diff --git a/flake.nix b/flake.nix
index 7d75c10..7fc072d 100644
--- a/flake.nix
+++ b/flake.nix
@@ -57,7 +57,7 @@
               cp build/release/${pname}_bin $out/bin/${pname}
             '';
             hardeningDisable = [ "all" ];
-            __noChroot = true;
+            __noChroot = false;
           };
       in {
         packages.default = cargoxx-bin;
diff --git a/src/cli/cmd_build.cpp b/src/cli/cmd_build.cpp
index c5e4d0a..9294cba 100644
--- a/src/cli/cmd_build.cpp
+++ b/src/cli/cmd_build.cpp
@@ -35,6 +35,34 @@ auto write_text(const fs::path& path, std::string_view content) -> util::Result<
     return {};
 }
 
+auto query_flake_rev(std::string_view flake_ref) -> std::optional<std::string> {
+    auto r = exec::run("nix",
+                       {"--extra-experimental-features",
+                        "nix-command flakes", "flake", "metadata", "--json",
+                        std::string{flake_ref}},
+                       exec::ExecOptions{
+                           .cwd = fs::current_path(),
+                           .env_overrides = {},
+                           .timeout = std::chrono::seconds{30},
+                           .inherit_stdio = false,
+                       });
+    if (!r || r->exit_code != 0) {
+        return std::nullopt;
+    }
+    std::string_view body = r->stdout_text;
+    constexpr std::string_view key = "\"rev\":\"";
+    auto pos = body.find(key);
+    if (pos == std::string_view::npos) {
+        return std::nullopt;
+    }
+    pos += key.size();
+    auto end = body.find('"', pos);
+    if (end == std::string_view::npos) {
+        return std::nullopt;
+    }
+    return std::string{body.substr(pos, end - pos)};
+}
+
 // Builds the lockfile from the manifest + resolved recipes, **preserving**
 // `nixpkgs_rev` for any (name, version) entry that already exists in
 // `prior` with a matching key. This is what makes `cargoxx build`
@@ -59,6 +87,13 @@ auto merge_lockfile(const manifest::Manifest& m,
 
     lockfile::Lockfile lock;
     lock.version = 1;
+    lock.nixpkgs_rev_pin = prior.nixpkgs_rev_pin.has_value()
+                               ? prior.nixpkgs_rev_pin
+                               : query_flake_rev("github:NixOS/nixpkgs/nixos-unstable");
+    lock.flake_utils_rev_pin =
+        prior.flake_utils_rev_pin.has_value()
+            ? prior.flake_utils_rev_pin
+            : query_flake_rev("github:numtide/flake-utils");
 
     lockfile::LockfilePackage root{
         .name = m.package.name,
@@ -114,7 +149,9 @@ namespace {
 
 auto run_nix_cmake(const fs::path& project_root, const std::vector<std::string>& cmake_args,
                    std::string_view phase) -> util::Result<void> {
-    std::vector<std::string> args{"develop", "path:./build", "--command", "cmake"};
+    std::vector<std::string> args{"--extra-experimental-features",
+                                   "nix-command flakes", "develop",
+                                   "path:./build", "--command", "cmake"};
     args.insert(args.end(), cmake_args.begin(), cmake_args.end());
 
     auto r = exec::run("nix", args, exec::ExecOptions{
diff --git a/src/cli/cmd_test.cpp b/src/cli/cmd_test.cpp
index a2fd8d7..f659f32 100644
--- a/src/cli/cmd_test.cpp
+++ b/src/cli/cmd_test.cpp
@@ -19,7 +19,8 @@ auto cmd_test(const fs::path& project_root, bool release,
     const auto build_dir = std::format("build/{}", profile);
 
     auto r = exec::run("nix",
-                       {"develop", "path:./build", "--command", "ctest",
+                       {"--extra-experimental-features", "nix-command flakes",
+                        "develop", "path:./build", "--command", "ctest",
                         "--test-dir", build_dir, "--output-on-failure"},
                        exec::ExecOptions{
                            .cwd = project_root,
diff --git a/src/codegen/flake.cpp b/src/codegen/flake.cpp
index 09ecd73..3b07657 100644
--- a/src/codegen/flake.cpp
+++ b/src/codegen/flake.cpp
@@ -99,20 +99,26 @@ auto pinned_inputs_dedup(const std::vector<DepBinding>& bindings)
     return out;
 }
 
-auto emit_inputs_block(const std::vector<const DepBinding*>& pinned)
-    -> std::string {
-    // Always emit the shared toolchain `nixpkgs` and `flake-utils`
-    // inputs. Per-pinned-dep inputs land between them so the output
-    // diff stays stable across reruns.
+auto emit_inputs_block(const std::vector<const DepBinding*>& pinned,
+                        const lockfile::Lockfile& lock) -> std::string {
+    auto nixpkgs_url =
+        lock.nixpkgs_rev_pin && !lock.nixpkgs_rev_pin->empty()
+            ? std::format("github:NixOS/nixpkgs/{}", *lock.nixpkgs_rev_pin)
+            : std::string{"github:NixOS/nixpkgs/nixos-unstable"};
+    auto flake_utils_url =
+        lock.flake_utils_rev_pin && !lock.flake_utils_rev_pin->empty()
+            ? std::format("github:numtide/flake-utils/{}",
+                          *lock.flake_utils_rev_pin)
+            : std::string{"github:numtide/flake-utils"};
     std::string out =
         "  inputs = {\n"
-        "    nixpkgs.url = \"github:NixOS/nixpkgs/nixos-unstable\";\n";
+        + std::format("    nixpkgs.url = \"{}\";\n", nixpkgs_url);
     for (const auto* b : pinned) {
         out += std::format("    {}.url = \"github:NixOS/nixpkgs/{}\";\n",
                            b->sanitized, *b->rev);
     }
-    out += "    flake-utils.url = \"github:numtide/flake-utils\";\n"
-           "  };\n";
+    out += std::format("    flake-utils.url = \"{}\";\n", flake_utils_url);
+    out += "  };\n";
     return out;
 }
 
@@ -165,7 +171,7 @@ auto flake_nix(const GenerateInputs& in) -> std::string {
     out += "{\n";
     out += std::format("  description = \"{}\";\n\n", in.manifest.package.name);
 
-    out += emit_inputs_block(pinned);
+    out += emit_inputs_block(pinned, in.lock);
 
     const bool any_pkg_config =
         std::ranges::any_of(in.recipes,
diff --git a/src/lockfile/lockfile.cpp b/src/lockfile/lockfile.cpp
index 889c67d..6bfce0d 100644
--- a/src/lockfile/lockfile.cpp
+++ b/src/lockfile/lockfile.cpp
@@ -131,6 +131,12 @@ auto parse(const std::filesystem::path& path) -> util::Result<Lockfile> {
     if (auto v = root["version"].value<int>()) {
         lock.version = *v;
     }
+    if (auto v = root["nixpkgs_rev"].value<std::string>()) {
+        lock.nixpkgs_rev_pin = *v;
+    }
+    if (auto v = root["flake_utils_rev"].value<std::string>()) {
+        lock.flake_utils_rev_pin = *v;
+    }
 
     if (const auto* arr = root["package"].as_array()) {
         lock.packages.reserve(arr->size());
@@ -154,6 +160,12 @@ auto parse(const std::filesystem::path& path) -> util::Result<Lockfile> {
 auto write(const Lockfile& lock, const std::filesystem::path& path) -> util::Result<void> {
     toml::table root;
     root.insert_or_assign("version", lock.version);
+    if (lock.nixpkgs_rev_pin) {
+        root.insert_or_assign("nixpkgs_rev", *lock.nixpkgs_rev_pin);
+    }
+    if (lock.flake_utils_rev_pin) {
+        root.insert_or_assign("flake_utils_rev", *lock.flake_utils_rev_pin);
+    }
 
     toml::array packages;
     for (const auto& p : lock.packages) {
diff --git a/src/lockfile/lockfile.cppm b/src/lockfile/lockfile.cppm
index 863000c..e2b1f49 100644
--- a/src/lockfile/lockfile.cppm
+++ b/src/lockfile/lockfile.cppm
@@ -24,12 +24,12 @@ struct LockfilePackage {
 
 struct Lockfile {
     int version = 1;
+    std::optional<std::string> nixpkgs_rev_pin;
+    std::optional<std::string> flake_utils_rev_pin;
     std::vector<LockfilePackage> packages;
 
     bool operator==(const Lockfile&) const = default;
 
-    // The nixpkgs revision is shared across every dep package per SPEC §5.
-    // Returns the first non-empty rev seen, or nullopt if no deps are pinned.
     [[nodiscard]] auto nixpkgs_rev() const -> std::optional<std::string>;
 };
 
diff --git a/tests/cmd_build.cpp b/tests/cmd_build.cpp
index e975ad1..c002171 100644
--- a/tests/cmd_build.cpp
+++ b/tests/cmd_build.cpp
@@ -75,7 +75,7 @@ TEST_CASE("cmd_build generates files for a no-deps binary project",
 
     auto flake_text = read_file(root / "build" / "flake.nix");
     REQUIRE(flake_text.find("description = \"hello\";") != std::string::npos);
-    REQUIRE(flake_text.find("github:NixOS/nixpkgs/nixos-unstable") != std::string::npos);
+    REQUIRE(flake_text.find("github:NixOS/nixpkgs/") != std::string::npos);
 }
 
 TEST_CASE("cmd_build generates files for a library project", "[cli][build]") {
diff --git a/tests/codegen_flake.cpp b/tests/codegen_flake.cpp
index 3e21fe7..34f70f7 100644
--- a/tests/codegen_flake.cpp
+++ b/tests/codegen_flake.cpp
@@ -77,7 +77,7 @@ TEST_CASE("flake_nix adds pkgs.pkg-config to nativeBuildInputs only when needed"
           "[codegen][flake]") {
     Manifest m{pkg("app"), {dep("sqlite", "*")}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {root_pkg("app", "0.1.0")}};
+    Lockfile lock{.version = 1, .packages = {root_pkg("app", "0.1.0")}};
     std::vector<Recipe> recipes = {Recipe{
         .nixpkgs_attr = "sqlite",
         .find_package = "PkgConfig REQUIRED",
@@ -96,7 +96,7 @@ TEST_CASE("flake_nix omits pkgs.pkg-config when no recipe needs it",
           "[codegen][flake]") {
     Manifest m{pkg("hello"), {dep("fmt", "*")}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {root_pkg("hello", "0.1.0")}};
+    Lockfile lock{.version = 1, .packages = {root_pkg("hello", "0.1.0")}};
     std::vector<Recipe> recipes = {recipe("fmt_10")};
     GenerateInputs in{m, layout, lock, recipes, {}, "/tmp/hello"};
 
@@ -108,7 +108,7 @@ TEST_CASE("flake_nix always emits the shared nixos-unstable nixpkgs input",
           "[codegen][flake]") {
     Manifest m{pkg("hello"), {}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {root_pkg("hello", "0.1.0")}};
+    Lockfile lock{.version = 1, .packages = {root_pkg("hello", "0.1.0")}};
     GenerateInputs in{m, layout, lock, {}, {}, "/tmp/hello"};
 
     auto out = flake_nix(in);
@@ -122,7 +122,7 @@ TEST_CASE("flake_nix always emits the shared nixos-unstable nixpkgs input",
 TEST_CASE("flake_nix emits a per-pinned-dep nixpkgs input", "[codegen][flake]") {
     Manifest m{pkg("app"), {dep("fmt", "10.2.1")}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {
+    Lockfile lock{.version = 1, .packages = {
                          root_pkg("app", "0.1.0"),
                          dep_pkg("fmt", "10.2.1", "abc123def456"),
                      }};
@@ -146,7 +146,7 @@ TEST_CASE("flake_nix uses shared `pkgs` for unpinned deps",
           "[codegen][flake]") {
     Manifest m{pkg("app"), {dep("fmt", "*")}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {root_pkg("app", "0.1.0"), dep_pkg("fmt", "*", std::nullopt)}};
+    Lockfile lock{.version = 1, .packages = {root_pkg("app", "0.1.0"), dep_pkg("fmt", "*", std::nullopt)}};
     std::vector<Recipe> recipes = {recipe("fmt_10")};
     GenerateInputs in{m, layout, lock, recipes, {}, "/tmp/app"};
 
@@ -158,7 +158,7 @@ TEST_CASE("flake_nix uses shared `pkgs` for unpinned deps",
 TEST_CASE("flake_nix mixes pinned and unpinned deps", "[codegen][flake]") {
     Manifest m{pkg("app"), {dep("fmt", "10.2.1"), dep("zlib", "*")}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {
+    Lockfile lock{.version = 1, .packages = {
                          root_pkg("app", "0.1.0"),
                          dep_pkg("fmt", "10.2.1", "abc"),
                          dep_pkg("zlib", "*", std::nullopt),
@@ -175,7 +175,7 @@ TEST_CASE("flake_nix emits an empty buildInputs list when there are no deps",
           "[codegen][flake]") {
     Manifest m{pkg("hello"), {}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {root_pkg("hello", "0.1.0")}};
+    Lockfile lock{.version = 1, .packages = {root_pkg("hello", "0.1.0")}};
     GenerateInputs in{m, layout, lock, {}, {}, "/tmp/hello"};
 
     auto out = flake_nix(in);
@@ -188,7 +188,7 @@ TEST_CASE("flake_nix dedupes deps that share input + attr",
                {dep("boost", "1.84.0"), dep("boost", "1.84.0")},
                {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {
+    Lockfile lock{.version = 1, .packages = {
                          root_pkg("app", "0.1.0"),
                          dep_pkg("boost", "1.84.0", "rev42"),
                      }};
@@ -205,7 +205,7 @@ TEST_CASE("flake_nix dedupes deps that share input + attr",
 TEST_CASE("flake_nix produces deterministic output", "[codegen][flake]") {
     Manifest m{pkg("app"), {dep("fmt", "10.2.1"), dep("spdlog", "*")}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {
+    Lockfile lock{.version = 1, .packages = {
                          root_pkg("app", "0.1.0"),
                          dep_pkg("fmt", "10.2.1", "abc"),
                          dep_pkg("spdlog", "*", std::nullopt),
@@ -219,7 +219,7 @@ TEST_CASE("flake_nix produces deterministic output", "[codegen][flake]") {
 TEST_CASE("flake_nix output ends with a newline", "[codegen][flake]") {
     Manifest m{pkg("hello"), {}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {root_pkg("hello", "0.1.0")}};
+    Lockfile lock{.version = 1, .packages = {root_pkg("hello", "0.1.0")}};
     GenerateInputs in{m, layout, lock, {}, {}, "/tmp/hello"};
 
     auto out = flake_nix(in);
@@ -231,7 +231,7 @@ TEST_CASE("flake_nix sanitizes hyphens and dots in dep names",
           "[codegen][flake]") {
     Manifest m{pkg("app"), {dep("range-v3", "0.12.0")}, {}};
     DiscoveredLayout layout{};
-    Lockfile lock{1, {
+    Lockfile lock{.version = 1, .packages = {
                          root_pkg("app", "0.1.0"),
                          dep_pkg("range-v3", "0.12.0", "rev123"),
                      }};
diff --git a/tests/lockfile_round_trip.cpp b/tests/lockfile_round_trip.cpp
index 1de3335..1555483 100644
--- a/tests/lockfile_round_trip.cpp
+++ b/tests/lockfile_round_trip.cpp
@@ -54,14 +54,14 @@ auto round_trip(const Lockfile& l) -> Lockfile {
 } // namespace
 
 TEST_CASE("write round-trips a minimal lockfile", "[lockfile]") {
-    Lockfile l{1, {root_pkg("my-project", "0.1.0")}};
+    Lockfile l{.version = 1, .packages = {root_pkg("my-project", "0.1.0")}};
     REQUIRE(round_trip(l) == l);
 }
 
 TEST_CASE("write round-trips a lockfile with deps", "[lockfile]") {
     Lockfile l{
-        1,
-        {
+        .version = 1,
+        .packages = {
             root_pkg("my-project", "0.1.0", {"fmt 10.2.1", "spdlog 1.13.0"}),
             dep_pkg("fmt", "10.2.1", "fmt_10", "8a3f...c2d1"),
             dep_pkg("spdlog", "1.13.0", "spdlog", "8a3f...c2d1"),
@@ -72,8 +72,8 @@ TEST_CASE("write round-trips a lockfile with deps", "[lockfile]") {
 
 TEST_CASE("write round-trips lockfile recipe fields", "[lockfile]") {
     Lockfile l{
-        1,
-        {
+        .version = 1,
+        .packages = {
             LockfilePackage{
                 .name = "fmt",
                 .version = "10.2.1",
@@ -120,8 +120,8 @@ TEST_CASE("write round-trips lockfile recipe fields", "[lockfile]") {
 
 TEST_CASE("Lockfile::nixpkgs_rev returns the shared rev", "[lockfile]") {
     Lockfile l{
-        1,
-        {
+        .version = 1,
+        .packages = {
             root_pkg("p", "0.1.0", {"fmt 10.2.1"}),
             dep_pkg("fmt", "10.2.1", "fmt_10", "abc123"),
         },
@@ -129,8 +129,19 @@ TEST_CASE("Lockfile::nixpkgs_rev returns the shared rev", "[lockfile]") {
     REQUIRE(l.nixpkgs_rev() == "abc123");
 }
 
+TEST_CASE("write round-trips top-level nixpkgs_rev + flake_utils_rev pins",
+          "[lockfile]") {
+    Lockfile l{
+        .version = 1,
+        .nixpkgs_rev_pin = "549bd84d6279f9852cae6225e372cc67fb91a4c1",
+        .flake_utils_rev_pin = "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        .packages = {root_pkg("p", "0.1.0")},
+    };
+    REQUIRE(round_trip(l) == l);
+}
+
 TEST_CASE("Lockfile::nixpkgs_rev is nullopt when no deps", "[lockfile]") {
-    Lockfile l{1, {root_pkg("p", "0.1.0")}};
+    Lockfile l{.version = 1, .packages = {root_pkg("p", "0.1.0")}};
     REQUIRE_FALSE(l.nixpkgs_rev().has_value());
 }