@@ -880,3 +880,46 @@ effectIt.effect(
     }).pipe(Effect.provide(layer));
   },
 );
+
+effectIt.effect("ignores a partial Windows netstat snapshot when one query exits nonzero", () => {
+  const fetchFn = (() =>
+    Promise.resolve(
+      new Response("hello", { headers: { "content-type": "text/html" } }),
+    )) as typeof globalThis.fetch;
+  const layer = PortScanner.layer.pipe(
+    Layer.provide(
+      Layer.mergeAll(
+        layerNoProc,
+        Layer.succeed(ProcessRunner.ProcessRunner, {
+          run: (input) => {
+            const isIpv6 = input.args.includes("tcpv6");
+            return Effect.succeed({
+              stdout: isIpv6
+                ? ""
+                : "  TCP    127.0.0.1:5173         0.0.0.0:0              LISTENING       1234\r\n",
+              stderr: "",
+              code: isIpv6 ? ChildProcessSpawner.ExitCode(1) : null,
+              timedOut: false,
+              stdoutTruncated: false,
+              stderrTruncated: false,
+              stdoutInvalidUtf8: false,
+              stderrInvalidUtf8: false,
+            });
+          },
+        }),
+        Layer.succeed(HostProcess.Platform, "win32"),
+        FetchHttpClient.layer.pipe(Layer.provide(Layer.succeed(FetchHttpClient.Fetch, fetchFn))),
+      ),
+    ),
+  );
+
+  return Effect.gen(function* () {
+    const scanner = yield* PortScanner.PortDiscovery;
+    yield* scanner.registerTerminalProcesses({
+      threadId: "scanner-thread",
+      terminalId: "scanner-terminal",
+      processIds: [1234],
+    });
+    expect(yield* scanner.scan()).toEqual([]);
+  }).pipe(Effect.provide(layer));
+});
@@ -304,6 +304,10 @@ const parseWindowsListenerLine = (
   };
 };
 
+/** `ProcessRunner` reports a nonzero exit as a result, not a failure. */
+const exitedWithError = (result: ProcessRunner.ProcessRunOutput): boolean =>
+  result.code !== null && result.code !== 0;
+
 /** Reads the image name from `tasklist.exe /FO CSV /NH /FI "PID eq <pid>"` output. */
 const parseTasklistProcessName = (raw: string, pid: number): string | null => {
   for (const line of raw.split(/\r?\n/g)) {
@@ -694,7 +698,7 @@ export const make = Effect.gen(function* PortDiscoveryMake() {
           .pipe(
             // Only a completed lookup is cached; failures retry on the next scan.
             Effect.map((result) =>
-              result.code !== null && result.code !== 0
+              exitedWithError(result)
                 ? []
                 : [
                     [
@@ -752,8 +756,12 @@ export const make = Effect.gen(function* PortDiscoveryMake() {
       const listeners = yield* Effect.all([runNetstat("tcp"), runNetstat("tcpv6")], {
         concurrency: 2,
       }).pipe(
+        // One failed query leaves a partial snapshot that could hide another process sharing a
+        // port across IPv4/IPv6, so fall back to configured URLs instead.
         Effect.map(([tcp4, tcp6]) =>
-          parseWindowsListenerOutput(`${tcp4.stdout}\n${tcp6.stdout}`, terminalByProcessId),
+          exitedWithError(tcp4) || exitedWithError(tcp6)
+            ? null
+            : parseWindowsListenerOutput(`${tcp4.stdout}\n${tcp6.stdout}`, terminalByProcessId),
         ),
         Effect.catchTags({
           ProcessSpawnError: recoverWindowsProbeFailure,