diff --git a/src/AquaSynth.Faust/AquaSynthDaemonService.cs b/src/AquaSynth.Faust/AquaSynthDaemonService.cs
index 0021895..13c01f4 100644
--- a/src/AquaSynth.Faust/AquaSynthDaemonService.cs
+++ b/src/AquaSynth.Faust/AquaSynthDaemonService.cs
@@ -1132,7 +1132,12 @@ public sealed class AquaSynthDaemonService : IDisposable
 
     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('\\', '/')}";
+    // `C:\a\b` -> file:///C:/a/b and `/a/b` -> file:///a/b; a bare "file:///" prefix made POSIX paths file:////a/b, which Uri reads as a UNC host.
+    private static string ToArtifactUri(string path)
+    {
+        var full = Path.GetFullPath(path).Replace('\\', '/');
+        return full.StartsWith('/') ? $"file://{full}" : $"file:///{full}";
+    }
 
     private static string Now() => DateTimeOffset.UtcNow.ToString("O");
 
diff --git a/tests/AquaSynth.Dsl.Tests/AquaSynthDaemonServiceTests.cs b/tests/AquaSynth.Dsl.Tests/AquaSynthDaemonServiceTests.cs
index 129491f..89da3ad 100644
--- a/tests/AquaSynth.Dsl.Tests/AquaSynthDaemonServiceTests.cs
+++ b/tests/AquaSynth.Dsl.Tests/AquaSynthDaemonServiceTests.cs
@@ -52,7 +52,7 @@ public sealed class AquaSynthDaemonServiceTests
         Assert.True(File.Exists(UriToPath(result.RenderReceipt.Float32Uri)));
         Assert.True(File.Exists(UriToPath(result.RenderReceipt.WavUri)));
         Assert.Equal(result.RenderReceipt.SampleCount * sizeof(float), new FileInfo(UriToPath(result.RenderReceipt.Float32Uri)).Length);
-        Assert.True(File.Exists(Path.Combine(storeRoot, "sessions", $"{result.CompileReceipt.SessionId}.cc")));
+        Assert.True(File.Exists(Path.Combine(storeRoot, "sessions", $"{StoreFileName.From(result.CompileReceipt.SessionId)}.cc")));
     }
 
     [Fact]
@@ -154,7 +154,7 @@ public sealed class AquaSynthDaemonServiceTests
         Assert.True(File.Exists(UriToPath(quiet.Float32Uri)));
         Assert.True(File.Exists(UriToPath(loud.Float32Uri)));
         Assert.Equal(128 * sizeof(float), new FileInfo(UriToPath(loud.Float32Uri)).Length);
-        Assert.True(File.Exists(Path.Combine(storeRoot, "live", $"{close.ReceiptId}.cc")));
+        Assert.True(File.Exists(Path.Combine(storeRoot, "live", $"{StoreFileName.From(close.ReceiptId)}.cc")));
         var liveState = await ReadLiveSessionStateAsync(storeRoot, open.SessionId);
         Assert.Equal(open.SessionId, liveState.SessionId);
         Assert.Equal("closed", liveState.Status);
@@ -210,7 +210,7 @@ public sealed class AquaSynthDaemonServiceTests
     private static async Task<AquaSynthLiveInstrumentSessionState> ReadLiveSessionStateAsync(string storeRoot, string sessionId)
     {
         using var cache = await CultCacheMessagePack.OpenAsync(
-            Path.Combine(storeRoot, "live-sessions", $"{sessionId}.cc")).ConfigureAwait(false);
+            Path.Combine(storeRoot, "live-sessions", $"{StoreFileName.From(sessionId)}.cc")).ConfigureAwait(false);
         return Assert.Single(cache.GetAll<AquaSynthLiveInstrumentSessionState>());
     }
 }