diff --git a/Semantics.Test/Quantities/GeneratorOutputInvariantTests.cs b/Semantics.Test/Quantities/GeneratorOutputInvariantTests.cs
new file mode 100644
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+++ b/Semantics.Test/Quantities/GeneratorOutputInvariantTests.cs
@@ -0,0 +1,157 @@
+// Copyright (c) ktsu.dev
+// All rights reserved.
+// Licensed under the MIT license.
+
+namespace ktsu.Semantics.Test.Quantities;
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using ktsu.Semantics.Quantities;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Locks in invariants on the source generator's output. Issue #57 raised the concern that
+/// the generator's dedup keys could let two methods (or operators) with the same name and
+/// parameter types land on the same type — which the C# compiler already rejects, but this
+/// test makes the property explicit and regression-proof if the dedup logic changes.
+/// </summary>
+[TestClass]
+public sealed class GeneratorOutputInvariantTests
+{
+	/// <summary>
+	/// For every generated quantity type in <c>ktsu.Semantics.Quantities</c>, no two public
+	/// static methods (including operators) share both the same name and the same parameter
+	/// type list. Walks the runtime assembly rather than re-parsing the .g.cs files because
+	/// the compiled types are the source of truth — anything the test sees is what consumers
+	/// would call.
+	/// </summary>
+	[TestMethod]
+	public void NoDuplicatePublicStaticMethodsOrOperatorsPerGeneratedType()
+	{
+		Assembly assembly = typeof(Mass<>).Assembly;
+		List<Type> generatedQuantityTypes = [.. CollectGeneratedQuantityTypes(assembly)];
+
+		// Sanity: we should be looking at a non-trivial set, otherwise the test is silently
+		// vacuous (e.g. namespace got renamed and the filter dropped everything).
+		Assert.IsTrue(
+			generatedQuantityTypes.Count > 50,
+			$"Expected to find many generated quantity types (got {generatedQuantityTypes.Count}). The filter likely needs updating.");
+
+		List<string> failures = [];
+		foreach (Type type in generatedQuantityTypes)
+		{
+			MethodInfo[] staticMethods = type.GetMethods(BindingFlags.Public | BindingFlags.Static | BindingFlags.DeclaredOnly);
+
+			IEnumerable<IGrouping<string, MethodInfo>> groups = staticMethods.GroupBy(SignatureKey);
+			foreach (IGrouping<string, MethodInfo> group in groups)
+			{
+				int count = group.Count();
+				if (count > 1)
+				{
+					failures.Add($"{type.Name}: {group.Key} appears {count} times");
+				}
+			}
+		}
+
+		if (failures.Count > 0)
+		{
+			Assert.Fail(
+				$"Found duplicate public static method signatures on generated quantity types:\n  " +
+				string.Join("\n  ", failures));
+		}
+	}
+
+	/// <summary>
+	/// Cross-dimensional <c>operator *</c> overloads should exist in both operand orders so
+	/// either-order user code (<c>mass * accel</c> and <c>accel * mass</c>) compiles. The
+	/// generator's <c>CollectAllOperators</c> emits both directions; this test asserts the
+	/// commutativity property explicitly so a regression in the dedup keys would fail here
+	/// before it reaches a downstream consumer.
+	/// </summary>
+	[TestMethod]
+	public void EveryCrossDimensionalMultiplicationHasBothOperandOrders()
+	{
+		Assembly assembly = typeof(Mass<>).Assembly;
+		List<Type> types = [.. CollectGeneratedQuantityTypes(assembly)];
+
+		// Collect every observed operator * signature as a tuple (left, right, returnType).
+		HashSet<(string Left, string Right, string Result)> observed = [];
+		foreach (Type type in types)
+		{
+			foreach (MethodInfo m in type.GetMethods(BindingFlags.Public | BindingFlags.Static | BindingFlags.DeclaredOnly))
+			{
+				if (m.Name != "op_Multiply")
+				{
+					continue;
+				}
+
+				ParameterInfo[] pars = m.GetParameters();
+				if (pars.Length != 2)
+				{
+					continue;
+				}
+
+				observed.Add((pars[0].ParameterType.Name, pars[1].ParameterType.Name, m.ReturnType.Name));
+			}
+		}
+
+		// For every cross-dimensional product (operands of distinct types), the swapped pair
+		// should also be present with the same return. Same-type products (T * T) are exempt
+		// because there is no swap — they're idempotent under reorder.
+		List<string> missing = [];
+		foreach ((string left, string right, string result) in observed)
+		{
+			if (left == right)
+			{
+				continue;
+			}
+
+			if (!observed.Contains((right, left, result)))
+			{
+				missing.Add($"missing reverse pair: {right} * {left} -> {result} (forward {left} * {right} -> {result} exists)");
+			}
+		}
+
+		if (missing.Count > 0)
+		{
+			Assert.Fail(
+				"Cross-dimensional multiplication should be emitted in both operand orders, but found " +
+				$"{missing.Count} unmatched forward(s):\n  " +
+				string.Join("\n  ", missing));
+		}
+	}
+
+	private static IEnumerable<Type> CollectGeneratedQuantityTypes(Assembly assembly)
+	{
+		foreach (Type type in assembly.GetTypes())
+		{
+			if (type.Namespace != "ktsu.Semantics.Quantities")
+			{
+				continue;
+			}
+
+			if (!type.IsGenericTypeDefinition)
+			{
+				continue;
+			}
+
+			// Generated quantity types implement one of IVector0..IVector4 (closed over TSelf, T).
+			bool isQuantity = type.GetInterfaces().Any(static i =>
+				i.IsGenericType && i.Name.StartsWith("IVector", StringComparison.Ordinal));
+			if (!isQuantity)
+			{
+				continue;
+			}
+
+			yield return type;
+		}
+	}
+
+	private static string SignatureKey(MethodInfo m)
+	{
+		string parameterList = string.Join(",", m.GetParameters().Select(static p => p.ParameterType.FullName ?? p.ParameterType.Name));
+		return $"{m.Name}({parameterList})";
+	}
+}