A0

A0

The programming language built for AI, not for people. a0lang.com · Docs

A0 is a small programming language that AI agents edit through checked, structured edits and compile to native code, wasm, JavaScript, the JVM, .NET, Metal and SystemVerilog from one source.

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1. Agent skill (Claude Code, Codex, Cursor, Copilot, Gemini CLI and many other agents, through the skills CLI):

npx skills add Joe-Simo/a0

2. MCP server (needs the a0 binary on PATH, see the next item). Paste into Claude Desktop (claude_desktop_config.json), Cursor (.cursor/mcp.json) or any client that takes mcpServers; Claude Code: claude mcp add a0 -- a0 mcp .; VS Code: .vscode/mcp.json from integrations/vscode.mcp.json.

{ "mcpServers": { "a0": { "command": "a0", "args": ["mcp", "."] } } }

3. The binary (one file, no Node, SHA-256 verified against the release's checksums.txt):

curl -fsSL https://raw.githubusercontent.com/Joe-Simo/a0/main/install.sh | sh     # macOS / Linux

irm https://raw.githubusercontent.com/Joe-Simo/a0/main/install.ps1 | iex # Windows (PowerShell)

Honest status: A0 has no floating point, no heap and no recursion, and strings are bytes. It is a kernel and edit-loop language, not a general-purpose one. The numbers below are measured on one machine that was not quiet, and the loss ledger in STATUS.md lists where A0 is slower. Correctness is split: every target is checked by execution against one oracle, and the optimizer is proved equivalent by Z3 on 48 corpus functions; the rest is tested, not proved. License: Apache 2.0. More: a0lang.com, a0lang.com/benchmarks.

A0 is a compact, exactly specified language that models write and edit through revision-checked structured edits. A model reads only what an edit touches, writes only the changed lines, and nothing invalid lands. One program compiles to native machine code (A0's own AArch64 code generator, or C), the browser (wasm32), JavaScript, the JVM, .NET, Metal GPU kernels, and clocked SystemVerilog, and every target is verified against one oracle.

Measured on the repository's benchmarks (Apple M3, 8 cores, results/*.json). The machine was not quiet: results/exec-benchmark.json records a 1/5/15-minute load average of 6.4-8.0 for the main and arm64 runs and 32-40 for the JavaScript remeasurement; a quiet-machine rerun is pending:

The site a0lang.com is itself two A0 programs (site/page.a0, site/docs.a0).

See DESIGN.md for intent and semantics, MODEL_GUIDE.txt for the AI-facing language instructions, STATUS.md for the current results, loss ledger, known limits, and next actions (session history in docs/history/), and results/ for machine-readable evidence.

Install

A0 is a single self-contained binary: no Node, no Bun.

# macOS / Linux: Homebrew (this repository is the tap; no separate tap repository)

brew tap Joe-Simo/a0 https://github.com/Joe-Simo/a0 brew install a0

macOS / Linux: script (detects OS/arch, verifies SHA-256, installs to ~/.local/bin)

curl -fsSL https://raw.githubusercontent.com/Joe-Simo/a0/main/install.sh | sh

# Windows (installs to %LOCALAPPDATA%\Programs\a0 and adds it to your user PATH)

irm https://raw.githubusercontent.com/Joe-Simo/a0/main/install.ps1 | iex

Both scripts accept A0_VERSION=v0.8.16 to pin a release and A0_INSTALL_DIR to change the destination. Or download a binary from the latest release by hand and check it against checksums.txt. a0 --version prints the release and compiler version. Then:

printf 'fn sq u32 -> u32\na mul p0 p0\nret a\nend\n' > sq.a0

a0 run sq.a0 sq 12 # 144 a0 emit arm64 sq.a0 # A0's own machine code; or c, js, java, sv a0 check sq.a0 # diagnostics with codes; --json for fields, --fix applies the exact fixes a0 explain A0102 # what a diagnostic means, with a failing and a fixed example

Binaries: a0-darwin-arm64, a0-darwin-x64, a0-linux-x64, a0-linux-arm64, a0-windows-x64.exe. A C compiler (clang or gcc) is needed only to link native output on your machine.

MCP server

a0 mcp serves A0 to AI agents over stdio (Model Context Protocol), so they edit through tools instead of text files: a0_open (function view with a handle), a0_program (program handle, optionally scoped to a target), a0_apply (edit under a handle; returns the new view or a diagnostic with code (class), id (A0nnnn), message, expected/actual, fix and applicability; the reply fix all applies every exact fix of the last rejected edit, atomically), a0_check, a0_run (reference interpreter, fuel-bounded), a0_emit (any target), a0_save (only after a successful apply). Paths are confined to the launch root (symlink escapes and use escapes rejected); no shell is run; output is bounded.

Language server

a0 lsp [root] is a Language Server Protocol server over stdio (built on vscode-languageserver; --stdio is accepted and ignored). Point any LSP client at it for .a0 files: diagnostics on open and change from the linker and checker (the diagnostic code is the class, data carries the table code id, the fix and its applicability, and an exact fix is a quick fix), hover (function signatures, op docs), go-to-definition across use files, document symbols, completion of ops and in-scope functions, and formatting through the canonical printer. Files are confined to root (default: the working directory) exactly as in the MCP server; a document outside it gets one limit diagnostic and nothing else.

Use with AI agents

Put a0 on your PATH first (see Install). Every entry below runs the same local stdio server, a0 mcp , with no hosting and no account. Ready-to-copy configs are in integrations/. The Agent Skill in skills/a0/ (install with npx skills add Joe-Simo/a0) teaches the language and loads the primer and the edit protocol only when they are needed.

Replace . with the folder the server may read and write when your agent does not start servers in the project folder.

Project rules, hooks and CI

• a0 init [dir] writes AGENTS.md (read by Codex, Cursor, Copilot, Jules and others) with the MCP tool workflow, the edit protocol and the primer path (.a0/MODEL_GUIDE.txt, copied in when available, otherwise the GitHub URL), plus @AGENTS.md pointers in CLAUDE.md and GEMINI.md, glob rules for Cursor (`.cursor/rules/a0