diff --git a/internal/rosa/rosa.go b/internal/rosa/rosa.go
index d7a6105..ebad810 100644
--- a/internal/rosa/rosa.go
+++ b/internal/rosa/rosa.go
@@ -55,6 +55,51 @@ func linuxArch() string {
 	}
 }
 
+// triplet returns the Rosa OS host triple corresponding to [runtime.GOARCH].
+func triplet() string {
+	return linuxArch() + "-rosa-linux-musl"
+}
+
+const (
+	// EnvTriplet holds the return value of triplet.
+	EnvTriplet = "ROSA_TRIPLE"
+	// EnvRefCFLAGS holds toolchain-specific reference CFLAGS.
+	EnvRefCFLAGS = "ROSA_CFLAGS"
+	// EnvRefCXXFLAGS holds toolchain-specific reference CXXFLAGS.
+	EnvRefCXXFLAGS = "ROSA_CXXFLAGS"
+)
+
+// ldflags returns LDFLAGS corresponding to triplet.
+func ldflags(static bool) string {
+	s := "LDFLAGS=" +
+		"-fuse-ld=lld " +
+		"-L/system/lib -Wl,-rpath=/system/lib " +
+		"-L/system/lib/" + triplet() + " " +
+		"-Wl,-rpath=/system/lib/" + triplet() + " " +
+		"-rtlib=compiler-rt " +
+		"-unwindlib=libunwind " +
+		"-Wl,--as-needed"
+	if !static {
+		s += " -Wl,--dynamic-linker=/system/lib/ld-musl-x86_64.so.1"
+	}
+	return s
+}
+
+// cflags is reference CFLAGS for the Rosa OS toolchain.
+const cflags = "-Qunused-arguments " +
+	"-isystem/system/include"
+
+// cxxflags returns reference CXXFLAGS for the Rosa OS toolchain corresponding
+// to [runtime.GOARCH].
+func cxxflags() string {
+	return "--start-no-unused-arguments " +
+		"-stdlib=libc++ " +
+		"--end-no-unused-arguments " +
+		"-isystem/system/include/c++/v1 " +
+		"-isystem/system/include/" + triplet() + "/c++/v1 " +
+		"-isystem/system/include "
+}
+
 // Toolchain denotes the infrastructure to compile a [pkg.Artifact] on.
 type Toolchain uintptr
 
@@ -62,6 +107,10 @@ const (
 	// toolchainBusybox denotes a busybox installation from the busyboxBin
 	// binary distribution. This is for decompressing unsupported formats.
 	toolchainBusybox Toolchain = iota
+
+	// toolchainStage3 denotes the Gentoo stage3 toolchain. Special care must be
+	// taken to compile correctly against this toolchain.
+	toolchainStage3
 )
 
 // lastIndexFunc is like [strings.LastIndexFunc] but for [slices].
@@ -109,6 +158,8 @@ func (t Toolchain) New(
 
 	paths ...pkg.ExecPath,
 ) pkg.Artifact {
+	const lcMessages = "LC_MESSAGES=C.UTF-8"
+
 	var (
 		path    = AbsSystem.Append("bin", "busybox")
 		args    = []string{"hush", absCureScript.String()}
@@ -119,6 +170,39 @@ func (t Toolchain) New(
 		support = slices.Concat([]pkg.Artifact{newBusyboxBin()}, extra)
 		env = fixupEnviron(env, nil, "/system/bin")
 
+	case toolchainStage3:
+		const (
+			version  = "20260111T160052Z"
+			checksum = "c5_FwMnRN8RZpTdBLGYkL4RR8ampdaZN2JbkgrFLe8-QHQAVQy08APVvIL6eT7KW"
+		)
+		path = fhs.AbsRoot.Append("bin", "bash")
+		args[0] = "bash"
+		support = slices.Concat([]pkg.Artifact{
+			cureEtc{},
+			toolchainBusybox.New("stage3-"+version, nil, nil, nil, `
+tar -C /work -xf /usr/src/stage3.tar.xz
+rm -rf /work/dev/ /work/proc/
+ln -vs ../usr/bin /work/bin
+`, pkg.Path(AbsUsrSrc.Append("stage3.tar.xz"), false,
+				pkg.NewHTTPGet(
+					nil, "https://distfiles.gentoo.org/releases/"+
+						runtime.GOARCH+"/autobuilds/"+version+
+						"/stage3-"+runtime.GOARCH+"-musl-llvm-"+version+".tar.xz",
+					mustDecode(checksum),
+				),
+			)),
+		}, extra)
+		env = fixupEnviron(env, []string{
+			EnvTriplet + "=" + triplet(),
+			lcMessages,
+
+			EnvRefCFLAGS + "=" + cflags,
+			EnvRefCXXFLAGS + "=" + cxxflags(),
+			ldflags(true),
+		}, "/system/bin",
+			"/usr/bin",
+			"/usr/lib/llvm/21/bin",
+		)
 	default:
 		panic("unsupported toolchain " + strconv.Itoa(int(t)))
 	}