The custom_id must be unique within the file. Duplicate IDs cause the batch to fail validation, and you will not find out until after upload.
Step 2: upload the file and create the batch
Upload the file with purpose batch, then reference its ID when you create the batch job.
Where noburn fits
The tools compared in this article handle observability, routing, or evaluation — all of which operate after the LLM call completes. noburn operates before it. It wraps your existing OpenAI, Anthropic, LangChain, and the Vercel AI SDK client, estimates the token cost of each call, and blocks it if the calling user or project has exceeded their budget. Nothing in this comparison does that at a self-serve price point.
Per-user metering lets you enforce separate limits per end-customer, and Stripe passthrough lets you bill them for their LLM usage without writing a billing layer yourself. The free tier covers 100 requests per month. Documentation and SDKs are at noburn.dev/docs.