link: tell the phone how full an agent's context is

lib/chat/panels/context_usage_panel.dart

@@ -46,8 +46,7 @@ class ContextUsagePanel extends StatefulWidget {
   /// Claude Code's categories (by their English names, as it reports
   /// them) as the usage views' parts: an agent's definition is in the
   /// Agent tool's, the servers' instructions with their tools, the slash
-  /// commands with the skills.
-  @visibleForTesting
+  /// commands with the skills. The phone is told them so too.
   static String? partOf(String category) =>
       switch (category.toLowerCase().trim()) {
         'system prompt' => 'system',

lib/link/workspace_bridge.dart

@@ -12,6 +12,7 @@ import '../bots/bots.dart';
 import '../chat/chat_models.dart';
 import '../chat/chat_session.dart';
 import '../chat/composer/composer_files.dart' show fileReferenceText;
+import '../chat/panels/context_usage_panel.dart' show ContextUsagePanel;
 import '../chat/rewind_files_setting.dart';
 import '../customize/customization_store.dart';
 import '../customize/customizations.dart';
@@ -1287,9 +1288,11 @@ class _Follow {
         ? null
         : _from > 0 || session.hasEarlierHistory || session.loadingEarlier;
     final goal = _goal(session.goal);
+    final context = _context(session.context);
     final restText = jsonEncode([
       earlier,
       goal,
+      context,
       session.isStreaming,
       interaction,
       [for (final choice in choices) choice.toJson()],
@@ -1328,9 +1331,31 @@ class _Follow {
       tasks: tasks,
       earlier: earlier,
       goal: goal,
+      context: context,
     );
   }
 
+  /// [usage] as the phone reads it (see [LinkSessionUpdate.context]).
+  static Map<String, Object?>? _context(ContextUsage? usage) {
+    if (usage == null || usage.window <= 0) return null;
+    final parts = <String, int>{};
+    for (final segment in usage.segments) {
+      if (segment.kind != ContextKind.used || segment.tokens <= 0) continue;
+      final part = ContextUsagePanel.partOf(segment.label) ?? segment.label;
+      parts[part] = (parts[part] ?? 0) + segment.tokens;
+    }
+    return {
+      'window': usage.window,
+      'used': usage.used,
+      if (parts.isNotEmpty)
+        'parts': [
+          for (final MapEntry(:key, :value) in parts.entries)
+            {'part': key, 'tokens': value},
+        ],
+      'compactAt': ?usage.detail?.autoCompactAt,
+    };
+  }
+
   /// [goal] as the phone reads it (see [LinkSessionUpdate.goal]).
   static Map<String, Object?>? _goal(KernelGoal? goal) => switch (goal) {
     null => null,

packages/bao_link/lib/src/protocol.dart

@@ -749,6 +749,7 @@ class LinkSessionUpdate {
     this.tasks = const [],
     this.earlier,
     this.goal,
+    this.context,
   });
 
   factory LinkSessionUpdate.fromJson(Map<String, Object?> json) =>
@@ -779,6 +780,10 @@ class LinkSessionUpdate {
           final Map<String, Object?> map => map,
           _ => null,
         },
+        context: switch (json['context']) {
+          final Map<String, Object?> map => map,
+          _ => null,
+        },
       );
 
   final String session;
@@ -814,6 +819,13 @@ class LinkSessionUpdate {
   /// "duration", "tokens"}`, its times in milliseconds.
   final Map<String, Object?>? goal;
 
+  /// How full the agent's context is, if its kernel says: `{"window",
+  /// "used", "parts": [{"part", "tokens"}], "compactAt"}`, a part by the
+  /// desktop's key for it (`system`, `tools`, `mcp`, `memory`, `skills`,
+  /// `messages`) or the kernel's own name, `compactAt` where it is
+  /// compacted, in tokens.
+  final Map<String, Object?>? context;
+
   Map<String, Object?> toJson() => {
     'session': session,
     'start': start,
@@ -826,6 +838,7 @@ class LinkSessionUpdate {
     if (tasks.isNotEmpty) 'tasks': [for (final task in tasks) task.toJson()],
     'earlier': ?earlier,
     'goal': ?goal,
+    'context': ?context,
   };
 }
 

packages/bao_link/test/link_test.dart

@@ -55,6 +55,32 @@ void main() {
     }
   });
 
+  test('a session update carries how full the context is, when told', () {
+    const plain = LinkSessionUpdate(
+      session: 's',
+      start: 0,
+      items: [],
+      count: 0,
+    );
+    expect(plain.toJson().containsKey('context'), isFalse);
+    const context = {
+      'window': 200000,
+      'used': 52000,
+      'parts': [
+        {'part': 'messages', 'tokens': 40000},
+      ],
+      'compactAt': 167000,
+    };
+    const update = LinkSessionUpdate(
+      session: 's',
+      start: 0,
+      items: [],
+      count: 0,
+      context: context,
+    );
+    expect(LinkSessionUpdate.fromJson(update.toJson()).context, context);
+  });
+
   test('a bot\'s picture is told from an emoji by its file\'s name', () {
     expect(isAvatarImage('avatar-0a1b2c3d4e5f.jpg'), isTrue);
     expect(isAvatarImage('avatar-0a1b2c3d4e5f.PNG'), isFalse);