GPT-5.4 API: Complete Developer Guide 2026

GPT-5.4 is OpenAI's latest flagship model in 2026. It brings significant improvements in reasoning, instruction following, and function calling over GPT-4o. But at $3.75/$22.50 per million tokens, knowing when to use it and when to save money with alternatives is critical for any developer.

GPT-5.4 Key Capabilities

Getting Started with GPT-5.4

from openai import OpenAI

client = OpenAI(
    base_url="https://api.aipower.me/v1",
    api_key="YOUR_AIPOWER_KEY",
)

response = client.chat.completions.create(
    model="openai/gpt-5.4",
    messages=[
        {"role": "system", "content": "You are a senior software architect."},
        {"role": "user", "content": "Design a rate limiter for a distributed system."},
    ],
    temperature=0.7,
)
print(response.choices[0].message.content)

Function Calling with GPT-5.4

GPT-5.4's improved function calling supports parallel tool execution and more reliable structured arguments:

tools = [
    {
        "type": "function",
        "function": {
            "name": "get_weather",
            "description": "Get current weather for a location",
            "parameters": {
                "type": "object",
                "properties": {
                    "location": {"type": "string", "description": "City name"},
                    "unit": {"type": "string", "enum": ["celsius", "fahrenheit"]},
                },
                "required": ["location"],
            },
        },
    }
]

response = client.chat.completions.create(
    model="openai/gpt-5.4",
    messages=[{"role": "user", "content": "What's the weather in Tokyo and Paris?"}],
    tools=tools,
    tool_choice="auto",
)

# GPT-5.4 can call multiple tools in parallel
for tool_call in response.choices[0].message.tool_calls:
    print(f"Function: {tool_call.function.name}")
    print(f"Args: {tool_call.function.arguments}")

Structured JSON Output

response = client.chat.completions.create(
    model="openai/gpt-5.4",
    messages=[{
        "role": "user",
        "content": "Extract key info from: Apple reported $94.9B revenue in Q1 2026, up 8% YoY."
    }],
    response_format={"type": "json_object"},
)
# Returns: {"company": "Apple", "revenue": "$94.9B", "period": "Q1 2026", "growth": "8%"}

When to Use GPT-5.4 vs Cheaper Alternatives

Node.js Example

import OpenAI from "openai";

const client = new OpenAI({
  baseURL: "https://api.aipower.me/v1",
  apiKey: "YOUR_AIPOWER_KEY",
});

const response = await client.chat.completions.create({
  model: "openai/gpt-5.4",
  messages: [{ role: "user", content: "Explain microservices vs monolith" }],
  stream: true,
});

for await (const chunk of response) {
  process.stdout.write(chunk.choices[0]?.delta?.content || "");
}

Cost Optimization Strategy

Use GPT-5.4 strategically: route simple requests to cheaper models and reserve GPT-5.4 for tasks that truly benefit from it.

# Smart routing — let AIPower auto-select
response = client.chat.completions.create(
    model="auto",  # Routes to best model per task (often NOT GPT-5.4)
    messages=[{"role": "user", "content": prompt}],
)
# Simple queries → DeepSeek V3, Complex → GPT-5.4 or Claude

Access GPT-5.4 and 15 other models at aipower.me — 50 free API calls, one API key for everything.