3.2MCP Token Cost: The Tool Schema Tax

Every MCP tool exposes a JSON schema that's loaded into every API request:

{
  "name": "browser_click",
  "description": "Click an element on the page",
  "inputSchema": {
    "type": "object",
    "properties": {
      "selector": {
        "type": "string",
        "description": "CSS selector for the element to click"
      },
      "timeout": {
        "type": "number",
        "description": "Maximum wait time in milliseconds",
        "default": 30000
      },
      "force": {
        "type": "boolean",
        "description": "Force click even if element is not actionable",
        "default": false
      }
    },
    "required": ["selector"]
  }
}

A typical Playwright MCP server exposes 15-25 tools. Each tool definition costs 200-500 tokens. That's 3,000-12,500 tokens just for the tool schemas — loaded on EVERY API call.

Plus, MCP responses often include accessibility trees (structured DOM representations):

[role="main"] Main Content
  [role="navigation"] Nav
    [role="link"] "Home" [ref=1]
    [role="link"] "About" [ref=2]
  [role="heading"] "Welcome" [ref=3]
  [role="textbox"] "Search..." [ref=4]
  [role="button"] "Submit" [ref=5]
  ... (hundreds of elements)

A single accessibility snapshot can cost 2,000-10,000 tokens depending on page complexity.

MCP Total Cost Per Turn

Tool schemas:          ~5,000 tokens  (fixed, every turn)
Accessibility tree:    ~5,000 tokens  (per page interaction)
Tool call/response:    ~200 tokens    (per command)
─────────────────────────────────────
Total per interaction: ~10,200 tokens

Over a 20-step test:

20 steps × 10,200 tokens = ~204,000 tokens (may exceed context window!)