diff --git a/crates/irtt-client/src/client/tests/mod.rs b/crates/irtt-client/src/client/tests/mod.rs
index 8212f33..7e4ebc8 100644
--- a/crates/irtt-client/src/client/tests/mod.rs
+++ b/crates/irtt-client/src/client/tests/mod.rs
@@ -1,8 +1,8 @@
 use super::*;
 use crate::config::{NegotiationPolicy, DEFAULT_OPEN_TIMEOUTS};
 use irtt_proto::{
-    echo_packet_len, flags::FLAG_HMAC, flags::FLAG_OPEN, flags::FLAG_REPLY, verify_hmac, Clock,
-    ReceivedStats, StampAt, HMAC_SIZE, MAGIC,
+    flags::FLAG_HMAC, flags::FLAG_OPEN, flags::FLAG_REPLY, verify_hmac, Clock, ReceivedStats,
+    StampAt, HMAC_SIZE, MAGIC,
 };
 use std::{thread, time::SystemTime};
 
diff --git a/crates/irtt-client/src/client/tests/probes_replies.rs b/crates/irtt-client/src/client/tests/probes_replies.rs
index c4daaca..aa5a1f7 100644
--- a/crates/irtt-client/src/client/tests/probes_replies.rs
+++ b/crates/irtt-client/src/client/tests/probes_replies.rs
@@ -1,4 +1,5 @@
 use super::*;
+use support::echo_packet_len;
 
 #[cfg(not(all(target_os = "linux", feature = "ancillary")))]
 fn assert_packet_meta_unavailable(packet_meta: &crate::event::PacketMeta) {
diff --git a/crates/irtt-client/src/client/tests/support.rs b/crates/irtt-client/src/client/tests/support.rs
index 27a8f60..a3cddd7 100644
--- a/crates/irtt-client/src/client/tests/support.rs
+++ b/crates/irtt-client/src/client/tests/support.rs
@@ -1,7 +1,8 @@
 use super::*;
 use irtt_proto::{
-    compute_hmac_in_place, echo_packet_len, flags::FLAG_HMAC, flags::FLAG_OPEN, flags::FLAG_REPLY,
-    layout::PacketLayout, Clock, ReceivedStats, StampAt, HMAC_SIZE, MAGIC,
+    compute_hmac_in_place, echo_packet_len as checked_echo_packet_len, flags::FLAG_HMAC,
+    flags::FLAG_OPEN, flags::FLAG_REPLY, layout::PacketLayout, Clock, ReceivedStats, StampAt,
+    HMAC_SIZE, MAGIC,
 };
 use std::{
     net::UdpSocket,
@@ -74,6 +75,11 @@ pub(super) fn open_reply(
     packet
 }
 
+pub(crate) fn echo_packet_len(hmac: bool, params: &Params) -> usize {
+    checked_echo_packet_len(hmac, params)
+        .expect("test params must have a non-negative packet length")
+}
+
 pub(super) fn echo_reply_packet(
     token: u64,
     seq: u32,
diff --git a/crates/irtt-client/src/managed/runner.rs b/crates/irtt-client/src/managed/runner.rs
index 3fc15db..7df4742 100644
--- a/crates/irtt-client/src/managed/runner.rs
+++ b/crates/irtt-client/src/managed/runner.rs
@@ -311,8 +311,10 @@ mod tests {
     use super::*;
     use crate::{config::NegotiationPolicy, SubscriberOverflow};
     use irtt_proto::{
-        echo_packet_len, flags, flags::FLAG_OPEN, flags::FLAG_REPLY, layout::PacketLayout, Clock,
-        Params, ReceivedStats, StampAt, TimestampFields, MAGIC, PROTOCOL_VERSION,
+        echo_packet_len,
+        flags::{self, FLAG_OPEN, FLAG_REPLY},
+        layout::PacketLayout,
+        Clock, Params, ReceivedStats, StampAt, TimestampFields, MAGIC, PROTOCOL_VERSION,
     };
     use std::{
         net::{SocketAddr, UdpSocket},
@@ -331,6 +333,11 @@ mod tests {
         }
     }
 
+    fn test_echo_packet_len(hmac: bool, params: &Params) -> usize {
+        echo_packet_len(hmac, params)
+            .expect("managed runner test params must have a non-negative packet length")
+    }
+
     fn test_params(duration: Option<Duration>, interval: Duration) -> Params {
         Params {
             protocol_version: PROTOCOL_VERSION,
diff --git a/crates/irtt-client/tests/hmac.rs b/crates/irtt-client/tests/hmac.rs
index 42ddcb4..d1534a9 100644
--- a/crates/irtt-client/tests/hmac.rs
+++ b/crates/irtt-client/tests/hmac.rs
@@ -13,6 +13,8 @@ use support::{
     start_hmac_required_open_drop_server, BackendPeer, ServerObservation, TOKEN,
 };
 
+use crate::support::test_echo_packet_len;
+
 #[test]
 fn hmac_open_success_negotiates_without_warnings() {
     let key = b"compat-secret".to_vec();
diff --git a/crates/irtt-client/tests/large_packets.rs b/crates/irtt-client/tests/large_packets.rs
index aa06109..084ef65 100644
--- a/crates/irtt-client/tests/large_packets.rs
+++ b/crates/irtt-client/tests/large_packets.rs
@@ -1,10 +1,12 @@
 mod support;
 
 use irtt_client::{Client, ClientConfig, ClientEvent, NegotiationPolicy};
-use irtt_proto::{echo_header_len, echo_packet_len, Params, TimestampFields};
+use irtt_proto::{echo_header_len, Params, TimestampFields};
 
 use support::{config_for_params, default_params, run_one_probe, BackendPeer, ServerObservation};
 
+use crate::support::test_echo_packet_len;
+
 fn params_with_length(length: i64) -> Params {
     Params {
         length,
diff --git a/crates/irtt-client/tests/protocol_options.rs b/crates/irtt-client/tests/protocol_options.rs
index 6e1c12b..19e6a8a 100644
--- a/crates/irtt-client/tests/protocol_options.rs
+++ b/crates/irtt-client/tests/protocol_options.rs
@@ -14,6 +14,8 @@ use support::{
     ServerObservation, RECV_COUNT, RECV_WINDOW, TOKEN,
 };
 
+use crate::support::test_echo_packet_len;
+
 struct ReplyView<'a> {
     seq: u32,
     rtt: &'a RttSample,
diff --git a/crates/irtt-client/tests/server_fill.rs b/crates/irtt-client/tests/server_fill.rs
index 021f29f..1dfdb08 100644
--- a/crates/irtt-client/tests/server_fill.rs
+++ b/crates/irtt-client/tests/server_fill.rs
@@ -1,13 +1,15 @@
 mod support;
 
 use irtt_client::{Client, ClientConfig, ClientError, ClientEvent, NegotiationPolicy};
-use irtt_proto::{echo_packet_len, Params, TimestampFields};
+use irtt_proto::{Params, TimestampFields};
 
 use support::{
     config_for_params, default_params, run_one_probe, run_one_probe_with_config, server_fill,
     start_open_server, OneProbeRun, ServerObservation,
 };
 
+use crate::support::test_echo_packet_len;
+
 fn open_observation_params(run: &OneProbeRun) -> &Params {
     match &run.observations[0] {
         ServerObservation::Open { params, .. } => params,
diff --git a/crates/irtt-client/tests/support/mod.rs b/crates/irtt-client/tests/support/mod.rs
index 92e5d0f..4620c51 100644
--- a/crates/irtt-client/tests/support/mod.rs
+++ b/crates/irtt-client/tests/support/mod.rs
@@ -520,6 +520,11 @@ fn open_reply(flags: u8, token: u64, params: &Params, hmac_key: Option<&[u8]>) -
     packet
 }
 
+pub(super) fn test_echo_packet_len(hmac: bool, params: &Params) -> usize {
+    echo_packet_len(hmac, params)
+        .expect("managed runner test params must have a non-negative packet length")
+}
+
 fn echo_reply_packet(
     token: u64,
     seq: u32,