Anthropic Messages API: A Beginner’s Walkthrough

Wire formatsEN2026-10-25

The Anthropic Messages API accepts a conversation and returns the next assistant message. Start with one short request, inspect the response, then add conversation history. This walkthrough focuses on that basic cycle before introducing more advanced features. ([platform.claude.com](https://platform.claude.com/docs/en/api/messages/create))

§ Step 1: Choose your access route

First, distinguish the protocol from the provider. The Messages API defines the request and response format. Your provider determines which credentials, models and access rules apply. Direct Anthropic access requires a Claude Console account and an API key. ([platform.claude.com](https://platform.claude.com/docs/en/get-started))

Vana API provides a shared address for different client protocols, but check its current Claude access requirements before configuring a custom application. Its published guidance currently restricts Claude models to supported official clients. Create a Vana API account if you want to explore the available integrations. ([vanaapi.com](https://vanaapi.com/))

§ Step 2: Prepare authentication

For a direct Anthropic HTTP request, send your API key using the Authorization header with the Bearer scheme. The x-api-key header is also supported. Include content-type: application/json and an anthropic-version header using a documented version. The official SDK handles these standard headers automatically. ([platform.claude.com](https://platform.claude.com/docs/en/api/overview?utm_source=openai))

Keep your key in an environment variable or secrets manager, never in public source code or browser-delivered JavaScript. Treat credentials from different providers separately: an Anthropic key and a gateway key are not interchangeable. ([platform.claude.com](https://platform.claude.com/docs/en/manage-claude/authentication?utm_source=openai))

§ Step 3: Build a minimal request

The Messages endpoint accepts POST requests at /v1/messages. Begin with three fields: model, max_tokens and messages. Each message needs a role and content. For the first test, use one user message and leave optional features out. ([platform.claude.com](https://platform.claude.com/docs/en/api/messages/create))

model: Copy the exact identifier supported by your provider. For example, select Claude Sonnet 5.5 rather than guessing an identifier from its display name.
max_tokens: Set an output ceiling, such as 256, for a short test. This is a maximum, not a requested answer length.
messages: Supply a list containing one object with role set to user and content containing your question. ([platform.claude.com](https://platform.claude.com/docs/en/api/messages/create))

Try the prompt: “Explain an HTTP request in three short sentences for a beginner.” Specifying the audience and desired format gives you a clear result to evaluate. ([platform.claude.com](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/claude-prompting-best-practices))

§ Step 4: Add stable instructions

If your application needs a consistent role or tone, use the top-level system parameter. For example: “You are a patient programming tutor. Define unfamiliar terms before using them.” Keep the immediate question in the user message. Anthropic documents the top-level system field for this purpose. ([platform.claude.com](https://platform.claude.com/docs/en/api/messages/create))

Make the first test deliberately narrow. Ask one question, inspect the answer, and change one instruction at a time. This makes it easier to identify whether an unexpected result comes from your prompt or your integration.

§ Step 5: Read the response correctly

Do not assume the response is a single text string. The content field contains blocks. Inspect each block’s type and collect text from text blocks. Anthropic’s introductory SDK example uses this pattern rather than printing only the first block. ([platform.claude.com](https://platform.claude.com/docs/en/get-started))

Also inspect stop_reason. An end_turn value indicates a natural stopping point; max_tokens means generation reached a token limit. Check this before treating a shortened answer as complete. Record the usage fields when monitoring token consumption. ([platform.claude.com](https://platform.claude.com/docs/en/api/cli/beta/messages/create?utm_source=openai))

§ Step 6: Continue the conversation

Basic Messages requests are stateless. For a follow-up, send the earlier user message, the returned assistant content and the new user message in chronological order. Asking “Give me an example” without supplying the earlier exchange does not recreate that context. ([platform.claude.com](https://platform.claude.com/docs/en/api/messages/create?utm_source=openai))

§ Step 7: Handle failures before expanding

Read the error body before retrying. A 400 can indicate an invalid request, while a 401 points to authentication. A 429 may indicate a rate limit or spending cap, so immediate repeated retries are not a reliable fix. Preserve the request ID for troubleshooting. ([platform.claude.com](https://platform.claude.com/docs/en/api/errors))

Your first milestone is simple: one authenticated request, readable text output, a contextual follow-up and useful error reporting. Once those work consistently, expand the integration one feature at a time.