公式WebアプリとAPIで結果が異なるのはなぜですか？

from openai import OpenAI

client = OpenAI(
    api_key="YOUR_API_KEY",
    base_url="https://api.apiyi.com/v1"
)

SYSTEM_PROMPT = """You are a professional technical assistant.
- Lead with the conclusion, then the reasoning
- Use Markdown headings to structure the answer
- Code must be runnable and include key comments
- Flag anything uncertain; never fabricate"""

messages = [{"role": "system", "content": SYSTEM_PROMPT}]

def chat(user_input):
    messages.append({"role": "user", "content": user_input})
    resp = client.chat.completions.create(
        model="claude-opus-5",
        messages=messages,
    )
    reply = resp.choices[0].message.content
    messages.append({"role": "assistant", "content": reply})
    return reply