diff --git a/scripts/runner.node.mjs b/scripts/runner.node.mjs
index c223ca3d2c4e..d67fa4d6549b 100755
--- a/scripts/runner.node.mjs
+++ b/scripts/runner.node.mjs
@@ -218,6 +218,12 @@ if (isBuildkite) {
 
 let coresDir;
 
+// Declared before the first top-level `await spawnSafe(...)` below — spawnSafe
+// is a hoisted function declaration that references this; with the const lower
+// in the file the Linux-only coredump sysctl call hit the TDZ.
+/** @type {Set<import("node:child_process").ChildProcess>} */
+const activeSubprocesses = new Set();
+
 if (options["coredump-upload"]) {
   // this sysctl is set in bootstrap.sh to /var/bun-cores-$distro-$release-$arch
   const sysctl = await spawnSafe({ command: "sysctl", args: ["-n", "kernel.core_pattern"] });
@@ -945,6 +951,7 @@ async function spawnSafe(options) {
         subprocess.kill(9);
       }
     }
+    activeSubprocesses.delete(subprocess);
     resolve();
   };
   await new Promise(resolve => {
@@ -975,6 +982,7 @@ async function spawnSafe(options) {
         env,
       });
       subprocess.on("spawn", () => {
+        activeSubprocesses.add(subprocess);
         timestamp = Date.now();
         timer = setTimeout(() => done(resolve), timeout);
       });
@@ -2350,6 +2358,18 @@ function isAlwaysFailure(error) {
 function onExit(signal) {
   const label = `${getAnsi("red")}Received ${signal}, exiting...${getAnsi("reset")}`;
   startGroup(label, () => {
+    // Windows: children spawned without `detached` are assigned to this
+    // process's Job Object with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE, so
+    // process.exit() below already terminates the whole tree — nothing to
+    // do here. POSIX: Buildkite cancel sends SIGTERM to the runner PID only
+    // (not the group), so orphans survive unless we kill them explicitly.
+    if (!isWindows) {
+      for (const proc of activeSubprocesses) {
+        try {
+          proc.kill(9);
+        } catch {}
+      }
+    }
     process.exit(getExitCode("cancel"));
   });
 }