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OpenAI · Frontier

GPT 5.1 Codex Max

Added Dec 4, 2025

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Use in the app

Top-tier GPT-5.1 Codex route used when direct OpenAI is unavailable.

  • Reasoning
  • Tools
  • Vision
  • Structured output
  • PDF input
Input
Text, Images
Output
Text
Reasoning effort
none · minimal · low · medium · high · xhigh
Discovery tier
Frontier

Pricing

USD per 1M tokens
Input
$2.50
Output
$20.00
Cache read
$0.25

Reference prices as published by the model provider. API example

Specifications

Context window
400K
Max output
128K

Side by side

GPT 5.1 Codex Max compared

Specifications, prices, speed and benchmark scores next to each other. Benchmarks below show every compared model too.

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GPT 5.1 Codex MaxOpenAIThis modelKimi K2.5MoonshotRemove
ProviderOpenAIMoonshot
Discovery tierFrontierOpen weights
AddedDec 4, 2025Jan 26, 2026
Limits
Context window400KBest262.14K
Max output128KBest65.54K
Average output—437 tokens
Pricing, USD per 1M tokens
Input$2.50$0.38Best
Output$20.00$2.00Best
Cache read$0.25$0.07Best
Performance
Throughput—98.6 tok/s
Time to first token—2.0 s
Request success (24h)—99.8%
Benchmark indexes
Intelligence Index23.7Best23.5
Coding Index—46.8
Math Index95.7—
Capabilities
ReasoningYesNo
ToolsYesYes
VisionYesYes
Structured outputYesNo
PDF inputYesNo
Reasoning effortnone, minimal, low, medium, high, xhigh—

Public evaluations

Benchmarks

Independent evaluation results, with how each score ranks among the models in the Noumen catalog.

  • GPT 5.1 Codex Max
  • Kimi K2.5

Intelligence Index

  • GPT 5.1 Codex Max23.7
  • Kimi K2.523.5

GPT 5.1 Codex Max tested at high reasoning effort

GPT 5.1 Codex Max: Better than 58% of 326 scored models

3.8Catalog models by score55.1

Indexes

  • Coding Index

    Composite of the coding evaluations

    • GPT 5.1 Codex Max: —
    • Kimi K2.5: 46.8

  • Math Index

    Composite of the math evaluations

    • GPT 5.1 Codex Max: 95.7
    • Kimi K2.5: —

Agentic work

  • GDPval-AAv2.1

    Economically valuable occupational tasks, rated head to head

    • GPT 5.1 Codex Max: —
    • Kimi K2.5: 839 Elo

Document reasoning

  • AA-LCRv1.1

    Reasoning across long documents

    • GPT 5.1 Codex Max: 69.3%
    • Kimi K2.5: 78.0%Top

Reasoning and knowledge

  • Humanity’s Last Exam

    Expert-level questions across academic fields

    • GPT 5.1 Codex Max: 25.7%
    • Kimi K2.5: 30.7%Top

  • GPQA Diamond

    Graduate-level science questions

    • GPT 5.1 Codex Max: 86.0%
    • Kimi K2.5: 87.9%Top

  • MMLU-Pro

    Multi-subject knowledge and reasoning

    • GPT 5.1 Codex Max: 86.0%
    • Kimi K2.5: —

  • IFBench

    Following precise output instructions

    • GPT 5.1 Codex Max: 70.0%
    • Kimi K2.5: 70.2%Top

Coding

  • LiveCodeBench

    Recent competitive programming problems

    • GPT 5.1 Codex Max: 84.9%
    • Kimi K2.5: —

Math

  • AIME 2025

    Competition mathematics, 2025 problems

    • GPT 5.1 Codex Max: 95.7%
    • Kimi K2.5: —

Legacy evaluations

  • Terminal-Bench Hard

    Earlier terminal task suite

    • GPT 5.1 Codex Max: 34.8%Top
    • Kimi K2.5: 34.8%Top

  • τ²-Bench Telecom

    Tool use in a customer-support simulation

    • GPT 5.1 Codex Max: 83.0%
    • Kimi K2.5: 95.9%Top

  • AA-LCR (unversioned)

    Long-context reasoning, earlier grading

    • GPT 5.1 Codex Max: 69.3%
    • Kimi K2.5: 78.0%Top

Scores from Artificial Analysis, updated Oct 26, 2026. GPT 5.1 Codex Max was tested at high reasoning effort; compare scores measured at the same effort.Artificial Analysis

Developers

Use it through the API

Noumen’s API is OpenAI-compatible: any Chat Completions client works with a base URL and a key.

Read the quickstart
curl https://www.noumen.xyz/api/v1/chat/completions \
  -H "Authorization: Bearer $NOUMEN_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-5.1-codex-max",
    "reasoning_effort": "medium",
    "messages": [{ "role": "user", "content": "Explain this stack trace." }]
  }'
import os
from openai import OpenAI

client = OpenAI(base_url="https://www.noumen.xyz/api/v1", api_key=os.environ["NOUMEN_API_KEY"])
reply = client.chat.completions.create(
    model="openai/gpt-5.1-codex-max",
    messages=[{"role": "user", "content": "Explain this stack trace."}],
)
print(reply.choices[0].message.content)

OpenAI

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