diff --git a/project.cpp b/project.cpp
index 3c0915c..38db3a4 100644
--- a/project.cpp
+++ b/project.cpp
@@ -258,6 +258,7 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         cfg.AddTest("GitSubmodules").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("LinkFlagsChange").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ExternalArtifactTime").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("ExternalReconfigure").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ConcurrentExternalFetch").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("SharedExternalFetch").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("StandardArgs").Dependencies({ CrafterBuildLib.get() });
diff --git a/tests/ExternalReconfigure/main.cpp b/tests/ExternalReconfigure/main.cpp
new file mode 100644
index 0000000..57d50ba
--- /dev/null
+++ b/tests/ExternalReconfigure/main.cpp
@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// A CMake dependency configured with other inputs than this build's (another
+// compiler or --sysroot, the flags crafter-build injects, which it records in
+// <build>/.crafter-options) has to be built again from nothing. It was
+// reconfigured in place, and CMake does not recompile what a changed
+// --sysroot compiled (the Makefiles generator writes it into build.make,
+// which make does not compare) and keeps the check results of the old
+// configure. A mingw dependency built by a release that gave clang the
+// host's /usr as the sysroot (issue #43) kept its objects reading
+// __libc_single_threaded after the update.
+//
+// The dependency is a local repository with a one-file C library. A file
+// planted in its build directory stands for what the old configure left,
+// and the recorded inputs are rewritten the way another crafter-build would
+// have left them. Everything is local; XDG_CACHE_HOME keeps the clone out of
+// the real cache.
+
+extern "C" int setenv(const char* name, const char* value, int overwrite);
+
+namespace {
+    std::int32_t failures = 0;
+
+    void Check(bool cond, std::string_view msg) {
+        if (!cond) {
+            std::println(std::cerr, "FAIL: {}", msg);
+            ++failures;
+        }
+    }
+
+    void Git(const fs::path& dir, std::string_view args) {
+        const std::string command = std::format("git -C {} {}", ShellQuote(dir.string()), args);
+        auto result = RunCommandWithTimeout(command, std::chrono::seconds(60));
+        if (result.exitCode != 0) throw std::runtime_error(std::format("{} failed: {}", command, result.output));
+    }
+
+    void Write(const fs::path& file, std::string_view text) {
+        std::ofstream(file, std::ios::binary | std::ios::trunc) << text;
+    }
+}
+
+int main() {
+    const fs::path scratch = fs::temp_directory_path() / "crafter-build-external-reconfigure";
+    fs::remove_all(scratch);
+    fs::create_directories(scratch);
+    setenv("XDG_CACHE_HOME", (scratch / "cache").c_str(), 1);
+    setenv("GIT_CONFIG_NOSYSTEM", "1", 1);
+    setenv("GIT_CONFIG_GLOBAL", "/dev/null", 1);
+    for (const char* name : { "GIT_AUTHOR_NAME", "GIT_COMMITTER_NAME" }) setenv(name, "crafter-build test", 1);
+    for (const char* name : { "GIT_AUTHOR_EMAIL", "GIT_COMMITTER_EMAIL" }) setenv(name, "test@example.invalid", 1);
+
+    const fs::path repo = scratch / "reconf";
+    fs::create_directories(repo);
+    Write(repo / "lib.c", "int Reconfigured(void) { return 1; }\n");
+    Write(repo / "CMakeLists.txt",
+          "cmake_minimum_required(VERSION 3.16)\n"
+          "project(reconf C)\n"
+          "add_library(reconf STATIC lib.c)\n");
+    Git(repo, "init -q -b main");
+    Git(repo, "add .");
+    Git(repo, "commit -q -m reconf");
+
+    ExternalDependency dep;
+    dep.name = "reconf";
+    dep.source.url = std::format("file://{}", repo.string());
+    dep.builder = ExternalBuilder::CMake;
+    dep.libs = { "reconf" };
+    std::string error = BuildExternal(dep, HostTarget());
+    Check(error.empty(), std::format("the dependency builds: {}", error));
+
+    const fs::path build = ExternalBuildDir(dep, HostTarget());
+    const fs::path leftover = build / "left-by-the-old-configure";
+    Write(leftover, "");
+    error = BuildExternal(dep, HostTarget());
+    Check(error.empty(), std::format("and builds again: {}", error));
+    Check(fs::exists(leftover), "with the same inputs its build directory is kept");
+
+    Write(build / ".crafter-options", " -DCMAKE_SYSROOT=/usr\n");
+    error = BuildExternal(dep, HostTarget());
+    Check(error.empty(), std::format("configured with other inputs, it builds: {}", error));
+    Check(!fs::exists(leftover), "from an empty build directory");
+    Check(fs::exists(build / "libreconf.a"), "and the library is built again");
+
+    std::error_code ec;
+    fs::remove_all(scratch, ec);
+    if (failures > 0) {
+        std::println(std::cerr, "{} assertions failed", failures);
+        return 1;
+    }
+    return 0;
+}