# Host Adapter **Domain:** Software Architecture / Portability **Doc Type:** Canonical Concept Node **Maturity:** Core Design Principle **Related:** [[wiki/Portable Contract|Portable Contract]], [[wiki/Capability Registry|Capability Registry]], [[wiki/Cross-Runtime Capability Parity|Cross-Runtime Capability Parity]] --- ## Definition **A host adapter is a narrow boundary that exposes environment-specific facilities to portable core logic.** Browsers, Node, PHP servers, Obsidian, command-line tools, filesystems, databases, and network clients do not offer identical powers; adapters make those differences explicit. ## Project Context X Core should own deterministic transformations. X Host Kits should own controlled access to files, HTTP, editor state, databases, and interface regions. This separation prevents a reusable slug or parser from becoming entangled with one deployment environment. ## Key Insight **The host supplies effects; the portable core supplies meaning.** ## See Also [[projects/X and XApps - Drop the Folder and It Runs|X and XApps]], [[wiki/Permissioned Capability|Permissioned Capability]], [[wiki/Deterministic Transformation|Deterministic Transformation]], [[wiki/Technical Terms|Technical Terms]]