What is the Batch API and when should you use it?

from openai import OpenAI
import jsonlines

client = OpenAI()

# 1. Create a JSONL file with requests
batch_requests = [
    {
        "custom_id": "review-001",
        "method": "POST",
        "url": "/v1/responses",
        "body": {
            "model": "gpt-5.5",
            "input": "Review: def fib(n): return fib(n-1) + fib(n-2)",
            "max_output_tokens": 500,
        }
    },
    # ... thousands more requests
]

# Write JSONL
with open("batch_input.jsonl", "w") as f:
    for req in batch_requests:
        f.write(json.dumps(req) + "\n")

# 2. Upload the file
batch_file = client.files.create(
    file=open("batch_input.jsonl", "rb"),
    purpose="batch"
)

# 3. Create the batch
batch = client.batches.create(
    input_file_id=batch_file.id,
    endpoint="/v1/responses",
    completion_window="24h",
)
print(f"Batch ID: {batch.id}")

# 4. Poll for completion (or use webhooks)
batch_status = client.batches.retrieve(batch.id)
if batch_status.status == "completed":
    results = client.files.content(batch_status.output_file_id)
    # Parse JSONL results