 @@ struct ActiveSession {
     completed: CompletedSet,
 }
 
+#[derive(Debug)]
+pub(crate) struct PreparedProbe {
+    pub(crate) bytes: Box<[u8]>,
+    pub(crate) seq: u32,
+}
+
+#[derive(Debug)]
+pub(crate) struct ProbeCommit {
+    pending: PendingProbe,
+    next_wire_seq: u32,
+    pub(crate) next_packets_sent: u64,
+}
+
 #[derive(Debug, Clone, Copy)]
-pub(crate) struct SendProbeResult {
+pub(crate) struct ProbeSent {
+    pub(crate) seq: u32,
     pub(crate) sent_at: ClientTimestamp,
     pub(crate) bytes: usize,
-    pub(crate) send_call: Duration,
 }
 
 impl SessionMachine {
@@ -160,110 +172,79 @@ impl SessionMachine {
         self.config.probe_timeout
     }
 
-    pub(crate) fn send_probe_with<F>(
-        &mut self,
-        schedule: &mut ProbeSchedule,
-        override_ts: Option<ClientTimestamp>,
-        send: F,
-    ) -> Result<Vec<ClientEvent>, ClientError>
-    where
-        F: FnOnce(&[u8]) -> Result<SendProbeResult, ClientError>,
-    {
-        self.send_probe_inner(schedule, override_ts, None, false, send)
-    }
-
-    pub(crate) fn send_probe_for_deadline<F>(
-        &mut self,
-        schedule: &mut ProbeSchedule,
-        override_ts: Option<ClientTimestamp>,
-        scheduled_at: Instant,
-        send: F,
-    ) -> Result<Vec<ClientEvent>, ClientError>
-    where
-        F: FnOnce(&[u8]) -> Result<SendProbeResult, ClientError>,
-    {
-        self.send_probe_inner(schedule, override_ts, Some(scheduled_at), true, send)
-    }
-
-    fn send_probe_inner<F>(
-        &mut self,
-        schedule: &mut ProbeSchedule,
-        override_ts: Option<ClientTimestamp>,
-        scheduled_at_override: Option<Instant>,
-        skip_missed_slots: bool,
-        send: F,
-    ) -> Result<Vec<ClientEvent>, ClientError>
-    where
-        F: FnOnce(&[u8]) -> Result<SendProbeResult, ClientError>,
-    {
-        let now = override_ts.unwrap_or_else(ClientTimestamp::now);
-        if !schedule.permit_probe_at(now.mono) {
-            return Ok(vec![]);
-        }
-
-        let (config, state) = (&self.config, &mut self.state);
-        let probe_timeout = config.probe_timeout;
-        let hmac_key = config.hmac_key.as_deref();
-        let session = match state {
-            MachineState::Open(session) => session,
-            MachineState::Closed { .. } => return Err(ClientError::AlreadyClosed),
-            MachineState::Connected => return Err(ClientError::NotOpen),
-            MachineState::NoTestCompleted => return Err(ClientError::AlreadyCompleted),
-        };
-
-        session.pending.preflight_insert(session.next_wire_seq)?;
-
-        let wire_seq = session.next_wire_seq;
-
+    pub(crate) fn prepare_probe(&self) -> Result<Option<PreparedProbe>, ClientError> {
+        let session = self.open_session()?;
         let request = EchoRequest {
             token: session.token,
-            sequence: wire_seq,
+            sequence: session.next_wire_seq,
             payload: vec![],
         };
-        let packet = encode_echo_request(&request, &session.negotiated.params, hmac_key)?;
-        let send_result = send(&packet)?;
+        let bytes = encode_echo_request(
+            &request,
+            &session.negotiated.params,
+            self.config.hmac_key.as_deref(),
+        )?;
+        Ok(Some(PreparedProbe {
+            bytes: bytes.into_boxed_slice(),
+            seq: session.next_wire_seq,
+        }))
+    }
 
-        let pending = PendingProbe {
-            wire_seq,
-            sent_at: send_result.sent_at,
-            timeout_at: send_result
-                .sent_at
-                .mono
-                .checked_add(probe_timeout)
-                .ok_or(ClientError::DurationOverflow)?,
-        };
-        session.pending.commit_insert(pending);
-
-        session.next_wire_seq = session.next_wire_seq.wrapping_add(1);
-        session.packets_sent =
+    pub(crate) fn preflight_probe_commit(
+        &mut self,
+        prepared: &PreparedProbe,
+        sent_at: ClientTimestamp,
+    ) -> Result<ProbeCommit, ClientError> {
+        let probe_timeout = self.config.probe_timeout;
+        let session = self.open_session_mut()?;
+        if prepared.seq != session.next_wire_seq {
+            return Err(ClientError::StalePreparedProbe {
+                prepared_seq: prepared.seq,
+                next_wire_seq: session.next_wire_seq,
+            });
+        }
+        session.pending.preflight_insert(prepared.seq)?;
+        let next_packets_sent =
             session
                 .packets_sent
                 .checked_add(1)
                 .ok_or(ClientError::CounterOverflow {
                     counter: "packets_sent",
                 })?;
+        let timeout_at = sent_at
+            .mono
+            .checked_add(probe_timeout)
+            .ok_or(ClientError::DurationOverflow)?;
 
-        let schedule_commit = if skip_missed_slots {
-            schedule.preflight_managed_commit(
-                scheduled_at_override.expect("managed sends include a scheduled deadline"),
-                send_result.sent_at.mono,
-            )?
-        } else {
-            schedule.preflight_caller_commit(send_result.sent_at.mono, session.packets_sent)?
+        Ok(ProbeCommit {
+            pending: PendingProbe {
+                wire_seq: prepared.seq,
+                sent_at,
+                timeout_at,
+            },
+            next_wire_seq: prepared.seq.wrapping_add(1),
+            next_packets_sent,
+        })
+    }
+
+    pub(crate) fn commit_probe_sent(&mut self, commit: ProbeCommit, bytes: usize) -> ProbeSent {
+        let session = match &mut self.state {
+            MachineState::Open(session) => session,
+            _ => unreachable!("probe commits are only created for an open session"),
         };
-        let scheduled_at = schedule_commit.scheduled_at;
-        let timer_error = schedule_commit.timer_error;
-        schedule.commit(schedule_commit);
+        let seq = commit.pending.wire_seq;
+        let sent_at = commit.pending.sent_at;
+        session.timed_out.remove(seq);
+        session.completed.remove(seq);
+        session.pending.commit_insert(commit.pending);
+        session.next_wire_seq = commit.next_wire_seq;
+        session.packets_sent = commit.next_packets_sent;
 
-        Ok(vec![ClientEvent::EchoSent {
-            seq: wire_seq,
-            remote: self.remote,
-            scheduled_at,
-            sent_at: send_result.sent_at,
-            bytes: send_result.bytes,
-            send_call: send_result.send_call,
-            timer_error,
-        }])
+        ProbeSent {
+            seq,
+            sent_at,
+            bytes,
+        }
     }
 
     pub(crate) fn process_received_echo_packet(
@@ -830,3 +811,320 @@ fn config_duration_to_ns(field: &str, duration: Duration) -> Result<i64, ClientE
         reason: format!("{field} is too large to encode as nanoseconds"),
     })
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::client::{schedule::ProbeSchedule, validate_datagram_length};
+
+    fn open_machine(max_pending_probes: usize, probe_timeout: Duration) -> SessionMachine {
+        let config = ClientConfig {
+            max_pending_probes,
+            probe_timeout,
+            ..ClientConfig::default()
+        };
+        let remote = "127.0.0.1:2112".parse().unwrap();
+        let mut machine = SessionMachine::new(config, remote).unwrap();
+        let negotiated = NegotiatedParams {
+            params: machine.requested.clone(),
+            restrictions: Vec::new(),
+        };
+        machine.state = MachineState::Open(Box::new(ActiveSession {
+            token: 0x0102_0304_0506_0708,
+            negotiated,
+            next_wire_seq: 0,
+            highest_received_seq: None,
+            packets_sent: 0,
+            pending: PendingMap::new(max_pending_probes),
+            timed_out: TimedOutMap::new(max_pending_probes),
+            completed: CompletedSet::new(max_pending_probes),
+        }));
+        machine
+    }
+
+    fn active(machine: &SessionMachine) -> &ActiveSession {
+        match &machine.state {
+            MachineState::Open(session) => session,
+            _ => panic!("test machine must be open"),
+        }
+    }
+
+    fn active_mut(machine: &mut SessionMachine) -> &mut ActiveSession {
+        match &mut machine.state {
+            MachineState::Open(session) => session,
+            _ => panic!("test machine must be open"),
+        }
+    }
+
+    fn timestamp(mono: Instant) -> ClientTimestamp {
+        ClientTimestamp {
+            mono,
+            wall: SystemTime::now(),
+        }
+    }
+
+    #[test]
+    fn dropping_prepared_probe_changes_nothing() {
+        let machine = open_machine(4, Duration::from_secs(1));
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+        assert_eq!(prepared.seq, 0);
+        drop(prepared);
+
+        let session = active(&machine);
+        assert_eq!(session.next_wire_seq, 0);
+        assert_eq!(session.packets_sent, 0);
+        assert_eq!(session.pending.len(), 0);
+    }
+
+    #[test]
+    fn repeated_uncommitted_preflight_changes_no_logical_state() {
+        let mut machine = open_machine(4, Duration::from_secs(1));
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+        let sent_at = timestamp(Instant::now());
+
+        let _first_preflight = machine.preflight_probe_commit(&prepared, sent_at).unwrap();
+        let _second_preflight = machine.preflight_probe_commit(&prepared, sent_at).unwrap();
+
+        let session = active(&machine);
+        assert_eq!(session.next_wire_seq, 0);
+        assert_eq!(session.packets_sent, 0);
+        assert_eq!(session.pending.len(), 0);
+    }
+
+    #[test]
+    fn stale_prepared_probe_is_rejected_without_changing_state() {
+        let mut machine = open_machine(4, Duration::from_secs(1));
+        let stale = machine.prepare_probe().unwrap().unwrap();
+        let accepted = machine.prepare_probe().unwrap().unwrap();
+        let sent_at = timestamp(Instant::now());
+        let commit = machine.preflight_probe_commit(&accepted, sent_at).unwrap();
+        machine.commit_probe_sent(commit, accepted.bytes.len());
+
+        let capacity = active(&machine).pending.capacity();
+        assert!(matches!(
+            machine.preflight_probe_commit(&stale, sent_at),
+            Err(ClientError::StalePreparedProbe {
+                prepared_seq: 0,
+                next_wire_seq: 1,
+            })
+        ));
+
+        let session = active(&machine);
+        assert_eq!(session.next_wire_seq, 1);
+        assert_eq!(session.packets_sent, 1);
+        assert_eq!(session.pending.len(), 1);
+        assert_eq!(session.pending.capacity(), capacity);
+    }
+
+    #[test]
+    fn simulated_send_error_leaves_machine_and_schedule_unchanged() {
+        let mut machine = open_machine(4, Duration::from_secs(1));
+        let start = Instant::now();
+        let mut schedule = ProbeSchedule::new(start, &active(&machine).negotiated).unwrap();
+        assert!(schedule.permit_probe_at(start));
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+        let sent_at = timestamp(start);
+        let machine_commit = machine.preflight_probe_commit(&prepared, sent_at).unwrap();
+        let schedule_commit = schedule
+            .preflight_caller_commit(sent_at.mono, machine_commit.next_packets_sent)
+            .unwrap();
+
+        let _uncommitted_machine = machine_commit;
+        let _uncommitted_schedule = schedule_commit;
+
+        let session = active(&machine);
+        assert_eq!(session.next_wire_seq, 0);
+        assert_eq!(session.packets_sent, 0);
+        assert_eq!(session.pending.len(), 0);
+        assert_eq!(schedule.next_send_deadline(), Some(start));
+    }
+
+    #[test]
+    fn short_success_commits_before_reporting_transport_invariant() {
+        let mut machine = open_machine(4, Duration::from_secs(1));
+        let start = Instant::now();
+        let mut schedule = ProbeSchedule::new(start, &active(&machine).negotiated).unwrap();
+        assert!(schedule.permit_probe_at(start));
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+        let machine_commit = machine
+            .preflight_probe_commit(&prepared, timestamp(start))
+            .unwrap();
+        let schedule_commit = schedule.preflight_managed_commit(start, start).unwrap();
+        let expected = prepared.bytes.len();
+        let actual = expected - 1;
+
+        machine.commit_probe_sent(machine_commit, actual);
+        schedule.commit(schedule_commit);
+        let result = validate_datagram_length(expected, actual);
+
+        assert!(matches!(
+            result,
+            Err(ClientError::DatagramLengthMismatch { expected, actual })
+                if actual + 1 == expected
+        ));
+        let session = active(&machine);
+        assert_eq!(session.next_wire_seq, 1);
+        assert_eq!(session.packets_sent, 1);
+        assert_eq!(session.pending.len(), 1);
+        assert_eq!(
+            schedule.next_send_deadline(),
+            Some(start + machine.config.interval)
+        );
+    }
+
+    #[test]
+    fn repeated_would_block_style_preflight_commits_once() {
+        let mut machine = open_machine(4, Duration::from_secs(1));
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+        let sent_at = timestamp(Instant::now());
+
+        for _ in 0..3 {
+            let _would_block_commit = machine.preflight_probe_commit(&prepared, sent_at).unwrap();
+        }
+        let commit = machine.preflight_probe_commit(&prepared, sent_at).unwrap();
+        let sent = machine.commit_probe_sent(commit, prepared.bytes.len());
+
+        assert_eq!(sent.seq, 0);
+        let session = active(&machine);
+        assert_eq!(session.next_wire_seq, 1);
+        assert_eq!(session.packets_sent, 1);
+        assert_eq!(session.pending.len(), 1);
+    }
+
+    #[test]
+    fn probe_commit_does_not_require_presentation_timing() {
+        let mut machine = open_machine(4, Duration::from_secs(1));
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+        let sent_at = timestamp(Instant::now());
+        let commit = machine.preflight_probe_commit(&prepared, sent_at).unwrap();
+
+        let sent = machine.commit_probe_sent(commit, prepared.bytes.len());
+
+        assert_eq!(sent.seq, prepared.seq);
+        assert_eq!(sent.sent_at, sent_at);
+        assert_eq!(sent.bytes, prepared.bytes.len());
+    }
+
+    #[test]
+    fn commit_uses_reserved_capacity_and_prevalidated_counter() {
+        let mut machine = open_machine(2, Duration::from_secs(1));
+        active_mut(&mut machine).packets_sent = u64::MAX - 1;
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+        let commit = machine
+            .preflight_probe_commit(&prepared, timestamp(Instant::now()))
+            .unwrap();
+        let reserved_capacity = active(&machine).pending.capacity();
+
+        machine.commit_probe_sent(commit, prepared.bytes.len());
+
+        let session = active(&machine);
+        assert_eq!(session.pending.capacity(), reserved_capacity);
+        assert_eq!(session.pending.len(), 1);
+        assert_eq!(session.packets_sent, u64::MAX);
+        assert_eq!(session.next_wire_seq, 1);
+    }
+
+    #[test]
+    fn pending_capacity_exhaustion_is_detected_before_commit() {
+        let mut machine = open_machine(1, Duration::from_secs(1));
+        let first = machine.prepare_probe().unwrap().unwrap();
+        let sent_at = timestamp(Instant::now());
+        let commit = machine.preflight_probe_commit(&first, sent_at).unwrap();
+        machine.commit_probe_sent(commit, first.bytes.len());
+
+        let second = machine.prepare_probe().unwrap().unwrap();
+        assert!(matches!(
+            machine.preflight_probe_commit(&second, sent_at),
+            Err(ClientError::PendingLimitExceeded { limit: 1 })
+        ));
+        assert_eq!(active(&machine).packets_sent, 1);
+    }
+
+    #[test]
+    fn pending_sequence_collision_is_detected_before_commit() {
+        let mut machine = open_machine(2, Duration::from_secs(1));
+        let first = machine.prepare_probe().unwrap().unwrap();
+        let sent_at = timestamp(Instant::now());
+        let commit = machine.preflight_probe_commit(&first, sent_at).unwrap();
+        machine.commit_probe_sent(commit, first.bytes.len());
+        active_mut(&mut machine).next_wire_seq = 0;
+
+        let reused = machine.prepare_probe().unwrap().unwrap();
+        assert!(matches!(
+            machine.preflight_probe_commit(&reused, sent_at),
+            Err(ClientError::PendingSequenceCollision { seq: 0 })
+        ));
+        assert_eq!(active(&machine).packets_sent, 1);
+    }
+
+    #[test]
+    fn counter_overflow_is_detected_before_commit() {
+        let mut machine = open_machine(2, Duration::from_secs(1));
+        active_mut(&mut machine).packets_sent = u64::MAX;
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+
+        assert!(matches!(
+            machine.preflight_probe_commit(&prepared, timestamp(Instant::now())),
+            Err(ClientError::CounterOverflow {
+                counter: "packets_sent"
+            })
+        ));
+        assert_eq!(active(&machine).pending.len(), 0);
+    }
+
+    #[test]
+    fn timeout_overflow_is_detected_before_commit() {
+        let mut machine = open_machine(2, Duration::MAX);
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+
+        assert!(matches!(
+            machine.preflight_probe_commit(&prepared, timestamp(Instant::now())),
+            Err(ClientError::DurationOverflow)
+        ));
+        assert_eq!(active(&machine).pending.len(), 0);
+    }
+
+    #[test]
+    fn wrapping_sequence_from_max_to_zero_remains_valid() {
+        let mut machine = open_machine(2, Duration::from_secs(1));
+        active_mut(&mut machine).next_wire_seq = u32::MAX;
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+        let commit = machine
+            .preflight_probe_commit(&prepared, timestamp(Instant::now()))
+            .unwrap();
+        machine.commit_probe_sent(commit, prepared.bytes.len());
+
+        assert_eq!(active(&machine).next_wire_seq, 0);
+        assert_eq!(machine.prepare_probe().unwrap().unwrap().seq, 0);
+    }
+
+    #[test]
+    fn successful_wrapped_reuse_purges_obsolete_history_only_on_commit() {
+        let mut machine = open_machine(3, Duration::from_secs(1));
+        let now = Instant::now();
+        let obsolete = PendingProbe {
+            wire_seq: 0,
+            sent_at: timestamp(now - Duration::from_secs(1)),
+            timeout_at: now,
+        };
+        let session = active_mut(&mut machine);
+        session.next_wire_seq = 0;
+        session.timed_out.insert(obsolete);
+        session.completed.insert(0);
+
+        let prepared = machine.prepare_probe().unwrap().unwrap();
+        let commit = machine
+            .preflight_probe_commit(&prepared, timestamp(now))
+            .unwrap();
+        assert!(active(&machine).timed_out.contains(0));
+        assert!(active(&machine).completed.contains(0));
+
+        machine.commit_probe_sent(commit, prepared.bytes.len());
+        let session = active(&machine);
+        assert!(!session.timed_out.contains(0));
+        assert!(!session.completed.contains(0));
+        assert!(session.pending.contains(0));
+        assert_eq!(session.next_wire_seq, 1);
+        assert_eq!(session.packets_sent, 1);
+    }
+}