# Migration: harn-hostlib host contracts

> harn-hostlib began as a migration path for code-intelligence and tool surfaces that had lived in an external IDE host's Swift code-intelligence modules. That history explains...

Website: https://harnlang.com/migrations/harn-hostlib-host-contracts.html

This page documents Harn, which is pre-1.0. Language, standard library, and CLI APIs may change. If the intended version is unclear, clarify before using this page.

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`harn-hostlib` began as a migration path for code-intelligence and tool
surfaces that had lived in an external IDE host's Swift code-intelligence
modules. That history explains the early parity tests and the initial schema
names, but it is no longer the ownership model.

The current contract is Harn-owned:

- JSON schemas under `crates/harn-hostlib/schemas/` define request and
  response compatibility for every hostlib method.
- `HostlibRegistry` is the authoritative runtime catalog of registered
  modules and methods.
- Consumer repositories should treat their bridge tests as compatibility
  checks against Harn's published contract, not as the source of truth for
  hostlib behavior.

During migration, keep any consumer-specific bridge notes in this page or
in the consumer repository. Public hostlib docs, schema descriptions, and
module comments should describe the neutral Harn contract first.

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## Read next

- [Rust connectors → Harn packages](https://harnlang.com/migrations/rust-connectors-to-harn-packages.md)
- [Language basics](https://harnlang.com/language-basics.md)