diff --git a/tools/add-license-headers.sh b/tools/add-license-headers.sh
new file mode 100755
index 0000000..f823524
--- /dev/null
+++ b/tools/add-license-headers.sh
@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+# Prepend SPDX license identifiers to every Rust and Lua source file.
+# Idempotent: files already containing an `SPDX-License-Identifier:` line are skipped.
+# Run from the repository root: `./tools/add-license-headers.sh`
+
+set -euo pipefail
+
+readonly LICENSE_ID="AGPL-3.0-only"
+readonly RUST_HEADER="// SPDX-License-Identifier: ${LICENSE_ID}"
+readonly LUA_HEADER="-- SPDX-License-Identifier: ${LICENSE_ID}"
+
+# Directories to scan, relative to the repository root.
+readonly RUST_DIRS=("crates")
+readonly LUA_DIRS=("assets/scripts" "mods")
+
+added=0
+skipped=0
+
+prepend_if_missing() {
+    local file="$1"
+    local header="$2"
+
+    if grep -q "SPDX-License-Identifier:" "$file"; then
+        skipped=$((skipped + 1))
+        return
+    fi
+
+    # Use a temp file in the same directory so the rename is atomic on the same filesystem.
+    local tmp
+    tmp=$(mktemp "${file}.XXXXXX")
+    {
+        printf '%s\n\n' "$header"
+        cat "$file"
+    } >"$tmp"
+    mv "$tmp" "$file"
+    added=$((added + 1))
+}
+
+scan_dir() {
+    local dir="$1"
+    local ext="$2"
+    local header="$3"
+
+    [[ -d "$dir" ]] || return 0
+
+    while IFS= read -r -d '' file; do
+        prepend_if_missing "$file" "$header"
+    done < <(find "$dir" -type f -name "*.${ext}" -print0)
+}
+
+for dir in "${RUST_DIRS[@]}"; do
+    scan_dir "$dir" "rs" "$RUST_HEADER"
+done
+
+for dir in "${LUA_DIRS[@]}"; do
+    scan_dir "$dir" "lua" "$LUA_HEADER"
+done
+
+printf 'License headers — added: %d, already present: %d\n' "$added" "$skipped"