---
title: "Chunked Prefill & Choosing the Chunk Size"
description: "Why a long prompt blocks a step, how chunked prefill splits it against the scheduler's token budget, the default setting, and the startup error it fixes."
canonical_url: "https://fanout.sh/inference-eng/curriculum/10-02"
md_url: "https://fanout.sh/inference-eng/curriculum/10-02.md"
access: "pro-preview"
---

# Chunked Prefill & Choosing the Chunk Size

Why a long prompt blocks a step, how chunked prefill splits it against the scheduler's token budget, the default setting, and the startup error it fixes.

- Access: Fanout Pro (public preview)

## Public preview

A prompt is computed before its first token comes back, and so far that computation has been indivisible. A request either entered a pass with room for the whole prompt, or it waited for a pass that had room. The scheduler budgets lesson fixed the size of that room, and named the one place where a request's length and a…

## In this lesson

- A prompt against a budget
- The clamp
- What splitting changes
- The default and the platform
- Back to the startup error

## Continue learning

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