A Look at Antml: The Anthropic Markup Language (karashiiro.leaflet.pub)

Researchers and prompt engineers have uncovered ANTML — likely “Anthropic Markup Language” — in Anthropic’s Claude system prompts and API behavior. ANTML uses XML-like tags (e.g., <antml:thinking>, <antml:thinking_mode>, <antml:max_thinking_length>, <antml:function_calls>) to mark internal chain-of-thought, tool-call structure, and thinking configuration. When the API’s extended-thinking feature is enabled, the model emits these thinking blocks which the UI can render as collapsible internal reasoning; when disabled, the API strips or prefix-removes ANTML tags from both inputs and outputs. ANTML appears in Bedrock-hosted Claude too, and it’s part of Anthropic’s API design — not the model’s training format — parsed permissively (regex-based) rather than as strict XML. For AI/ML practitioners this is significant because ANTML exposes how agentic behavior and tool-calling are orchestrated at the API/system-prompt level. Tags like thinking_mode (e.g., interleaved) and max_thinking_length map to runtime parameters (thinking.budget_tokens) and influence how the model interleaves tool calls and internal reasoning. Practically, this means more predictable agent workflows and new prompt-engineering levers — but also API-level interception limits what outputs you can inject or surface, and raises considerations around transparency, debugging, and potential leakage of system-level instructions.