# Engine MCP

Execution-time MCP registries, facades, session handling, schema discovery, and tool calls.

For user-facing provider and tool config objects, see [MCP configuration](../config/mcp.md).

## Parallel Structure

| Model layer | MCP layer | Purpose |
|-------------|-----------|---------|
| `ModelProviderRegistry` | `MCPProviderRegistry` | Holds provider configurations. |
| `ModelRegistry` | `MCPRegistry` | Manages configs by alias and lazily creates facades. |
| `ModelFacade` | `MCPFacade` | Provides a lightweight runtime facade scoped to one config. |
| `ModelConfig.alias` | `ToolConfig.tool_alias` | Alias referenced by column configs. |

## Registry

### `MCPToolDefinition` {#data_designer.engine.mcp.registry.MCPToolDefinition}

::: data_designer.engine.mcp.registry.MCPToolDefinition
    options:
      show_root_toc_entry: false

### `MCPToolResult` {#data_designer.engine.mcp.registry.MCPToolResult}

::: data_designer.engine.mcp.registry.MCPToolResult
    options:
      show_root_toc_entry: false

### `MCPRegistry` {#data_designer.engine.mcp.registry.MCPRegistry}

::: data_designer.engine.mcp.registry.MCPRegistry
    options:
      show_root_toc_entry: false

### `create_mcp_registry` {#data_designer.engine.mcp.factory.create_mcp_registry}

::: data_designer.engine.mcp.factory.create_mcp_registry
    options:
      show_root_toc_entry: false

## Facade

`ModelFacade.generate()` accepts a `tool_alias` parameter. When it is provided, `ModelFacade` looks up the matching `MCPFacade` from `MCPRegistry`, fetches tool schemas, passes them to the model, processes tool calls after each completion, tracks tool-call turns, and returns messages that include tool results for trace capture.

### `MCPFacade` {#data_designer.engine.mcp.facade.MCPFacade}

::: data_designer.engine.mcp.facade.MCPFacade
    options:
      show_root_toc_entry: false

## I/O Service

The I/O service owns a background event loop, pools MCP sessions by provider config, coalesces concurrent tool schema lookups, and executes parallel tool calls.

### `MCPIOService` {#data_designer.engine.mcp.io.MCPIOService}

::: data_designer.engine.mcp.io.MCPIOService
    options:
      show_root_toc_entry: false

### Runtime Helpers

::: data_designer.engine.mcp.io.list_tools
    options:
      show_root_toc_entry: false

::: data_designer.engine.mcp.io.list_tool_names
    options:
      show_root_toc_entry: false

::: data_designer.engine.mcp.io.call_tools
    options:
      show_root_toc_entry: false

::: data_designer.engine.mcp.io.clear_provider_caches
    options:
      show_root_toc_entry: false

::: data_designer.engine.mcp.io.clear_tools_cache
    options:
      show_root_toc_entry: false

::: data_designer.engine.mcp.io.get_cache_info
    options:
      show_root_toc_entry: false

::: data_designer.engine.mcp.io.clear_session_pool
    options:
      show_root_toc_entry: false

::: data_designer.engine.mcp.io.get_session_pool_info
    options:
      show_root_toc_entry: false