Chapter L6.6 — Integrating via API

Level 6 — Advanced. Product details verified on 24/06/2026 against official sources.

Goal

By the end you’ll know when to move to the API, what a call to the messages endpoint looks like, and how tool use works. It’s the bridge from someone who uses Claude to someone who integrates it into their own software.

Prerequisites

• An API key from the Console (ch. L2.3, F.3).
• Familiarity with a programming language.

When you need the API

The interfaces (chat, Cowork, Code) are enough for personal work. The API is for when you want to put Claude inside a product of yours: an app that answers customers, a process that classifies documents, a service that generates text on demand. It’s programmatic access, pay-as-you-go by token (ch. F.3).

The messages endpoint

The heart of the API is one endpoint: you send a list of messages, Claude generates the next one. The raw call, with curl:

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-4-8",
    "max_tokens": 1024,
    "messages": [
      {"role": "user",
       "content": "Hello"}
    ]
  }'

Three headers are mandatory: x-api-key (your key), anthropic-version (e.g. 2023-06-01), and content-type. The body declares at least model, max_tokens, and messages. (VOLATILE: the model string changes, see the ledger.)

With the SDK

In practice you don’t use curl, but an official SDK (Python or TypeScript), which handles headers and formats. In Python:

import anthropic

# Reads the key from ANTHROPIC_API_KEY
client = anthropic.Anthropic()

msg = client.messages.create(
    model="claude-opus-4-8",
    max_tokens=1024,
    messages=[
        {"role": "user",
         "content": "Hello, who are you?"},
    ],
)
print(msg.content[0].text)

Tool use, in brief

Tool use lets Claude use tools you provide — a function that queries a database, calls an API, runs a calculation. The flow is a back-and-forth in three steps:

1. You define the tools in the request and send the message.
2. If a tool is needed, Claude responds with stop_reason: "tool_use" and a block that says which tool and with which arguments.
3. Your code runs the operation and returns the result as a tool_result; Claude uses it for the final answer.

Figure L6.6.1 — The tool use cycle. Alt text: vertical diagram from the message to the tool request, to execution in your code, to the result, to the final answer.

The key point: the tool you run yourself, in your software. Claude decides when to use it and with which arguments, but the code stays under your control.

In practice: your first call

1. Generate an API key in the Console and set it as an environment variable (ch. L2.3).
2. Install the SDK for your language (Python or TypeScript).
3. Make a minimal call to the messages endpoint and read the response.
4. To integrate actions, define a tool and handle the tool_use → tool_result cycle.
5. Keep the key out of the code: use environment variables (ch. L2.3).

Common mistakes

• Missing headers. You need x-api-key, anthropic-version, and content-type.
• Hardcoded model string. It changes over time: take it from the ledger or the official sources.
• Key in the code. Never: use an environment variable (ch. L2.3).
• Expecting Claude to run the tool. You run it; Claude decides when and how to call it.

Summary

1. The API is for integrating Claude inside your software; pay-as-you-go.
2. Endpoint POST /v1/messages with headers x-api-key, anthropic-version, content-type.
3. In practice you use an SDK (Python/TS), not curl.
4. Tool use is a cycle: Claude asks for a tool, your code runs it and returns the result.
5. The key lives in an environment variable, never in the code.

Next step

You’ve completed Level 6. In the Closing, ch. C.1 — End-to-end project runs through all the levels on a real case, from design to code to automation.

Data on the endpoint, headers, and tool use verified on 24/06/2026 against platform.claude.com/docs (messages, tool use). The examples were not run here; they require a valid API key.