diff --git a/.gitignore b/.gitignore
index 776b630011..b0f8b3eeb0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -27,8 +27,7 @@
 /FMDC_V03/
 /Duet3_MB6HC_no_SD/
 /Duet3_MB6HC_no_S_curve/
-/INDX/
+/tests/build/
 /.clangd
 /.clang-format
 /src/Temp/
-/.settings/
diff --git a/tests/Makefile b/tests/Makefile
new file mode 100644
index 0000000000..d7bf27283f
--- /dev/null
+++ b/tests/Makefile
@@ -0,0 +1,102 @@
+# RepRapFirmware host-native test build
+
+CXX ?= g++-15
+
+ifeq ($(V),1)
+	Q :=
+else
+	Q := @
+endif
+
+ROOT := ..
+WORKSPACE := ../..
+BUILD_DIR := build
+TARGET := $(BUILD_DIR)/rrf_host_tests
+
+CPPFLAGS := \
+	-I$(CURDIR)/support/shims \
+	-I$(ROOT) \
+	-I$(ROOT)/src \
+	-I$(WORKSPACE)/CANlib/src \
+	-I$(WORKSPACE)/RRFLibraries/src \
+	-I$(CURDIR)/support
+
+# RRFLibraries/src/General must NOT be on the include path: it holds String.h, which on a
+# case-insensitive filesystem (macOS) libc++'s <cstring> finds in place of the real <string.h>.
+# Sources reach those headers through the General/ and Math/ prefixes instead.
+CXXFLAGS ?= -std=gnu++20 -O2 -Wall -Wextra -Wpedantic
+
+# unit/test_fopdt.cpp is excluded: it targets the pre-3.7.0-beta.3 FopDt API (CorrectPwmForVoltage,
+# SetM301PidParameters, ArePidParametersOverridden, AppendM301Command, 3-argument EstimateRequiredPwm),
+# all of which have since changed. It needs updating by whoever owns the heater model.
+TEST_SOURCES := \
+	support/TestMain.cpp \
+	unit/test_bitmap.cpp \
+	unit/test_deviation.cpp \
+	unit/test_isqrt.cpp
+
+LIB_SOURCES := \
+	$(WORKSPACE)/RRFLibraries/src/General/SafeVsnprintf.cpp \
+	$(WORKSPACE)/RRFLibraries/src/General/StringFunctions.cpp \
+	$(WORKSPACE)/RRFLibraries/src/General/StringRef.cpp \
+	$(WORKSPACE)/RRFLibraries/src/General/Strnlen.cpp \
+	$(WORKSPACE)/RRFLibraries/src/Math/Deviation.cpp \
+	$(WORKSPACE)/RRFLibraries/src/Math/Isqrt.cpp
+
+OBJECTS := $(patsubst %.cpp,$(BUILD_DIR)/%.o,$(TEST_SOURCES)) \
+	$(BUILD_DIR)/rrflibraries/SafeVsnprintf.o \
+	$(BUILD_DIR)/rrflibraries/StringFunctions.o \
+	$(BUILD_DIR)/rrflibraries/StringRef.o \
+	$(BUILD_DIR)/rrflibraries/Strnlen.o \
+	$(BUILD_DIR)/rrflibraries/Deviation.o \
+	$(BUILD_DIR)/rrflibraries/Isqrt.o
+
+.PHONY: all clean run
+
+all: run
+
+run: $(TARGET)
+	$(Q)./$(TARGET)
+
+$(TARGET): $(OBJECTS)
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CXXFLAGS) $^ -o $@
+
+$(BUILD_DIR)/support/%.o: support/%.cpp support/TestRunner.h
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+$(BUILD_DIR)/unit/%.o: unit/%.cpp support/TestRunner.h
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+$(BUILD_DIR)/rrflibraries/Isqrt.o: $(WORKSPACE)/RRFLibraries/src/Math/Isqrt.cpp
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+$(BUILD_DIR)/rrflibraries/SafeVsnprintf.o: $(WORKSPACE)/RRFLibraries/src/General/SafeVsnprintf.cpp
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+$(BUILD_DIR)/rrflibraries/StringFunctions.o: $(WORKSPACE)/RRFLibraries/src/General/StringFunctions.cpp
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+$(BUILD_DIR)/rrflibraries/StringRef.o: $(WORKSPACE)/RRFLibraries/src/General/StringRef.cpp
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+$(BUILD_DIR)/rrflibraries/Strnlen.o: $(WORKSPACE)/RRFLibraries/src/General/Strnlen.cpp
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+$(BUILD_DIR)/rrflibraries/Deviation.o: $(WORKSPACE)/RRFLibraries/src/Math/Deviation.cpp
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+$(BUILD_DIR)/reprapfirmware/FOPDT.o: $(ROOT)/src/Heating/FOPDT.cpp
+	$(Q)mkdir -p $(dir $@)
+	$(Q)$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+clean:
+	$(Q)rm -rf $(BUILD_DIR)
\ No newline at end of file
diff --git a/tests/README.md b/tests/README.md
new file mode 100644
index 0000000000..19efdac0a1
--- /dev/null
+++ b/tests/README.md
@@ -0,0 +1,51 @@
+# Host-Native Tests
+
+This directory contains Linux-hosted tests for code that can be validated without running on a Duet board.
+
+The initial scope is intentionally narrow:
+
+- pure math and algorithm code that does not require MCU peripherals
+- selected firmware-side model code compiled against narrow test-only shims
+- deterministic tests that run with the system C++ compiler
+- no dependency on the ARM cross-toolchain
+
+Run the tests from the RepRapFirmware root:
+
+```sh
+make test-host
+```
+
+Current contents:
+
+- `support/` - minimal in-repo test runner
+- `support/shims/` - narrow host-only shims for firmware types needed by selected model code
+- `unit/` - host-native unit tests
+- `make test-host` from the repository root builds and runs the suite with `g++`
+
+Current coverage:
+
+- RRFLibraries helpers: bitmap, deviation, integer square root, fast square root
+- RepRapFirmware heater model: `FOPDT`
+
+Planned next steps:
+
+- add deterministic time and captured-output test support
+- add more RepRapFirmware-side tests for kinematics and control logic
+## Why the motion system is not testable here
+
+`Move.cpp` needs 98 RepRapFirmware headers across 18 subsystems just to parse,
+and its body calls `reprap.GetGCodes()` 91 times, plus `GetPlatform()`,
+`GetExpansion()` and `GetPortControl()`. `Move` and `DDA` are not separable from
+the `reprap` singleton, so reaching the planner from here would mean either a
+large fake Platform/GCodes layer or refactoring `Move` onto interfaces.
+
+Until that changes, host tests are limited to leaf code. Validating motion
+behaviour - junction blending, speed ceilings, deceleration on loss of input -
+needs a peripheral-level emulator such as Renode.
+
+Measure it again before believing it:
+
+```sh
+make Duet3_MB6HC ...                       # produces the .d files
+tr ' ' '\n' < Duet3_MB6HC/src/Movement/Move.d | grep -E '^src/.*\.h$' | sort -u | wc -l
+```
diff --git a/tests/support/TestMain.cpp b/tests/support/TestMain.cpp
new file mode 100644
index 0000000000..e4d4290f50
--- /dev/null
+++ b/tests/support/TestMain.cpp
@@ -0,0 +1,8 @@
+#include <cstdio>
+
+#include "TestRunner.h"
+
+int main()
+{
+	return test::RunAllTests();
+}
\ No newline at end of file
diff --git a/tests/support/TestRunner.h b/tests/support/TestRunner.h
new file mode 100644
index 0000000000..5fe4697537
--- /dev/null
+++ b/tests/support/TestRunner.h
@@ -0,0 +1,129 @@
+#ifndef RRF_TEST_RUNNER_H
+#define RRF_TEST_RUNNER_H
+
+#include <cmath>
+#include <cstdio>
+#include <cstddef>
+#include <cstdint>
+#include <functional>
+#include <sstream>
+#include <stdexcept>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace test
+{
+	using TestFunction = void (*)();
+
+	struct TestCase
+	{
+		const char* name;
+		TestFunction function;
+	};
+
+	inline std::vector<TestCase>& Registry()
+	{
+		static std::vector<TestCase> registry;
+		return registry;
+	}
+
+	class Registrar
+	{
+	public:
+		Registrar(const char* name, TestFunction function)
+		{
+			Registry().push_back({name, function});
+		}
+	};
+
+	class Failure : public std::runtime_error
+	{
+	public:
+		explicit Failure(const std::string& message)
+			: std::runtime_error(message)
+		{
+		}
+	};
+
+	[[noreturn]] inline void Fail(const char* file, int line, const std::string& message)
+	{
+		std::ostringstream stream;
+		stream << file << ':' << line << ": " << message;
+		throw Failure(stream.str());
+	}
+
+	template <typename T, typename U>
+	void ExpectEqual(const T& actual, const U& expected, const char* file, int line, const char* actualExpr, const char* expectedExpr)
+	{
+		if (!(actual == expected))
+		{
+			std::ostringstream stream;
+			stream << "expected " << actualExpr << " == " << expectedExpr << ", got " << actual << " and " << expected;
+			Fail(file, line, stream.str());
+		}
+	}
+
+	template <typename T, typename U, typename V>
+	void ExpectNear(const T& actual, const U& expected, const V& tolerance, const char* file, int line, const char* actualExpr, const char* expectedExpr, const char* toleranceExpr)
+	{
+		const auto delta = std::fabs(static_cast<double>(actual) - static_cast<double>(expected));
+		if (delta > static_cast<double>(tolerance))
+		{
+			std::ostringstream stream;
+			stream << "expected " << actualExpr << " ~= " << expectedExpr << " within " << toleranceExpr
+				   << ", got delta " << delta;
+			Fail(file, line, stream.str());
+		}
+	}
+
+	inline void ExpectTrue(bool condition, const char* file, int line, const char* expression)
+	{
+		if (!condition)
+		{
+			std::ostringstream stream;
+			stream << "expected true: " << expression;
+			Fail(file, line, stream.str());
+		}
+	}
+
+	inline int RunAllTests()
+	{
+		int failures = 0;
+		for (const TestCase& testCase : Registry())
+		{
+			try
+			{
+				testCase.function();
+			}
+			catch (const Failure& failure)
+			{
+				++failures;
+				std::fprintf(stderr, "[FAIL] %s\n  %s\n", testCase.name, failure.what());
+				continue;
+			}
+			catch (const std::exception& exception)
+			{
+				++failures;
+				std::fprintf(stderr, "[FAIL] %s\n  unexpected exception: %s\n", testCase.name, exception.what());
+				continue;
+			}
+
+			std::fprintf(stdout, "[PASS] %s\n", testCase.name);
+		}
+
+		std::fprintf(stdout, "Ran %zu test(s), %d failure(s)\n", Registry().size(), failures);
+		return failures;
+	}
+}
+
+#define TEST_CASE(name) \
+	static void name(); \
+	static test::Registrar name##_registrar(#name, &name); \
+	static void name()
+
+#define EXPECT_TRUE(expr) test::ExpectTrue((expr), __FILE__, __LINE__, #expr)
+#define EXPECT_EQ(actual, expected) test::ExpectEqual((actual), (expected), __FILE__, __LINE__, #actual, #expected)
+#define EXPECT_NEAR(actual, expected, tolerance) test::ExpectNear((actual), (expected), (tolerance), __FILE__, __LINE__, #actual, #expected, #tolerance)
+
+#endif
\ No newline at end of file
diff --git a/tests/support/shims/ObjectModel/ObjectModel.h b/tests/support/shims/ObjectModel/ObjectModel.h
new file mode 100644
index 0000000000..71bb01fa3b
--- /dev/null
+++ b/tests/support/shims/ObjectModel/ObjectModel.h
@@ -0,0 +1,6 @@
+#ifndef RRF_HOST_TEST_OBJECTMODEL_H
+#define RRF_HOST_TEST_OBJECTMODEL_H
+
+#include "../RepRapFirmware.h"
+
+#endif
\ No newline at end of file
diff --git a/tests/support/shims/RepRapFirmware.h b/tests/support/shims/RepRapFirmware.h
new file mode 100644
index 0000000000..ea8eb4a9dc
--- /dev/null
+++ b/tests/support/shims/RepRapFirmware.h
@@ -0,0 +1,37 @@
+#ifndef RRF_HOST_TEST_REPRAPFIRMWARE_H
+#define RRF_HOST_TEST_REPRAPFIRMWARE_H
+
+#include <cstdarg>
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <string>
+
+#include "../../../../RRFLibraries/src/ecv_duet3d.h"
+#include "../../../../RRFLibraries/src/General/SimpleMath.h"
+#include "../../../../RRFLibraries/src/General/String.h"
+#include "../../../../RRFLibraries/src/General/StringRef.h"
+
+#define SUPPORT_OBJECT_MODEL 0
+#define HAS_MASS_STORAGE 0
+#define HAS_SBC_INTERFACE 0
+#define SUPPORT_CAN_EXPANSION 0
+
+#ifndef THROWS
+#define THROWS(...) noexcept(false)
+#endif
+
+#ifndef INHERIT_OBJECT_MODEL
+#define INHERIT_OBJECT_MODEL
+#endif
+
+#ifndef DECLARE_OBJECT_MODEL
+#define DECLARE_OBJECT_MODEL
+#endif
+
+constexpr float NormalAmbientTemperature = 25.0;
+constexpr unsigned int MaxFloatDigitsDisplayedAfterPoint = 3;
+
+#define DEGREE_SYMBOL " "
+
+#endif
\ No newline at end of file
diff --git a/tests/unit/test_bitmap.cpp b/tests/unit/test_bitmap.cpp
new file mode 100644
index 0000000000..ab8ffb540c
--- /dev/null
+++ b/tests/unit/test_bitmap.cpp
@@ -0,0 +1,52 @@
+#include "../../../RRFLibraries/src/General/Bitmap.h"
+
+#include "../support/TestRunner.h"
+
+TEST_CASE(BitmapSetsClearsAndCombinesBits)
+{
+	Bitmap<uint32_t> bits;
+	EXPECT_TRUE(bits.IsEmpty());
+
+	bits.SetBit(1);
+	bits.SetBit(4);
+	EXPECT_TRUE(bits.IsBitSet(1));
+	EXPECT_TRUE(bits.IsBitSet(4));
+	EXPECT_EQ(bits.GetRaw(), 18u);
+
+	bits.ClearBit(1);
+	EXPECT_TRUE(!bits.IsBitSet(1));
+	EXPECT_EQ(bits.GetRaw(), 16u);
+
+	Bitmap<uint32_t> other;
+	other.SetBit(2);
+	other.SetBit(4);
+
+	EXPECT_TRUE(bits.Intersects(other));
+	EXPECT_TRUE(!bits.Disjoint(other));
+	EXPECT_EQ((bits | other).GetRaw(), 20u);
+	EXPECT_EQ((bits & other).GetRaw(), 16u);
+	EXPECT_EQ((other - bits).GetRaw(), 4u);
+}
+
+TEST_CASE(BitmapContainsAndShiftOperations)
+{
+	Bitmap<uint16_t> bits;
+	bits.SetBit(0);
+	bits.SetBit(3);
+
+	Bitmap<uint16_t> subset;
+	subset.SetBit(3);
+
+	EXPECT_TRUE(bits.Contains(subset));
+	EXPECT_TRUE(bits.IsAnyBitSet(0, 1));
+	EXPECT_TRUE(bits.IsAnyBitSet(1, 2, 3));
+	EXPECT_EQ(bits.ShiftUp(2).GetRaw(), static_cast<uint16_t>(36));
+}
+
+TEST_CASE(ExtractBitHelpersMoveBitsCorrectly)
+{
+	EXPECT_EQ(ExtractBit<uint32_t>(0b0010u, 1, 4), 0b10000u);
+	EXPECT_EQ(ExtractBit<uint32_t>(0b1000u, 3, 0), 0b0001u);
+	EXPECT_EQ(ExtractTwoBits<uint32_t>(0b1100u, 2, 0), 0b0011u);
+	EXPECT_EQ(ExtractTwoBits<uint32_t>(0b0011u, 0, 4), 0b110000u);
+}
\ No newline at end of file
diff --git a/tests/unit/test_deviation.cpp b/tests/unit/test_deviation.cpp
new file mode 100644
index 0000000000..9af31d9e00
--- /dev/null
+++ b/tests/unit/test_deviation.cpp
@@ -0,0 +1,32 @@
+#include <cmath>
+
+#include "../../../RRFLibraries/src/Math/Deviation.h"
+
+#include "../support/TestRunner.h"
+
+TEST_CASE(DeviationCalculatesMeanAndSpread)
+{
+	Deviation deviation;
+	deviation.Set(30.0f, 10.0f, 4);
+
+	EXPECT_NEAR(deviation.GetMean(), 2.5, 1e-6);
+	EXPECT_NEAR(deviation.GetDeviationFromMean(), std::sqrt(1.25), 1e-6);
+}
+
+TEST_CASE(DeviationHandlesEmptyInput)
+{
+	Deviation deviation;
+	deviation.Set(123.0f, 456.0f, 0);
+
+	EXPECT_EQ(deviation.GetMean(), 0.0f);
+	EXPECT_EQ(deviation.GetDeviationFromMean(), 0.0f);
+}
+
+TEST_CASE(DeviationClampsTinyNegativeVariance)
+{
+	Deviation deviation;
+	deviation.Set(4.0f, 4.0f, 4);
+
+	EXPECT_EQ(deviation.GetMean(), 1.0f);
+	EXPECT_EQ(deviation.GetDeviationFromMean(), 0.0f);
+}
\ No newline at end of file
diff --git a/tests/unit/test_fopdt.cpp b/tests/unit/test_fopdt.cpp
new file mode 100644
index 0000000000..d04e392c53
--- /dev/null
+++ b/tests/unit/test_fopdt.cpp
@@ -0,0 +1,78 @@
+#include "../../src/Heating/FOPDT.h"
+
+#include "../support/TestRunner.h"
+
+TEST_CASE(FopDtRejectsInvalidParameters)
+{
+	FopDt model;
+	String<512> reply;
+
+	EXPECT_TRUE(!model.SetParameters(0.01f, 1.0f, 0.0f, 1.2f, 5.0f, 0.5f, 24.0f, true, false, reply.GetRef()));
+	EXPECT_TRUE(std::string(reply.c_str()).find("estimated temperature rise too small") != std::string::npos);
+	EXPECT_TRUE(!model.IsEnabled());
+}
+
+TEST_CASE(FopDtAcceptsValidParametersAndComputesHeatingModel)
+{
+	FopDt model;
+	String<512> reply;
+
+	EXPECT_TRUE(model.SetParameters(2.4f, 0.4f, 0.2f, 1.2f, 4.0f, 0.8f, 24.0f, true, false, reply.GetRef()));
+	EXPECT_TRUE(model.IsEnabled());
+	EXPECT_NEAR(model.GetMaxPwm(), 0.8, 1e-6);
+	EXPECT_NEAR(model.EstimateRequiredPwm(100.0f, 0.5f), model.GetNetHeatingRate(100.0f, 0.5f, 0.0f) / -model.GetHeatingRate(), 1e-5);
+	EXPECT_NEAR(model.GetNetHeatingRate(60.0f, 0.0f, 0.5f), model.GetHeatingRate() * 0.5f - model.EstimateRequiredPwm(60.0f, 0.0f) * model.GetHeatingRate(), 1e-5);
+	EXPECT_TRUE(reply.IsEmpty());
+}
+
+TEST_CASE(FopDtCorrectsPwmForVoltageWithinLimits)
+{
+	FopDt model;
+	String<512> reply;
+	EXPECT_TRUE(model.SetParameters(2.0f, 0.5f, 0.0f, 1.35f, 5.0f, 0.9f, 24.0f, true, false, reply.GetRef()));
+
+	const float corrected = model.CorrectPwmForVoltage(0.4f, 20.0f);
+	EXPECT_NEAR(corrected, 0.4 * (24.0 / 20.0) * (24.0 / 20.0), 1e-5);
+	EXPECT_EQ(model.CorrectPwmForVoltage(1.0f, 20.0f), 0.9f);
+}
+
+TEST_CASE(FopDtCalculatesPidParametersAndOverridesThem)
+{
+	FopDt model;
+	String<512> reply;
+	EXPECT_TRUE(model.SetParameters(2.4f, 0.4f, 0.2f, 1.2f, 4.0f, 0.8f, 24.0f, true, false, reply.GetRef()));
+
+	model.CalcPidConstants(220.0f);
+	const PidParameters loadParams = model.GetPidParameters(true);
+	const PidParameters setpointParams = model.GetPidParameters(false);
+	EXPECT_TRUE(loadParams.kP > 0.0f);
+	EXPECT_TRUE(loadParams.recipTi > 0.0f);
+	EXPECT_TRUE(loadParams.tD > 0.0f);
+	EXPECT_TRUE(setpointParams.kP > 0.0f);
+
+	const M301PidParameters overrideParams{255.0f, 10.0f, 50.0f};
+	model.SetM301PidParameters(overrideParams);
+	EXPECT_TRUE(model.ArePidParametersOverridden());
+	const M301PidParameters roundTripped = model.GetM301PidParameters(false);
+	EXPECT_NEAR(roundTripped.kP, overrideParams.kP, 1e-4);
+	EXPECT_NEAR(roundTripped.kI, overrideParams.kI, 1e-4);
+	EXPECT_NEAR(roundTripped.kD, overrideParams.kD, 1e-4);
+}
+
+TEST_CASE(FopDtAppendsModelCommands)
+{
+	FopDt model;
+	String<512> reply;
+	EXPECT_TRUE(model.SetParameters(2.4f, 0.4f, 0.2f, 1.2f, 4.0f, 0.8f, 24.0f, true, true, reply.GetRef()));
+	EXPECT_TRUE(model.IsInverted());
+
+	String<512> command;
+	model.AppendM307Command(1, command.GetRef(), true);
+	EXPECT_TRUE(std::string(command.c_str()).find("M307 H1") != std::string::npos);
+	EXPECT_TRUE(std::string(command.c_str()).find("V24.0") != std::string::npos);
+
+	model.SetM301PidParameters({200.0f, 5.0f, 25.0f});
+	String<512> pidCommand;
+	model.AppendM301Command(1, pidCommand.GetRef());
+	EXPECT_TRUE(std::string(pidCommand.c_str()).find("M301 H1") != std::string::npos);
+}
\ No newline at end of file
diff --git a/tests/unit/test_isqrt.cpp b/tests/unit/test_isqrt.cpp
new file mode 100644
index 0000000000..e967333c11
--- /dev/null
+++ b/tests/unit/test_isqrt.cpp
@@ -0,0 +1,35 @@
+#include <limits>
+
+#include "../../../RRFLibraries/src/Math/Isqrt.h"
+
+#include "../support/TestRunner.h"
+
+TEST_CASE(Isqrt64Handles32BitInputs)
+{
+	EXPECT_EQ(isqrt64(0), 0u);
+	EXPECT_EQ(isqrt64(1), 1u);
+	EXPECT_EQ(isqrt64(15), 3u);
+	EXPECT_EQ(isqrt64(16), 4u);
+	EXPECT_EQ(isqrt64(17), 4u);
+	EXPECT_EQ(isqrt64(4294967295ULL), 65535u);
+}
+
+TEST_CASE(Isqrt64HandlesLarge62BitInputs)
+{
+	EXPECT_EQ(isqrt64(4611686014132420609ULL), 2147483647u);
+	EXPECT_EQ(isqrt64(4611686018427387903ULL), 2147483647u);
+	EXPECT_EQ(isqrt64(1125899906842624ULL), 33554432u);
+}
+
+TEST_CASE(Isqrt64RejectsOutOfRangeInputs)
+{
+	EXPECT_EQ(isqrt64(0xC000000000000000ULL), std::numeric_limits<uint32_t>::max());
+}
+
+TEST_CASE(FastSqrtfMatchesReferenceForNormalInputs)
+{
+	EXPECT_NEAR(fastSqrtf(0.25f), 0.5, 1e-6);
+	EXPECT_NEAR(fastSqrtf(2.0f), std::sqrt(2.0), 1e-6);
+	EXPECT_NEAR(fastSqrtf(1234.5f), std::sqrt(1234.5), 1e-4);
+	EXPECT_EQ(fastSqrtf(-1.0f), 0.0f);
+}
\ No newline at end of file