GPT-5.3 Codex
Provider list prices: no platform markup, pay-as-you-go. These are official list prices. Logged-in customers may see effective prices including workspace discounts on /console/pricing. Effective input at a 70% cache-hit rate:$1.1375/M. Automatic prefix caching once a prompt passes the minimum length, with no code changes; cached reads are discounted (up to 90% on current models) and there is no write fee.
Use GPT-5.3 Codex in 30 seconds
OpenAI-compatible: swap the base_url, keep your SDK. POST /v1/chat/completions
from openai import OpenAI
client = OpenAI(
base_url="https://synthorai.io/v1",
api_key="sk-syn-...",
)
resp = client.chat.completions.create(
model="gpt-5.3-codex",
messages=[{"role": "user", "content": "Summarize this diff"}],
reasoning_effort="medium",
)
print(resp.choices[0].message.content)import OpenAI from "openai";
const client = new OpenAI({
baseURL: "https://synthorai.io/v1",
apiKey: "sk-syn-...",
});
const resp = await client.chat.completions.create({
model: "gpt-5.3-codex",
messages: [{ role: "user", content: "Summarize this diff" }],
reasoning_effort: "medium",
});
console.log(resp.choices[0].message.content);curl https://synthorai.io/v1/chat/completions \
-H "Authorization: Bearer sk-syn-..." \
-H "Content-Type: application/json" \
-d '{
"model": "gpt-5.3-codex",
"messages": [{"role": "user", "content": "Hello"}],
"reasoning_effort": "medium"
}'package main
import (
"context"
"fmt"
"github.com/openai/openai-go/v3"
"github.com/openai/openai-go/v3/option"
)
func main() {
client := openai.NewClient(
option.WithBaseURL("https://synthorai.io/v1"),
option.WithAPIKey("sk-syn-..."),
)
resp, _ := client.Chat.Completions.New(context.TODO(), openai.ChatCompletionNewParams{
Model: "gpt-5.3-codex",
Messages: []openai.ChatCompletionMessageParamUnion{
openai.UserMessage("Summarize this diff"),
},
ReasoningEffort: openai.ReasoningEffortMedium,
})
fmt.Println(resp.Choices[0].Message.Content)
}import com.openai.client.OpenAIClient;
import com.openai.client.okhttp.OpenAIOkHttpClient;
import com.openai.models.chat.completions.*;
import com.openai.models.ReasoningEffort;
OpenAIClient client = OpenAIOkHttpClient.builder()
.baseUrl("https://synthorai.io/v1")
.apiKey("sk-syn-...")
.build();
ChatCompletion resp = client.chat().completions().create(
ChatCompletionCreateParams.builder()
.model("gpt-5.3-codex")
.addUserMessage("Summarize this diff")
.reasoningEffort(ReasoningEffort.MEDIUM)
.build());
System.out.println(resp.choices().get(0).message().content().orElse(""));About GPT-5.3 Codex
GPT-5.3 Codex is OpenAI's "most capable agentic coding model to date," succeeding GPT-5.2 Codex as the default engine for autonomous coding workflows.
- Tuned specifically for agentic coding in Codex environments, it retains the 400K-token context window and 128K max output of its predecessor at the same per-token pricing, with four reasoning-effort levels plus streaming, function calling, and structured outputs.
- For agent frameworks already speaking the OpenAI protocol, Synthorai serves GPT-5.3 Codex on its compatible chat endpoint out of the box.
Specs & limits
| Max output (vendor spec) | 128,000 |
| Modalities | text + image → text |
| Features | tools · parallel_tool_calls · structured_output · streaming · vision · batch · reasoning · caching |
| Notable | Described as the most capable agentic coding model to date; 400k context; reasoning effort low-xhigh. |
| Prompt caching | automatic · min 1,024-token prefix · TTL 5–10m, up to 1h |
FAQ
Is the GPT-5.3 Codex API free to try?
Yes: new accounts get 10 trial calls and up to $1 in free credit, no card required. At $1.75/M input tokens, that credit alone covers roughly 71 requests of ~8K tokens against GPT-5.3 Codex.
What is GPT-5.3 Codex best at?
Most capable agentic coding model to date, plus retains 400K context at unchanged pricing and default engine for autonomous coding workflows. See the About section for the full picture from the vendor's own release notes.
How much does GPT-5.3 Codex cost?
GPT-5.3 Codex costs $1.75 per million input tokens and $14 per million output tokens on Synthorai. That is the provider's list price, with no platform markup. Cached input tokens bill at $0.875/M.
Does GPT-5.3 Codex support prompt caching?
Yes, automatically: OpenAI-served prompts cache with no code changes. Cached input tokens bill at $0.875/M vs $1.75/M uncached; prompts need a 1,024-token stable prefix to cache (TTL 5–10m, up to 1h). Prompt caching guide →
How do I get access to GPT-5.3 Codex?
Point your existing OpenAI SDK at base_url="https://synthorai.io/v1", set model="gpt-5.3-codex", and you're done. One API key covers every model on the gateway.
What is GPT-5.3 Codex's knowledge cutoff?
GPT-5.3 Codex's knowledge cutoff is 2025-08, per the vendor's official documentation (as of 2026-07-09).