diff --git a/packaging/dev-shell.nix b/packaging/dev-shell.nix
index 37e92e3637f..7eec45bfb18 100644
--- a/packaging/dev-shell.nix
+++ b/packaging/dev-shell.nix
@@ -124,6 +124,7 @@ pkgs.nixComponents2.nix-util.overrideAttrs (
             (pkgs.writeScriptBin "pre-commit-hooks-install" modular.pre-commit.settings.installationScript)
             pkgs.buildPackages.nixfmt-rfc-style
             pkgs.buildPackages.shellcheck
+            pkgs.buildPackages.include-what-you-use
             pkgs.buildPackages.gdb
           ]
           ++ lib.optional (stdenv.cc.isClang && stdenv.hostPlatform == stdenv.buildPlatform) (
diff --git a/src/libflake-tests/flakeref.cc b/src/libflake-tests/flakeref.cc
index e2cb91bb85f..34d281c52d9 100644
--- a/src/libflake-tests/flakeref.cc
+++ b/src/libflake-tests/flakeref.cc
@@ -1,8 +1,15 @@
 #include <gtest/gtest.h>
+#include <set>
+#include <string>
+#include <utility>
 
 #include "nix/fetchers/fetch-settings.hh"
 #include "nix/flake/flakeref.hh"
 #include "nix/fetchers/attrs.hh"
+#include "nix/fetchers/fetchers.hh"
+#include "nix/util/configuration.hh"
+#include "nix/util/error.hh"
+#include "nix/util/experimental-features.hh"
 
 namespace nix {
 
diff --git a/src/libflake-tests/nix_api_flake.cc b/src/libflake-tests/nix_api_flake.cc
index f7e0cb71980..da7f01401ad 100644
--- a/src/libflake-tests/nix_api_flake.cc
+++ b/src/libflake-tests/nix_api_flake.cc
@@ -1,15 +1,17 @@
+#include <gtest/gtest.h>
+#include <filesystem>
+#include <string>
+
 #include "nix/util/file-system.hh"
 #include "nix_api_store.h"
 #include "nix_api_util.h"
 #include "nix_api_expr.h"
 #include "nix_api_value.h"
 #include "nix_api_flake.h"
-
-#include "nix/expr/tests/nix_api_expr.hh"
 #include "nix/util/tests/string_callback.hh"
-
-#include <gmock/gmock.h>
-#include <gtest/gtest.h>
+#include "nix/store/tests/nix_api_store.hh"
+#include "nix/util/tests/nix_api_util.hh"
+#include "nix_api_fetchers.h"
 
 namespace nixC {
 
diff --git a/src/libflake-tests/url-name.cc b/src/libflake-tests/url-name.cc
index 81ba516c8e4..64cbe5c9db0 100644
--- a/src/libflake-tests/url-name.cc
+++ b/src/libflake-tests/url-name.cc
@@ -1,6 +1,8 @@
-#include "nix/flake/url-name.hh"
 #include <gtest/gtest.h>
 
+#include "nix/flake/url-name.hh"
+#include "nix/util/url.hh"
+
 namespace nix {
 
 /* ----------- tests for url-name.hh --------------------------------------------------*/
diff --git a/src/libflake/config.cc b/src/libflake/config.cc
index c9071f601f9..c248ed0a66a 100644
--- a/src/libflake/config.cc
+++ b/src/libflake/config.cc
@@ -1,9 +1,30 @@
+#include <nlohmann/json.hpp>
+#include <assert.h>
+#include <stdint.h>
+#include <nlohmann/json_fwd.hpp>
+#include <cctype>
+#include <filesystem>
+#include <map>
+#include <memory>
+#include <optional>
+#include <string>
+#include <utility>
+#include <variant>
+#include <vector>
+#include <format>
+
 #include "nix/util/users.hh"
 #include "nix/util/config-global.hh"
 #include "nix/flake/settings.hh"
 #include "nix/flake/flake.hh"
-
-#include <nlohmann/json.hpp>
+#include "nix/util/ansicolor.hh"
+#include "nix/util/configuration.hh"
+#include "nix/util/file-system.hh"
+#include "nix/util/fmt.hh"
+#include "nix/util/logging.hh"
+#include "nix/util/strings.hh"
+#include "nix/util/types.hh"
+#include "nix/util/util.hh"
 
 namespace nix::flake {
 
diff --git a/src/libflake/flake-primops.cc b/src/libflake/flake-primops.cc
index 7c5ce01b269..eeff9a96623 100644
--- a/src/libflake/flake-primops.cc
+++ b/src/libflake/flake-primops.cc
@@ -1,8 +1,34 @@
+#include <stdint.h>
+#include <functional>
+#include <map>
+#include <optional>
+#include <set>
+#include <string>
+#include <utility>
+#include <variant>
+#include <vector>
+
 #include "nix/flake/flake-primops.hh"
 #include "nix/expr/eval.hh"
 #include "nix/flake/flake.hh"
 #include "nix/flake/flakeref.hh"
 #include "nix/flake/settings.hh"
+#include "nix/expr/attr-set.hh"
+#include "nix/expr/eval-error.hh"
+#include "nix/expr/eval-inline.hh"
+#include "nix/expr/eval-settings.hh"
+#include "nix/expr/symbol-table.hh"
+#include "nix/expr/value.hh"
+#include "nix/fetchers/attrs.hh"
+#include "nix/fetchers/fetchers.hh"
+#include "nix/util/configuration.hh"
+#include "nix/util/error.hh"
+#include "nix/util/experimental-features.hh"
+#include "nix/util/pos-idx.hh"
+#include "nix/util/pos-table.hh"
+#include "nix/util/source-path.hh"
+#include "nix/util/types.hh"
+#include "nix/util/util.hh"
 
 namespace nix::flake::primops {
 
diff --git a/src/libflake/flake.cc b/src/libflake/flake.cc
index 147bff820ea..8e7e2be26b1 100644
--- a/src/libflake/flake.cc
+++ b/src/libflake/flake.cc
@@ -1,3 +1,24 @@
+#include <nlohmann/json.hpp>
+#include <assert.h>
+#include <stdint.h>
+#include <boost/container/detail/std_fwd.hpp>
+#include <boost/core/pointer_traits.hpp>
+#include <boost/unordered/detail/foa/table.hpp>
+#include <algorithm>
+#include <filesystem>
+#include <functional>
+#include <map>
+#include <memory>
+#include <optional>
+#include <set>
+#include <span>
+#include <string>
+#include <tuple>
+#include <utility>
+#include <variant>
+#include <vector>
+#include <format>
+
 #include "nix/util/terminal.hh"
 #include "nix/util/ref.hh"
 #include "nix/util/environment-variables.hh"
@@ -6,7 +27,6 @@
 #include "nix/expr/eval-cache.hh"
 #include "nix/expr/eval-settings.hh"
 #include "nix/flake/lockfile.hh"
-#include "nix/expr/primops.hh"
 #include "nix/expr/eval-inline.hh"
 #include "nix/store/store-api.hh"
 #include "nix/fetchers/fetchers.hh"
@@ -14,14 +34,36 @@
 #include "nix/fetchers/fetch-settings.hh"
 #include "nix/flake/settings.hh"
 #include "nix/expr/value-to-json.hh"
-#include "nix/store/local-fs-store.hh"
 #include "nix/fetchers/fetch-to-store.hh"
 #include "nix/util/memory-source-accessor.hh"
 #include "nix/fetchers/input-cache.hh"
-
-#include <nlohmann/json.hpp>
+#include "nix/expr/attr-set.hh"
+#include "nix/expr/eval-error.hh"
+#include "nix/expr/nixexpr.hh"
+#include "nix/expr/symbol-table.hh"
+#include "nix/expr/value.hh"
+#include "nix/expr/value/context.hh"
+#include "nix/fetchers/attrs.hh"
+#include "nix/fetchers/registry.hh"
+#include "nix/flake/flakeref.hh"
+#include "nix/store/path.hh"
+#include "nix/util/canon-path.hh"
+#include "nix/util/configuration.hh"
+#include "nix/util/error.hh"
+#include "nix/util/experimental-features.hh"
+#include "nix/util/file-system.hh"
+#include "nix/util/fmt.hh"
+#include "nix/util/hash.hh"
+#include "nix/util/logging.hh"
+#include "nix/util/pos-idx.hh"
+#include "nix/util/pos-table.hh"
+#include "nix/util/position.hh"
+#include "nix/util/source-path.hh"
+#include "nix/util/types.hh"
+#include "nix/util/util.hh"
 
 namespace nix {
+struct SourceAccessor;
 
 using namespace flake;
 
@@ -865,7 +907,7 @@ static ref<SourceAccessor> makeInternalFS()
     internalFS->setPathDisplay("«flakes-internal»", "");
     internalFS->addFile(
         CanonPath("call-flake.nix"),
-#include "call-flake.nix.gen.hh"
+#include "call-flake.nix.gen.hh" // IWYU pragma: keep
     );
     return internalFS;
 }
diff --git a/src/libflake/flakeref.cc b/src/libflake/flakeref.cc
index 38979783d5e..a26f269c323 100644
--- a/src/libflake/flakeref.cc
+++ b/src/libflake/flakeref.cc
@@ -1,10 +1,39 @@
+#include <assert.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <filesystem>
+#include <ostream>
+#include <string_view>
+#include <vector>
+#include <optional>
+#include <regex>
+#include <string>
+#include <tuple>
+#include <utility>
+
 #include "nix/flake/flakeref.hh"
-#include "nix/store/store-api.hh"
 #include "nix/util/url.hh"
 #include "nix/util/url-parts.hh"
 #include "nix/fetchers/fetchers.hh"
+#include "nix/util/error.hh"
+#include "nix/util/file-system.hh"
+#include "nix/util/fmt.hh"
+#include "nix/util/logging.hh"
+#include "nix/util/strings.hh"
+#include "nix/util/util.hh"
+#include "nix/fetchers/attrs.hh"
+#include "nix/fetchers/registry.hh"
+#include "nix/store/outputs-spec.hh"
+#include "nix/util/ref.hh"
+#include "nix/util/types.hh"
 
 namespace nix {
+class Store;
+struct SourceAccessor;
+
+namespace fetchers {
+struct Settings;
+} // namespace fetchers
 
 #if 0
 // 'dir' path elements cannot start with a '.'. We also reject
diff --git a/src/libflake/include/nix/flake/flake-primops.hh b/src/libflake/include/nix/flake/flake-primops.hh
index 35a7128f4fd..b333e33d706 100644
--- a/src/libflake/include/nix/flake/flake-primops.hh
+++ b/src/libflake/include/nix/flake/flake-primops.hh
@@ -1,7 +1,12 @@
 #pragma once
 
 #include "nix/expr/eval.hh"
-#include "nix/flake/settings.hh"
+
+namespace nix {
+namespace flake {
+struct Settings;
+} // namespace flake
+} // namespace nix
 
 namespace nix::flake::primops {
 
diff --git a/src/libflake/include/nix/flake/flakeref.hh b/src/libflake/include/nix/flake/flakeref.hh
index c8c536bce6f..1af8c5afdba 100644
--- a/src/libflake/include/nix/flake/flakeref.hh
+++ b/src/libflake/include/nix/flake/flakeref.hh
@@ -2,9 +2,11 @@
 ///@file
 
 #include <regex>
+#include <iosfwd>
+#include <string>
+#include <tuple>
+#include <utility>
 
-#include "nix/util/types.hh"
-#include "nix/fetchers/fetchers.hh"
 #include "nix/store/outputs-spec.hh"
 #include "nix/fetchers/registry.hh"
 
@@ -12,6 +14,10 @@ namespace nix {
 
 class Store;
 
+namespace fetchers {
+struct Settings;
+} // namespace fetchers
+
 typedef std::string FlakeId;
 
 /**
diff --git a/src/libflake/include/nix/flake/settings.hh b/src/libflake/include/nix/flake/settings.hh
index 618ed4d38ef..7187a3294a3 100644
--- a/src/libflake/include/nix/flake/settings.hh
+++ b/src/libflake/include/nix/flake/settings.hh
@@ -1,9 +1,10 @@
 #pragma once
 ///@file
 
-#include "nix/util/configuration.hh"
-
 #include <sys/types.h>
+#include <string>
+
+#include "nix/util/configuration.hh"
 
 namespace nix {
 // Forward declarations
diff --git a/src/libflake/include/nix/flake/url-name.hh b/src/libflake/include/nix/flake/url-name.hh
index b95d2dff616..d313db33bb9 100644
--- a/src/libflake/include/nix/flake/url-name.hh
+++ b/src/libflake/include/nix/flake/url-name.hh
@@ -1,9 +1,8 @@
-#include "nix/util/url.hh"
-#include "nix/util/url-parts.hh"
-#include "nix/util/util.hh"
-#include "nix/util/split.hh"
+#include <optional>
+#include <string>
 
 namespace nix {
+struct ParsedURL;
 
 /**
  * Try to extract a reasonably unique and meaningful, human-readable
diff --git a/src/libflake/lockfile.cc b/src/libflake/lockfile.cc
index f381a57e6bf..d3dac19c5e0 100644
--- a/src/libflake/lockfile.cc
+++ b/src/libflake/lockfile.cc
@@ -1,15 +1,49 @@
-#include "nix/fetchers/fetch-settings.hh"
-#include "nix/flake/settings.hh"
-#include "nix/flake/lockfile.hh"
-#include "nix/store/store-api.hh"
-#include "nix/util/strings.hh"
-
+#include <boost/unordered/unordered_flat_set.hpp>
+#include <nlohmann/json.hpp>
+#include <assert.h>
+#include <boost/unordered/unordered_flat_set_fwd.hpp>
+#include <nlohmann/detail/iterators/iter_impl.hpp>
+#include <nlohmann/detail/iterators/iteration_proxy.hpp>
+#include <nlohmann/json_fwd.hpp>
 #include <algorithm>
 #include <iomanip>
-
-#include <boost/unordered/unordered_flat_set.hpp>
 #include <iterator>
-#include <nlohmann/json.hpp>
+#include <compare>
+#include <ctime>
+#include <format>
+#include <functional>
+#include <map>
+#include <memory>
+#include <optional>
+#include <ostream>
+#include <regex>
+#include <set>
+#include <string>
+#include <string_view>
+#include <utility>
+#include <variant>
+#include <vector>
+
+#include "nix/fetchers/fetch-settings.hh"
+#include "nix/flake/lockfile.hh"
+#include "nix/util/strings.hh"
+#include "nix/fetchers/attrs.hh"
+#include "nix/fetchers/fetchers.hh"
+#include "nix/flake/flakeref.hh"
+#include "nix/store/path.hh"
+#include "nix/util/ansicolor.hh"
+#include "nix/util/configuration.hh"
+#include "nix/util/error.hh"
+#include "nix/util/fmt.hh"
+#include "nix/util/hash.hh"
+#include "nix/util/logging.hh"
+#include "nix/util/ref.hh"
+#include "nix/util/types.hh"
+#include "nix/util/util.hh"
+
+namespace nix {
+class Store;
+} // namespace nix
 
 namespace nix::flake {
 
diff --git a/src/libflake/settings.cc b/src/libflake/settings.cc
index e77bded306a..52fa1b49d51 100644
--- a/src/libflake/settings.cc
+++ b/src/libflake/settings.cc
@@ -1,5 +1,9 @@
+#include <vector>
+
 #include "nix/flake/settings.hh"
 #include "nix/flake/flake-primops.hh"
+#include "nix/expr/eval-settings.hh"
+#include "nix/expr/eval.hh"
 
 namespace nix::flake {
 
diff --git a/src/libflake/url-name.cc b/src/libflake/url-name.cc
index 3bba3692eb1..f4b5c6a7f2f 100644
--- a/src/libflake/url-name.cc
+++ b/src/libflake/url-name.cc
@@ -1,6 +1,10 @@
-#include "nix/flake/url-name.hh"
 #include <regex>
-#include <iostream>
+#include <optional>
+#include <string>
+
+#include "nix/flake/url-name.hh"
+#include "nix/util/strings.hh"
+#include "nix/util/url.hh"
 
 namespace nix {