Reference#

The complete technical reference for Jac -- pure lookup, organized by subject. If you're new, start with Build Anything and the guided "I like to build…" tracks, or work through the Learn tutorials. Come here when you need the exact syntax, option, or API.

How to use this reference#

Looking up language syntax? → Language below.
Looking up a jac command? → CLI Commands.
Configuring a project? → Configuration (jac.toml).
Wiring AI, deployment, or the full-stack client? → Capabilities and Plugins.

Language#

The core language -- syntax, types, objects, graphs, concurrency, and native compilation.

Foundation -- syntax, types, literals, variables, scoping, operators, control flow, pattern matching
Primitives & Codespace Semantics -- values, the sv/cl/na codespaces
Functions & Objects -- can vs def, OOP, inheritance, enums, impl blocks
Access Modifiers -- :pub / :protect / :priv across member, module, and service contexts
Object-Spatial Programming -- nodes, edges, walkers, visit/report/disengage, graph queries
Concurrency -- async/await, flow/wait, parallel operations
Comprehensions & Filters -- filter/assign comprehensions, typed filters
Walker Patterns -- the .reports array, response patterns, nested spawning
Appendices -- keyword & operator reference, grammar, gotchas, migration
Syntax Cheatsheet -- one-page lookup
Native Compilation -- compiling to native binaries and C-ABI shared libraries

Capabilities and Plugins#

AI, deployment, and the full-stack frameworks. byLLM and Scale are built into the jac binary (capability-gated); jac-client and jac-desktop are first-party plugins that ship with core.

byLLM -- by llm(), model config, tool calling, streaming, multimodal, agentic patterns
Scale -- production serving, storage, and Kubernetes (built into jaclang core):
Overview · HTTP API & Walkers · Data & Storage · Kubernetes & Operations
jac-client -- codespaces, components, state, routing, auth, npm packages, web/PWA/mobile targets
jac-desktop -- native desktop window, sidecar bundling, [desktop] config

Python Integration#

Interoperability -- importing and using Python packages in Jac, the adoption patterns
Library Mode -- using Jac from pure Python (jaclang.lib, jac2py)
Import Anything -- importing from PyPI, npm, and C across the codespaces

Developer Workflow#

CLI Commands -- every jac subcommand with options and examples
MCP Server -- expose your project to AI coding assistants via jac mcp
Agent Skills & MCP -- jac guide, exportable skills, and when to use each
Plugin Authoring -- write your own CLI/runtime/config plugin
Configuration -- jac.toml, profiles, environments
Publishing Packages -- building wheels and npm tarballs
Persistence & Schema Migration -- the root graph, schema drift, migrations
Errors & Warnings -- diagnostic codes
Code Organization · Testing

Quick start#

# 1. Install the jac binary
curl -fsSL https://raw.githubusercontent.com/jaseci-labs/jaseci/main/scripts/install.sh | bash

# 2. Scaffold a new project (pick a kind -- see Build Anything)
jac create myapp --kind web-app

# 3. Run it
jac start

main.jac is the default entry point; pass a different name explicitly (e.g. jac start app.jac). See Installation for details.

Using Jac with AI coding assistants#

Jac is a young language, so AI assistants may hallucinate outdated syntax. The Jaseci team maintains an official condensed reference sized for LLM context windows: jaseci-llmdocs. Add it to your assistant's persistent context:

curl -LO https://github.com/jaseci-labs/jaseci-llmdocs/releases/latest/download/candidate.txt

Tool	Context file	Quick setup
Claude Code	`CLAUDE.md` (or `~/.claude/CLAUDE.md`)	`cat candidate.txt >> CLAUDE.md`
Gemini CLI / Antigravity	`GEMINI.md`	`cat candidate.txt >> GEMINI.md`
Cursor	`.cursor/rules/jac-reference.mdc`	`mkdir -p .cursor/rules && cp candidate.txt .cursor/rules/jac-reference.mdc`
OpenAI Codex	`AGENTS.md` (or `~/.codex/AGENTS.md`)	`cat candidate.txt >> AGENTS.md`

For a live, always-current option, point your assistant at the built-in MCP server (jac mcp) instead. Pull a fresh candidate.txt when you upgrade Jac.