diff --git a/CMakeLists.txt b/CMakeLists.txt
index 46b1708..79ca7c8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -99,3 +99,25 @@ target_compile_options(fourq-qubic PRIVATE -DBUILD_4Q_LIB)
 install(TARGETS fourq-qubic LIBRARY)
 install(TARGETS qubic-cli RUNTIME)
 
+
+# Opt-in unit tests (off by default, so normal and CI builds are unchanged):
+#   cmake -DQUBIC_CLI_BUILD_TESTS=ON -S . -B build
+#   cmake --build build --target qubic-cli-tests && ctest --test-dir build
+option(QUBIC_CLI_BUILD_TESTS "Build qubic-cli unit tests" OFF)
+if(QUBIC_CLI_BUILD_TESTS)
+    include(FetchContent)
+    FetchContent_Declare(
+        googletest
+        URL https://github.com/google/googletest/archive/refs/tags/v1.15.2.zip
+    )
+    set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+    FetchContent_MakeAvailable(googletest)
+
+    enable_testing()
+    add_executable(qubic-cli-tests tests/sanity_check_test.cpp)
+    target_include_directories(qubic-cli-tests PRIVATE ${CMAKE_SOURCE_DIR})
+    target_link_libraries(qubic-cli-tests PRIVATE GTest::gtest_main)
+
+    include(GoogleTest)
+    gtest_discover_tests(qubic-cli-tests)
+endif()
diff --git a/tests/sanity_check_test.cpp b/tests/sanity_check_test.cpp
new file mode 100644
index 0000000..d3cc955
--- /dev/null
+++ b/tests/sanity_check_test.cpp
@@ -0,0 +1,113 @@
+// Unit tests for the input validators in sanity_check.h.
+//
+// sanity_check.h expects a few symbols to be visible before it is included (the
+// CLI pulls them in via earlier headers). We provide them here so the validators
+// can be tested in isolation, without dragging in the crypto / core-submodule
+// dependency graph: <cstring> for the str/mem calls, DEFAULT_SEED, and a stubbed
+// checkSumIdentity (the identity-format tests below exit before the checksum, so
+// the stub is never relied upon for a result).
+#include <cstring>
+#include <cstdlib>
+#include <string>
+
+#include "gtest/gtest.h"
+
+#define DEFAULT_SEED "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+static bool checkSumIdentity(const char*) { return true; }
+
+#include "sanity_check.h"
+
+TEST(IsValidIpAddress, AcceptsWellFormedIPv4)
+{
+    char ip[] = "192.168.1.1";
+    EXPECT_TRUE(isValidIpAddress(ip));
+    char zero[] = "0.0.0.0";
+    EXPECT_TRUE(isValidIpAddress(zero));
+    char max[] = "255.255.255.255";
+    EXPECT_TRUE(isValidIpAddress(max));
+}
+
+TEST(IsValidIpAddress, RejectsMalformedOrOutOfRange)
+{
+    char over[] = "256.1.1.1";
+    EXPECT_FALSE(isValidIpAddress(over));
+    char neg[] = "-1.2.3.4";
+    EXPECT_FALSE(isValidIpAddress(neg));
+    char tooFew[] = "1.2.3";
+    EXPECT_FALSE(isValidIpAddress(tooFew));
+    char text[] = "not.an.ip.addr";
+    EXPECT_FALSE(isValidIpAddress(text));
+}
+
+// The remaining validators abort the process with exit(1) on bad input and
+// simply return on good input, so they are exercised with death tests.
+
+TEST(SanityCheckValidAssetName, AcceptsValidNames)
+{
+    sanityCheckValidAssetName("QX");    // upper-case start
+    sanityCheckValidAssetName("CFB123"); // letters + digits, <= 7 chars
+    SUCCEED();
+}
+
+TEST(SanityCheckValidAssetName, RejectsInvalidNames)
+{
+    EXPECT_EXIT(sanityCheckValidAssetName("qx"), ::testing::ExitedWithCode(1), "");        // lower-case start
+    EXPECT_EXIT(sanityCheckValidAssetName("TOOLONG1"), ::testing::ExitedWithCode(1), "");  // 8 chars
+    EXPECT_EXIT(sanityCheckValidAssetName("1ABC"), ::testing::ExitedWithCode(1), "");      // digit start
+    EXPECT_EXIT(sanityCheckValidAssetName(nullptr), ::testing::ExitedWithCode(1), "");     // null
+}
+
+TEST(SanityCheckTxType, AcceptsKnownRejectsUnknown)
+{
+    sanityCheckTxType(0);
+    sanityCheckTxType(1);
+    SUCCEED();
+    EXPECT_EXIT(sanityCheckTxType(2), ::testing::ExitedWithCode(1), "");
+}
+
+TEST(CheckValidEpoch, BoundsAreEnforced)
+{
+    checkValidEpoch(0);
+    checkValidEpoch(999);
+    SUCCEED();
+    EXPECT_EXIT(checkValidEpoch(-1), ::testing::ExitedWithCode(1), "");
+    EXPECT_EXIT(checkValidEpoch(1000), ::testing::ExitedWithCode(1), "");
+}
+
+TEST(SanityCheckMainAuxStatus, OnlyMainOrAux)
+{
+    sanityCheckMainAuxStatus("MAIN");
+    sanityCheckMainAuxStatus("AUX");
+    SUCCEED();
+    EXPECT_EXIT(sanityCheckMainAuxStatus("FOO"), ::testing::ExitedWithCode(1), "");
+    EXPECT_EXIT(sanityCheckMainAuxStatus("MAIX"), ::testing::ExitedWithCode(1), "");
+    EXPECT_EXIT(sanityCheckMainAuxStatus(nullptr), ::testing::ExitedWithCode(1), "");
+}
+
+TEST(SanityCheckUnitOfMeasurement, AcceptsSevenChars)
+{
+    sanityCheckUnitofMeasurement("1234567"); // length 7 is accepted
+    SUCCEED();
+}
+
+TEST(SanityCheckUnitOfMeasurement, RejectsWrongLengthOrNull)
+{
+    EXPECT_EXIT(sanityCheckUnitofMeasurement("SHORT"), ::testing::ExitedWithCode(1), "");
+    EXPECT_EXIT(sanityCheckUnitofMeasurement(nullptr), ::testing::ExitedWithCode(1), "");
+}
+
+TEST(SanityCheckLoggingMode, ZeroToTwoOnly)
+{
+    sanityCheckLoggingMode(0);
+    sanityCheckLoggingMode(2);
+    SUCCEED();
+    EXPECT_EXIT(sanityCheckLoggingMode(3), ::testing::ExitedWithCode(1), "");
+    EXPECT_EXIT(sanityCheckLoggingMode((char)-1), ::testing::ExitedWithCode(1), "");
+}
+
+TEST(SanityCheckExtraDataSize, RejectsOverLimit)
+{
+    sanityCheckExtraDataSize(1024);
+    SUCCEED();
+    EXPECT_EXIT(sanityCheckExtraDataSize(1025), ::testing::ExitedWithCode(1), "");
+}