Hirð

A hirð is a Norse king's household guard: sworn retainers, each with a named duty, answerable to one lord. Illustration by Erik Werenskiold for Magnús Erlingsson's saga in Snorri's Heimskringla (public domain, via Wikimedia Commons).

A typed language for long-running agent systems on BEAM: effect-row tracking, auditable tool effects, typed actors, and OTP supervision. Python agent frameworks hide side effects in coroutine soup; Hirð makes every tool call, every actor message, and every supervisor boundary visible in the types and queryable by tooling.

What that buys: deterministic replay of real agent traffic. Every tool call is recorded unconditionally in a canonical wire format, so a recorded run is a file you can replay — the same calls, in the same order, each served the result the recorded run got back, with no service contacted. That is a regression test with no oracle to maintain, a bug report that reproduces, and a fixed environment to evaluate a change in. It is also not something you can retrofit onto a framework that hides its side effects: it needs the effects in the types and a single dispatch path underneath them.

Status: pre-1.0 and experimental. The v0.1 compiler pipeline works end to end (the demos below type-check, compile to Erlang, and run on BEAM), but the language surface is unstable, nothing is published to crates.io, and breaking changes land without deprecation cycles. The roadmap lives in the in-repo issue tracker (see .beads/README.md).

Install

Prebuilt binaries for Linux, macOS, and Windows are attached to every release: extract the archive for your platform and put hird (the compiler), hird-lsp, and hird-mcp on your PATH.

From source, with Rust 1.97 or newer:

cargo install --git https://github.com/no-materials/hird hird-cli
cargo install --git https://github.com/no-materials/hird hird-lsp  # optional
cargo install --git https://github.com/no-materials/hird hird-mcp  # optional

With Nix, the same three binaries are flake outputs (nix run github:no-materials/hird#hird-mcp).

Compiling and running programs needs Erlang/OTP on PATH (apt install erlang, brew install erlang, …); hird check works without it.

Quick start

Hirð has no ambient print. Anything a program tells the outside world goes through a tool — a declared, typed, audited external operation — so the smallest observable program is a tool call. Save this as hello.hird:

module Hello

effect Tool<t>

tool Say : { message: String } → ()

fn quiet_say(args: { message: String }) → () = ()

fn main() → () ! {} =
  handle {
    Tool<Say> → quiet_say,
  } in say({ message: "hello, world" })

hird run hello.hird

{"schema_version":1,"tool":"Say","args":{"message":"hello, world"},"result":{"ok":null},"timestamp":"…","caller":"Hello.main"}

Three things happened. Declaring tool Say created the effect Tool<Say> and a callable say. The handle block supplied an implementation and discharged that effect, so main is honestly ! {}. And the call was recorded on the audit stream — unconditionally, because mocked and real tool calls audit identically. ASCII operator spellings (->) normalise to their Unicode forms (→) at lex time, so either is legal input.

Command	What it does
hird check <file-or-dir>	type- and effect-check; coded diagnostics
hird build <file>	emit readable Erlang, compile it to .beam
hird run <file>	build, then execute fn main on BEAM
hird emit-ast <file> --json	the typed IR of every definition
hird emit-effect-graph <file> --json	actors, mailboxes, handler rows, supervisors, tools

docs/writing-hird-human.md is the guided tour, and phrasebook.md the dense syntax reference.

The v0.1 demo: a supervised agent planner

demo/agent_planner.hird is the flagship v0.1 program. A Planner actor receives a repository path, reads repository state through Tool<ReadRepo>, analyzes it (pure computation), files tickets through Tool<CreateTicket>, and logs progress through Tool<Log>; a PlannerSup supervisor declares it as a one_for_one child. The entry point installs the demo's tool handlers in the runtime registry — handler maps never cross the supervision boundary, so the install block is how the supervised planner's tool calls resolve — starts the tree with supervise(PlannerSup), reaches the running child with child(PlannerSup, planner), and drives one planning round end to end: the planner it messages is a supervised OTP process, restarted by PlannerSup if it crashes.

Build it (requires Erlang/OTP on PATH):

hird build demo/agent_planner.hird

This type-checks the program, emits human-readable Erlang source (hird_agent_planner.erl, hird_planner.erl, hird_planner_sup.erl, plus the hand-written runtime), and compiles it all with erlc into _build/hird/.

Run it on BEAM:

hird run demo/agent_planner.hird

Every tool invocation — mocked or real — is recorded unconditionally on the audit stream, one canonical JSON line per call:

{"schema_version":1,"tool":"CreateTicket","args":{"body":"The parser has no fuzz harness.","title":"Fuzz the parser"},"result":{"ok":{"ctor":"TicketId","args":["Fuzz the parser"]}},"timestamp":"2026-07-28T06:44:42.893Z","caller":"AgentPlanner.file_tickets"}

Query the actor/effect graph as JSON (or drop --json for text):

hird emit-effect-graph demo/agent_planner.hird --json

The graph shows the Planner actor with its full effect summary, its mailbox sum type (PlanRepo | GetStatus | Shutdown), the PlannerSup supervisor with its strategy and children, and each tool declaration with structured argument and return types.

The dry-run test harness lives in crates/hird-cli/tests/demo.rs: it re-runs the same demo with mock handlers swapped into the install block and asserts on the audit JSON lines — the same program, the same unconditional audit stream, differing only in the installed handler set.

Record and replay a run

Because the stream is complete — every call, full arguments, tagged result — a recorded run is a replayable environment. Record one to a file instead of stdout:

hird run demo/agent_planner.hird --audit-file run.jsonl

Then feed it back as the tool implementation:

hird run demo/agent_planner.hird --replay run.jsonl

The replay cursor outranks every handle and install block, so no tool runs and no service is contacted; each call receives its logged result, failures included. Matching is strict: the call at each position must be the one the log recorded there, or the run crashes with a replay_divergence naming the position, the recorded call and the offered one — and a log the run did not read to the end fails too.

So a checked-in recording is a regression test with no oracle to maintain: demo/agent_planner.golden.jsonl is one run of the planner, replayed by the demo suite in CI, and the build fails the moment the program's decisions drift from it. And because the log serves every result, one recording is a fixed environment to compare variants of a program in — every arm meets a byte-identical world, so what differs is attributable to the programs:

baseline        agreed with all 7 calls
announce-first  parted at call 2 (tool_mismatch)
eager           parted at call 4 (args_mismatch)

docs/audit-evidence.md states what the stream guarantees and what it does not; docs/tool-effects.md is the normative format and replay specification.

LLM tooling (MCP)

hird-mcp is a Model Context Protocol server over the same compiler pipeline, speaking stdio. It gives LLM agents structured compiler queries instead of source-reading guesswork: infer_type, lookup_definition, explain_effect_row, render_ir_fragment, explain_actor_protocol, emit_actor_effect_graph, get_context_for_symbol (token-budget-aware symbol summaries), and get_context_budget. Errors come back structured — undefined names list the available ones, parse and type errors carry coded diagnostics — so agents can self-correct from tool output alone.

The repository ships a project-scoped .mcp.json, so Claude Code sessions started here pick the server up automatically (it launches nix run .#hird-mcp; run nix build .#hird-mcp once so the first session start doesn't wait on a cold build). Any other MCP client can launch the hird-mcp binary directly, with no arguments.

Things worth asking an agent wired to it:

• "What does the Planner actor in demo/agent_planner.hird do? Ask the compiler instead of reading the source."
• "If the Planner crashes mid-plan, who restarts it, and what's the restart budget?"
• "Give me a 50-token summary of the Planner actor. Now 400 tokens. What got dropped?"
• "Write a new Hirð module with a supervised actor, and iterate with the hird tools until they confirm it's clean."

demo/counter_demo.hird is that last prompt's output: a supervised counter written by an LLM agent that verified itself against the MCP tools alone — it type-checks and runs on BEAM unmodified. docs/writing-hird-llm.md is the agent-facing guide.

Editor support

Language server

hird-lsp is a Language Server Protocol server over the compiler front end, speaking stdio. Point any LSP client at the hird-lsp binary, with no arguments.

v0.1 capabilities:

• Diagnostics on file open and save: parse errors, then type errors and warnings, with source spans.
• Hover: the inferred type of the identifier or expression under the cursor, including the effect row for functions (read_file : Path → String ! {Tool<ReadFile>}).
• Go-to-definition for top-level declarations: functions, types and their constructors, effects, tools (by marker or generated function name), actors and their message types, and supervisors.

Known limitations (real, by design for v0.1):

• No completion, rename/refactor, or code actions.
• No workspace-wide analysis: each file is compiled alone, so use imports of other modules report as unresolved and definitions resolve only within the current file.
• No incremental compilation: every change recompiles the whole file.

Syntax highlighting

tree-sitter-hird/ is a tree-sitter grammar for the v0.1 surface, with highlights.scm, indents.scm and folds.scm under queries/. Both operator spellings parse identically, so → and -> highlight the same. The flake builds it as a package output, next to hird-lsp:

nix build github:no-materials/hird#tree-sitter-hird

The result holds the compiled parser and a copy of queries/. A flake-based Neovim configuration takes this repository as an input and hands the grammar to nvim-treesitter, which wants the parser and the queries under the names it looks them up by:

# inputs.hird.url = "github:no-materials/hird";
{
  plugins = [
    (pkgs.neovimUtils.grammarToPlugin
      inputs.hird.packages.${pkgs.system}.tree-sitter-hird)
  ];
}

Neovim needs the file type registered too, whichever route below you take, since .hird is not one it knows:

vim.filetype.add({ extension = { hird = "hird" } })

Without nix, nvim-treesitter builds the grammar itself, given the tree-sitter CLI on PATH (npm i -g tree-sitter-cli). src/parser.c is generated rather than committed — grammar.js is the only source — so requires_generate_from_grammar is the part that matters: it makes :TSInstall hird generate the parser before compiling it.

require('nvim-treesitter.parsers').get_parser_configs().hird = {
  install_info = {
    url = "https://github.com/no-materials/hird",
    location = "tree-sitter-hird",
    files = { "src/parser.c", "src/scanner.c" },
    requires_generate_from_grammar = true,
  },
  filetype = "hird",
}

That installs the parser but not the queries: nvim-treesitter ships those only for the languages it supports, so copy this grammar's onto the runtime path by hand. It is the one step the nix package does for you.

mkdir -p ~/.config/nvim/queries/hird
cp tree-sitter-hird/queries/*.scm ~/.config/nvim/queries/hird/

With no plugin at all, build the parser straight onto the runtime path next to those queries (tree-sitter build -o ~/.config/nvim/parser/hird.so) and call vim.treesitter.start() from a FileType hird autocommand.

Working on the grammar itself needs no global tree-sitter CLI — the dev shell ships one, and nix flake check runs the corpus tests and parses every .hird source in the repository:

cd tree-sitter-hird && tree-sitter generate && tree-sitter test

Repository layout

• crates/ — the Rust compiler workspace (lexer, parser, checker, IR, codegen, CLI, LSP and MCP servers).
• tree-sitter-hird/ — the tree-sitter grammar and editor queries.
• runtime/ — the hand-written Erlang runtime support library (tool dispatch, audit sink, handler registry).
• demo/ — the v0.1 demo programs.
• conformance/ — golden files for the audit-log wire format.
• docs/ — normative specifications (grammar, error model, tool effects wire format) and the audit stream's guarantees.
• phrasebook.md — dense surface-syntax reference.
• DECISIONS.md — architecture decision records.
• .beads/README.md — the issue tracker and roadmap, driven by bd.

Development

MSRV is Rust 1.97 (edition 2024). Before sending changes:

cargo fmt --all
cargo clippy --workspace --all-targets --all-features -- -D warnings
cargo test --workspace --all-features

BEAM-dependent tests skip themselves when erlc is not on PATH.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.