長文promptと2段階生成：32K制限はどこから来るのか

import os
from openai import OpenAI

client = OpenAI(
    api_key=os.environ["APIYI_API_KEY"],
    base_url="https://api.apiyi.com/v1",
)

DISTILL_SYSTEM = """You are an ad-creative prompt engineer. Read all the material the user provides and output one prompt that can be sent directly to an image model.

Write in this order, one line per section:
1 Hard constraints: logo not distorted, packaging text verbatim, number of people, aspect ratio, background colour value
2 Purpose and placement
3 Subject and packaging elements that must be kept
4 Composition and negative space: product position, people position, framing, area reserved for copy
5 Visual: setting, palette, one identifiable key light, materials, photographic style
6 Do not

Rules:
- At most 2500 characters. Output only the prompt body, no explanation, no headings
- Do not invent anything absent from the material, especially brand names and packaging text
- Do not use vague quality words such as 8K, ultra HD, masterpiece, perfect
- Keep anything the material specifies explicitly, do not rewrite it"""

PROMPT_LIMIT = 10000  # conservative character gate: the measured limit is 8,192 tokens (~11,000 Chinese chars), and exceeding it raises no error


def distill(brief: str, model: str = "gpt-5.6") -> str:
    resp = client.chat.completions.create(
        model=model,
        messages=[
            {"role": "system", "content": DISTILL_SYSTEM},
            {"role": "user", "content": brief},
        ],
    )
    prompt = resp.choices[0].message.content.strip()
    if len(prompt) > PROMPT_LIMIT:
        # rare; compress one more round if it happens
        prompt = distill(f"Compress the following prompt to under 2500 characters, keeping every hard constraint:\n\n{prompt}", model)
    return prompt


def generate(prompt: str, size: str = "1536x1024", quality: str = "high") -> bytes:
    import base64
    resp = client.images.generate(
        model="gpt-image-2.5-sunburst",
        prompt=prompt,
        size=size,
        quality=quality,
        timeout=600,
    )
    return base64.b64decode(resp.data[0].b64_json)


if __name__ == "__main__":
    brief = open("brief.md", encoding="utf-8").read()   # brand book + packaging spec + ad requirements, any length
    prompt = distill(brief)
    open("prompt.txt", "w", encoding="utf-8").write(prompt)   # persist; generation replays only this
    open("ad.png", "wb").write(generate(prompt))