Claude Architect · Lesson

Anatomy of an API Request

model, max_tokens, system, messages, tools, tool_choice.

The Single Endpoint

Every call to Claude is one request to the Messages API. As an architect, you don't memorize syntax — you reason about six fields that shape the whole interaction: model, max_tokens, system, messages, tools, and tool_choice.

Get these right and everything downstream — agents, tool loops, structured output — falls into place. This lesson walks through each field and the decision it represents.

from anthropic import Anthropic

client = Anthropic()
response = client.messages.create(
    model="claude-opus-4-8",
    max_tokens=1024,
    system="You are a concise assistant.",
    messages=[{"role": "user", "content": "Hello!"}],
)
print(response.content[0].text)

model — Which Brain

The model field selects which Claude does the work. It's a single string, and the choice is a real architectural trade-off: capability versus latency versus cost.

Opus — most capable, best for long-horizon agentic and hard reasoning.
Sonnet — strong balance of speed and intelligence.
Haiku — fastest and cheapest for simple, high-volume tasks.

You can change the model per request, so route easy tasks to a cheaper model and hard ones to a stronger one.

# Same request shape, different routing decision
response = client.messages.create(
    model="claude-opus-4-8",  # swap to a cheaper model for simple tasks
    max_tokens=1024,
    messages=[{"role": "user", "content": "Summarize this ticket."}],
)

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