# Grok 4.20 Reasoning - Core.Today AI API

> Grok 4.20 reasoning variant (0309 snapshot) - chain-of-thought reasoning with a 1M-token context window. Reasoning tokens are billed additively on top of completion tokens.

- **Provider**: xAI
- **Model ID**: grok-4.20-0309-reasoning
- **Category**: LLM
- **Credits**: 3 per 1K tokens (avg)
- **Speed**: Medium
- **Quality**: Ultra

## Model Specifications
- **Context Window**: 1M tokens
- **Max Output**: 33K tokens
- **Training Cutoff**: 2025-11
- **Supported Formats**: text, json, markdown
- **Compatible SDK**: OpenAI

### Capabilities
- Function Calling
- Streaming
- JSON Mode
- System Prompt

### Token Pricing (per 1M tokens)
- **Input Tokens**: 2,322.5 credits ($1.55)
- **Output Tokens**: 4,645 credits ($3.10)
- **Cached Tokens**: 371.6 credits ($0.25)

## Features
- Additive reasoning-token billing (accurate metering)
- 1M context window
- OpenAI-compatible API (chat completions)
- Prompt caching with discounted cache reads
- Long-context tier: prompts reaching 200K tokens bill the whole request at 2x rates

## Use Cases
- Long-document analysis and summarization
- Coding assistants and agents
- Multi-turn chat with large shared context

## API Endpoint

Base URL: https://api.core.today
Endpoint: POST /llm/grok/v1/chat/completions

## Authentication
Header: Authorization: Bearer YOUR_API_KEY

Note: LLM endpoints use OpenAI-compatible format with Authorization Bearer token.

## Input Parameters

### Required
- **model**: string - Model ID, e.g. "grok-4.20-0309-reasoning".
- **messages**: array - Chat messages in OpenAI format (system/user/assistant roles).

### Optional
- **temperature**: number (default: 0.7) - Sampling temperature (0-2).
- **max_tokens**: integer (default: 2048) - Maximum completion tokens.
- **stream**: boolean (default: false) - Stream the response as server-sent events.

## Examples

### Chat completion
OpenAI-compatible chat completion request
```json
{
  "model": "grok-4.20-0309-reasoning",
  "messages": [
    {
      "role": "user",
      "content": "Summarize the attached report in 5 bullets."
    }
  ],
  "max_tokens": 1024
}
```

## Response Format
```json
{
  "id": "chatcmpl-abc123",
  "object": "chat.completion",
  "choices": [
    {
      "index": 0,
      "message": {
        "role": "assistant",
        "content": "Response text here"
      },
      "finish_reason": "stop"
    }
  ],
  "usage": {
    "prompt_tokens": 100,
    "completion_tokens": 50,
    "total_tokens": 150
  }
}
```

## Tips
- Works with the OpenAI SDK - set base_url to https://ai.api.core.today/llm/grok/v1
- Keep prompts under 200K tokens to stay on the standard rate (2x above that)
- Cache reads are billed at a discounted rate - reuse long system prompts

## Documentation
https://docs.x.ai/docs/models