diff --git a/build-cthulhu.sh b/build-cthulhu.sh
index e889b8c..e18e015 100755
--- a/build-cthulhu.sh
+++ b/build-cthulhu.sh
@@ -1,9 +1,132 @@
 #!/usr/bin/env bash
-
+# Build both Cthulhu binaries without replacing the system Go installation.
 set -euo pipefail
 
+projectDir=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)
 
-go build -o cthulhu ./cmd/cthulhu
-go build -tags brlapi -o cthulhu-braille ./cmd/cthulhu-braille
-echo "Cthulhu has been built."
-exit 0
+fail() {
+    echo "Error: $*" >&2
+    exit 1
+}
+
+usage() {
+    cat <<'EOF'
+Usage: build-cthulhu.sh [-o DIR | --output-dir DIR] [-- GO_BUILD_FLAGS...]
+
+Build cthulhu and cthulhu-braille, obtaining a suitable Go toolchain if needed.
+
+  -o, --output-dir DIR  Write both binaries to DIR (default: checkout root).
+                        Relative paths start at the caller's directory.
+                        Missing directories are created.
+  -h, --help            Show help without checking build dependencies.
+  --                    Pass remaining flags to both go build commands.
+
+GOTOOLCHAIN defaults to auto. Set GOTOOLCHAIN=local to require installed Go
+and prevent toolchain downloads. Other Go settings, compiler/linker flags,
+and cache paths are inherited. CGO must be enabled (the default here).
+If Go is absent or older than 1.21, the bootstrap toolchain is cached under
+${XDG_CACHE_HOME:-$HOME/.cache}/cthulhu/toolchains; curl, tar and sha256sum
+are needed only for this download. Installation belongs in the package recipe.
+EOF
+}
+
+outputDir=$projectDir
+buildFlags=()
+while (( $# )); do
+    case $1 in
+        -o|--output-dir)
+            (( $# >= 2 )) || fail "$1 requires a directory."
+            [[ -n $2 && $2 != -* ]] || fail "$1 requires a directory (use ./ for a name starting with '-')."
+            outputDir=$2
+            shift 2
+            ;;
+        -h|--help)
+            usage
+            exit 0
+            ;;
+        --)
+            shift
+            buildFlags=("$@")
+            break
+            ;;
+        *) fail "Unknown argument: $1. See --help." ;;
+    esac
+done
+if [[ $outputDir != /* ]]; then
+    outputDir="$PWD/$outputDir"
+fi
+[[ ! -e $outputDir || -d $outputDir ]] || fail "Output directory is an existing file: $outputDir"
+for binary in cthulhu cthulhu-braille; do
+    [[ ! -d $outputDir/$binary ]] || fail "Binary output path is a directory: $outputDir/$binary"
+done
+cd -- "$projectDir"
+
+[[ $(uname -s) == Linux ]] || fail "Cthulhu requires Linux."
+export CGO_ENABLED=${CGO_ENABLED:-1}
+[[ $CGO_ENABLED == 1 ]] || fail "Cthulhu requires CGO_ENABLED=1 and a C compiler."
+command -v pkg-config >/dev/null || fail "Install pkg-config and the development libraries listed in CONTRIBUTING.md."
+missingLibraries=()
+for library in gtk+-3.0 atspi-2 gio-2.0 gstreamer-1.0 x11 xtst xfixes xres xkbcommon liblouis; do
+    if ! pkg-config --exists "$library"; then
+        missingLibraries+=("$library")
+    fi
+done
+if (( ${#missingLibraries[@]} )); then
+    fail "Missing development libraries (pkg-config names): ${missingLibraries[*]}. See CONTRIBUTING.md; BRLTTY/BrlAPI headers and library are also required."
+fi
+
+requiredVersion=$(awk '$1 == "go" { print $2; exit }' go.mod)
+[[ $requiredVersion =~ ^1\.[0-9]+\.[0-9]+$ ]] || fail "Cannot read the required Go version from go.mod."
+export GOTOOLCHAIN=${GOTOOLCHAIN:-auto}
+goCommand=""
+if command -v go >/dev/null; then
+    installedVersion=$(GOTOOLCHAIN=local go version 2>/dev/null || true)
+    if [[ $installedVersion =~ go1\.([0-9]+) ]] && (( 10#${BASH_REMATCH[1]} >= 21 )); then
+        goCommand=$(command -v go)
+    fi
+fi
+
+if [[ -z $goCommand ]]; then
+    [[ $GOTOOLCHAIN == auto ]] || fail "Install Go $requiredVersion or newer, or use GOTOOLCHAIN=auto to allow bootstrapping."
+    case $(uname -m) in
+        x86_64) archiveArch=amd64 ;;
+        aarch64|arm64) archiveArch=arm64 ;;
+        armv6l|armv7l) archiveArch=armv6l ;;
+        i386|i486|i586|i686) archiveArch=386 ;;
+        ppc64le|riscv64|s390x|loong64) archiveArch=$(uname -m) ;;
+        loongarch64) archiveArch=loong64 ;;
+        *) fail "No Go bootstrap archive configured for $(uname -m). Install Go 1.21 or newer."
+    esac
+    toolchainDir="${XDG_CACHE_HOME:-$HOME/.cache}/cthulhu/toolchains/go${requiredVersion}.linux-${archiveArch}"
+    goCommand="$toolchainDir/go/bin/go"
+    if [[ ! -x $goCommand ]]; then
+        for dependency in curl tar sha256sum; do
+            command -v "$dependency" >/dev/null || fail "Install $dependency to bootstrap Go, or install Go $requiredVersion or newer."
+        done
+        mkdir -p -- "$(dirname -- "$toolchainDir")"
+        downloadDir=$(mktemp -d "${toolchainDir}.download.XXXXXX")
+        trap 'rm -rf -- "$downloadDir"' EXIT
+        archiveURL="https://dl.google.com/go/go${requiredVersion}.linux-${archiveArch}.tar.gz"
+        echo "Downloading Go $requiredVersion for Linux/$archiveArch..."
+        curlFlags=(--fail --location --silent --show-error --proto '=https' --proto-redir '=https')
+        expectedChecksum=$(curl "${curlFlags[@]}" "$archiveURL.sha256")
+        [[ $expectedChecksum =~ ^[[:xdigit:]]{64}$ ]] || fail "Invalid Go archive checksum."
+        curl "${curlFlags[@]}" --output "$downloadDir/go.tar.gz" "$archiveURL"
+        printf '%s  %s\n' "$expectedChecksum" "$downloadDir/go.tar.gz" | sha256sum --check --status || fail "Go archive checksum mismatch."
+        tar -xzf "$downloadDir/go.tar.gz" -C "$downloadDir"
+        [[ -x $downloadDir/go/bin/go ]] || fail "The downloaded archive does not contain Go."
+        # Publish a complete tree; another build may have populated the cache.
+        if [[ ! -x $goCommand ]]; then
+            mkdir -p -- "$toolchainDir"
+            mv -T -- "$downloadDir/go" "$toolchainDir/go" || [[ -x $goCommand ]]
+        fi
+    fi
+    unset GOROOT
+fi
+
+goVersion=$("$goCommand" version) || fail "The selected Go toolchain could not run."
+echo "Using $goVersion"
+mkdir -p -- "$outputDir"
+"$goCommand" build "${buildFlags[@]}" -o "$outputDir/cthulhu" ./cmd/cthulhu
+"$goCommand" build "${buildFlags[@]}" -tags brlapi -o "$outputDir/cthulhu-braille" ./cmd/cthulhu-braille
+echo "Cthulhu has been built: $outputDir/cthulhu and $outputDir/cthulhu-braille"