Language Models API

Overview

The examples on this page use the OpenAI-compatible /v1/chat/completions endpoint. The same key also works with the native Anthropic Messages API at /v1/messages: point the Anthropic SDK (or Claude Code) at https://api.apimodels.app. Both support streaming, vision, tool use and extended thinking.

Endpoint

Available Models

Prices are per 1M tokens

Request Parameters

Basic Request

curl -X POST https://api.apimodels.app/v1/messages \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4-5-20250929",
    "max_tokens": 1024,
    "messages": [
      {"role": "user", "content": "Hello, Claude!"}
    ]
  }'

Response

{
  "id": "msg_01XFDUDYJgAACzvnptvVoYEL",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Hello! How can I help you today?"
    }
  ],
  "model": "claude-sonnet-4-5-20250929",
  "stop_reason": "end_turn",
  "usage": {
    "input_tokens": 12,
    "output_tokens": 10
  }
}

Extended Thinking

For *-thinking models, you can enable extended thinking for complex reasoning tasks.

curl -X POST https://api.apimodels.app/v1/messages \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4-5-20250929-thinking",
    "max_tokens": 16000,
    "thinking": {
      "type": "enabled",
      "budget_tokens": 10000
    },
    "messages": [
      {"role": "user", "content": "Solve this step by step: What is 15% of 340?"}
    ]
  }'

Streaming

Set stream: true to receive Server-Sent Events (SSE). Strongly recommended for long generations: non-streaming requests that take longer than ~100 seconds to produce a first byte hit the CDN gateway timeout and return an HTML 524 page even though the generation completes (and is billed) server-side. If you expect more than a few hundred output tokens or use thinking/reasoning models, stream — or poll asynchronously.

curl -X POST https://api.apimodels.app/v1/messages \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4-5-20250929",
    "max_tokens": 1024,
    "stream": true,
    "messages": [
      {"role": "user", "content": "Write a short poem about coding."}
    ]
  }'

Prompt Caching

Repeated prefixes — a system prompt, a codebase dump, a long document — can be cached upstream and re-read at a fraction of the input rate. Cache prices are per model; see the price on each model page.

Claude: mark the last block of the stable prefix with cache_control. Reads come back as cache_read_input_tokens and writes as cache_creation_input_tokens, with the 5-minute and 1-hour TTLs broken out under cache_creation. Both TTLs bill at the same cache-write rate here, so passing ttl:"1h" costs nothing extra.

The cached prefix must be byte-identical between calls — one changed character (a timestamp, a re-ordered tool list) misses the cache and you pay the full input rate. Put everything variable at the END of the prompt.

{
  "model": "claude-fable-5-1",
  "max_tokens": 1024,
  "system": [
    {
      "type": "text",
      "text": "<long, unchanging instructions or document>",
      "cache_control": { "type": "ephemeral" }
    }
  ],
  "messages": [{ "role": "user", "content": "<the part that changes>" }]
}

Billing

Credits are calculated from actual token usage, across four tiers: full-rate input, cached input, cache writes and output. Reasoning / thinking tokens are billed at the OUTPUT rate and are not part of the visible answer — on Claude they are counted in output_tokens, on Gemini they arrive as thoughtsTokenCount alongside candidatesTokenCount, and on GPT-5.6 as reasoning_tokens inside completion_tokens. A model that thinks a lot can bill several times what the visible reply suggests. Failed requests are never billed.

Cost = (input_tokens * input_price + cached_tokens * cache_read_price
      + cache_write_tokens * cache_write_price + output_tokens * output_price) / 1,000,000
Credits = Cost  // amounts are in USD ($)

Error Codes