diff --git a/jcp/jcp-rs/crates/jcp-gateway/tests/stdio_binary.rs b/jcp/jcp-rs/crates/jcp-gateway/tests/stdio_binary.rs
index 8677d904e..82b306373 100644
--- a/jcp/jcp-rs/crates/jcp-gateway/tests/stdio_binary.rs
+++ b/jcp/jcp-rs/crates/jcp-gateway/tests/stdio_binary.rs
@@ -6,7 +6,7 @@
 //!
 //! The server is awk, answering only the lines that carry an `id`, as an MCP
 //! server answers requests and never notifications. It runs on every host
-//! this crate builds on.
+//! this crate builds on, whichever awk the host has ([`awk_server`]).
 
 use serde_json::{json, Value};
 use std::io::{BufRead, BufReader, Read, Write};
@@ -18,6 +18,33 @@ use std::time::Duration;
 /// call takes, on a host at a load of 14 to 260.
 const LINE_DEADLINE: Duration = Duration::from_secs(120);
 
+/// The command line of an awk server running `script` over a pipe.
+///
+/// An awk reading a pipe must hand each line to the script as it arrives, or
+/// the server never answers a request until its client closes the pipe. mawk,
+/// the default awk on Debian and Ubuntu hosts, buffers piped input in blocks
+/// unless it is run with `-W interactive`; the other awks read lines as they
+/// come and reject or ignore that option. So the awk is asked what it is: a
+/// host whose awk is mawk once failed both tests at their deadline, while the
+/// same tests passed where the awk was another.
+fn awk_server(script: &str) -> Vec<String> {
+    const AWK: &str = "/usr/bin/awk";
+    let is_mawk = Command::new(AWK)
+        .args(["-W", "version"])
+        .stdin(Stdio::null())
+        .output()
+        .map(|o| {
+            String::from_utf8_lossy(&o.stdout).contains("mawk") || String::from_utf8_lossy(&o.stderr).contains("mawk")
+        })
+        .unwrap_or(false);
+    let mut argv = vec![AWK.to_string()];
+    if is_mawk {
+        argv.extend(["-W".to_string(), "interactive".to_string()]);
+    }
+    argv.push(script.to_string());
+    argv
+}
+
 /// An MCP-ish server in awk: a request (a line with an `id`) gets one
 /// response carrying that id; a line without one gets nothing.
 const ANSWERS_REQUESTS_ONLY: &str = r#"/"id"/ {
@@ -111,7 +138,7 @@ fn call(id: i64) -> Value {
 /// without waiting for a reply, or it never reads another line.
 #[test]
 fn the_binary_serves_calls_after_the_initialized_notification() {
-    let mut g = Gateway::spawn("smoke", &["/usr/bin/awk", ANSWERS_REQUESTS_ONLY], &[]);
+    let mut g = Gateway::spawn("smoke", &awk_server(ANSWERS_REQUESTS_ONLY).iter().map(String::as_str).collect::<Vec<_>>(), &[]);
     g.send(&json!({"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{}}}));
     let init = g.next("the initialize response");
     assert_eq!(init["id"], json!(1), "{init}");
@@ -166,7 +193,11 @@ const TALKS_MID_CALL: &str = r#"/"method":"tools\/call"/ {
 /// the call's id that ends its span.
 #[test]
 fn the_binary_relays_what_a_server_sends_in_the_middle_of_a_call() {
-    let mut g = Gateway::spawn("chatty", &["/usr/bin/awk", TALKS_MID_CALL], &["--upstream-self-contained"]);
+    let mut g = Gateway::spawn(
+        "chatty",
+        &awk_server(TALKS_MID_CALL).iter().map(String::as_str).collect::<Vec<_>>(),
+        &["--upstream-self-contained"],
+    );
     g.send(&json!({"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{}}}));
     assert_eq!(g.next("the initialize response")["id"], json!(1));
     g.send(&json!({"jsonrpc":"2.0","method":"notifications/initialized"}));