GPT-5.5 API

Overview

GPT-5.5 steps up reasoning and reliability over the base GPT-5 line, aimed at harder tasks that still want OpenAI's tool-use ergonomics. A good middle ground when GPT-5 isn't quite enough but you want to stay in the OpenAI ecosystem.

What GPT-5.5 is best for

• Harder reasoning within the OpenAI ecosystem
• Complex tool-use workflows
• Agentic tasks needing reliability
• When GPT-5 falls short on quality

Strengths and tradeoffs

Strengths: Higher capability than GPT-5, native function calling and JSON mode.

Tradeoff: 3x the input cost of GPT-5. For many tasks, GPT-5 or Claude Sonnet delivers similar quality for less.

Pricing

Estimate your own usage with the API cost calculator or count tokens with the token counter.

Quick start (Python)

GPT-5.5 runs through KissAPI's OpenAI-compatible endpoint, so any OpenAI SDK works. Just change the base URL and model id:

from openai import OpenAI

client = OpenAI(
    api_key="YOUR_KISSAPI_KEY",
    base_url="https://api.kissapi.ai/v1",
)

resp = client.chat.completions.create(
    model="gpt-5-5",
    messages=[{"role": "user", "content": "Explain async/await in one paragraph."}],
)
print(resp.choices[0].message.content)

Works with Cursor, Claude Code, Cline, Aider, and any OpenAI-compatible client.

Frequently asked questions

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