Markers are navigation aids for attention
The model uses distinctive section markers as anchors. Bare prose blends together; clearly marked sections (XML tags, all-caps headers, --- separators, fenced regions) give the model handles to navigate back to. This effect compounds with position bias: a buried section with a strong marker is more recoverable than the same content in unmarked prose.
Pick a syntax and stick with it
XML tags are the most reliable across providers. Markdown headers work but compete with body markdown. JSON is precise but harder for the model to write into. The choice matters less than consistency: the model learns within the prompt that <rules> means rules and <sources> means sources, and uses that mapping for the rest of the request.
Do not over-tag
Wrapping every paragraph in tags is noise. Tag the structural seams: rules, tools, sources, state, task. Inside each section, plain prose or markdown is fine.