# Harn language specification > Version: tracks the current workspace release series; derived from the implementation and updated alongside it. The language is still pre-1.0 — surface-level breaking changes... Website: https://harnlang.com/language-spec.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. --- # Harn language specification Version: tracks the current workspace release series; derived from the implementation and updated alongside it. The language is still pre-1.0 — surface-level breaking changes are possible between minor and patch releases. Run `harn --version` for the build you have. See the [changelog](https://github.com/burin-labs/harn/blob/main/CHANGELOG.md) for what changed and when, and the `Stability` column in subsections below for per-feature guarantees. Harn is a pipeline-oriented programming language for orchestrating AI agents. It is implemented as a Rust workspace with a lexer, parser, type checker, tree-walking VM, tree-sitter grammar, and CLI/runtime tooling. Programs consist of named pipelines containing imperative statements, expressions, and calls to registered builtins that perform I/O, LLM calls, and tool execution. The canonical specification is authored as per-chapter Markdown files in `spec/chapters/` (one file per top-level section). `spec/HARN_SPEC.md` is a generated single-file assembly of those chapters — do not edit it directly — and the hosted docs page `docs/src/language-spec.md` is generated alongside it by `scripts/sync_language_spec.harn`. Edit the chapter files and run `make sync-language-spec` (the pre-commit hook does this automatically). ## Specification chapters The language specification is organized into the chapters below. - [Lexical rules](./spec/language/01-lexical-rules.md) - [Grammar](./spec/language/02-grammar.md) - [Operator precedence table](./spec/language/03-operator-precedence-table.md) - [Scope rules](./spec/language/04-scope-rules.md) - [Destructuring patterns](./spec/language/05-destructuring-patterns.md) - [Evaluation order](./spec/language/06-evaluation-order.md) - [Runtime values](./spec/language/07-runtime-values.md) - [Binary operator semantics](./spec/language/08-binary-operator-semantics.md) - [Control flow](./spec/language/09-control-flow.md) - [Concurrency](./spec/language/10-concurrency.md) - [Pipeline lifecycle](./spec/language/11-pipeline-lifecycle.md) - [Error model](./spec/language/12-error-model.md) - [Functions and closures](./spec/language/13-functions-and-closures.md) - [Enums](./spec/language/14-enums.md) - [Structs](./spec/language/15-structs.md) - [Impl blocks](./spec/language/16-impl-blocks.md) - [Interfaces](./spec/language/17-interfaces.md) - [Attributes](./spec/language/18-attributes.md) - [Type annotations](./spec/language/19-type-annotations.md) - [Built-in methods](./spec/language/20-built-in-methods.md) - [Iterator protocol](./spec/language/21-iterator-protocol.md) - [Method-style builtins](./spec/language/22-method-style-builtins.md) - [Runtime errors](./spec/language/23-runtime-errors.md) - [OAuth](./spec/language/24-oauth.md) - [Persistent store](./spec/language/25-persistent-store.md) - [Checkpoint & resume](./spec/language/26-checkpoint-resume.md) - [Agent lifecycle (suspend/resume)](./spec/language/27-agent-lifecycle-suspend-resume.md) - [Host shell discovery](./spec/language/28-host-shell-discovery.md) - [Workspace manifest (`harn.toml`)](./spec/language/29-workspace-manifest-harn-toml.md) - [Sandbox mode](./spec/language/30-sandbox-mode.md) - [Test framework](./spec/language/31-test-framework.md) - [Environment variables](./spec/language/32-environment-variables.md) - [Known limitations and future work](./spec/language/33-known-limitations-and-future-work.md) --- ## Read next - [Filesystem host capabilities](https://harnlang.com/host-capabilities/fs.md) - [Lexical rules](https://harnlang.com/spec/language/01-lexical-rules.md)