diff --git a/test/services/observer_config_service_test.dart b/test/services/observer_config_service_test.dart
new file mode 100644
index 0000000..cacf313
--- /dev/null
+++ b/test/services/observer_config_service_test.dart
@@ -0,0 +1,128 @@
+// Adversarial + correctness tests for ObserverConfigService (#64 A2).
+//
+// TDD: the concurrency and secret-redaction tests are written to FAIL on the
+// current service — they document the Gemini BLOCKER (#1 race: "one request in
+// flight" is assumed, not enforced) and MINOR (#6: secret key names leak into
+// error strings). S1 (#73) hardens the service until these pass.
+//
+// The service only uses 4 members of MeshCoreConnector (receivedFrames,
+// sendFrame, firmwareVerCode, offbandCaps), so a fake subclass injects frames
+// without any production change.
+
+import 'dart:async';
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meshcore_open/connector/meshcore_connector.dart';
+import 'package:meshcore_open/connector/observer_config_client.dart';
+import 'package:meshcore_open/services/observer_config_service.dart';
+
+/// Test transport: overrides only what the service touches; injects responses.
+class _FakeConnector extends MeshCoreConnector {
+  final StreamController<Uint8List> _frames =
+      StreamController<Uint8List>.broadcast();
+  final List<Uint8List> sent = [];
+  int? ver = 14;
+  int? caps = 0x01;
+
+  @override
+  Stream<Uint8List> get receivedFrames => _frames.stream;
+
+  @override
+  int? get firmwareVerCode => ver;
+
+  @override
+  int? get offbandCaps => caps;
+
+  @override
+  Future<void> sendFrame(
+    Uint8List data, {
+    String? channelSendQueueId,
+    bool expectsGenericAck = false,
+  }) async {
+    sent.add(data);
+  }
+
+  void inject(Uint8List frame) => _frames.add(frame);
+  void close() => _frames.close();
+}
+
+Uint8List _respText(int sub, String text) => Uint8List.fromList([
+  ObserverConfigClient.respConfig,
+  sub,
+  ...utf8.encode(text),
+  0,
+]);
+
+Future<void> _tick() => Future<void>.delayed(const Duration(milliseconds: 10));
+
+void main() {
+  late _FakeConnector c;
+  late ObserverConfigService svc;
+
+  setUp(() {
+    c = _FakeConnector();
+    svc = ObserverConfigService(c);
+  });
+  tearDown(() => c.close());
+
+  test('supported reflects version gate + capability bit', () {
+    expect(svc.supported, isTrue);
+    c.caps = 0x00; // observer bit off
+    expect(svc.supported, isFalse);
+    c.caps = 0x01;
+    c.ver = 13; // below gate
+    expect(svc.supported, isFalse);
+  });
+
+  test('setFlat returns true on ACK (happy path)', () async {
+    final f = svc.setFlat('mqtt.iata', 'HAO');
+    await _tick();
+    c.inject(_respText(ObserverConfigClient.rAck, 'mqtt.iata = HAO'));
+    expect(await f, isTrue);
+    // The frame actually went out.
+    expect(c.sent.single[0], ObserverConfigClient.cmdConfig);
+  });
+
+  // ---- BLOCKER #1: single-flight is assumed but unenforced ----
+  test('overlapping requests must NOT cross responses (single-flight)', () async {
+    final getF = svc.getFlat('wifi.ssid');
+    final setF = svc.setFlat('mqtt.iata', 'HAO');
+    await _tick();
+    // Device answers the GET first, then the SET.
+    c.inject(_respText(ObserverConfigClient.rValue, 'wifi.ssid = MyNet'));
+    await _tick();
+    c.inject(_respText(ObserverConfigClient.rAck, 'mqtt.iata = HAO'));
+    await _tick();
+    expect(await getF, 'MyNet');
+    expect(
+      await setF,
+      isTrue,
+      reason:
+          'setFlat must receive its OWN ACK, not the GET value — cross-talk means single-flight is unenforced',
+    );
+  });
+
+  // ---- MINOR #6: secret key names leak into error messages ----
+  test('error for a secret key does not leak the key name or value', () async {
+    svc.timeout = const Duration(milliseconds: 30);
+    final res = await svc.setFlat(
+      'wifi.pwd',
+      'hunter2',
+    ); // no response -> timeout
+    expect(res, isFalse);
+    expect(svc.lastError, isNotNull);
+    final err = svc.lastError!.toLowerCase();
+    expect(
+      err,
+      isNot(contains('pwd')),
+      reason: 'secret key name (wifi.pwd) must be redacted from errors',
+    );
+    expect(
+      err,
+      isNot(contains('hunter2')),
+      reason: 'secret value must never appear in an error',
+    );
+  });
+}