mbun | Rewrite Bun in MC++

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mbun is a modular C++26 JavaScript runtime built with mcpp — a rewrite of Bun in MC++ that is compatible with both Node.js and Bun. Bun APIs and bun:test, node:* built-ins, CommonJS/ES modules, and the npm package ecosystem run in one executable.

Why rewrite Bun in MC++?

1. "Rewrite" performance art, reprised — Rewrite Bun in Rust
2. mcpp build tool + modular C++ engineering validation — stress-testing the mcpp build tool, C++26, and C++ Modules for real-world usability in a 90+-member module workspace.
3. Validating and exploring AI-agent-driven development & collaboration — AI agents participate in the whole development and collaboration loop, as an end-to-end experiment: validating ideas, surfacing the problems you actually hit, and finding out how humans and agents work together on a large codebase.

Caution

mbun is an experimental, source-first project — do not use it in production yet. The goal is useful compatibility in real applications, measured against the native Bun and Node.js test corpora rather than API checklists.

Quick start

1. Build from source and run a minimal script

Install xlings, then install mcpp with it (toolchains are downloaded automatically into an isolated sandbox):

curl -fsSL https://raw.githubusercontent.com/openxlings/xlings/main/tools/other/quick_install.sh | bash
xlings install mcpp -y

Build mbun and run the smallest example:

git clone https://github.com/Sunrisepeak/mbun
cd mbun
git submodule update --init --recursive
mcpp build
mcpp run -- --version   # mbun 2026.07.18.0 + the compatible bun/node versions
mcpp run -- examples/common/hello.ts

2. Minimal Node.js and Bun examples

The same executable runs both API families natively:

# Node.js: node:http built-in + CommonJS
mcpp run -- examples/node/http-server.js

# Bun: Bun.serve + TypeScript
mcpp run -- examples/bun/http-server.ts

Each starts a server on http://127.0.0.1:3000/.

3. Well-known framework demos: Express and Elysia

Express is the most widely used Node.js web framework. mbun installs its npm dependencies itself and runs it:

mcpp run -- --cwd examples/node/express install
mcpp run -- --cwd examples/node/express server.js

Elysia is a popular Bun-native web framework, running on the same executable:

mcpp run -- --cwd examples/bun/elysia install
mcpp run -- --cwd examples/bun/elysia server.ts

Open http://127.0.0.1:3000/ to see each demo page.

Contributing & collaboration — AI Agent open-source spec (hagent)

mbun's goal #3 is to explore AI-agent-driven development and collaboration. That model is written up as hagent — an AI Agent open-source collaboration spec (human + agent) that lives in hagent/ and is dogfooded here. Its core thesis: agents are the execution engine; the builder — the human who runs them — creates and judges (architecture, decisions, taste, norms). Every action is attributed to a builder, and no change is "done" without reproducible evidence.

Ways to contribute

Everything goes through your coding agent — start it in the repo root and it auto-loads AGENTS.md and the relevant skills, so it already knows the spec. Pick the level that fits you, and always review what the agent produced before opening a PR.

1. Report & discuss — hit a bug or have an idea? Have your agent file an issue or discussion following the reporting SOP: software versions, error output, initial analysis, and references — with local privacy scrubbed (usernames, tokens). → issue-reporting
2. Verify & review — reproduce reported bugs and confirm whether they're real, add findings; help verify issues and PRs; review others' PRs.
3. Develop — pick a task and build it, issue-first (bugfix / refactor / new feature — new features need an issue discussion and a design doc under .agents/docs/). → dev-process, tdd-workflow

More: hagent overview · charter · contributing · agents · labels.

Project maintainers

Elevated trust is a ladder — humans and agents earn it by the same rules (charter §4). Tiers above committer are still being defined.

Role	Permission	How to apply	Holders
Triager	Triage, label, verify issues & PRs, run CI	Self-apply after ≥ 3 merged PRs	—
Committer	Write (push / merge)	Module background + significant contributions; maintainer approval	—
Module maintainer	A module's review & decisions	—	—
Project maintainer	Governance, protected surfaces	—	@sunrisepeak

Compatibility data

Measured 2026-08-03 by the runners in tools/integration/, both corpora executed in full on one frozen binary (f65f960b7ac7d212…). Nothing unsupported is counted as a pass.

Target	Result	Rate
Node.js native tests (compat/node/test/parallel)	3,267 / 4,433 files pass	73.7%
Node.js native tests, excluding files that skip themselves	3,267 / 3,919 files pass	83.4%
Bun native full corpus (compat/bun/test)	1,076 / 1,902 files fully green	56.6%
Bun corpus, excluding files with no runnable tests here	1,076 / 1,806 files fully green	59.6%
Both corpora combined	4,343 / 6,335 files	68.6%
Both corpora combined, runnable only	4,343 / 5,725 files	75.9%
Elysia test suite (measured 2026-07-30, not re-run since)	1,522 pass / 3 fail	99.8%

The "excluding" rows are not a softer denominator: a file counts as excluded only when it reports no runnable tests on this platform — it skipped itself, it has no tests upstream, or it needs a service this machine does not provide. Those denominators grew this round (Node 3,898 → 3,919) because 21 files that used to skip themselves now run.

What stands between here and 100%, counted rather than estimated. 514 Node files decline to run, and the reasons are not interchangeable:

Self-skip reason	Files	Nature
QUIC not enabled	238	no node:quic subsystem; node's own build disables it by default too
V8 inspector disabled	180	needs a CDP backend; JSC speaks the WebInspector protocol instead
missing ESLint	29	node's own lint tests; ESLint is not vendored here
no OpenSSL CLI	10	crypto build configuration
requires Amaro	6	TypeScript loader
Windows-specific	6	unreachable on Linux by construction
other (Temporal, debug-only builds, cipher availability)	45	assorted

The two large blocks are priced. QUIC needs no protocol work — node builds it on vendored ngtcp2, and the linked OpenSSL 3.5.1 already exports everything that backend calls — but it does need node's ~19k-line binding re-expressed against JSC. The inspector block has no per-file gate: all of it hangs on one global capability flag, so no partial slice can score, and flipping it early converts skips into failures.

Related projects

• mcpp — the modular C++26 build tool and package manager mbun is built with
• Bun — the JavaScript runtime whose APIs, bun:test runner, and native test corpus mbun targets (bun.com)
• Node.js — the node:* built-ins, module systems, and native test corpus mbun targets (nodejs.org)
• Express — Node.js web framework used in examples/node/express (expressjs.com)
• Elysia — Bun-native web framework used in examples/bun/elysia (elysiajs.com)
• WebKit JavaScriptCore — the JavaScript engine embedded by mbun (as by Bun)

Limitations

• Linux x86_64 is the primary validated target: it is the only one the corpus numbers above were measured on, and the only one whose CI runs the workspace unit tests and the example-app smoke.
• macOS arm64 builds and runs. CI compiles the whole workspace and executes the binary on macos-latest; the wider test steps there are still being observed and are not yet gates. The numbers above have not been re-measured on it, so treat them as Linux figures until they are.
• Windows is not built or tested. The source carries _WIN32 branches, but nothing has ever been compiled for it here, so that is intent rather than support.
• JavaScriptCore and native dependencies are built through mcpp packages.
• Compatibility varies by API, package, and execution mode; passing the demos does not imply full Bun or Node.js compatibility.
• No prebuilt binary or stable release package is provided yet.

License

mbun code is licensed under MIT — the same license as Node.js and Bun. Third-party components (JavaScriptCore, OpenSSL, …) and the Bun / Node.js test corpora keep their upstream licenses — they are listed together at the end of LICENSE.