Anthropic Messages API

Use this API for models labelled anthropic in the Model Marketplace or the model list for your current key. Read the API and model capability matrix first: the model, key group, and protocol must all match.

Messages is not a universal endpointUse Chat Completions for OpenAI-compatible applications. Do not switch a current Anthropic-type Claude channel to /v1/responses; not implemented normally means a protocol mismatch, not a temporary error that retries can solve.

Endpoint and minimum request

POST https://nexusapi.link/v1/messages

curl 'https://nexusapi.link/v1/messages' \
  -H 'Content-Type: application/json' \
  -H 'Authorization: Bearer YOUR_API_KEY' \
  -H 'anthropic-version: 2023-06-01' \
  -d '{
    "model": "YOUR_ANTHROPIC_MODEL_ID",
    "max_tokens": 256,
    "messages": [{"role": "user", "content": "Hello"}]
  }'

Item	Required	Notes
`model`	Yes	Exact model ID visible to the current key and labelled `anthropic`
`max_tokens`	Yes	Maximum output tokens for this request
`messages`	Yes	Conversation in Anthropic Messages format
`anthropic-version`	Required by the native protocol	Managed by the SDK; use a compatible version for manual calls

Use the Apifox API reference and the protocol documentation for your Anthropic client for full fields, tools, streaming, and response schemas. Optional-field availability still depends on the selected model and channel.

Use the Anthropic SDK

The native SDK manages its required protocol headers. Pass a NexusAPI key to the SDK and set its Base URL without /v1:

from anthropic import Anthropic

client = Anthropic(
    api_key="YOUR_API_KEY",
    base_url="https://nexusapi.link",
)

message = client.messages.create(
    model="YOUR_ANTHROPIC_MODEL_ID",
    max_tokens=256,
    messages=[{"role": "user", "content": "Hello"}],
)

The SDK appends /v1/messages. Do not set base_url to https://nexusapi.link/v1/messages, which can create a repeated path or 404.

Claude Code versus your own app

Scenario	Configuration
Claude Code	Follow the Claude Code guide: set `ANTHROPIC_BASE_URL=https://nexusapi.link` and `ANTHROPIC_AUTH_TOKEN` in `~/.claude/settings.json`
Anthropic SDK / native app	Set Base URL to `https://nexusapi.link` and let the client send Messages
OpenAI SDK app	Do not mix this setup into it; use Chat Completions

Troubleshooting

401: Check that the key is correct, enabled, and has not been reset after exposure.
404: Check whether an SDK has appended a second /v1.
400: Verify with only model, max_tokens, and messages, then add optional fields gradually.
500, not implemented: Make sure an Anthropic-type model is not being sent through /v1/responses.

For persistent errors, keep the Request ID and a redacted request, then use the self-check guide.

Get started

Models and billing

Client tools

Troubleshooting

Developer API

Endpoint and minimum request

Use the Anthropic SDK

Claude Code versus your own app

Troubleshooting

​Endpoint and minimum request

​Use the Anthropic SDK

​Claude Code versus your own app

​Troubleshooting

Endpoint and minimum request

Use the Anthropic SDK

Claude Code versus your own app

Troubleshooting