Anthropic: Claude Opus 4.8 (Fast)

anthropic/claude-opus-4-8-fast

Fast-mode variant of Opus 4.8 - identical capabilities with higher output speed at 2x pricing relative to regular Opus 4.8.

Learn more in Anthropic's docs: https://platf...

visiontools

MODALITIES

→

INPUT PRICE

$10per 1M

OUTPUT PRICE

$50per 1M

CONTEXT

RELEASED

May 28, 2026

Provider				Cache	Uptime	Chat
FastAPI	—	$10	$50	Cache read$1

Capabilities

Input modalities

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Output modalities

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Features

include_reasoningmax_tokensreasoningreasoning_effortresponse_formatstopstructured_outputstool_choicetoolsverbosity

Get your API key

Create an API key from the Keys page, then set it as an environment variable:

export ONLIST_API_KEY=sk-...

Make your first request

Endpoints

POSThttps://onlist.io/v1/chat/completions

OpenAI Chat Completions format

Request Headers

Authorization:Bearer $ONLIST_API_KEY

Content-Type:application/json

Model:anthropic/claude-opus-4-8-fast

POSThttps://onlist.io/v1/responses

OpenAI Responses format

Request Headers

Authorization:Bearer $ONLIST_API_KEY

Content-Type:application/json

Model:anthropic/claude-opus-4-8-fast

POSThttps://onlist.io/v1/messages

Anthropic Messages format

Request Headers

Authorization:Bearer $ONLIST_API_KEY

Content-Type:application/json

Model:anthropic/claude-opus-4-8-fast

Code samples

curl https://onlist.io/v1/chat/completions \
  -H "Authorization: Bearer $ONLIST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
       "model": "anthropic/claude-opus-4-8-fast",
       "messages": [
         {
           "role": "user",
           "content": "Explain quantum entanglement in one paragraph."
         }
       ]
     }'

Replace $ONLIST_API_KEY with the API key from your Keys page.

Authentication

All requests must include an Authorization: Bearer <TOKEN> header. Generate keys from the Keys page; keys can be scoped to specific models, groups, IP ranges, and rate limits.

Enable streaming

Add "stream": true to receive partial responses as server-sent events in real time.

Streaming example

curl https://onlist.io/v1/chat/completions \
  -H "Authorization: Bearer $ONLIST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
       "model": "anthropic/claude-opus-4-8-fast",
       "messages": [
         {
           "role": "user",
           "content": "Write a haiku about recursion."
         }
       ],
       "stream": true
     }'

Supported parameters

Name	Type	Description
`include_reasoning`
`max_tokens`	integer	Maximum number of tokens to generate in the completion.
`reasoning`
`reasoning_effort`	string	Constrains effort on reasoning. Supported values: "low", "medium", "high".
`response_format`	object	Specifies the output format. Use {"type": "json_object"} for JSON mode.
`stop`	string \| array	Up to 4 sequences where the API will stop generating tokens.
`structured_outputs`
`tool_choice`	string \| object	Controls which tool is called. "auto", "none", "required", or a specific function.
`tools`	array	A list of tools the model may call. Currently supports functions.
`verbosity`

These are the request parameters this model accepts. Parameter semantics follow the OpenAI Chat Completions specification.