API Tips intermediate

Pre-Count Tokens Before Sending Requests

Use the free token counting endpoint to estimate costs before making expensive API calls, especially useful for dynamic prompts with variable-length content.

March 15, 2026

The token counting endpoint tells you exactly how many tokens a message will use before you spend money sending it. The endpoint itself is free.

API Usage

import anthropic
client = anthropic.Anthropic()

token_count = client.messages.count_tokens(
    model="claude-sonnet-4-6-20260301",
    messages=[{
        "role": "user",
        "content": "Your potentially long prompt here..."
    }],
    system="Your system prompt"
)

print(f"Input tokens: {token_count.input_tokens}")

# Budget enforcement
estimated_cost = token_count.input_tokens * 0.000003  # Sonnet input pricing
if estimated_cost > 0.10:
    print(f"Warning: ~${estimated_cost:.4f} in input tokens alone")

Use Cases

Dynamic prompts - when user content varies wildly in length
Budget enforcement - reject requests exceeding a cost threshold
Model routing - send cheap prompts to Opus, expensive ones to Sonnet
Monitoring - track token patterns before they become costs

Combined with Prompt Caching

Count tokens to decide if caching is worthwhile. Prompts under 1,024 tokens don’t benefit from caching (minimum cacheable length). Prompts over 10K tokens that repeat are prime candidates.