What is MCP Adapter

Purpose and Scope

This document explains the core purpose of MCP Adapter, its relationship to the WordPress Abilities API, and how it enables AI clients to interact with WordPress functionality. For detailed information about key concepts like tools, resources, prompts, and transport layers, see Key Concepts. For architectural details, see Architecture.

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What is MCP Adapter

MCP Adapter is a WordPress package that implements the Model Context Protocol (MCP) specification, exposing WordPress functionality to AI agents. It serves as a translation layer between WordPress's Abilities API and the MCP specification, automatically converting registered WordPress abilities into MCP-compliant tools, resources, and prompts.

The adapter is implemented as a singleton registry (McpAdapter) that manages multiple MCP servers, each capable of exposing different sets of abilities through standardized communication protocols.

Key characteristics:

• Implements MCP specification 2025-06-18
• Supports both HTTP and STDIO transport protocols
• Provides automatic ability-to-MCP component conversion
• Offers multi-server architecture with isolated configurations
• Includes built-in error handling and observability

The MCP Adapter Position

Sources: README.md1-501 includes/Core/McpAdapter.php1-500

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Relationship to WordPress Abilities API

MCP Adapter depends entirely on the WordPress Abilities API for its functionality. The Abilities API provides:

Abilities API Component	Purpose	MCP Adapter Usage
wp_register_ability()	Registers abilities with metadata	Source of all MCP components
wp_get_ability()	Retrieves registered abilities	Used by handlers to access abilities
input_schema	Defines ability input structure	Converted to MCP tool schemas
output_schema	Defines ability output structure	Used for response validation
execute_callback	Executes ability logic	Invoked by MCP handlers
permission_callback	Checks ability permissions	Enforced during MCP requests

The relationship is one-way: WordPress abilities are registered independently of MCP Adapter, and the adapter automatically discovers and exposes them based on metadata configuration.

Ability Registration Flow

Sources: README.md176-185 includes/Core/McpAdapter.php1-500 includes/Domain/Tools/RegisterAbilityAsMcpTool.php1-200

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The Bridge Architecture

MCP Adapter acts as a bridge with three distinct layers:

1. Transport Layer

Handles communication between AI clients and WordPress:

Transport	Class	Protocol	Use Case
HTTP	HttpTransport	JSON-RPC 2.0 over HTTP	Production AI client connections
STDIO	StdioServerBridge	JSON-RPC 2.0 over stdin/stdout	Local development and WP-CLI testing

Sources: includes/Transport/HttpTransport.php1-500 includes/Cli/StdioServerBridge.php1-300

2. Handler Layer

Processes MCP protocol methods:

Handler	File	MCP Methods
InitializeHandler	includes/Handlers/Initialize/InitializeHandler.php	initialize
ToolsHandler	includes/Handlers/Tools/ToolsHandler.php	tools/list, tools/call
ResourcesHandler	includes/Handlers/Resources/ResourcesHandler.php	resources/list, resources/read
PromptsHandler	includes/Handlers/Prompts/PromptsHandler.php	prompts/list, prompts/get

Sources: includes/Handlers/ README.md102-108

3. Domain Layer

Transforms WordPress abilities into MCP components:

Conversion Class	Source	Target	Responsibility
RegisterAbilityAsMcpTool	WordPress Ability	McpTool	Schema transformation, execution wrapping
RegisterAbilityAsMcpResource	WordPress Ability	McpResource	URI-based identification, read operations
RegisterAbilityAsMcpPrompt	WordPress Ability	McpPrompt	Message generation, argument templates

Sources: includes/Domain/Tools/RegisterAbilityAsMcpTool.php includes/Domain/Resources/RegisterAbilityAsMcpResource.php includes/Domain/Prompts/RegisterAbilityAsMcpPrompt.php

Request Flow Through the Bridge

Sources: includes/Transport/HttpTransport.php1-500 includes/Transport/Infrastructure/RequestRouter.php1-300 includes/Handlers/Tools/ToolsHandler.php1-400

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Core Components and Their Roles

McpAdapter Singleton

The central registry at includes/Core/McpAdapter.php1-500 manages all MCP servers:

Key methods:

• instance(): Returns singleton instance
• init(): Initializes WordPress integration, creates default server
• create_server(): Creates new MCP server instances
• get_server(): Retrieves server by ID
• register_routes(): Registers REST API endpoints

Initialization hook: mcp_adapter_init fires after default server creation, allowing plugins to create custom servers.

McpServer

Individual server management at includes/Core/McpServer.php1-500:

Properties:

• id: Unique server identifier (e.g., mcp-adapter-default-server)
• namespace: REST API namespace (e.g., mcp)
• route: REST API route (e.g., mcp-adapter-default-server)
• component_registry: Stores tools, resources, prompts

Key methods:

• register_abilities(): Converts abilities to MCP components
• get_tools(): Returns registered tools
• get_resources(): Returns registered resources
• get_prompts(): Returns registered prompts

McpComponentRegistry

Component storage at includes/Core/McpComponentRegistry.php1-300:

Stores:

• Tools as McpTool instances
• Resources as McpResource instances
• Prompts as McpPrompt instances

Methods:

• register_tool(), register_resource(), register_prompt(): Add components
• get_tool(), get_resource(), get_prompt(): Retrieve by name/URI

Sources: includes/Core/McpAdapter.php1-500 includes/Core/McpServer.php1-500 includes/Core/McpComponentRegistry.php1-300

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Transport Protocols

HTTP Transport

The HttpTransport class at includes/Transport/HttpTransport.php1-500 implements MCP over HTTP:

Request handling:

1. Parse JSON-RPC 2.0 request body
2. Validate or create session (via HttpSessionValidator)
3. Check transport permissions
4. Route to appropriate handler (via RequestRouter)
5. Build JSON-RPC response (via JsonRpcResponseBuilder)

Endpoint pattern: /wp-json/{namespace}/{route}

Example: /wp-json/mcp/mcp-adapter-default-server

STDIO Transport

The StdioServerBridge class at includes/Cli/StdioServerBridge.php1-300 implements MCP over stdin/stdout:

Invocation: wp mcp-adapter serve --server=server-id --user=username

Request handling:

1. Read JSON-RPC from stdin
2. Route to handler
3. Write JSON-RPC response to stdout

Use case: Local development, WP-CLI testing, process-based MCP clients

Sources: includes/Transport/HttpTransport.php1-500 includes/Cli/StdioServerBridge.php1-300 includes/Cli/McpCommand.php1-200

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MCP Component Types

MCP Adapter exposes three types of components as defined in the MCP specification:

Component Type	MCP Spec	WordPress Ability	Determines By
Tool	Executable function	Ability with execute_callback	meta.mcp.type = 'tool' or default
Resource	Readable data source	Ability with execute_callback returning data	meta.mcp.type = 'resource'
Prompt	Template for AI guidance	Ability returning message structure	meta.mcp.type = 'prompt'

Automatic Component Conversion

Sources: includes/Domain/Tools/RegisterAbilityAsMcpTool.php1-200 includes/Domain/Resources/RegisterAbilityAsMcpResource.php1-200 includes/Domain/Prompts/RegisterAbilityAsMcpPrompt.php1-200

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Default Server and Built-in Abilities

MCP Adapter automatically creates a default server with ID mcp-adapter-default-server at includes/Servers/DefaultServerFactory.php1-200 This server includes three built-in abilities for system introspection:

Built-in Ability	Class	Purpose
mcp-adapter-discover-abilities	DiscoverAbilitiesAbility	List all registered WordPress abilities
mcp-adapter-execute-ability	ExecuteAbilityAbility	Execute any ability by name
mcp-adapter-get-ability-info	GetAbilityInfoAbility	Retrieve detailed ability information

These abilities enable AI agents to discover and interact with WordPress functionality dynamically.

Sources: includes/Servers/DefaultServerFactory.php1-200 includes/Abilities/DiscoverAbilitiesAbility.php1-200 includes/Abilities/ExecuteAbilityAbility.php1-300

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Summary

MCP Adapter is a translation layer that:

1. Implements the Model Context Protocol specification 2025-06-18
2. Bridges WordPress Abilities API to MCP-compliant components
3. Provides HTTP and STDIO transport protocols for AI client communication
4. Manages multiple isolated MCP servers with independent configurations
5. Automatically converts WordPress abilities into tools, resources, and prompts
6. Exposes functionality through REST API endpoints and WP-CLI commands

The adapter enables AI agents to interact with WordPress using standardized protocols, with all functionality derived from the WordPress Abilities API. For detailed information about creating abilities, see Creating Abilities. For security and permissions, see Security and Permissions.

Sources: README.md1-501 includes/Core/McpAdapter.php1-500 includes/Core/McpServer.php1-500