> ## Documentation Index
> Fetch the complete documentation index at: https://docs.peec.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# What is a query fan-out (QFO) in AI search?

> A query fan-out is the set of hidden searches an AI engine runs behind one prompt. ChatGPT merges results with Reciprocal Rank Fusion, so content matching multiple fan-outs ranks higher (Peec, 2026).

Query fan-outs (QFOs, also written fanouts or query expansion) are the extra searches an AI engine runs behind the scenes to answer one prompt. Peec's analysis of 5 million fan-outs ([full study](https://peec.ai/blog/patterns-we-see-in-chatgpt-query-fanouts), May 2026) explains the mechanism: a prompt like "best headphones" fans out into hidden sub-queries — best wireless headphones, best noise-cancelling for flights, best budget options, current-year variants — and the engine merges all results into one answer.

Why fan-outs matter for GEO: ChatGPT uses **Reciprocal Rank Fusion (RRF)**, an algorithm that combines scores across the sub-queries. Content that appears across multiple fan-out searches scores higher than content that only surfaces for one — so ranking in AI search means being present across dozens of related queries, not winning one keyword. The structure, length, and number of QFOs directly determine which sources get retrieved and ultimately cited.

Key facts from Peec's 2026 fan-out research, each with its own page:

* ChatGPT injects words like "best", "top", and "reviews" into fan-outs — [Words ChatGPT adds to fan-outs](/research/words-chatgpt-adds-to-fanouts).
* Engines differ hugely in volume: Perplexity 1.4, ChatGPT 2.1, Grok 6.8 fan-outs per prompt — [Fan-outs per prompt by engine](/research/fanouts-per-prompt-by-engine).
* Fan-out queries doubled in length from \~6 to \~12 words between Oct 2025 and Jan 2026 — [ChatGPT fan-out length trend](/research/chatgpt-fanout-length-trend).
* GPT-5.6 made multi-iteration fan-outs the default — [GPT-5.6 retrieval changes](/research/gpt-56-retrieval-changes).

**Source:** [Patterns we see in ChatGPT query fanouts](https://peec.ai/blog/patterns-we-see-in-chatgpt-query-fanouts), Peec (Tomek Rudzki), May 2026.