XML-tagged prompt
XML-tagged prompts use XML-style tags to delineate sections of a prompt, <instructions>...</instructions>, <context>...</context>, <example>...</example>, so the model can clearly distinguish parts. Anthropic's prompt guidance specifically recommends XML tags for Claude; the pattern is also effective with other models.
The tag-based structure helps the model parse complex prompts that mix multiple kinds of content: static instructions, dynamic context, few-shot examples, user input. Without tags, the model has to infer where each section starts and ends from formatting and content; with tags, the boundary is explicit and the model is less likely to confuse, say, an example for the actual input. The tags don't need to be valid XML, the model treats them as semantic markers, but consistency matters (use the same tag names across the prompt and across related prompts).