diff --git a/dwds/lib/src/servers/extension_debugger.dart b/dwds/lib/src/servers/extension_debugger.dart
new file mode 100644
index 000000000..5961b3363
--- /dev/null
+++ b/dwds/lib/src/servers/extension_debugger.dart
@@ -0,0 +1,117 @@
+// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:built_collection/built_collection.dart';
+import 'package:dwds/data/extension_request.dart';
+import 'package:sse/server/sse_handler.dart';
+import 'package:webkit_inspection_protocol/webkit_inspection_protocol.dart';
+
+import '../../data/serializers.dart';
+
+/// A debugger backed by the Dart Debug Extension.
+class ExtensionDebugger implements WipDebugger {
+  @override
+  WipConnection get connection => throw UnimplementedError();
+
+  /// A connection between the debugger and the background of
+  /// Dart Debug Extension
+  final SseConnection _connection;
+
+  /// A map from id to a completer associated with an [ExtensionRequest]
+  final _completers = <int, Completer>{};
+
+  var _completerId = 0;
+
+  ExtensionDebugger(this._connection) {
+    _connection.stream.listen((data) {
+      var message = serializers.deserialize(jsonDecode(data));
+      if (message is ExtensionResponse) {
+        var encodedResult = json.decode(message.result);
+        if (_completers[message.id] == null) {
+          throw StateError('Missing completer.');
+        }
+        _completers[message.id]
+            .complete(WipResponse(encodedResult as Map<String, dynamic>));
+      }
+    }, onError: (e) {
+      close();
+    });
+  }
+
+  /// Sends a [command] with optional [params] to Dart Debug Extension
+  /// over the SSE connection.
+  @override
+  Future<WipResponse> sendCommand(String command,
+      {Map<String, dynamic> params}) {
+    var completer = Completer<WipResponse>();
+    var id = newId();
+    _completers[id] = completer;
+    _connection.sink.add(jsonEncode(serializers.serialize(ExtensionRequest(
+        (b) => b
+          ..id = id
+          ..command = command
+          ..commandParams =
+              BuiltMap<String, Object>(params ?? {}).toBuilder()))));
+    return completer.future;
+  }
+
+  int newId() => _completerId++;
+
+  Future<void> close() async {
+    await _connection.sink.close();
+  }
+
+  @override
+  Future disable() => throw UnimplementedError();
+
+  @override
+  Future enable() => throw UnimplementedError();
+
+  @override
+  Stream<T> eventStream<T>(String method, WipEventTransformer<T> transformer) =>
+      throw UnimplementedError();
+
+  @override
+  Future<String> getScriptSource(String scriptId) => throw UnimplementedError();
+
+  @override
+  Stream<WipDomain> get onClosed => throw UnimplementedError();
+
+  @override
+  Stream<GlobalObjectClearedEvent> get onGlobalObjectCleared =>
+      throw UnimplementedError();
+
+  @override
+  Stream<DebuggerPausedEvent> get onPaused => throw UnimplementedError();
+
+  @override
+  Stream<DebuggerResumedEvent> get onResumed => throw UnimplementedError();
+
+  @override
+  Stream<ScriptParsedEvent> get onScriptParsed => throw UnimplementedError();
+
+  @override
+  Future pause() => throw UnimplementedError();
+
+  @override
+  Future resume() => throw UnimplementedError();
+
+  @override
+  Map<String, WipScript> get scripts => throw UnimplementedError();
+
+  @override
+  Future setPauseOnExceptions(PauseState state) => throw UnimplementedError();
+
+  @override
+  Future stepInto() => throw UnimplementedError();
+
+  @override
+  Future stepOut() => throw UnimplementedError();
+
+  @override
+  Future stepOver() => throw UnimplementedError();
+}
diff --git a/dwds/test/extension_debugger_test.dart b/dwds/test/extension_debugger_test.dart
new file mode 100644
index 000000000..73d954647
--- /dev/null
+++ b/dwds/test/extension_debugger_test.dart
@@ -0,0 +1,42 @@
+// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:dwds/data/extension_request.dart';
+import 'package:dwds/data/serializers.dart';
+import 'package:dwds/src/servers/extension_debugger.dart';
+import 'package:pedantic/pedantic.dart';
+import 'package:test/test.dart';
+
+import 'fixtures/fakes.dart';
+
+FakeSseConnection connection;
+ExtensionDebugger extensionDebugger;
+
+void main() async {
+  setUp(() async {
+    connection = FakeSseConnection();
+    extensionDebugger = ExtensionDebugger(connection);
+  });
+
+  test('can send a command & receive a response', () async {
+    var extensionResponse = ExtensionResponse((b) => b
+      ..result = jsonEncode({
+        'result': {'value': 3.14}
+      })
+      ..id = 0
+      ..success = true);
+    var resultCompleter = Completer();
+    unawaited(extensionDebugger.sendCommand('Runtime.evaluate',
+        params: {'expression': '\$pi'}).then((response) {
+      resultCompleter.complete(response);
+    }));
+    connection.controller
+        .add(jsonEncode(serializers.serialize(extensionResponse)));
+    var response = await resultCompleter.future;
+    expect(response.result['value'], 3.14);
+  });
+}
diff --git a/dwds/test/fixtures/fakes.dart b/dwds/test/fixtures/fakes.dart
index 5fa63fba7..2c86f4610 100644
--- a/dwds/test/fixtures/fakes.dart
+++ b/dwds/test/fixtures/fakes.dart
@@ -2,8 +2,14 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:async';
+
+import 'package:async/src/stream_sink_transformer.dart';
 import 'package:dwds/src/debugging/inspector.dart';
 import 'package:dwds/src/utilities/domain.dart';
+import 'package:sse/server/sse_handler.dart';
+import 'package:stream_channel/src/stream_channel_transformer.dart';
+import 'package:stream_channel/stream_channel.dart';
 
 /// A library of fake/stub implementations of our classes and their supporting
 /// classes (e.g. WipConnection) for unit testing.
@@ -89,3 +95,46 @@ class FakeRuntime extends WipRuntime {
     return results[resultsReturned++];
   }
 }
+
+class FakeSseConnection implements SseConnection {
+  @override
+  StreamChannel<S> cast<S>() => null;
+
+  final _controller = StreamController<String>();
+
+  StreamController<String> get controller => _controller;
+
+  @override
+  StreamChannel<String> changeSink(
+          StreamSink<String> Function(StreamSink<String> sink) change) =>
+      null;
+
+  @override
+  StreamChannel<String> changeStream(
+          Stream<String> Function(Stream<String> stream) change) =>
+      null;
+
+  @override
+  void pipe(StreamChannel<String> other) {}
+
+  @override
+  StreamSink<String> get sink => controller.sink;
+
+  @override
+  Stream<String> get stream => controller.stream;
+
+  @override
+  StreamChannel<S> transform<S>(
+          StreamChannelTransformer<S, String> transformer) =>
+      null;
+
+  @override
+  StreamChannel<String> transformSink(
+          StreamSinkTransformer<String, String> transformer) =>
+      null;
+
+  @override
+  StreamChannel<String> transformStream(
+          StreamTransformer<String, String> transformer) =>
+      null;
+}