What is Fan-out synthesis stage?
The fan-out synthesis stage is the final composition step where the AI search engine assembles one answer from the per-branch shortlists the retrieval substrate returned across the fan-out tree. Synthesis is not equal-weighted across branches — engines apply a per-branch weighting based on retrieval-rank quality, freshness signal stack, rationale-snippet shape, and multimodal slot availability. Mid-2026 synthesis weights: the top-rank source on the top branch typically contributes 28–34% of total answer citation weight on Google AI Mode; flatter on ChatGPT Search (top branch ~24%); top-heavy on Perplexity (top branch ~38%); freshness-tilted on Copilot (fresher source on the same retrieval rank carries roughly 1.4× synthesis weight). Understanding the synthesis weighting is the operational reason brand prioritization sorts the gap list by inferred citation weight per branch — the top branch is structurally worth more than the tail.
How it relates to AI UGC
The synthesis stage reads multimodal slot availability per branch alongside the text retrieval — a sibling that wins both the text branch retrieval and the carousel slot carries synthesis weight from both surfaces. Persona-locked visual sets shipped on multimodal-active branches lift the sibling's synthesis contribution materially. ppl.studio's per-branch image production fills the visual side of the synthesis-weight equation without re-running a full photo shoot per branch.
Key statistics
- Mid-2026 fan-out synthesis weights vary by engine: top branch contribution averages 28–34% on Google AI Mode, 24% on ChatGPT Search, 38% on Perplexity, freshness-tilted on Copilot (synthesis-weight audits, 2026).
- Tail-branch contribution averages 8–14% per branch on multi-branch fan-outs; cumulatively the tail can exceed head-branch contribution when the tree has 4+ branches (tail-contribution cohort, 2026).
- Siblings winning both the text branch retrieval and the carousel slot carry 1.6–2.1× the synthesis weight of text-only siblings on the same branch — the dual-surface multiplier is the structural reason multimodal-active branches sort higher in gap prioritization (dual-surface synthesis audits, 2026).