Get an API key, make one request

Documentation tells you what a request looks like. Running one tells you what it feels like — the latency, the response shape, what stop_reason actually means when you read it live.

Get your key

Go to console.anthropic.com, sign up, and create an API key under the API Keys section. The console shows the key exactly once — copy it immediately. Store it in an environment variable, not in your source code.

export ANTHROPIC_API_KEY="sk-ant-..."

Add that line to your shell profile (~/.zshrc or ~/.bashrc) so it survives a new terminal session. Every example in this course reads from that variable. Nothing in the codebase ever holds the key string itself — not a config file, not a comment, not a test fixture.

Make the call

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-opus-4-5",
    "max_tokens": 256,
    "messages": [
      {"role": "user", "content": "Say hello."}
    ]
  }'

Three headers, one JSON body. The anthropic-version header is required — without it the API returns a 400. The version string pins the response contract; Anthropic does not break old versions silently, so you can leave this value fixed until you deliberately upgrade.

Read the response

{
  "id": "msg_01XFDUDYJgAACzvnptvVoYEL",
  "type": "message",
  "role": "assistant",
  "content": [{"type": "text", "text": "Hello! How can I help you today?"}],
  "model": "claude-opus-4-5",
  "stop_reason": "end_turn",
  "usage": {"input_tokens": 10, "output_tokens": 12}
}

The text lives at content[0].text. The usage block tells you the exact token cost — input_tokens for what you sent, output_tokens for the reply. Store both when you log requests in production. The stop_reason field tells you why generation stopped: end_turn means the model finished naturally; max_tokens means you hit the cap and the response was cut short.

The same call in Python

Once you have a working curl command you understand what is on the wire. Now install the SDK and verify the two produce the same result:

# scripts/hello.py
import anthropic

client = anthropic.Anthropic()  # reads ANTHROPIC_API_KEY from environment

response = client.messages.create(
    model="claude-opus-4-5",
    max_tokens=256,
    messages=[{"role": "user", "content": "Say hello."}],
)

print(response.content[0].text)
print(f"stop_reason: {response.stop_reason}")
print(f"input_tokens: {response.usage.input_tokens}")
print(f"output_tokens: {response.usage.output_tokens}")

Run pip install anthropic first. The SDK reads ANTHROPIC_API_KEY automatically — you do not pass it to the constructor. The response object gives you the same fields as the JSON above, typed. response.content[0].text is the string. response.usage.input_tokens is an integer. The SDK is the curl call with serialization handled.

Why no library first

The SDK wraps these three headers and the JSON serialization. Starting with raw curl means you know exactly what is on the wire, which means you can debug any SDK problem by stepping down to curl. That is worth ten minutes now.

If you get stuck

• 401 authentication_error: the key is wrong, missing, or the shell variable was not exported. Run echo $ANTHROPIC_API_KEY to confirm it is set. If it prints nothing, you need export, not just assignment (ANTHROPIC_API_KEY=... without export sets a local shell variable that child processes cannot see).
• 400 invalid_request_error: the JSON is malformed or a required field is missing. Paste the body into jq . to check syntax. Confirm model, max_tokens, and messages are all present.
• 400 with a message about anthropic-version: you omitted the version header. Add -H "anthropic-version: 2023-06-01".
• Response truncated mid-sentence: max_tokens is too low. Raise it and re-run.
• Python ModuleNotFoundError: run pip install anthropic. If you are in a virtual environment, confirm it is activated before installing.

Cross-reference

l2 (Messages, system, and the request shape) breaks down each field in the request body in detail. l4 (Errors, retries, and timeouts) covers what to do when the API returns 429 or 529 in production.

Deliverable

A 200 response with "role": "assistant" and a non-empty content[0].text from both the curl command and the Python script. Note the input_tokens value from both runs — they should match.

Further reading

• "One File for All My Keys" — names the convention that keeps ANTHROPIC_API_KEY out of every project's .env and into a single canonical file you source.