Model Context Protocol (MCP)

Also Known As

TL;DR

Explanation

MCP defines a client-server protocol where: MCP servers expose tools (functions the LLM can call), resources (data the LLM can read), and prompts (reusable prompt templates). MCP clients (Claude, IDE plugins, custom agents) connect to servers and make tools available to the LLM. The protocol uses JSON-RPC over stdio or HTTP/SSE. MCP servers can be written in any language — PHP can both consume MCP servers and act as an MCP server exposing PHP application data to AI agents.

Common Misconception

Why It Matters

Common Mistakes

• MCP servers with excessive permissions — an MCP server that can delete production data should require confirmation, not execute blindly.
• Not validating tool inputs — MCP server tools receive LLM-generated arguments; validate them as strictly as any user input.
• No authentication on HTTP-based MCP servers — anyone who can reach the server can invoke your tools.
• Stateful MCP servers — MCP servers should be stateless; state belongs in the resources they read.

Code Examples

// MCP tool with no input validation:
$tools = [[
    'name' => 'run_sql',
    'description' => 'Run a SQL query',
    'inputSchema' => ['query' => 'string'],
]];
// Handler executes whatever SQL the LLM generates — SQL injection from AI

// MCP tool with restricted, validated operations:
$tools = [[
    'name' => 'get_user_orders',
    'description' => 'Get orders for a specific user ID',
    'inputSchema' => [
        'type' => 'object',
        'properties' => ['user_id' => ['type' => 'integer', 'minimum' => 1]],
        'required' => ['user_id'],
    ],
]];
// Handler uses parameterised query, read-only DB user:
$orders = $pdo->prepare('SELECT * FROM orders WHERE user_id = ?');
$orders->execute([$input['user_id']]);

References

• ↗ https://modelcontextprotocol.io/

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