diff --git a/src/AquaSynth.Core/StoreFileName.cs b/src/AquaSynth.Core/StoreFileName.cs
new file mode 100644
index 0000000..c18bda1
--- /dev/null
+++ b/src/AquaSynth.Core/StoreFileName.cs
@@ -0,0 +1,38 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace AquaSynth.Dsl;
+
+/// <summary>
+/// The one mapping from a caller-chosen id to a single file-name component in a store.
+/// Ids already within [A-Za-z0-9._-] pass through unchanged. Anything else is sanitised and
+/// suffixed with the first 8 hex of SHA-256 of the original id, so distinct ids stay distinct.
+/// The result is never a path: no separators, never "." or "..".
+/// </summary>
+public static class StoreFileName
+{
+    public static string From(string id)
+    {
+        ArgumentNullException.ThrowIfNull(id);
+        if (id.Length == 0 || id.All(c => c == '.'))
+        {
+            throw new ArgumentException($"Id '{id}' cannot name a store file.", nameof(id));
+        }
+
+        var builder = new StringBuilder(id.Length);
+        foreach (var c in id)
+        {
+            builder.Append(c is >= 'a' and <= 'z' or >= 'A' and <= 'Z' or >= '0' and <= '9' or '-' or '_' or '.' ? c : '_');
+        }
+
+        var safe = builder.ToString();
+        // A trailing dot is dropped by Windows, so "a." would alias "a"; hash it like any other rewrite.
+        if (safe == id && !id.EndsWith('.'))
+        {
+            return safe;
+        }
+
+        var hash = Convert.ToHexString(SHA256.HashData(Encoding.UTF8.GetBytes(id)))[..8].ToLowerInvariant();
+        return $"{safe}-{hash}";
+    }
+}
diff --git a/src/AquaSynth.Faust/AquaSynthDaemonService.cs b/src/AquaSynth.Faust/AquaSynthDaemonService.cs
index 28ae62d..74f6830 100644
--- a/src/AquaSynth.Faust/AquaSynthDaemonService.cs
+++ b/src/AquaSynth.Faust/AquaSynthDaemonService.cs
@@ -556,9 +556,9 @@ public sealed class AquaSynthDaemonService : IDisposable
         session.Stream.ProcessBlock([], outputs, command.FrameCount, session.Controls);
         var mono = MixMono(outputs, command.FrameCount, command.Gain * session.Gain);
         var blockId = BlockId(command.CommandId);
-        var blockDir = Path.Combine(options.StoreRoot, "live", SafeFileName(command.SessionId));
+        var blockDir = Path.Combine(options.StoreRoot, "live", StoreFileName.From(command.SessionId));
         Directory.CreateDirectory(blockDir);
-        var floatPath = Path.Combine(blockDir, $"{SafeFileName(blockId)}.f32");
+        var floatPath = Path.Combine(blockDir, $"{StoreFileName.From(blockId)}.f32");
         await WriteFloat32Async(floatPath, mono).ConfigureAwait(false);
 
         var sequence = session.NextSequence();
@@ -703,8 +703,8 @@ public sealed class AquaSynthDaemonService : IDisposable
 
         var runId = $"{command.CommandId}-stream";
         var verseId = "aquasynth.instrument";
-        var audioStreamId = $"aquasynth.audio.{SafeFileName(command.CommandId)}";
-        var controlStreamId = $"aquasynth.controls.{SafeFileName(command.CommandId)}";
+        var audioStreamId = $"aquasynth.audio.{StoreFileName.From(command.CommandId)}";
+        var controlStreamId = $"aquasynth.controls.{StoreFileName.From(command.CommandId)}";
 
         if (!string.Equals(compile.Status, "succeeded", StringComparison.Ordinal))
         {
@@ -741,7 +741,7 @@ public sealed class AquaSynthDaemonService : IDisposable
             .GroupBy(frame => frame.Block)
             .ToDictionary(group => group.Key, group => NormalizeControls(group.Last().Controls), EqualityComparer<int>.Default);
 
-        var streamDir = Path.Combine(options.StoreRoot, "streams", SafeFileName(runId));
+        var streamDir = Path.Combine(options.StoreRoot, "streams", StoreFileName.From(runId));
         Directory.CreateDirectory(streamDir);
         var packets = new List<AquaSynthAutomationPacketReceipt>();
         using (var stream = patch.CreateStreamingPatch())
@@ -918,7 +918,7 @@ public sealed class AquaSynthDaemonService : IDisposable
     private async Task WriteReceiptAsync<T>(string family, string id, T document, bool global = false)
         where T : class
     {
-        var filePath = Path.Combine(options.StoreRoot, family, $"{SafeFileName(id)}.cc");
+        var filePath = Path.Combine(options.StoreRoot, family, $"{StoreFileName.From(id)}.cc");
         Directory.CreateDirectory(Path.GetDirectoryName(filePath)!);
         // Opening hydrates; a receipt file holds exactly one document, so everything else goes before the write.
         using var cache = CultCacheMessagePack.Create(filePath);
@@ -1129,18 +1129,7 @@ public sealed class AquaSynthDaemonService : IDisposable
 
     private static string BlockId(string commandId) => $"{commandId}-block";
 
-    private static string SessionId(string patchId, string compileKey) => $"{SafeFileName(patchId)}-{compileKey[..Math.Min(12, compileKey.Length)]}";
-
-    private static string SafeFileName(string value)
-    {
-        var builder = new StringBuilder(value.Length);
-        foreach (var c in value)
-        {
-            builder.Append(char.IsLetterOrDigit(c) || c is '-' or '_' or '.' ? c : '-');
-        }
-
-        return builder.ToString().Trim('-');
-    }
+    private static string SessionId(string patchId, string compileKey) => $"{StoreFileName.From(patchId)}-{compileKey[..Math.Min(12, compileKey.Length)]}";
 
     private static string ToArtifactUri(string path) => $"file:///{Path.GetFullPath(path).Replace('\\', '/')}";
 
diff --git a/src/AquaSynth.Faust/NativeFaustRuntime.cs b/src/AquaSynth.Faust/NativeFaustRuntime.cs
index e6b44c4..99bc288 100644
--- a/src/AquaSynth.Faust/NativeFaustRuntime.cs
+++ b/src/AquaSynth.Faust/NativeFaustRuntime.cs
@@ -633,18 +633,10 @@ public sealed class AquaSynthNativeCompiler : IDisposable
         }
 
         Directory.CreateDirectory(outputRoot);
-        var path = Path.Combine(outputRoot, $"{SafeFileName(patchId)}.dsp");
+        var path = Path.Combine(outputRoot, $"{StoreFileName.From(patchId)}.dsp");
         File.WriteAllText(path, source);
         return path;
     }
-
-    private static string SafeFileName(string value)
-    {
-        var invalid = Path.GetInvalidFileNameChars().ToHashSet();
-        var chars = value.Select(ch => invalid.Contains(ch) ? '_' : ch).ToArray();
-        var name = new string(chars).Trim();
-        return string.IsNullOrWhiteSpace(name) ? "aquasynth_patch" : name;
-    }
 }
 
 public static class AquaSynthFaustToolchain
diff --git a/tests/AquaSynth.Dsl.Tests/StoreFileNameTests.cs b/tests/AquaSynth.Dsl.Tests/StoreFileNameTests.cs
new file mode 100644
index 0000000..24ad104
--- /dev/null
+++ b/tests/AquaSynth.Dsl.Tests/StoreFileNameTests.cs
@@ -0,0 +1,68 @@
+using AquaSynth.Faust;
+
+namespace AquaSynth.Dsl.Tests;
+
+public sealed class StoreFileNameTests
+{
+    [Fact]
+    public void DistinctIdsGetDistinctNames()
+    {
+        var ids = new[] { "a/b", "a b", "a:b", "a-b", "a_b", "a\\b", "a?b" };
+        var names = ids.Select(StoreFileName.From).ToArray();
+        Assert.Equal(ids.Length, names.Distinct(StringComparer.OrdinalIgnoreCase).Count());
+    }
+
+    [Theory]
+    [InlineData("dings.warm-bell")]
+    [InlineData("demo-stream-block")]
+    [InlineData("A_b.c-1")]
+    public void CleanIdsPassThroughUnchanged(string id) => Assert.Equal(id, StoreFileName.From(id));
+
+    [Fact]
+    public void RewrittenIdsCarryHashOfTheOriginal()
+    {
+        // sha256("a b") begins c2a5..., checked independently of the implementation.
+        var expected = Convert.ToHexString(System.Security.Cryptography.SHA256.HashData("a b"u8.ToArray()))[..8].ToLowerInvariant();
+        Assert.Equal($"a_b-{expected}", StoreFileName.From("a b"));
+    }
+
+    [Theory]
+    [InlineData(".")]
+    [InlineData("..")]
+    [InlineData("")]
+    public void DotsAndEmptyAreRefused(string id) => Assert.Throws<ArgumentException>(() => StoreFileName.From(id));
+
+    [Theory]
+    [InlineData("../..")]
+    [InlineData("../../etc/passwd")]
+    [InlineData("..\\..\\x")]
+    [InlineData("/abs/path")]
+    [InlineData("C:\\abs")]
+    [InlineData("a.")]
+    [InlineData("a/..")]
+    public void NoIdCanMakePathCombineEscapeTheRoot(string id)
+    {
+        var root = Path.Combine(Path.GetTempPath(), "store-root-probe", "store");
+        var name = StoreFileName.From(id);
+        Assert.Equal(name, Path.GetFileName(name));
+        Assert.NotEqual(".", name);
+        Assert.NotEqual("..", name);
+        var full = Path.GetFullPath(Path.Combine(root, name));
+        Assert.Equal(Path.GetFullPath(root), Path.GetDirectoryName(full));
+    }
+
+    [Fact]
+    public async Task DaemonReceiptForTraversalCommandIdStaysInsideStore()
+    {
+        var parent = Path.Combine(Path.GetTempPath(), "aquasynth-sfn-" + Guid.NewGuid().ToString("N"));
+        var storeRoot = Path.Combine(parent, "store");
+        using var service = new AquaSynthDaemonService(new AquaSynthDaemonOptions(storeRoot));
+
+        await service.SampleAsync(new AquaSynthInstrumentSampleCommand(
+            "../../escape", "test.invalid", "test_invalid", "this is not aqua syntax", DurationSeconds: 0.1f));
+
+        Assert.Empty(Directory.GetFiles(parent));
+        Assert.Empty(Directory.GetDirectories(parent).Where(d => Path.GetFileName(d) != "store"));
+        Assert.NotEmpty(Directory.GetFiles(Path.Combine(storeRoot, "compile")));
+    }
+}