---
title: "Bun OS feature qualification matrix"
description: "Source inventory and platform constraints for the fused HTML, CSS and browser engines."
---

# Bun OS feature qualification matrix

The source inventory is `scripts/aphrody/fusion-features.ts`, with declared
qualification combinations in `tools/config/fusion-features.json`. It reads
every explicit root Cargo workspace member, records declared feature edges,
optional dependencies, dependency aliases and target expressions, and hashes
the inspected manifests and build/runtime sources. It performs no build,
network request, host mutation or feature activation.

```sh
bun scripts/aphrody/fusion-features.ts --root .
bun scripts/aphrody/fusion-features.ts --root . --strict
bun scripts/aphrody/fusion-features.ts --strict --build-selection pgoGenerate --build-selection pgoUse
```

`--strict` exits nonzero for unsupported selections. `supported` means the
declaration exists and passes the configured static constraints.
`requiring-dependencies` identifies native linkage, tooling or behavior gates
that remain necessary. Every row reports `qualified: false`; source analysis
cannot establish stability. Undeclared features, absent root packages and a
JSC-only selection that still reaches V8 produce `unsupported`.

## Linux, Windows and macOS

| Combination             | Source path                                                  | Qualification needed on each target                                   |
| ----------------------- | ------------------------------------------------------------ | --------------------------------------------------------------------- |
| Native HTML and CSS     | `bun_runtime`, `bun_css`, `bun_browser_dom`                  | Complete Bun/JSC native link; candidate HTML/CSS and MCP parser tests |
| Portable paint          | `bun_browser_render/paint`                                   | Pinned font/text/image libraries; deterministic screenshots           |
| Shared CSS and paint    | `bun_browser_render/paint,bun_browser_render/bun-css`        | Bun native symbols plus renderer cascade/layout/screenshots           |
| Page render and stealth | browser/CDP/MCP/service `render,stealth`, renderer `bun-css` | Page engine migration and full browser behavior; pinned TLS stack     |
| Coreutils               | `bun_runtime/coreutils`                                      | Runtime build-planner unit graph, native applet/platform tests        |

The page engine's current `bun_browser_js` manifest depends on `deno_core`
in both normal and build dependencies. Activating `render` or `stealth` does
not change its JavaScript engine. The inventory checks the current source
dependency graph for this blocker rather than marking a JSC migration complete.
The page build rejects cross-compilation when a matching `cross-mksnapshot`
producer is absent. That rejection prevents embedding a host architecture V8
snapshot; successful cross-target build and execution receipts remain required.
Native `Bun.HTML` and the inert MCP HTML/CSS parsers already use Bun's shared
libraries; this does not establish an interactive page runtime or Chromium
compatibility. Chromium-level browser support requires DOM APIs, navigation,
events, CSS/layout/paint, networking and web-platform behavior qualification.

`wreq` has target-specific dependency declarations: Linux/Android enable
`prefix-symbols`; Windows and macOS omit it. Preserve these platform guards.
Rendering uses the existing pinned text stack and font policy. Shared CSS
requires Bun's native symbols, so a standalone Cargo test link is insufficient.
The factory's portable browser Nextest suite excludes `bun-css`; candidate
native CSS tests and the full renderer linkage gate provide separate evidence.

## Build flags and runtime toggles

Cargo features, `bun bd` profiles and runtime environment flags are separate
controls. The inventory lists build profiles from `scripts/build/profiles.ts`
and build option declarations from `PartialConfig`, plus runtime feature
declarations from `src/bun_core/env_var.rs`, including ordinary boolean/string
`BUN_FEATURE_FLAG_*` declarations and `new_feature_flag!`; source hashes
also cover `scripts/build/config.ts`, `scripts/build/rust.ts` and compile-time
`src/bun_core/feature_flags.rs`. Target-dependent constants are not switches
that can safely be enabled universally.

Cargo's unit graph, produced by the existing native build planner, decides
which features reach the final executable. The static traversal preserves
weak optional dependency edges and target expressions; it does not replace
Cargo resolution or evaluate all target `cfg` expressions. Mandatory edges and
defaults and dependency feature requests are traversed. Disabled dependency
defaults stay disabled; weak child features are followed only after their own
alias is active. Optional aliases are not activated by another owner's selection
of the same package. Target-dependent exclusions, development-only feature
unification and resolver behavior require review in Cargo's actual unit graph.
Review the emitted
unit graph on the qualified factory for each native build. A `cargo check`
selection is a proposed source gate, not evidence that `bun bd` forwards it.

`buildFeatureForwarding` audits the runtime invocation separately from the
Windows shim invocation. It reports whether `--features` and `coreutils` occur
in that source and whether configuration rejects concurrent PGO generation/use.
Those source observations require actual unit-graph and final-binary checks;
a `shim_standalone` flag cannot establish runtime feature forwarding.

The native build owner exposes `coreutils` through its existing configuration,
enables it by default and permits `--coreutils=off`. Its Cargo runtime invocation
forwards `--features coreutils` when enabled. Source forwarding does not establish
that every applet executes correctly on each platform.

Do not select `--all-features` blindly. PGO generation and PGO use are different
build operations; local and prebuilt WebKit are alternative dependency modes.
The configuration records incompatible selections explicitly; repeated
`--build-selection` arguments audit the proposed selection and report conflicts.
ASAN, assertions,
exception verification, optimization and architecture settings retain their
existing target/toolchain validation in the build owner.

`BUN_LIVE_WEBSITE_TEST=1` enables only the explicit HTTPS qualification fixture.
`BUN_JSC_validateExceptionChecks` and `BUN_JSC_dumpSimulatedThrows` are debug
verification controls. Docker mutation and native MCP host capability flags
require an explicit owner policy. Unclassified runtime flags preserve their
defaults until reviewed. No matrix selection grants remote pages Bun or host
privileges, bypasses SSRF/origin rules or disables a security check.

## Evidence required for stable delivery

The source-only tests in `scripts/aphrody/fusion-features.test.ts` cover aliases,
target guards, disabled defaults, dependency requests, alias activation, weak
edges, cycles, missing features, V8 blockers, the missing cross-snapshot rejection
and incompatible
selections. Run them on the SSH VPS through the existing factory resource
owner. No test or build for this matrix has been executed locally.

Use the root Cargo workspace, `browser-tests` profile, serialized Cargo wrapper,
existing native build lock and declared factory cache. Linux VPS gates require
live resource admission and a maximum of six build jobs. Native Windows and
macOS packaging and runtime results require their respective qualified runners;
a Linux pass cannot qualify them. Stable status requires actual source-matched
check/test/Clippy, native runtime, renderer and platform CI receipts. See
[fusion verification](./fusion-verification.mdx) for the orchestrator and
coverage contract.