diff --git a/packages/powersync_core/analysis_options.yaml b/packages/powersync_core/analysis_options.yaml
index dee8927a..4be9ff7b 100644
--- a/packages/powersync_core/analysis_options.yaml
+++ b/packages/powersync_core/analysis_options.yaml
@@ -13,6 +13,12 @@
 
 include: package:lints/recommended.yaml
 
+analyzer:
+  language:
+    strict-casts: true
+    strict-inference: true
+    strict-raw-types: true
+
 # Uncomment the following section to specify additional rules.
 
 # linter:
diff --git a/packages/powersync_core/example/getting_started.dart b/packages/powersync_core/example/getting_started.dart
index eb291b3b..13f43642 100644
--- a/packages/powersync_core/example/getting_started.dart
+++ b/packages/powersync_core/example/getting_started.dart
@@ -91,7 +91,7 @@ Future<String> getDatabasePath() async {
   return join(dir, dbFilename);
 }
 
-openDatabase() async {
+Future<void> openDatabase() async {
   // Setup the database.
   final psFactory = PowerSyncDartOpenFactory(path: await getDatabasePath());
   db = PowerSyncDatabase.withFactory(psFactory, schema: schema);
diff --git a/packages/powersync_core/lib/src/bucket_storage.dart b/packages/powersync_core/lib/src/bucket_storage.dart
index 804a278c..6815e8d9 100644
--- a/packages/powersync_core/lib/src/bucket_storage.dart
+++ b/packages/powersync_core/lib/src/bucket_storage.dart
@@ -21,7 +21,7 @@ class BucketStorage {
     _init();
   }
 
-  _init() {}
+  void _init() {}
 
   // Use only for read statements
   Future<ResultSet> select(String query,
@@ -36,7 +36,8 @@ class BucketStorage {
         'SELECT name as bucket, cast(last_op as TEXT) as op_id FROM ps_buckets WHERE pending_delete = 0 AND name != \'\$local\'');
     return [
       for (var row in rows)
-        BucketState(bucket: row['bucket'], opId: row['op_id'])
+        BucketState(
+            bucket: row['bucket'] as String, opId: row['op_id'] as String)
     ];
   }
 
@@ -157,14 +158,16 @@ class BucketStorage {
       Checkpoint checkpoint) async {
     final rs = await select("SELECT powersync_validate_checkpoint(?) as result",
         [jsonEncode(checkpoint)]);
-    final result = jsonDecode(rs[0]['result']);
-    if (result['valid']) {
+    final result =
+        jsonDecode(rs[0]['result'] as String) as Map<String, dynamic>;
+    if (result['valid'] as bool) {
       return SyncLocalDatabaseResult(ready: true);
     } else {
       return SyncLocalDatabaseResult(
           checkpointValid: false,
           ready: false,
-          checkpointFailures: result['failed_buckets'].cast<String>());
+          checkpointFailures:
+              (result['failed_buckets'] as List).cast<String>());
     }
   }
 
@@ -232,7 +235,7 @@ class BucketStorage {
       // Nothing to update
       return false;
     }
-    int seqBefore = rs.first['seq'];
+    int seqBefore = rs.first['seq'] as int;
     var opId = await checkpointCallback();
 
     return await writeTransaction((tx) async {
@@ -244,7 +247,7 @@ class BucketStorage {
           .execute('SELECT seq FROM sqlite_sequence WHERE name = \'ps_crud\'');
       assert(rs.isNotEmpty);
 
-      int seqAfter = rs.first['seq'];
+      int seqAfter = rs.first['seq'] as int;
       if (seqAfter != seqBefore) {
         // New crud data may have been uploaded since we got the checkpoint. Abort.
         return false;
@@ -362,12 +365,13 @@ class SyncBucketData {
       this.nextAfter});
 
   SyncBucketData.fromJson(Map<String, dynamic> json)
-      : bucket = json['bucket'],
-        hasMore = json['has_more'] ?? false,
-        after = json['after'],
-        nextAfter = json['next_after'],
-        data =
-            (json['data'] as List).map((e) => OplogEntry.fromJson(e)).toList();
+      : bucket = json['bucket'] as String,
+        hasMore = json['has_more'] as bool? ?? false,
+        after = json['after'] as String?,
+        nextAfter = json['next_after'] as String?,
+        data = (json['data'] as List)
+            .map((e) => OplogEntry.fromJson(e as Map<String, dynamic>))
+            .toList();
 
   Map<String, dynamic> toJson() {
     return {
@@ -407,16 +411,25 @@ class OplogEntry {
       required this.checksum});
 
   OplogEntry.fromJson(Map<String, dynamic> json)
-      : opId = json['op_id'],
-        op = OpType.fromJson(json['op']),
-        rowType = json['object_type'],
-        rowId = json['object_id'],
-        checksum = json['checksum'],
-        data = json['data'] is String ? json['data'] : jsonEncode(json['data']),
-        subkey = json['subkey'] is String ? json['subkey'] : null;
+      : opId = json['op_id'] as String,
+        op = OpType.fromJson(json['op'] as String),
+        rowType = json['object_type'] as String?,
+        rowId = json['object_id'] as String?,
+        checksum = json['checksum'] as int,
+        data = switch (json['data']) {
+          String data => data,
+          var other => jsonEncode(other),
+        },
+        subkey = switch (json['subkey']) {
+          String subkey => subkey,
+          _ => null,
+        };
 
   Map<String, dynamic>? get parsedData {
-    return data == null ? null : jsonDecode(data!);
+    return switch (data) {
+      final data? => jsonDecode(data) as Map<String, dynamic>,
+      null => null,
+    };
   }
 
   /// Key to uniquely represent a source entry in a bucket.
@@ -463,8 +476,8 @@ class SyncLocalDatabaseResult {
 
   @override
   int get hashCode {
-    return Object.hash(
-        ready, checkpointValid, const ListEquality().hash(checkpointFailures));
+    return Object.hash(ready, checkpointValid,
+        const ListEquality<String?>().hash(checkpointFailures));
   }
 
   @override
@@ -472,7 +485,7 @@ class SyncLocalDatabaseResult {
     return other is SyncLocalDatabaseResult &&
         other.ready == ready &&
         other.checkpointValid == checkpointValid &&
-        const ListEquality()
+        const ListEquality<String?>()
             .equals(other.checkpointFailures, checkpointFailures);
   }
 }
diff --git a/packages/powersync_core/lib/src/connector.dart b/packages/powersync_core/lib/src/connector.dart
index f8d58460..acb7b961 100644
--- a/packages/powersync_core/lib/src/connector.dart
+++ b/packages/powersync_core/lib/src/connector.dart
@@ -90,13 +90,13 @@ class PowerSyncCredentials {
   }
 
   factory PowerSyncCredentials.fromJson(Map<String, dynamic> parsed) {
-    String token = parsed['token'];
+    String token = parsed['token'] as String;
     DateTime? expiresAt = getExpiryDate(token);
 
     return PowerSyncCredentials(
-        endpoint: parsed['endpoint'],
-        token: parsed['token'],
-        userId: parsed['user_id'],
+        endpoint: parsed['endpoint'] as String,
+        token: token,
+        userId: parsed['user_id'] as String?,
         expiresAt: expiresAt);
   }
 
@@ -110,9 +110,9 @@ class PowerSyncCredentials {
         // dart:convert doesn't like missing padding
         final rawData = base64Url.decode(base64.normalize(parts[1]));
         final text = Utf8Decoder().convert(rawData);
-        Map<String, dynamic> payload = jsonDecode(text);
-        if (payload.containsKey('exp') && payload['exp'] is int) {
-          return DateTime.fromMillisecondsSinceEpoch(payload['exp'] * 1000);
+        final payload = jsonDecode(text) as Map<String, dynamic>;
+        if (payload['exp'] case int exp) {
+          return DateTime.fromMillisecondsSinceEpoch(exp * 1000);
         }
       }
       return null;
@@ -131,7 +131,7 @@ class PowerSyncCredentials {
     return Uri.parse(endpoint).resolve(path);
   }
 
-  _validateEndpoint() {
+  void _validateEndpoint() {
     final parsed = Uri.parse(endpoint);
     if ((!parsed.isScheme('http') && !parsed.isScheme('https')) ||
         parsed.host.isEmpty) {
@@ -162,14 +162,14 @@ class DevCredentials {
 
   factory DevCredentials.fromJson(Map<String, dynamic> parsed) {
     return DevCredentials(
-        endpoint: parsed['endpoint'],
-        token: parsed['token'],
-        userId: parsed['user_id']);
+        endpoint: parsed['endpoint'] as String,
+        token: parsed['token'] as String?,
+        userId: parsed['user_id'] as String?);
   }
 
   factory DevCredentials.fromString(String credentials) {
     var parsed = jsonDecode(credentials);
-    return DevCredentials.fromJson(parsed);
+    return DevCredentials.fromJson(parsed as Map<String, dynamic>);
   }
 
   static DevCredentials? fromOptionalString(String? credentials) {
@@ -255,10 +255,12 @@ class DevConnector extends PowerSyncBackendConnector {
 
     if (res.statusCode == 200) {
       var parsed = jsonDecode(res.body);
+      var data = parsed['data'] as Map<String, dynamic>;
+
       storeDevCredentials(DevCredentials(
           endpoint: endpoint,
-          token: parsed['data']['token'],
-          userId: parsed['data']['user_id']));
+          token: data['token'] as String?,
+          userId: data['user_id'] as String?));
     } else {
       throw http.ClientException(res.reasonPhrase ?? 'Request failed', uri);
     }
@@ -281,7 +283,8 @@ class DevConnector extends PowerSyncBackendConnector {
       throw http.ClientException(res.reasonPhrase ?? 'Request failed', uri);
     }
 
-    return PowerSyncCredentials.fromJson(jsonDecode(res.body)['data']);
+    return PowerSyncCredentials.fromJson(
+        jsonDecode(res.body)['data'] as Map<String, dynamic>);
   }
 
   /// Upload changes using the PowerSync dev API.
@@ -319,7 +322,7 @@ class DevConnector extends PowerSyncBackendConnector {
     final body = jsonDecode(response.body);
     // writeCheckpoint is optional, but reduces latency between writing,
     // and reading back the same change.
-    final String? writeCheckpoint = body['data']['write_checkpoint'];
+    final writeCheckpoint = body['data']['write_checkpoint'] as String?;
     await batch.complete(writeCheckpoint: writeCheckpoint);
   }
 }
diff --git a/packages/powersync_core/lib/src/crud.dart b/packages/powersync_core/lib/src/crud.dart
index 5a11ae4d..3cf6afaf 100644
--- a/packages/powersync_core/lib/src/crud.dart
+++ b/packages/powersync_core/lib/src/crud.dart
@@ -81,9 +81,15 @@ class CrudEntry {
       this.opData);
 
   factory CrudEntry.fromRow(sqlite.Row row) {
-    final data = jsonDecode(row['data']);
-    return CrudEntry(row['id'], UpdateType.fromJsonChecked(data['op'])!,
-        data['type'], data['id'], row['tx_id'], data['data']);
+    final data = jsonDecode(row['data'] as String);
+    return CrudEntry(
+      row['id'] as int,
+      UpdateType.fromJsonChecked(data['op'] as String)!,
+      data['type'] as String,
+      data['id'] as String,
+      row['tx_id'] as int,
+      data['data'] as Map<String, Object?>?,
+    );
   }
 
   /// Converts the change to JSON format, as required by the dev crud API.
@@ -111,13 +117,13 @@ class CrudEntry {
         other.op == op &&
         other.table == table &&
         other.id == id &&
-        const MapEquality().equals(other.opData, opData));
+        const MapEquality<String, dynamic>().equals(other.opData, opData));
   }
 
   @override
   int get hashCode {
     return Object.hash(transactionId, clientId, op.toJson(), table, id,
-        const MapEquality().hash(opData));
+        const MapEquality<String, dynamic>().hash(opData));
   }
 }
 
diff --git a/packages/powersync_core/lib/src/database/native/native_powersync_database.dart b/packages/powersync_core/lib/src/database/native/native_powersync_database.dart
index a2a6d028..0b61898c 100644
--- a/packages/powersync_core/lib/src/database/native/native_powersync_database.dart
+++ b/packages/powersync_core/lib/src/database/native/native_powersync_database.dart
@@ -143,10 +143,10 @@ class PowerSyncDatabaseImpl
     await isInitialized;
     final dbRef = database.isolateConnectionFactory();
     ReceivePort rPort = ReceivePort();
-    StreamSubscription? crudUpdateSubscription;
+    StreamSubscription<UpdateNotification>? crudUpdateSubscription;
     rPort.listen((data) async {
       if (data is List) {
-        String action = data[0];
+        String action = data[0] as String;
         if (action == "getCredentials") {
           await (data[1] as PortCompleter).handle(() async {
             final token = await connector.getCredentialsCached();
@@ -159,7 +159,7 @@ class PowerSyncDatabaseImpl
             await connector.prefetchCredentials();
           });
         } else if (action == 'init') {
-          SendPort port = data[1];
+          SendPort port = data[1] as SendPort;
           var crudStream =
               database.onChange(['ps_crud'], throttle: crudThrottleTime);
           crudUpdateSubscription = crudStream.listen((event) {
@@ -173,7 +173,7 @@ class PowerSyncDatabaseImpl
             await connector.uploadData(this);
           });
         } else if (action == 'status') {
-          final SyncStatus status = data[1];
+          final SyncStatus status = data[1] as SyncStatus;
           setStatus(status);
         } else if (action == 'close') {
           // Clear status apart from lastSyncedAt
@@ -181,7 +181,7 @@ class PowerSyncDatabaseImpl
           rPort.close();
           crudUpdateSubscription?.cancel();
         } else if (action == 'log') {
-          LogRecord record = data[1];
+          LogRecord record = data[1] as LogRecord;
           logger.log(
               record.level, record.message, record.error, record.stackTrace);
         }
@@ -290,7 +290,7 @@ Future<void> _powerSyncDatabaseIsolate(
 
   rPort.listen((message) async {
     if (message is List) {
-      String action = message[0];
+      String action = message[0] as String;
       if (action == 'update') {
         crudUpdateController.add('update');
       } else if (action == 'close') {
diff --git a/packages/powersync_core/lib/src/database/powersync_db_mixin.dart b/packages/powersync_core/lib/src/database/powersync_db_mixin.dart
index a5fb93a5..39bf8014 100644
--- a/packages/powersync_core/lib/src/database/powersync_db_mixin.dart
+++ b/packages/powersync_core/lib/src/database/powersync_db_mixin.dart
@@ -87,7 +87,7 @@ mixin PowerSyncDatabaseMixin implements SqliteConnection {
     try {
       final row =
           await database.get('SELECT powersync_rs_version() as version');
-      version = row['version'];
+      version = row['version'] as String;
     } catch (e) {
       throw SqliteException(
           1, 'The powersync extension is not loaded correctly. Details: $e');
@@ -291,7 +291,7 @@ mixin PowerSyncDatabaseMixin implements SqliteConnection {
   /// A connection factory that can be passed to different isolates.
   ///
   /// Use this to access the database in background isolates.
-  isolateConnectionFactory() {
+  IsolateConnectionFactory<CommonDatabase> isolateConnectionFactory() {
     return database.isolateConnectionFactory();
   }
 
@@ -309,10 +309,10 @@ mixin PowerSyncDatabaseMixin implements SqliteConnection {
       final row = await getOptional(
           'SELECT SUM(cast(data as blob) + 20) as size, count(*) as count FROM ps_crud');
       return UploadQueueStats(
-          count: row?['count'] ?? 0, size: row?['size'] ?? 0);
+          count: row?['count'] as int? ?? 0, size: row?['size'] as int? ?? 0);
     } else {
       final row = await getOptional('SELECT count(*) as count FROM ps_crud');
-      return UploadQueueStats(count: row?['count'] ?? 0);
+      return UploadQueueStats(count: row?['count'] as int? ?? 0);
     }
   }
 
@@ -331,7 +331,7 @@ mixin PowerSyncDatabaseMixin implements SqliteConnection {
   /// This method does include transaction ids in the result, but does not group
   /// data by transaction. One batch may contain data from multiple transactions,
   /// and a single transaction may be split over multiple batches.
-  Future<CrudBatch?> getCrudBatch({limit = 100}) async {
+  Future<CrudBatch?> getCrudBatch({int limit = 100}) async {
     final rows = await getAll(
         'SELECT id, tx_id, data FROM ps_crud ORDER BY id ASC LIMIT ?',
         [limit + 1]);
@@ -384,7 +384,7 @@ mixin PowerSyncDatabaseMixin implements SqliteConnection {
       if (first == null) {
         return null;
       }
-      final int? txId = first['tx_id'];
+      final txId = first['tx_id'] as int?;
       List<CrudEntry> all;
       if (txId == null) {
         all = [CrudEntry.fromRow(first)];
diff --git a/packages/powersync_core/lib/src/exceptions.dart b/packages/powersync_core/lib/src/exceptions.dart
index 82d55d1f..983876a2 100644
--- a/packages/powersync_core/lib/src/exceptions.dart
+++ b/packages/powersync_core/lib/src/exceptions.dart
@@ -87,8 +87,8 @@ String? _stringOrFirst(Object? details) {
     return null;
   } else if (details is String) {
     return details;
-  } else if (details is List && details[0] is String) {
-    return details[0];
+  } else if (details case [final String first, ...]) {
+    return first;
   } else {
     return null;
   }
diff --git a/packages/powersync_core/lib/src/isolate_completer.dart b/packages/powersync_core/lib/src/isolate_completer.dart
index 7307ce8a..7192a59c 100644
--- a/packages/powersync_core/lib/src/isolate_completer.dart
+++ b/packages/powersync_core/lib/src/isolate_completer.dart
@@ -26,7 +26,7 @@ class IsolateResult<T> {
     return PortCompleter(receivePort.sendPort);
   }
 
-  close() {
+  void close() {
     receivePort.close();
   }
 }
@@ -43,10 +43,10 @@ class PortCompleter<T> {
   }
 
   void completeError(Object error, [StackTrace? stackTrace]) {
-    sendPort.send(PortResult.error(error, stackTrace));
+    sendPort.send(PortResult<void>.error(error, stackTrace));
   }
 
-  addExitHandler() {
+  void addExitHandler() {
     Isolate.current.addOnExitListener(sendPort, response: abortedResponse);
   }
 
diff --git a/packages/powersync_core/lib/src/setup_web.dart b/packages/powersync_core/lib/src/setup_web.dart
index 6e62f980..f1e72c0f 100644
--- a/packages/powersync_core/lib/src/setup_web.dart
+++ b/packages/powersync_core/lib/src/setup_web.dart
@@ -116,8 +116,8 @@ bool coreVersionIsInRange(String tag) {
 }
 
 dynamic getPackageFromConfig(dynamic packageConfig, String packageName) {
-  final pkg = (packageConfig['packages'] ?? []).firstWhere(
-    (e) => e['name'] == packageName,
+  final pkg = (packageConfig['packages'] as List? ?? <dynamic>[]).firstWhere(
+    (dynamic e) => e['name'] == packageName,
     orElse: () => null,
   );
   if (pkg == null) {
@@ -128,7 +128,8 @@ dynamic getPackageFromConfig(dynamic packageConfig, String packageName) {
 
 String getPubspecVersion(
     File packageConfigFile, dynamic package, String packageName) {
-  final rootUri = packageConfigFile.uri.resolve(package['rootUri'] ?? '');
+  final rootUri =
+      packageConfigFile.uri.resolve(package['rootUri'] as String? ?? '');
   print('Using package:$packageName from ${rootUri.toFilePath()}');
 
   String pubspec =
@@ -150,7 +151,7 @@ Future<List<String>> getLatestTagsFromRelease(HttpClient httpClient) async {
   var response = await request.close();
   if (response.statusCode == HttpStatus.ok) {
     var res = await response.transform(utf8.decoder).join();
-    List<dynamic> jsonObj = json.decode(res);
+    var jsonObj = json.decode(res) as List<dynamic>;
     List<String> tags = [];
     for (dynamic obj in jsonObj) {
       final tagName = obj['tag_name'] as String;
diff --git a/packages/powersync_core/lib/src/stream_utils.dart b/packages/powersync_core/lib/src/stream_utils.dart
index 37aee207..46be2f42 100644
--- a/packages/powersync_core/lib/src/stream_utils.dart
+++ b/packages/powersync_core/lib/src/stream_utils.dart
@@ -75,19 +75,19 @@ Stream<String> newlines(ByteStream input) {
   return lineInput;
 }
 
-void pauseAll(List<StreamSubscription> subscriptions) {
+void pauseAll(List<StreamSubscription<void>> subscriptions) {
   for (var sub in subscriptions) {
     sub.pause();
   }
 }
 
-void resumeAll(List<StreamSubscription> subscriptions) {
+void resumeAll(List<StreamSubscription<void>> subscriptions) {
   for (var sub in subscriptions) {
     sub.resume();
   }
 }
 
-Future<void> cancelAll(List<StreamSubscription> subscriptions) async {
+Future<void> cancelAll(List<StreamSubscription<void>> subscriptions) async {
   final futures = subscriptions.map((sub) => sub.cancel());
   await Future.wait(futures);
 }
diff --git a/packages/powersync_core/lib/src/streaming_sync.dart b/packages/powersync_core/lib/src/streaming_sync.dart
index 6f4191ff..f8a15566 100644
--- a/packages/powersync_core/lib/src/streaming_sync.dart
+++ b/packages/powersync_core/lib/src/streaming_sync.dart
@@ -43,7 +43,7 @@ class StreamingSyncImplementation implements StreamingSync {
   final StreamController<Null> _internalCrudTriggerController =
       StreamController<Null>.broadcast();
 
-  final Stream crudUpdateTriggerStream;
+  final Stream<void> crudUpdateTriggerStream;
 
   final StreamController<SyncStatus> _statusStreamController =
       StreamController<SyncStatus>.broadcast();
@@ -505,7 +505,7 @@ class StreamingSyncImplementation implements StreamingSync {
   /// Delays the standard `retryDelay` Duration, but exits early if
   /// an abort has been requested.
   Future<void> _delayRetry() async {
-    await Future.any([Future.delayed(retryDelay), _abort!.onAbort]);
+    await Future.any([Future<void>.delayed(retryDelay), _abort!.onAbort]);
   }
 }
 
diff --git a/packages/powersync_core/lib/src/sync_types.dart b/packages/powersync_core/lib/src/sync_types.dart
index 46eeb959..5ff51de0 100644
--- a/packages/powersync_core/lib/src/sync_types.dart
+++ b/packages/powersync_core/lib/src/sync_types.dart
@@ -9,10 +9,10 @@ class Checkpoint {
       {required this.lastOpId, required this.checksums, this.writeCheckpoint});
 
   Checkpoint.fromJson(Map<String, dynamic> json)
-      : lastOpId = json['last_op_id'],
-        writeCheckpoint = json['write_checkpoint'],
+      : lastOpId = json['last_op_id'] as String,
+        writeCheckpoint = json['write_checkpoint'] as String?,
         checksums = (json['buckets'] as List)
-            .map((b) => BucketChecksum.fromJson(b))
+            .map((b) => BucketChecksum.fromJson(b as Map<String, dynamic>))
             .toList();
 
   Map<String, dynamic> toJson() {
@@ -41,10 +41,10 @@ class BucketChecksum {
       this.lastOpId});
 
   BucketChecksum.fromJson(Map<String, dynamic> json)
-      : bucket = json['bucket'],
-        checksum = json['checksum'],
-        count = json['count'],
-        lastOpId = json['last_op_id'];
+      : bucket = json['bucket'] as String,
+        checksum = json['checksum'] as int,
+        count = json['count'] as int?,
+        lastOpId = json['last_op_id'] as String?;
 }
 
 class StreamingSyncCheckpoint {
@@ -66,12 +66,12 @@ class StreamingSyncCheckpointDiff {
       this.lastOpId, this.updatedBuckets, this.removedBuckets);
 
   StreamingSyncCheckpointDiff.fromJson(Map<String, dynamic> json)
-      : lastOpId = json['last_op_id'],
-        writeCheckpoint = json['write_checkpoint'],
+      : lastOpId = json['last_op_id'] as String,
+        writeCheckpoint = json['write_checkpoint'] as String?,
         updatedBuckets = (json['updated_buckets'] as List)
-            .map((e) => BucketChecksum.fromJson(e))
+            .map((e) => BucketChecksum.fromJson(e as Map<String, Object?>))
             .toList(),
-        removedBuckets = List<String>.from(json['removed_buckets']);
+        removedBuckets = (json['removed_buckets'] as List).cast();
 }
 
 class StreamingSyncCheckpointComplete {
@@ -80,7 +80,7 @@ class StreamingSyncCheckpointComplete {
   StreamingSyncCheckpointComplete(this.lastOpId);
 
   StreamingSyncCheckpointComplete.fromJson(Map<String, dynamic> json)
-      : lastOpId = json['last_op_id'];
+      : lastOpId = json['last_op_id'] as String;
 }
 
 class StreamingSyncKeepalive {
@@ -89,19 +89,20 @@ class StreamingSyncKeepalive {
   StreamingSyncKeepalive(this.tokenExpiresIn);
 
   StreamingSyncKeepalive.fromJson(Map<String, dynamic> json)
-      : tokenExpiresIn = json['token_expires_in'];
+      : tokenExpiresIn = json['token_expires_in'] as int;
 }
 
 Object? parseStreamingSyncLine(Map<String, dynamic> line) {
   if (line.containsKey('checkpoint')) {
-    return Checkpoint.fromJson(line['checkpoint']);
+    return Checkpoint.fromJson(line['checkpoint'] as Map<String, dynamic>);
   } else if (line.containsKey('checkpoint_diff')) {
-    return StreamingSyncCheckpointDiff.fromJson(line['checkpoint_diff']);
+    return StreamingSyncCheckpointDiff.fromJson(
+        line['checkpoint_diff'] as Map<String, dynamic>);
   } else if (line.containsKey('checkpoint_complete')) {
     return StreamingSyncCheckpointComplete.fromJson(
-        line['checkpoint_complete']);
+        line['checkpoint_complete'] as Map<String, dynamic>);
   } else if (line.containsKey('data')) {
-    return SyncBucketData.fromJson(line['data']);
+    return SyncBucketData.fromJson(line['data'] as Map<String, dynamic>);
   } else if (line.containsKey('token_expires_in')) {
     return StreamingSyncKeepalive.fromJson(line);
   } else {
diff --git a/packages/powersync_core/lib/src/web/sync_worker.dart b/packages/powersync_core/lib/src/web/sync_worker.dart
index 59c0d9a9..ebe6d586 100644
--- a/packages/powersync_core/lib/src/web/sync_worker.dart
+++ b/packages/powersync_core/lib/src/web/sync_worker.dart
@@ -9,6 +9,7 @@ import 'dart:js_interop';
 
 import 'package:async/async.dart';
 import 'package:fetch_client/fetch_client.dart';
+import 'package:logging/logging.dart';
 import 'package:powersync_core/powersync_core.dart';
 import 'package:powersync_core/sqlite_async.dart';
 import 'package:powersync_core/src/database/powersync_db_mixin.dart';
@@ -59,7 +60,7 @@ class _ConnectedClient {
   final _SyncWorker _worker;
 
   _SyncRunner? _runner;
-  StreamSubscription? _logSubscription;
+  StreamSubscription<LogRecord>? _logSubscription;
 
   _ConnectedClient(MessagePort port, this._worker) {
     channel = WorkerCommunicationChannel(
diff --git a/packages/powersync_core/lib/src/web/sync_worker_protocol.dart b/packages/powersync_core/lib/src/web/sync_worker_protocol.dart
index 9d8b8484..d5281e3d 100644
--- a/packages/powersync_core/lib/src/web/sync_worker_protocol.dart
+++ b/packages/powersync_core/lib/src/web/sync_worker_protocol.dart
@@ -201,8 +201,8 @@ final class WorkerCommunicationChannel {
   final Map<int, Completer<JSAny?>> _pendingRequests = {};
   int _nextRequestId = 0;
   bool _hasError = false;
-  StreamSubscription? _incomingMessages;
-  StreamSubscription? _incomingErrors;
+  StreamSubscription<MessageEvent>? _incomingMessages;
+  StreamSubscription<Event>? _incomingErrors;
 
   final MessagePort port;
   final FutureOr<(JSAny?, JSArray?)> Function(SyncWorkerMessageType, JSAny)
diff --git a/packages/powersync_core/test/bucket_storage_test.dart b/packages/powersync_core/test/bucket_storage_test.dart
index beb05d16..95d2e58c 100644
--- a/packages/powersync_core/test/bucket_storage_test.dart
+++ b/packages/powersync_core/test/bucket_storage_test.dart
@@ -486,7 +486,7 @@ void main() {
       await expectLater(() async {
         await powersync.execute('SELECT * FROM assets');
       },
-          throwsA((e) =>
+          throwsA((dynamic e) =>
               e is SqliteException && e.message.contains('no such table')));
 
       await powersync.close();
@@ -521,7 +521,7 @@ void main() {
 
       await expectLater(
           () async => await powersync.execute('SELECT * FROM assets'),
-          throwsA((e) =>
+          throwsA((dynamic e) =>
               e is SqliteException && e.message.contains('no such table')));
 
       await powersync.close();
diff --git a/packages/powersync_core/test/connected_test.dart b/packages/powersync_core/test/connected_test.dart
index ff6a9b0c..d42afc52 100644
--- a/packages/powersync_core/test/connected_test.dart
+++ b/packages/powersync_core/test/connected_test.dart
@@ -49,7 +49,7 @@ void main() {
       addTearDown(() => {db.close()});
       await db.initialize();
 
-      final connectedCompleter = Completer();
+      final connectedCompleter = Completer<void>();
 
       db.statusStream.listen((status) {
         if (status.connected) {
@@ -69,7 +69,7 @@ void main() {
 
     test('should trigger uploads when connection is re-established', () async {
       int uploadCounter = 0;
-      Completer uploadTriggeredCompleter = Completer();
+      var uploadTriggeredCompleter = Completer<void>();
       final testServer = await createTestServer();
       final connector = TestConnector(() async {
         return PowerSyncCredentials(
@@ -99,9 +99,9 @@ void main() {
       uploadTriggeredCompleter = Completer();
 
       // Connect the PowerSync instance
-      final connectedCompleter = Completer();
+      final connectedCompleter = Completer<void>();
       // The first connection attempt will fail
-      final connectedErroredCompleter = Completer();
+      final connectedErroredCompleter = Completer<void>();
 
       db.statusStream.listen((status) {
         if (status.connected && !connectedCompleter.isCompleted) {
@@ -185,7 +185,7 @@ void main() {
       await db.connect(connector: connector);
       await db.statusStream
           .firstWhere((status) => status.connected && status.downloading);
-      await Future.delayed(Duration(milliseconds: 20));
+      await Future<void>.delayed(Duration(milliseconds: 20));
       expect(
           await db.getAll('select name from customers'),
           equals([
@@ -241,7 +241,7 @@ void main() {
       await db.connect(connector: connector);
       await db.statusStream
           .firstWhere((status) => status.connected && status.downloading);
-      await Future.delayed(Duration(milliseconds: 20));
+      await Future<void>.delayed(Duration(milliseconds: 20));
       expect(await db.getAll('select name from customers'), equals([]));
     });
   });
diff --git a/packages/powersync_core/test/crud_test.dart b/packages/powersync_core/test/crud_test.dart
index 3a4ab635..e73a961b 100644
--- a/packages/powersync_core/test/crud_test.dart
+++ b/packages/powersync_core/test/crud_test.dart
@@ -72,7 +72,7 @@ void main() {
             'INSERT INTO assets(id, description) VALUES(?, ?)',
             [testId, 'test3']);
       },
-          throwsA((e) =>
+          throwsA((dynamic e) =>
               e is SqliteException &&
               e.message.contains('UNIQUE constraint failed')));
     });
@@ -132,7 +132,7 @@ void main() {
             'INSERT INTO assets(id, description) VALUES(?, ?) ON CONFLICT DO UPDATE SET description = ?',
             [testId, 'test2', 'test3']);
       },
-          throwsA((e) =>
+          throwsA((dynamic e) =>
               e is SqliteException &&
               e.message.contains('cannot UPSERT a view')));
     });
diff --git a/packages/powersync_core/test/offline_online_test.dart b/packages/powersync_core/test/offline_online_test.dart
index 2fb1ed6d..b51766a6 100644
--- a/packages/powersync_core/test/offline_online_test.dart
+++ b/packages/powersync_core/test/offline_online_test.dart
@@ -118,7 +118,7 @@ void main() {
       });
 
       final crud = (await db.getAll('SELECT data FROM ps_crud ORDER BY id'))
-          .map((d) => jsonDecode(d['data']))
+          .map((d) => jsonDecode(d['data'] as String))
           .toList();
       expect(
           crud,
diff --git a/packages/powersync_core/test/performance_shared_test.dart b/packages/powersync_core/test/performance_shared_test.dart
index f4a20431..1d5222f3 100644
--- a/packages/powersync_core/test/performance_shared_test.dart
+++ b/packages/powersync_core/test/performance_shared_test.dart
@@ -77,7 +77,7 @@ void main() {
       final timer = Stopwatch()..start();
 
       await db.writeTransaction((tx) async {
-        List<Future> futures = [];
+        List<Future<void>> futures = [];
         for (var i = 0; i < 1000; i++) {
           var future = tx.execute(
               'INSERT INTO customers(id, name, email) VALUES(uuid(), ?, ?)',
diff --git a/packages/powersync_core/test/powersync_native_test.dart b/packages/powersync_core/test/powersync_native_test.dart
index a94aaa10..0be588bf 100644
--- a/packages/powersync_core/test/powersync_native_test.dart
+++ b/packages/powersync_core/test/powersync_native_test.dart
@@ -70,7 +70,7 @@ void main() {
       await expectLater(() async {
         await db.getAll('INSERT INTO assets(id) VALUES(?)', ['test']);
       },
-          throwsA((e) =>
+          throwsA((dynamic e) =>
               e is SqliteException &&
               e.message
                   .contains('attempt to write in a read-only transaction')));
@@ -83,7 +83,7 @@ void main() {
         await db.getAll(
             "WITH test AS (SELECT 1 AS one) INSERT INTO assets(id) SELECT one FROM test");
       },
-          throwsA((e) =>
+          throwsA((dynamic e) =>
               e is SqliteException &&
               e.message
                   .contains('attempt to write in a read-only transaction')));
diff --git a/packages/powersync_core/test/powersync_shared_test.dart b/packages/powersync_core/test/powersync_shared_test.dart
index 34405eb6..145cd3df 100644
--- a/packages/powersync_core/test/powersync_shared_test.dart
+++ b/packages/powersync_core/test/powersync_shared_test.dart
@@ -27,7 +27,9 @@ void main() {
       await db.writeTransaction((tx) async {
         await expectLater(() async {
           await db.execute('INSERT INTO assets(id) VALUES(?)', ['test']);
-        }, throwsA((e) => e is LockError && e.message.contains('tx.execute')));
+        },
+            throwsA((dynamic e) =>
+                e is LockError && e.message.contains('tx.execute')));
       });
     });
 
@@ -41,7 +43,9 @@ void main() {
         // allow it by default.
         await expectLater(() async {
           await db.getAll('SELECT * FROM assets');
-        }, throwsA((e) => e is LockError && e.message.contains('tx.getAll')));
+        },
+            throwsA((dynamic e) =>
+                e is LockError && e.message.contains('tx.getAll')));
       });
 
       await db.readTransaction((tx) async {
@@ -51,7 +55,9 @@ void main() {
         // opens another connection, but doesn't use it.
         await expectLater(() async {
           await db.getAll('SELECT * FROM assets');
-        }, throwsA((e) => e is LockError && e.message.contains('tx.getAll')));
+        },
+            throwsA((dynamic e) =>
+                e is LockError && e.message.contains('tx.getAll')));
       });
     });
 
@@ -63,16 +69,16 @@ void main() {
         await expectLater(() async {
           await db.getOptional('SELECT * FROM assets');
         },
-            throwsA(
-                (e) => e is LockError && e.message.contains('tx.getOptional')));
+            throwsA((dynamic e) =>
+                e is LockError && e.message.contains('tx.getOptional')));
       });
 
       await db.readLock((tx) async {
         await expectLater(() async {
           await db.getOptional('SELECT * FROM assets');
         },
-            throwsA(
-                (e) => e is LockError && e.message.contains('tx.getOptional')));
+            throwsA((dynamic e) =>
+                e is LockError && e.message.contains('tx.getOptional')));
       });
     });
 
diff --git a/packages/powersync_core/test/schema_test.dart b/packages/powersync_core/test/schema_test.dart
index efa03552..7c8b8f33 100644
--- a/packages/powersync_core/test/schema_test.dart
+++ b/packages/powersync_core/test/schema_test.dart
@@ -76,7 +76,7 @@ void main() {
 
       // Updated
       expect(versionAfter2['schema_version'],
-          greaterThan(versionAfter['schema_version']));
+          greaterThan(versionAfter['schema_version'] as int));
 
       final schema3 = Schema([
         Table('assets', [
@@ -106,7 +106,7 @@ void main() {
 
       // Updated again (index)
       expect(versionAfter3['schema_version'],
-          greaterThan(versionAfter2['schema_version']));
+          greaterThan(versionAfter2['schema_version'] as int));
     });
 
     /// The assets table is locked after performing the EXPLAIN QUERY
diff --git a/packages/powersync_core/test/stream_test.dart b/packages/powersync_core/test/stream_test.dart
index 3550122e..4806269a 100644
--- a/packages/powersync_core/test/stream_test.dart
+++ b/packages/powersync_core/test/stream_test.dart
@@ -19,7 +19,7 @@ void main() {
         [int count = 50, Object? error]) async* {
       for (var i = 0; i < count; i++) {
         yield "$prefix $i";
-        await Future.delayed(delay);
+        await Future<void>.delayed(delay);
       }
       if (error != null) {
         throw error;
@@ -70,7 +70,7 @@ void main() {
       Stream<String> stream2 =
           genStream('S2:', Duration(milliseconds: 20)).asBroadcastStream();
 
-      var controller = StreamController();
+      var controller = StreamController<String>();
       var stream1 = controller.stream;
 
       var merged = addBroadcast(stream1, stream2);
diff --git a/packages/powersync_core/test/streaming_sync_test.dart b/packages/powersync_core/test/streaming_sync_test.dart
index 5238dd05..ae9e95b1 100644
--- a/packages/powersync_core/test/streaming_sync_test.dart
+++ b/packages/powersync_core/test/streaming_sync_test.dart
@@ -14,7 +14,7 @@ import 'utils/test_utils_impl.dart';
 final testUtils = TestUtils();
 
 class TestConnector extends PowerSyncBackendConnector {
-  final Function _fetchCredentials;
+  final Future<PowerSyncCredentials?> Function() _fetchCredentials;
   final Future<void> Function(PowerSyncDatabase)? _uploadData;
 
   TestConnector(this._fetchCredentials,
@@ -63,7 +63,7 @@ void main() {
         var connector = TestConnector(credentialsCallback);
         pdb.connect(connector: connector);
 
-        await Future.delayed(Duration(milliseconds: random.nextInt(100)));
+        await Future<void>.delayed(Duration(milliseconds: random.nextInt(100)));
         if (random.nextBool()) {
           server.close();
         }
@@ -73,7 +73,8 @@ void main() {
         // Give some time for connections to close
         final watch = Stopwatch()..start();
         while (server.connectionCount != 0 && watch.elapsedMilliseconds < 100) {
-          await Future.delayed(Duration(milliseconds: random.nextInt(10)));
+          await Future<void>.delayed(
+              Duration(milliseconds: random.nextInt(10)));
         }
 
         expect(server.connectionCount, equals(0));
@@ -116,7 +117,7 @@ void main() {
         // var stream = impl.streamingSyncRequest(StreamingSyncRequest([]));
         // 2ms: HttpException: HttpServer is not bound to a socket
         // 20ms: Connection closed while receiving data
-        await Future.delayed(Duration(milliseconds: 20));
+        await Future<void>.delayed(Duration(milliseconds: 20));
         server.close();
       }
       await pdb.close();
@@ -142,16 +143,16 @@ void main() {
 
       // Wait for at least one connection
       while (server.connectionCount < 1 && watch.elapsedMilliseconds < 500) {
-        await Future.delayed(Duration(milliseconds: random.nextInt(10)));
+        await Future<void>.delayed(Duration(milliseconds: random.nextInt(10)));
       }
       // Give some time for a second connection if any
-      await Future.delayed(Duration(milliseconds: random.nextInt(50)));
+      await Future<void>.delayed(Duration(milliseconds: random.nextInt(50)));
 
       await pdb.close();
 
       // Give some time for connections to close
       while (server.connectionCount != 0 && watch.elapsedMilliseconds < 1000) {
-        await Future.delayed(Duration(milliseconds: random.nextInt(10)));
+        await Future<void>.delayed(Duration(milliseconds: random.nextInt(10)));
       }
 
       expect(server.connectionCount, equals(0));
diff --git a/packages/powersync_core/test/test_server.dart b/packages/powersync_core/test/test_server.dart
index 8054582e..6b456076 100644
--- a/packages/powersync_core/test/test_server.dart
+++ b/packages/powersync_core/test/test_server.dart
@@ -41,7 +41,7 @@ class TestServer {
       var blob = "*" * 5000;
       for (var i = 0; i < 50; i++) {
         yield {"token_expires_in": tokenExpiresIn, "blob": blob};
-        await Future.delayed(Duration(microseconds: 1));
+        await Future<void>.delayed(Duration(microseconds: 1));
       }
     }
 
diff --git a/packages/powersync_core/test/upload_test.dart b/packages/powersync_core/test/upload_test.dart
index 3044ec6a..62e952ed 100644
--- a/packages/powersync_core/test/upload_test.dart
+++ b/packages/powersync_core/test/upload_test.dart
@@ -15,7 +15,7 @@ const partialWarning =
     'Potentially previously uploaded CRUD entries are still present';
 
 class TestConnector extends PowerSyncBackendConnector {
-  final Function _fetchCredentials;
+  final Future<PowerSyncCredentials?> Function() _fetchCredentials;
   final Future<void> Function(PowerSyncDatabase database) _uploadData;
 
   TestConnector(this._fetchCredentials, this._uploadData);
@@ -78,7 +78,7 @@ void main() {
           'INSERT INTO assets(id, description) VALUES(?, ?)', [testId, 'test']);
 
       // Wait for the uploadData to be called.
-      await Future.delayed(Duration(milliseconds: 100));
+      await Future<void>.delayed(Duration(milliseconds: 100));
 
       // Create something else with CRUD in it.
       await powersync.execute(
diff --git a/packages/powersync_core/test/watch_test.dart b/packages/powersync_core/test/watch_test.dart
index 55bd4c86..4aa9a4d4 100644
--- a/packages/powersync_core/test/watch_test.dart
+++ b/packages/powersync_core/test/watch_test.dart
@@ -57,7 +57,7 @@ void main() {
           await powersync.execute(
               'INSERT INTO assets(id, make, customer_id) VALUES (uuid(), ?, ?)',
               ['test', id]);
-          await Future.delayed(
+          await Future<void>.delayed(
               Duration(milliseconds: Random().nextInt(baseTime * 2)));
         }
       }
@@ -74,7 +74,7 @@ void main() {
 
         var lastCount = 0;
         for (var r in results) {
-          final count = r.first['count'];
+          final count = r.first['count'] as int;
           // This is not strictly incrementing, since we can't guarantee the
           // exact order between reads and writes.
           // We can guarantee that there will always be a read after the last write,
@@ -84,7 +84,8 @@ void main() {
         }
 
         // The number of read queries must not be greater than the number of writes overall.
-        expect(numberOfQueries, lessThanOrEqualTo(results.last.first['count']));
+        expect(numberOfQueries,
+            lessThanOrEqualTo(results.last.first['count'] as int));
 
         DateTime? lastTime;
         for (var r in times) {
@@ -112,7 +113,7 @@ void main() {
         while (!done) {
           await powersync.execute(
               'INSERT INTO assets(id, make) VALUES (uuid(), ?)', ['test']);
-          await Future.delayed(
+          await Future<void>.delayed(
               Duration(milliseconds: Random().nextInt(baseTime)));
         }
       }