Anthropic Messages API

POST /v1/messages speaks the Anthropic Messages API. Change the base URL and the key of an Anthropic client and keep your request and response code. Behind it is the same path as chat completions: models and sort failover, presets, guardrails, and billing to your wallet.

Drop-in for the Anthropic SDK

The SDK adds /v1/messages to the base URL and sends your key as x-api-key. Authorization: Bearer works too. anthropic-version is accepted with any value.

import os
import anthropic

client = anthropic.Anthropic(base_url='https://api.plungeai.com', api_key=os.environ['PLUNGE_API_KEY'])
message = client.messages.create(
    model='anthropic/claude-haiku-4-5',
    max_tokens=64,
    messages=[{'role': 'user', 'content': 'hi'}],
)
print(message.content[0].text)

import Anthropic from '@anthropic-ai/sdk'

const client = new Anthropic({ baseURL: 'https://api.plungeai.com', apiKey: process.env.PLUNGE_API_KEY })
const message = await client.messages.create({
  model: 'anthropic/claude-haiku-4-5',
  max_tokens: 64,
  messages: [{ role: 'user', content: 'hi' }],
})
console.log(message.content)

curl -s https://api.plungeai.com/v1/messages \
  -H "x-api-key: $PLUNGE_API_KEY" \
  -H 'anthropic-version: 2023-06-01' \
  -H 'Content-Type: application/json' \
  -d '{"model":"anthropic/claude-haiku-4-5","max_tokens":64,"messages":[{"role":"user","content":"hi"}]}'

The same route is served at https://api.plungeai.com/api/v1/messages, the OpenRouter-compatible base. There fallbacks takes up to 3 models to try after model.

Anthropic models: your body, unchanged

On an anthropic/* model the gateway forwards your request body as you sent it and sets only two fields: model (the upstream id) and max_tokens (the capped value). cache_control, thinking, documents, system blocks and server tools reach Anthropic unchanged. Ocean's own fields (models, fallbacks, sort, max_cost) are removed first.

Every other model: translated

Any other slug, such as openai/gpt-4o-mini, gets the chat-completions shape:

Blocks with no chat-completions form (thinking, server tools) are not sent to these models.

Responses

You get an Anthropic message for every model, built from the completion that served the request. content holds text and tool_use blocks. model is the slug that actually answered, which can differ from the one you asked for after a failover. usage.input_tokens excludes cache reads and cache writes; those are in cache_read_input_tokens and cache_creation_input_tokens. The x-request-id and x-ocean-* response headers are kept.

Streaming

With stream: true the events come in Anthropic's order:

1. message_start
2. For each content block: content_block_start, one or more content_block_delta (text_delta for text, input_json_delta for tool input), content_block_stop
3. message_delta with stop_reason and the final usage
4. message_stop

ping events are keepalives. A failure after the response has started ends the stream with an error event. The SDK helpers (client.messages.stream) read this stream as they read Anthropic's.

Errors

Errors use Anthropic's envelope on both /v1/messages and /api/v1/messages:

{"type":"error","error":{"type":"authentication_error","message":"Invalid API key"},"request_id":"00000000-0000-4000-8000-000000000004"}

error.type follows the status: 400 invalid_request_error, 401 authentication_error, 402 billing_error, 403 permission_error, 404 not_found_error, 429 rate_limit_error, 504 timeout_error, any other api_error.

Not supported

• The anthropic-beta header is not forwarded, so features behind a beta flag do not switch on.
• Responses carry text and tool_use blocks only; stop_sequence is always null.
• /v1/messages/count_tokens and Message Batches are not served.

Reference