diff --git a/packages/cultnet-ts/package.json b/packages/cultnet-ts/package.json
index 5f87eccc..f94f0bfa 100644
--- a/packages/cultnet-ts/package.json
+++ b/packages/cultnet-ts/package.json
@@ -26,7 +26,7 @@
     "prepack": "node tools/generate-swarm-contracts.mjs && node ../../node_modules/typescript/bin/tsc -p tsconfig.json",
     "typecheck": "node tools/generate-swarm-contracts.mjs && node ../../node_modules/typescript/bin/tsc -p tsconfig.json --pretty false",
     "test": "node tools/generate-swarm-contracts.mjs && node ../../node_modules/typescript/bin/tsc -p tsconfig.json && node ../../node_modules/typescript/bin/tsc -p tsconfig.test.json && node --test dist-test/test/*.test.js",
-    "test:interop": "node tools/generate-swarm-contracts.mjs && node ../../node_modules/typescript/bin/tsc -p tsconfig.json && node ../../node_modules/typescript/bin/tsc -p tsconfig.test.json && node --test dist-test/test/interop/*.test.js"
+    "test:interop": "node tools/generate-swarm-contracts.mjs && node ../../node_modules/typescript/bin/tsc -p tsconfig.json && node ../../node_modules/typescript/bin/tsc -p tsconfig.test.json && node --test --test-force-exit dist-test/test/interop/*.test.js"
   },
   "keywords": [
     "messagepack",
diff --git a/packages/cultnet-ts/test/interop/cultnet-interop.test.ts b/packages/cultnet-ts/test/interop/cultnet-interop.test.ts
index bec86942..4353e132 100644
--- a/packages/cultnet-ts/test/interop/cultnet-interop.test.ts
+++ b/packages/cultnet-ts/test/interop/cultnet-interop.test.ts
@@ -414,6 +414,12 @@ test("CultNet TS/Rust/C#/Python peers discover each other and exchange raw state
   const advertiseHost = findAdvertiseHost();
 
   const servers: RunningServeProcess[] = [];
+  // Registered before anything is spawned. A peer whose readiness rejects
+  // would otherwise skip this and strand every peer already running.
+  t.after(async () => {
+    await Promise.all(servers.map(stopProcess));
+  });
+
   logInteropPhase("schema-v0", "start rust peer");
   servers.push(await spawnServeProcess("rust", {
     command: rustBinaryPath,
@@ -498,10 +504,6 @@ test("CultNet TS/Rust/C#/Python peers discover each other and exchange raw state
   }));
   await servers[servers.length - 1].ready;
 
-  t.after(async () => {
-    await Promise.all(servers.map(stopProcess));
-  });
-
   logInteropPhase("schema-v0", "typescript dials rust");
   const tsDial = await runJsonCommand("ts-dial", process.execPath, [
     tsPeerScript,
@@ -989,6 +991,12 @@ test("CultNet TypeScript and Rust full interop peers exchange schema-v0 over RUD
   const discoveryPort = await getFreePort();
   const advertiseHost = findAdvertiseHost();
   const servers: RunningServeProcess[] = [];
+  // Registered before anything is spawned. A peer whose readiness rejects
+  // would otherwise skip this and strand every peer already running.
+  t.after(async () => {
+    await Promise.all(servers.map(stopProcess));
+  });
+
   servers.push(await spawnServeProcess("rust-rudp", {
     command: rustBinaryPath,
     args: [
@@ -1030,10 +1038,6 @@ test("CultNet TypeScript and Rust full interop peers exchange schema-v0 over RUD
   }));
   await servers[servers.length - 1].ready;
 
-  t.after(async () => {
-    await Promise.all(servers.map(stopProcess));
-  });
-
   logInteropPhase("ts-rust-full-rudp", "typescript dials rust over rudp");
   const tsRudpDial = await runJsonCommand("ts-rust-rudp-dial", process.execPath, [
     tsPeerScript,
@@ -1139,6 +1143,7 @@ test("CultNet Python and TypeScript exchange schema frames when Python dials a T
     assert.equal(exitCode, 0, pythonClient.stderr.join(""));
   } finally {
     server.close();
+    closeUdpSocket(serverSocket);
     if (pythonClient.child.exitCode === null && !pythonClient.child.killed) {
       pythonClient.child.kill("SIGTERM");
       await once(pythonClient.child, "exit").catch(() => undefined);
@@ -1246,6 +1251,7 @@ test("CultNet Python and TypeScript exchange schema-v0 MessagePack messages over
     assert.equal(exitCode, 0, pythonClient.stderr.join(""));
   } finally {
     server.close();
+    closeUdpSocket(serverSocket);
     if (pythonClient.child.exitCode === null && !pythonClient.child.killed) {
       pythonClient.child.kill("SIGTERM");
       await once(pythonClient.child, "exit").catch(() => undefined);
@@ -1322,6 +1328,7 @@ test("CultNet Rust and TypeScript exchange schema frames when Rust dials a TypeS
     assert.equal(exitCode, 0, rustClient.stderr.join(""));
   } finally {
     server.close();
+    closeUdpSocket(serverSocket);
     if (rustClient.child.exitCode === null && !rustClient.child.killed) {
       rustClient.child.kill("SIGTERM");
       await once(rustClient.child, "exit").catch(() => undefined);
@@ -1431,6 +1438,7 @@ test("CultNet Rust and TypeScript exchange schema-v0 MessagePack messages over R
     assert.equal(exitCode, 0, rustClient.stderr.join(""));
   } finally {
     server.close();
+    closeUdpSocket(serverSocket);
     if (rustClient.child.exitCode === null && !rustClient.child.killed) {
       rustClient.child.kill("SIGTERM");
       await once(rustClient.child, "exit").catch(() => undefined);
@@ -1507,6 +1515,7 @@ test("CultNet C# and TypeScript exchange schema frames when C# dials a TypeScrip
     assert.equal(exitCode, 0, csharpClient.stderr.join(""));
   } finally {
     server.close();
+    closeUdpSocket(serverSocket);
     if (csharpClient.child.exitCode === null && !csharpClient.child.killed) {
       csharpClient.child.kill("SIGTERM");
       await once(csharpClient.child, "exit").catch(() => undefined);
@@ -1616,6 +1625,7 @@ test("CultNet C# and TypeScript exchange schema-v0 MessagePack messages over RUD
     assert.equal(exitCode, 0, csharpClient.stderr.join(""));
   } finally {
     server.close();
+    closeUdpSocket(serverSocket);
     if (csharpClient.child.exitCode === null && !csharpClient.child.killed) {
       csharpClient.child.kill("SIGTERM");
       await once(csharpClient.child, "exit").catch(() => undefined);
@@ -1692,6 +1702,7 @@ test("CultNet Kotlin and TypeScript exchange schema frames when Kotlin dials a T
     assert.equal(exitCode, 0, kotlinClient.stderr.join(""));
   } finally {
     server.close();
+    closeUdpSocket(serverSocket);
     if (kotlinClient.child.exitCode === null && !kotlinClient.child.killed) {
       kotlinClient.child.kill("SIGTERM");
       await once(kotlinClient.child, "exit").catch(() => undefined);
@@ -1801,6 +1812,7 @@ test("CultNet Kotlin and TypeScript exchange schema-v0 MessagePack messages over
     assert.equal(exitCode, 0, kotlinClient.stderr.join(""));
   } finally {
     server.close();
+    closeUdpSocket(serverSocket);
     if (kotlinClient.child.exitCode === null && !kotlinClient.child.killed) {
       kotlinClient.child.kill("SIGTERM");
       await once(kotlinClient.child, "exit").catch(() => undefined);
@@ -1823,6 +1835,12 @@ test("CultNet TypeScript and C# full interop peers exchange schema-v0 over RUDP"
   const discoveryPort = await getFreePort();
   const advertiseHost = findAdvertiseHost();
   const servers: RunningServeProcess[] = [];
+  // Registered before anything is spawned. A peer whose readiness rejects
+  // would otherwise skip this and strand every peer already running.
+  t.after(async () => {
+    await Promise.all(servers.map(stopProcess));
+  });
+
   servers.push(await spawnServeProcess("csharp-rudp", {
     command: dotnetCommand,
     args: [
@@ -1865,10 +1883,6 @@ test("CultNet TypeScript and C# full interop peers exchange schema-v0 over RUDP"
   }));
   await servers[servers.length - 1].ready;
 
-  t.after(async () => {
-    await Promise.all(servers.map(stopProcess));
-  });
-
   logInteropPhase("ts-csharp-full-rudp", "typescript dials csharp over rudp");
   const tsRudpDial = await runJsonCommand("ts-csharp-rudp-dial", process.execPath, [
     tsPeerScript,
@@ -1923,6 +1937,12 @@ test("CultNet TypeScript and Kotlin full interop peers exchange schema-v0 over R
   const discoveryPort = await getFreePort();
   const advertiseHost = findAdvertiseHost();
   const servers: RunningServeProcess[] = [];
+  // Registered before anything is spawned. A peer whose readiness rejects
+  // would otherwise skip this and strand every peer already running.
+  t.after(async () => {
+    await Promise.all(servers.map(stopProcess));
+  });
+
   servers.push(await spawnServeProcess("kotlin-rudp", {
     command: kotlinJavaCommand,
     args: [
@@ -1966,10 +1986,6 @@ test("CultNet TypeScript and Kotlin full interop peers exchange schema-v0 over R
   }));
   await servers[servers.length - 1].ready;
 
-  t.after(async () => {
-    await Promise.all(servers.map(stopProcess));
-  });
-
   logInteropPhase("ts-kotlin-full-rudp", "typescript dials kotlin over rudp");
   const tsRudpDial = await runJsonCommand("ts-kotlin-rudp-dial", process.execPath, [
     tsPeerScript,
@@ -3160,6 +3176,20 @@ function spawnRustRudpMessageClient(remotePort: number): Omit<RunningPythonRudpP
   return { child, stderr };
 }
 
+/** Closes a socket this file bound, tolerating a transport that adopted and
+ * already closed it. A socket left open holds the event loop forever, which is
+ * invisible while tests pass and fatal to the runner's exit. */
+function closeUdpSocket(socket: Socket | undefined): void {
+  if (!socket) {
+    return;
+  }
+  try {
+    socket.close();
+  } catch {
+    // already closed
+  }
+}
+
 async function bindUdpSocket(): Promise<Socket> {
   const socket = dgram.createSocket("udp4");
   await new Promise<void>((resolveBind, rejectBind) => {