fix(mobile): harden agent card loading and link sign-in

- Don't follow redirects for agent.json, check content-length up front, and
  stop buffering at the cap.
- Any unexpected failure while loading a card re-enables Create and shows a
  message instead of leaving the page stuck.
- A link that needs sign-in is dropped only after sign-in was seen running,
  not on an effect rerun before reconnectAccount reaches that stage.
- Table tests for fetch: request URL and redirects, 403/404, 302, 500,
  network error, declared and streamed oversize bodies, non-JSON, array.

Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>
Signed-off-by: Tom Brow <tomb@squareup.com>

mobile/lib/features/agents/agent_creation_draft.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 import 'dart:convert';
+import 'dart:typed_data';
 
 import 'package:hooks_riverpod/hooks_riverpod.dart';
 import 'package:http/http.dart' as http;
@@ -66,7 +67,10 @@ class AgentCreationDraft {
     try {
       response = await client
           .send(
-            http.Request('GET', url)..headers['accept'] = 'application/json',
+            http.Request('GET', url)
+              ..headers['accept'] = 'application/json'
+              // A redirect could lead off the CDN; treat it as unavailable.
+              ..followRedirects = false,
           )
           .timeout(_fetchTimeout);
       if (response.statusCode == 403 || response.statusCode == 404) {
@@ -77,6 +81,9 @@ class AgentCreationDraft {
           'Couldn’t load this agent card (HTTP ${response.statusCode}).',
         );
       }
+      if ((response.contentLength ?? 0) > maxAgentCardBytes) {
+        throw const AgentCardException('This agent card is too large.');
+      }
       body = await _readBounded(response.stream).timeout(_fetchTimeout);
     } on AgentCardException {
       rethrow;
@@ -159,13 +166,13 @@ class AgentCreationDraft {
   }
 
   static Future<List<int>> _readBounded(Stream<List<int>> stream) async {
-    final bytes = <int>[];
+    final bytes = BytesBuilder(copy: false);
     await for (final chunk in stream) {
-      bytes.addAll(chunk);
-      if (bytes.length > maxAgentCardBytes) {
+      if (bytes.length + chunk.length > maxAgentCardBytes) {
         throw const AgentCardException('This agent card is too large.');
       }
+      bytes.add(chunk);
     }
-    return bytes;
+    return bytes.takeBytes();
   }
 }

mobile/lib/features/agents/agents_page.dart

@@ -305,6 +305,11 @@ class AgentsPage extends HookConsumerWidget {
       () {
         final link = draft;
         if (link == null || draftStep.value == _DraftStep.done) return null;
+        // Sign-in counts as attempted only once it is seen running, so a
+        // rerun before reconnectAccount reaches its sign-in stage still waits.
+        if (signingIn && draftStep.value == _DraftStep.signInRequested) {
+          draftStep.value = _DraftStep.signingIn;
+        }
         if (community == null || controller.busy || signingIn) return null;
         if (!supportsRemoteAgents(community)) {
           drop('Remote agents aren’t available in this community.');
@@ -312,9 +317,16 @@ class AgentsPage extends HookConsumerWidget {
         }
         if (controller.account.account == null) {
           if (draftStep.value == _DraftStep.wait) {
-            draftStep.value = _DraftStep.signingIn;
-            unawaited(flow.reconnectAccount());
-          } else {
+            draftStep.value = _DraftStep.signInRequested;
+            unawaited(
+              flow.reconnectAccount().catchError((Object error) {
+                debugPrint('agents: link sign-in failed: $error');
+                if (context.mounted) {
+                  drop('Couldn’t start Builderlab sign-in. Reopen the link.');
+                }
+              }),
+            );
+          } else if (draftStep.value == _DraftStep.signingIn) {
             drop(
               'Sign in to Builderlab to create this agent. Reopen the link.',
             );
@@ -342,10 +354,17 @@ class AgentsPage extends HookConsumerWidget {
           final AgentCreationDraft resolved;
           try {
             resolved = await AgentCreationDraft.fetch(link, client);
-          } on AgentCardException catch (error) {
+          } catch (error) {
+            if (error is! AgentCardException) {
+              debugPrint('agents: agent card failed: $error');
+            }
             if (context.mounted) {
               creating.value = false;
-              drop(error.message);
+              drop(
+                error is AgentCardException
+                    ? error.message
+                    : 'Couldn’t open this agent card.',
+              );
             }
             return;
           } finally {
@@ -505,5 +524,6 @@ class AgentsPage extends HookConsumerWidget {
   }
 }
 
-/// Progress of opening a link's draft: waiting on gates, in sign-in, or done.
-enum _DraftStep { wait, signingIn, done }
+/// Progress of opening a link's draft: waiting on gates, sign-in requested,
+/// sign-in seen running, or done.
+enum _DraftStep { wait, signInRequested, signingIn, done }

mobile/test/features/agents/agents_harness.dart

@@ -19,7 +19,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:hooks_riverpod/hooks_riverpod.dart';
-import 'package:hooks_riverpod/misc.dart' show Override;
+import 'package:hooks_riverpod/misc.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import '../../helpers/remote_agents_fixture.dart';
@@ -209,7 +209,6 @@ class AgentsHarness {
     await tester.pumpWidget(
       ProviderScope(
         overrides: [
-          ...overrides,
           ownedAgentDirectoryProvider.overrideWith(
             (ref) =>
                 loadOwnedAgentDirectory(fakeOwner.public, relay.fetchHistory),
@@ -229,6 +228,7 @@ class AgentsHarness {
                 AgentDirectoryEntry(pubkey: pubkey),
             ],
           ),
+          ...overrides,
         ],
         child: MaterialApp(
           theme: AppTheme.light(),

mobile/test/shared/deeplink/agent_creation_link_test.dart

@@ -1,6 +1,11 @@
+import 'dart:convert';
+
 import 'package:buzz/features/agents/agent_creation_draft.dart';
 import 'package:buzz/shared/deeplink/deep_link.dart';
+import 'package:buzz/shared/hosted/hosted_environment.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:http/http.dart' as http;
+import 'package:http/testing.dart';
 
 void main() {
   group('buzz://agent/new links', () {
@@ -169,4 +174,85 @@ void main() {
       });
     });
   });
+
+  group('fetching agent.json', () {
+    const link = AgentCreationDeepLink('av_1');
+    final big = 'x' * (maxAgentCardBytes + 1);
+
+    Future<Object> outcome(MockClientStreamHandler handler) async {
+      try {
+        return await AgentCreationDraft.fetch(
+          link,
+          MockClient.streaming(handler),
+        );
+      } on AgentCardException catch (error) {
+        return error.message;
+      }
+    }
+
+    http.StreamedResponse respond(
+      int status, [
+      List<List<int>> chunks = const [],
+      int? length,
+    ]) => http.StreamedResponse(
+      Stream.fromIterable(chunks),
+      status,
+      contentLength: length,
+    );
+
+    test('requests the build CDN without following redirects', () async {
+      late http.BaseRequest seen;
+      final result = await outcome((request, _) async {
+        seen = request;
+        return respond(200, [utf8.encode('{"name":"Scout"}')]);
+      });
+      expect(result, isA<AgentCreationDraft>());
+      expect(
+        seen.url,
+        HostedEnvironment.avatarsCdnOrigin.replace(path: '/av_1/agent.json'),
+      );
+      expect(seen.followRedirects, isFalse);
+    });
+
+    final failures = <String, (MockClientStreamHandler, String)>{
+      '403 from S3': (
+        (_, _) async => respond(403),
+        'This agent card isn’t available.',
+      ),
+      '404': ((_, _) async => respond(404), 'This agent card isn’t available.'),
+      'redirect': (
+        (_, _) async => respond(302),
+        'Couldn’t load this agent card (HTTP 302).',
+      ),
+      '500': (
+        (_, _) async => respond(500),
+        'Couldn’t load this agent card (HTTP 500).',
+      ),
+      'network error': (
+        (_, _) async => throw http.ClientException('offline'),
+        'Couldn’t load this agent card. Check your connection and reopen the '
+            'link.',
+      ),
+      'declared length over the cap': (
+        (_, _) async => respond(200, const [], maxAgentCardBytes + 1),
+        'This agent card is too large.',
+      ),
+      'streamed body over the cap': (
+        (_, _) async =>
+            respond(200, [utf8.encode('{"name":"'), utf8.encode(big)]),
+        'This agent card is too large.',
+      ),
+      'not JSON': (
+        (_, _) async => respond(200, [utf8.encode('<html>')]),
+        'This agent card is invalid.',
+      ),
+      'JSON array': (
+        (_, _) async => respond(200, [utf8.encode('[]')]),
+        'This agent card is invalid.',
+      ),
+    };
+    failures.forEach((label, c) {
+      test(label, () async => expect(await outcome(c.$1), c.$2));
+    });
+  });
 }