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GPT-5.3 Codex API

openai/gpt-5.3-codex

openai

Industry-leading agentic coding model. 400K context, reasoning, tool use, and complex execution

Code Examples

from blockrun_llm import LLMClient

client = LLMClient()  # Uses BLOCKRUN_WALLET_KEY (never sent to server)
response = client.chat("openai/gpt-5.3-codex", "Hello!")
import { LLMClient } from '@blockrun/llm';

const client = new LLMClient();  // Uses BLOCKRUN_WALLET_KEY (never sent to server)
const response = await client.chat('openai/gpt-5.3-codex', 'Hello!');
curl -X POST https://blockrun.ai/api/v1/chat/completions \
  -H "Content-Type: application/json" \
  -H "PAYMENT-SIGNATURE: <payment_header>" \
  -d '{
    "model": "openai/gpt-5.3-codex",
    "messages": [{"role": "user", "content": "Hello!"}],
    "max_tokens": 1024
  }'

On BlockRun since 2026-03-15. Measured on BlockRun: fewer than 100 calls to GPT-5.3 Codex this week — price only.

Pricing

Input$1.75 / 1M tokens
Output$14.00 / 1M tokens
Context400K tokens
Max Output128K tokens

No markup on official OpenAI rates; $0.001 fee per call.

Payment

Network
Base
Currency
USDC
Protocol
x402

Pay per request with USDC on Base. No subscription required.

Try It

Send a message to try GPT-5.3 Codex

Connect your wallet to enable payments

About OpenAI GPT-5.3 Codex

GPT-5.3 Codex is a coding and reasoning model from OpenAI with a 400K-token context window and up to 128K tokens of output per call. It is billed per token at the provider's list rate, with no markup on chat tokens. It is served through BlockRun's OpenAI-compatible API, so it can be called without an account, an API key, or a subscription.

Use GPT-5.3 Codex for

  • Coding agents: writing, reviewing and refactoring code across a repository
  • Multi-step reasoning: planning, analysis and math that need thinking before an answer
  • Chat assistants and agents that answer, summarise and draft

What it costs

GPT-5.3 Codex is billed per token. The underlying rates are $1.75 per million input tokens and $14.00 per million output tokens with no platform margin on chat tokens; only a small per-transaction fee covering on-chain settlement is added on top, and the resulting price is quoted in the 402 before you pay. You are charged for the tokens a request actually consumes — there is no monthly commitment and no unused balance to forfeit. With a context window of 400,000 tokens and up to 128,000 tokens of output, a single call can carry a substantial working set.

Specifications

Context window
400,000 tokens
Maximum output
128,000 tokens
API compatibility
OpenAI-compatible
Payment
USDC via x402
Categories
coding, reasoning, chat

Calling it from your code

Pass openai/gpt-5.3-codex as the model field. Because the endpoint mirrors the OpenAI chat completions schema, any existing OpenAI client works by changing the base URL — streaming, tool use, and multi-turn messages all behave the same way.

The first request returns a 402 carrying the exact price; a signed retry runs it, and client libraries fold the two into one call — how the payment works. See the documentation for request and response shapes, the LLM API page for every model on this endpoint, or browse the full catalog of 90 chat models to compare alternatives.

GPT-5.3 Codex or another model

The closest alternatives by price that do the same kind of work. Each is called on the same endpoint with the same request, so comparing two is a one-line change.

ModelPriceContext
OpenAI GPT-5.3 Codex (this page)$1.75 in / $14.00 out per 1M400K context
OpenAI GPT-5.2$1.75 in / $14.00 out per 1M400K context
OpenAI GPT-5.4$2.50 in / $15.00 out per 1M1.05M context
Google Gemini 3.1 Pro$2.00 in / $12.00 out per 1M1.05M context

GPT-5.3 Codex API questions

How much does the GPT-5.3 Codex API cost?

$1.75 per million input tokens and $14.00 per million output tokens, with cached input at $0.17 per million. There is no markup on chat tokens; a small per-call transaction fee is added, and the price is quoted in the 402 before the call runs.

What is GPT-5.3 Codex's context window?

400,000 tokens of context, with up to 128,000 tokens of output per call.

Is the GPT-5.3 Codex API OpenAI-compatible?

Yes. Send an OpenAI chat-completions request with "model": "openai/gpt-5.3-codex" to the BlockRun endpoint. An existing OpenAI client works by changing the base URL; streaming and multi-turn messages behave the same way.

Do I need an API key to use GPT-5.3 Codex?

No. Each request carries its own payment, so an agent or script can call GPT-5.3 Codex with no account and no key. Teams that prefer one can use an API key with card or invoice billing instead.

What happens if GPT-5.3 Codex is unavailable?

A request that fails is not charged. If you set "allow_fallback": true, a failed call can be retried on OpenAI GPT-5.2 instead, never at a higher price than you were quoted.

Related models

Every model on BlockRun is reached through the same endpoint and the same payment flow, so switching is a one-line change to the model field. These are the closest alternatives to GPT-5.3 Codex:

Two ways to run this

Get an API key

Sign in, get a key, and call the same endpoints with usage billed to your account. For teams that want one invoice instead of one wallet.

Get an API key 

Or let your agent pay per call, in USDC

No account, no API key. Three steps, and the last one is the request this page is about.

  1. 01
    Point your agent at BlockRun

    Install ClawRouter, or add the BlockRun MCP server to Claude Code, Cursor, Codex or OpenClaw. Any OpenAI-compatible client works too — change the base URL, nothing else.

    curl -fsSL https://blockrun.ai/ClawRouter-update | bash
  2. 02
    Fund a wallet with USDC

    Send USDC to a wallet on Base, Solana or Arc. There is no account to create and no API key to issue: the wallet is the account, and it pays per call.

  3. 03
    Call GPT-5.3 Codex

    The first call returns HTTP 402 with the exact price, your agent signs it, and the answer comes back. Payment settles on-chain; nothing is charged if the call fails.

    curl -X POST https://blockrun.ai/api/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{"model": "openai/gpt-5.3-codex", "messages": [{"role": "user", "content": "Hello!"}]}'
ClawRouter setup →How paying per call works →

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