Anthropic Messages endpoint

Point the official Anthropic SDK at the Cloud Score gateway: POST /v1/messages takes x-api-key, routes when model is auto, and returns Messages-shaped bodies, SSE events and errors with a dex_routing block.

WhereAdminDeveloperNeeds a connected providerNeeds an enabled modelNeeds a gateway keyOpen in Cloud Score

On this page

What it is

POST /v1/messages accepts the Anthropic Messages API, so clients that speak it (the Anthropic SDK, agent frameworks, Claude Code) work against the gateway by changing the base URL and the key. The request is translated into the same pipeline as /v1/chat/completions: guarded, routed, cached, recorded and billed. The reply comes back in Messages shape with an extra dex_routing block.

It is a stateless subset: Cloud Score never runs your tools and keeps no conversation between calls. Send the full history each time.

Point the SDK at the gateway

Send the gateway key in x-api-key (the Anthropic SDK does this for you) or as Authorization: Bearer dex_sk_…. The model can be auto or a model key.

python

import os
import anthropic

client = anthropic.Anthropic(
    base_url="http://localhost:7001",
    api_key=os.environ["DEX_API_KEY"],  # sent as x-api-key
)

message = client.messages.create(
    model="auto",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Summarize this quarter in two sentences."}],
)
print(message.content[0].text)

Request shape

Field	What Cloud Score does
`model`	Required. `auto` routes; a model key pins.
`max_tokens`	Required, an integer of 1 or more.
`messages`	Required, `user` and `assistant` turns only.
`system`	A string or text blocks, sent as the first system message.
`tools`	Custom tools only (`input_schema`). Server tools get 400 `unsupported_tool_type`.
`tool_choice`	`auto`, `any`, `none`, `tool`; `disable_parallel_tool_use` maps to parallel calls.
`stop_sequences`	Forwarded as stop.
`temperature`	Forwarded.
`stream`	Streams Messages SSE events. See Streaming.
`metadata.user_id`	Mapped to the `end_user` attribution tag.
`mcp_servers`	`container` too: rejected with 400 `unsupported_parameter`.
`thinking`, `top_k`, `top_p`, `service_tier`	Ignored. Cloud Score chooses the reasoning effort.

Content blocks

Block	What Cloud Score does
`text`	Joined into the message.
`tool_use`	Becomes an assistant tool call. The arguments are your input object.
`tool_result`	Becomes a tool message. `is_error: true` adds an `Error:` prefix to the text.
`thinking`, `redacted_thinking`	Inbound only: passed to Claude models, dropped for other providers; the request trace records `thinking_blocks_dropped`. Replies carry `text` and `tool_use` blocks only, never thinking blocks (thinking is off on tool-loop turns after the first).
`cache_control`	Removed. Cloud Score sets its own cache breakpoints.
`image`, `document`, other types	400 `unsupported_content_type`.

Streaming

Text is sent once the answer has passed output checks, so it arrives after the check rather than token by token from the model. Each tool_use is sent whole in a single input_json_delta. Usage is in message_delta.

event order

event: message_start
event: content_block_start      (text, once the answer has passed checks)
event: content_block_delta      (text_delta)
event: content_block_stop
event: content_block_start      (tool_use, input {})
event: content_block_delta      (one input_json_delta with the whole arguments)
event: content_block_stop
event: message_delta            (stop_reason and usage)
event: message_stop

Errors and HTTP codes

Every error uses the Messages shape.

error body

{
  "type": "error",
  "error": { "type": "invalid_request_error", "message": "…", "code": "unsupported_parameter" }
}

Status	When
`400`	Bad request, an unsupported parameter, block or tool type, or `invalid_tool_history`.
`401`	Missing or invalid key.
`403`	Blocked by AI Security or a policy.
`409`	No candidate model, including `tools_unsupported`. Not retried by the SDK.
`502`	Provider failure.
`503`	Budget window unavailable; `Retry-After` says when to retry.
`504`	`deadline_exceeded`.

A failure after the stream has started arrives as an error event with the same body.

Good to know

Attribute spend with the X-Dex-* headers, as on the other endpoints. Message ids are msg_ plus the request id, and the request shows as Anthropic Messages under Object on /logs.