diff --git a/Semantics.Strings/ISemanticString.cs b/Semantics.Strings/ISemanticString.cs
index 1a96b4b..a63c8ac 100644
--- a/Semantics.Strings/ISemanticString.cs
+++ b/Semantics.Strings/ISemanticString.cs
@@ -27,8 +27,10 @@ public interface ISemanticString : IComparable, IComparable<ISemanticString>, IE
 	/// <remarks>
 	/// Use this property when you need to interoperate with APIs that expect regular strings.
 	/// The name "WeakString" emphasizes that this breaks the type safety guarantees.
+	/// The value is read-only through this interface: it is set when the semantic string is created,
+	/// which is where it is canonicalized and validated (#310).
 	/// </remarks>
-	public string WeakString { get; init; }
+	public string WeakString { get; }
 
 	// System.String interface
 
diff --git a/Semantics.Strings/SemanticString.cs b/Semantics.Strings/SemanticString.cs
index 4f319c2..2e3562e 100644
--- a/Semantics.Strings/SemanticString.cs
+++ b/Semantics.Strings/SemanticString.cs
@@ -71,7 +71,7 @@ public TDest As<TDest>()
 
 	// ISemanticString implementation
 	/// <summary>
-	/// Gets or initializes the underlying string value for interoperability with non-semantic string operations.
+	/// Gets the underlying string value for interoperability with non-semantic string operations.
 	/// </summary>
 	/// <value>
 	/// The raw string value contained within this semantic string. Defaults to <see cref="string.Empty"/>.
@@ -80,9 +80,12 @@ public TDest As<TDest>()
 	/// Use this property when you need to interoperate with APIs that expect regular strings.
 	/// The name "WeakString" emphasizes that accessing this property breaks the type safety guarantees
 	/// that semantic strings provide. Consider using implicit conversion to string instead when possible.
-	/// This property is init-only to support record initialization while maintaining immutability.
+	/// The init accessor is protected so that a value can only arrive through <c>Create</c>, which
+	/// canonicalizes and validates it. A public one let <c>uuid with { WeakString = "not a uuid" }</c>
+	/// build an instance that never passed either (#310). A derived type may still set it from its own
+	/// constructor.
 	/// </remarks>
-	public string WeakString { get; init; } = string.Empty;
+	public string WeakString { get; protected init; } = string.Empty;
 
 	/// <inheritdoc/>
 	public int Length => WeakString.Length;
@@ -635,6 +638,7 @@ private static TDest FromStringInternal<TDest>(string? value)
 
 	/// <summary>
 	/// The <see cref="WeakString"/> init accessor as a delegate, looked up once per type rather than per value (#444).
+	/// The accessor is protected (#310), so the lookup asks for non-public accessors too.
 	/// </summary>
 	/// <typeparam name="TDest">The semantic string type to set the value of.</typeparam>
 	private static class WeakStringSetter<TDest>
@@ -642,7 +646,7 @@ private static class WeakStringSetter<TDest>
 	{
 		internal static readonly Action<TDest, string> Set = (Action<TDest, string>)Delegate.CreateDelegate(
 			typeof(Action<TDest, string>),
-			Ensure.NotNull(typeof(TDest).GetProperty(name: nameof(WeakString))?.GetSetMethod()));
+			Ensure.NotNull(typeof(TDest).GetProperty(name: nameof(WeakString))?.GetSetMethod(nonPublic: true)));
 	}
 
 	/// <summary>
diff --git a/Semantics.Test/WeakStringAccessTests.cs b/Semantics.Test/WeakStringAccessTests.cs
new file mode 100644
index 0000000..0c92f42
--- /dev/null
+++ b/Semantics.Test/WeakStringAccessTests.cs
@@ -0,0 +1,66 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Semantics.Test;
+
+using System;
+using System.IO;
+using System.Reflection;
+using ktsu.Semantics.Paths;
+using ktsu.Semantics.Strings;
+using ktsu.Semantics.Strings.Identifiers;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// A semantic string's value only arrives through <c>Create</c>, which canonicalizes and validates it.
+/// A public init accessor let <c>uuid with { WeakString = "not a uuid" }</c> skip both (#310).
+/// </summary>
+[TestClass]
+public class WeakStringAccessTests
+{
+	[TestMethod]
+	public void WeakString_HasNoPublicSetter()
+	{
+		Type[] types =
+		[
+			typeof(MySemanticString),
+			typeof(Uuid),
+			typeof(AbsoluteDirectoryPath),
+			typeof(AbsoluteFilePath),
+			typeof(RelativeDirectoryPath),
+		];
+
+		foreach (Type type in types)
+		{
+			PropertyInfo property = type.GetProperty(nameof(ISemanticString.WeakString))!;
+
+			Assert.IsNotNull(property.GetGetMethod(), $"{type.Name}.WeakString should stay publicly readable.");
+			Assert.IsNull(property.GetSetMethod(), $"{type.Name}.WeakString should not be publicly settable.");
+		}
+	}
+
+	[TestMethod]
+	public void ISemanticString_WeakString_IsReadOnly()
+	{
+		PropertyInfo property = typeof(ISemanticString).GetProperty(nameof(ISemanticString.WeakString))!;
+
+		Assert.IsFalse(property.CanWrite);
+	}
+
+	[TestMethod]
+	public void Create_StillSetsTheValue()
+	{
+		const string value = "123e4567-e89b-12d3-a456-426614174000";
+
+		Assert.AreEqual(value, Uuid.Create(value).WeakString);
+		Assert.AreEqual("test", MySemanticString.Create("test").WeakString);
+	}
+
+	[TestMethod]
+	public void Create_StillSetsThePathValue()
+	{
+		AbsoluteDirectoryPath parent = AbsoluteDirectoryPath.Create(Path.GetTempPath());
+		AbsoluteDirectoryPath child = AbsoluteDirectoryPath.Create(Path.Join(parent.WeakString, "weakstring"));
+
+		Assert.AreEqual(parent, child.Parent);
+	}
+}