8.3MCP Token Breakdown: A Real Playwright Session

Tool Schema Cost (Per API Call)

An MCP server exposes 15-25 browser tools, each with a JSON schema (~150-280 tokens apiece). Total: roughly 3,000-4,000 tokens of schemas loaded into EVERY API request, even on turns where the agent isn't doing browser work.

Accessibility Tree Cost (Per Page Read)

A typical web page accessibility snapshot, streamed inline with actions:

Page Complexity	Elements	A11y Tree Tokens
Simple (landing page)	20-50	~500-1,500
Medium (form page)	50-200	~1,500-5,000
Complex (dashboard)	200-500	~5,000-15,000
Data-heavy (table)	500-2000	~15,000-50,000+

A Realistic 20-Step Login Test via MCP

Step 1:  browser_navigate           → 200 tokens (call) + 100 (response)
Step 2:  [a11y tree streamed]       → 3,000 tokens (login form)
Step 3:  browser_type (email)       → 180 tokens + 80
Step 4:  browser_type (password)    → 180 tokens + 80
Step 5:  browser_click (submit)     → 160 tokens + 80
Step 6:  [a11y tree streamed]       → 5,000 tokens (dashboard)
Step 7:  browser_get_text (heading) → 150 tokens + 80
Step 8:  browser_screenshot         → 150 tokens + inline image data
Step 9:  browser_navigate (profile) → 200 tokens + 100
Step 10: [a11y tree streamed]       → 4,000 tokens (profile page)
...     (edit form, save, verify — three more tree streams)
Step 20: browser_close              → 120 tokens + 60

Tool schemas (loaded every turn):    ~3,400 × 20 = ~68,000 tokens
A11y trees (streamed, no opt-out):   ~20,000 tokens
Tool calls + responses:              ~4,000 tokens
────────────────────────────────────────────────────
TOTAL:                               ~92,000 tokens

Our estimate lands in the same order as Microsoft's measured ~114k per task (their benchmark includes agent reasoning turns as well). Either way: more than half of usable context gone on a simple login test.