flutter-mcp-toolkit-custom-tools

Custom MCP Toolkit Tools & Resources (Dynamic Registry)

Use this when bundled MCP tools (screenshot, semantic snapshot, tap, …) are not enough and you need app-specific read surfaces or actions — e.g. cart totals, feature flags, curated debug snapshots of internal state. Entries are registered in the Flutter process and exposed to the agent through the dynamic registry.

Migration: The legacy call-entry type was removed in intentcall Phase 6b. Use AgentCallEntry or mcpToolkitTool / mcpToolkitResource. See flutter-mcp-toolkit-intentcall-migration.

Boundary: Change app discovery, Flutter VM-service extensions, or app-owned debug surfaces here. Change canonical registry/session semantics, schema policy, platform projection, or publish behavior in the upstream IntentCall repository.

Pick the right primitive

Need	Use
One-off read of a simple value	fmt_evaluate_dart_expression (no app code change).
Stable read-only payload (diagnostics, JSON snapshot, “current route”)	AgentCallEntry.resource + fmt_client_resource. Prefer resources when the contract is “GET-like” and idempotent.
Parameterized or mutating action, or reusable named operation	AgentCallEntry.tool + fmt_client_tool.

Handler signatures

Native AgentCallEntry (preferred for new code)

Handlers receive AgentArguments (Map<String, Object?>) and return AgentResult:

import 'package:mcp_toolkit/mcp_toolkit.dart';

final tool = AgentCallEntry.tool(
  namespace: 'app',
  name: 'cart_get_snapshot',
  description: 'Return current cart total and items for a user.',
  inputSchema: const {
    'type': 'object',
    'additionalProperties': false,
    'properties': {
      'userId': {'type': 'string'},
    },
    'required': ['userId'],
  },
  handler: (final args) async {
    final userId = args['userId']?.toString() ?? '';
    final cart = CartRepository.instance.forUser(userId);
    return AgentResult.success(
      message: 'ok',
      data: {
        'total': cart.total,
        'items': cart.items.map((final i) => i.toJson()).toList(),
      },
    );
  },
);

await MCPToolkitBinding.instance.addEntries(entries: {tool});

Legacy MCPToolDefinition + MCPCallResult (bridge)

Built-in toolkits still use mcpToolkitTool / mcpToolkitResource to adapt Map<String, String> service-extension handlers:

final tool = mcpToolkitTool(
  namespace: 'app',
  handler: (final request) async {
    final userId = request['userId'] ?? '';
    return MCPCallResult(
      message: 'ok',
      parameters: {'userId': userId},
    );
  },
  definition: MCPToolDefinition(
    name: 'cart_get_snapshot',
    description: 'Return current cart total and items for a user.',
    inputSchema: ObjectSchema(
      properties: {
        'userId': StringSchema(),
      },
      required: ['userId'],
    ),
  ),
);

• Tool arguments on the wire are strings keyed by schema property names — parse with int.tryParse, jsonDecode, etc.
• Do not use request.arguments on the app side.

Prefer MCPToolkitBinding.instance.addEntries(entries: { ... }) before bootstrapFlutter(runApp: ...) so tools/resources register in one place with zone/error setup.

Register once at bootstrap — not inside build, not per-widget initState.

Custom resources

final resource = AgentCallEntry.resource(
  namespace: 'app',
  name: 'app_cart_digest',
  description: 'Compact cart summary for agents (read-only).',
  mimeType: 'application/json',
  handler: (final args) async => AgentResult.success(
    message: 'Cart digest',
    data: {
      'itemCount': CartRepository.instance.visibleCount,
      'currency': CartRepository.instance.currencyCode,
    },
  ),
);

Or via bridge:

mcpToolkitResource(
  namespace: 'app',
  definition: MCPResourceDefinition(
    name: 'app_cart_digest',
    description: 'Compact cart summary for agents (read-only).',
    mimeType: 'application/json',
  ),
  handler: (final request) async => MCPCallResult(
    message: 'Cart digest',
    parameters: {'itemCount': 3},
  ),
),

• name must be snake_case. Published resource URIs follow visual://localhost/... conventions; agents use fmt_client_resource and listings from fmt_list_client_tools_and_resources.

Schema rules (tools)

The MCP server enforces strict JSON Schema:

• Prefer additionalProperties: false unless you intentionally accept arbitrary keys.
• Mark required for anything the handler reads unconditionally.
• AgentResult.data / MCPCallResult.parameters must be JSON-serializable.

Discovery from the agent side

1. fmt_list_client_tools_and_resources — enumerate app-registered tools and resources.
2. fmt_client_tool — invoke a dynamic tool by name with JSON args.
3. fmt_client_resource — fetch a registered resource URI from the listing.

If something should appear but does not: confirm addEntries completed (await), then hot restart.

Lifecycle gotchas

• Hot reload + addEntries from widget code → duplicate registrations. Register once in main() / bootstrap.
• Debug mode only — release builds do not expose VM service extensions.
• Naming: the app uses a flat global namespace. Prefix tools/resources (cart_, flags_, nav_). If a built-in entry already uses a name, the custom entry is skipped without warning; the built-in handles the call and the custom handler does not run.
• Naming the running instance is MCPToolkitBinding.instance.setAppIdentity(label: 'Fleet app · staging'), called again whenever the name changes (sign-in, workspace switch). Discovery reads that label, and a tool of your own called app_identity is one of the collisions above.

When the agent authors surfaces for the user’s app

1. Ensure mcp_toolkit is in pubspec.yaml.
2. Add lib/mcp_tools/<domain>_surfaces.dart returning Set<AgentCallEntry> or calling addEntries once.
3. Wire from addEntries(...) before bootstrapFlutter(...) — never from StatefulWidget lifecycle.
4. Tight schemas; hot restart; then fmt_list_client_tools_and_resources before first client invoke.

Safety and scope

• Treat handlers as powerful debug hooks: avoid secrets, unchecked IO.
• Keep handlers thin: delegate to existing app services.

Common traps

• Mixing AgentArguments with Map<String, String> — pick one API (native vs mcpToolkitTool).
• Missing await on addEntries → race before discovery.
• inputSchema out of sync with the handler → agents trust the schema; update both.

Related

• flutter-mcp-toolkit-intentcall-migration — CLI migrator, breaking upgrade
• flutter-mcp-toolkit-guide → inspect / control / debug skills
• Repository ARCHITECTURE.md → “Dynamic Registry Architecture”