diff --git a/pkg/smith/lib/configuration.dart b/pkg/smith/lib/configuration.dart
index 85a11f3f0ba..ec3167e37f8 100644
--- a/pkg/smith/lib/configuration.dart
+++ b/pkg/smith/lib/configuration.dart
@@ -622,6 +622,45 @@ class Architecture extends NamedEnum {
   }
 
   const Architecture._(String name) : super(name);
+
+  static final Architecture host = _computeHost();
+  static Architecture _computeHost() {
+    String? arch;
+    if (Platform.isWindows) {
+      arch = Platform.environment["PROCESSOR_ARCHITECTURE"];
+    } else {
+      arch = (Process.runSync("uname", ["-m"]).stdout as String).trim();
+    }
+
+    switch (arch) {
+      case "i386":
+      case "i686":
+      case "ia32":
+      case "x86":
+      case "X86":
+        return ia32;
+      case "x64":
+      case "x86-64":
+      case "x86_64":
+      case "amd64":
+      case "AMD64":
+        return x64;
+      case "armv7l":
+      case "ARM":
+        return arm;
+      case "aarch64":
+      case "arm64":
+      case "arm64e":
+      case "ARM64":
+        return arm64;
+      case "riscv32":
+        return riscv32;
+      case "riscv64":
+        return riscv64;
+    }
+
+    throw "Unknown host architecture: $arch";
+  }
 }
 
 class Compiler extends NamedEnum {
diff --git a/pkg/test_runner/lib/src/options.dart b/pkg/test_runner/lib/src/options.dart
index 3f8451a12c9..ed15fda4cb5 100644
--- a/pkg/test_runner/lib/src/options.dart
+++ b/pkg/test_runner/lib/src/options.dart
@@ -107,7 +107,7 @@ none:             No runtime, compile only.''')
     ..addMultiOption('arch',
         abbr: 'a',
         allowed: ['all', ...Architecture.names],
-        defaultsTo: [Architecture.x64.name],
+        defaultsTo: [Architecture.host.name],
         hide: true,
         help: '''The architecture to run tests for.
 
@@ -935,7 +935,7 @@ void findConfigurations(Map<String, dynamic> options) {
   var architectureOption = options['arch'] as List<String>;
   var architectures = [
     if (architectureOption.isEmpty)
-      Architecture.x64
+      Architecture.host
     else if (!architectureOption.contains('all'))
       ...architectureOption.map(Architecture.find)
   ];