diff --git a/src/Expressif.LanguageServer.Core.Tests/SyntaxServiceTests.cs b/src/Expressif.LanguageServer.Core.Tests/SyntaxServiceTests.cs
index 244a041..48821d4 100644
--- a/src/Expressif.LanguageServer.Core.Tests/SyntaxServiceTests.cs
+++ b/src/Expressif.LanguageServer.Core.Tests/SyntaxServiceTests.cs
@@ -21,6 +21,18 @@ public void Parse_ValidExpression_ReturnsSyntaxTree()
         });
     }
 
+    [Test]
+    public void Parse_UnknownFunctionName_ReturnsNoSyntaxErrors()
+    {
+        var result = service.Parse("this-function-does-not-exist(1)");
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(result.IsValid, Is.True);
+            Assert.That(result.Errors, Is.Empty);
+        });
+    }
+
     [Test]
     public void Parse_InvalidExpression_ReturnsSyntaxErrors()
     {
diff --git a/src/Expressif.LanguageServer.Tests/SyntaxDiagnosticMapperTests.cs b/src/Expressif.LanguageServer.Tests/SyntaxDiagnosticMapperTests.cs
new file mode 100644
index 0000000..3e330a9
--- /dev/null
+++ b/src/Expressif.LanguageServer.Tests/SyntaxDiagnosticMapperTests.cs
@@ -0,0 +1,49 @@
+using Expressif.LanguageServer.Diagnostics;
+using Expressif.Syntax;
+using NUnit.Framework;
+using Range = OmniSharp.Extensions.LanguageServer.Protocol.Models.Range;
+
+namespace Expressif.LanguageServer.Tests;
+
+[TestFixture]
+public sealed class SyntaxDiagnosticMapperTests
+{
+    [Test]
+    public void Map_ParserSpan_ProducesExactLspRange()
+    {
+        const string source = "add(1";
+        var diagnostic = SyntaxDiagnosticMapper.Map(
+            source, new SyntaxError("ERROR", new SourceSpan(4, 1), "1", false));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(diagnostic.Range, Is.EqualTo(new Range(0, 4, 0, 5)));
+            Assert.That(diagnostic.Message, Is.EqualTo("Unexpected syntax '1'."));
+            Assert.That(diagnostic.Source, Is.EqualTo("expressif"));
+        });
+    }
+
+    [Test]
+    public void Map_MultilineUtf8Span_UsesZeroBasedUtf16Position()
+    {
+        const string source = "é\nadd(";
+        var diagnostic = SyntaxDiagnosticMapper.Map(
+            source, new SyntaxError(")", new SourceSpan(7, 0), "", true));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(diagnostic.Range, Is.EqualTo(new Range(1, 4, 1, 4)));
+            Assert.That(diagnostic.Message, Is.EqualTo("Missing )."));
+        });
+    }
+
+    [Test]
+    public void Map_SpanPastEndOfDocument_ClampsToEnd()
+    {
+        const string source = "add(";
+        var diagnostic = SyntaxDiagnosticMapper.Map(
+            source, new SyntaxError(")", new SourceSpan(100, 2), "", true));
+
+        Assert.That(diagnostic.Range, Is.EqualTo(new Range(0, 4, 0, 4)));
+    }
+}
diff --git a/src/Expressif.LanguageServer/Diagnostics/SyntaxDiagnosticMapper.cs b/src/Expressif.LanguageServer/Diagnostics/SyntaxDiagnosticMapper.cs
new file mode 100644
index 0000000..481f5b9
--- /dev/null
+++ b/src/Expressif.LanguageServer/Diagnostics/SyntaxDiagnosticMapper.cs
@@ -0,0 +1,49 @@
+using System.Text;
+using Expressif.Syntax;
+using OmniSharp.Extensions.LanguageServer.Protocol.Models;
+using Range = OmniSharp.Extensions.LanguageServer.Protocol.Models.Range;
+
+namespace Expressif.LanguageServer.Diagnostics;
+
+internal static class SyntaxDiagnosticMapper
+{
+    public static Diagnostic Map(string source, SyntaxError error) => new()
+    {
+        Range = new Range(
+            ToPosition(source, error.Span.Start),
+            ToPosition(source, error.Span.End)),
+        Severity = DiagnosticSeverity.Error,
+        Source = "expressif",
+        Message = CreateMessage(error)
+    };
+
+    private static Position ToPosition(string source, int utf8Offset)
+    {
+        var bytes = Encoding.UTF8.GetBytes(source);
+        var clampedOffset = Math.Clamp(utf8Offset, 0, bytes.Length);
+        var textOffset = Encoding.UTF8.GetCharCount(bytes, 0, clampedOffset);
+        var line = 0;
+        var lineStart = 0;
+
+        for (var index = 0; index < textOffset; index++)
+        {
+            if (source[index] != '\n')
+                continue;
+
+            line++;
+            lineStart = index + 1;
+        }
+
+        return new(line, textOffset - lineStart);
+    }
+
+    private static string CreateMessage(SyntaxError error)
+    {
+        var node = error.NodeType.Replace('_', ' ');
+        if (error.IsMissing)
+            return $"Missing {node}.";
+        if (!string.IsNullOrWhiteSpace(error.Text))
+            return $"Unexpected syntax '{error.Text}'.";
+        return $"Invalid {node} syntax.";
+    }
+}
diff --git a/src/Expressif.LanguageServer/Handlers/TextDocumentSyncHandler.cs b/src/Expressif.LanguageServer/Handlers/TextDocumentSyncHandler.cs
index bfd3cb6..15bc23e 100644
--- a/src/Expressif.LanguageServer/Handlers/TextDocumentSyncHandler.cs
+++ b/src/Expressif.LanguageServer/Handlers/TextDocumentSyncHandler.cs
@@ -1,20 +1,24 @@
 using Expressif.LanguageServer.Core.Documents;
+using Expressif.LanguageServer.Diagnostics;
 using MediatR;
 using OmniSharp.Extensions.LanguageServer.Protocol;
 using OmniSharp.Extensions.LanguageServer.Protocol.Client.Capabilities;
 using OmniSharp.Extensions.LanguageServer.Protocol.Document;
 using OmniSharp.Extensions.LanguageServer.Protocol.Models;
+using OmniSharp.Extensions.LanguageServer.Protocol.Server;
 using OmniSharp.Extensions.LanguageServer.Protocol.Server.Capabilities;
 
 namespace Expressif.LanguageServer.Handlers;
 
-public sealed class TextDocumentSyncHandler(IDocumentStore documents) : TextDocumentSyncHandlerBase
+public sealed class TextDocumentSyncHandler(IDocumentStore documents, ILanguageServerFacade server) : TextDocumentSyncHandlerBase
 {
     public override TextDocumentAttributes GetTextDocumentAttributes(DocumentUri uri) => new(uri, "expressif");
 
     public override Task<Unit> Handle(DidOpenTextDocumentParams request, CancellationToken cancellationToken)
     {
-        documents.Open(request.TextDocument.Uri.ToUri(), request.TextDocument.Text, request.TextDocument.Version);
+        var document = documents.Open(
+            request.TextDocument.Uri.ToUri(), request.TextDocument.Text, request.TextDocument.Version);
+        PublishDiagnostics(request.TextDocument.Uri, document);
         return Unit.Task;
     }
 
@@ -22,13 +26,22 @@ public override Task<Unit> Handle(DidChangeTextDocumentParams request, Cancellat
     {
         var text = request.ContentChanges.LastOrDefault()?.Text;
         if (text is not null)
-            documents.Change(request.TextDocument.Uri.ToUri(), text, request.TextDocument.Version);
+        {
+            var document = documents.Change(
+                request.TextDocument.Uri.ToUri(), text, request.TextDocument.Version);
+            PublishDiagnostics(request.TextDocument.Uri, document);
+        }
         return Unit.Task;
     }
 
     public override Task<Unit> Handle(DidCloseTextDocumentParams request, CancellationToken cancellationToken)
     {
         documents.Close(request.TextDocument.Uri.ToUri());
+        server.TextDocument.PublishDiagnostics(new()
+        {
+            Uri = request.TextDocument.Uri,
+            Diagnostics = []
+        });
         return Unit.Task;
     }
 
@@ -41,4 +54,16 @@ protected override TextDocumentSyncRegistrationOptions CreateRegistrationOptions
             Change = TextDocumentSyncKind.Full,
             Save = false
         };
+
+    private void PublishDiagnostics(DocumentUri uri, DocumentSnapshot document)
+    {
+        server.TextDocument.PublishDiagnostics(new()
+        {
+            Uri = uri,
+            Version = document.Version,
+            Diagnostics = document.SyntaxErrors
+                .Select(error => SyntaxDiagnosticMapper.Map(document.Text, error))
+                .ToArray()
+        });
+    }
 }
diff --git a/src/Expressif.LanguageServer/Properties/AssemblyInfo.cs b/src/Expressif.LanguageServer/Properties/AssemblyInfo.cs
new file mode 100644
index 0000000..9e84e40
--- /dev/null
+++ b/src/Expressif.LanguageServer/Properties/AssemblyInfo.cs
@@ -0,0 +1,3 @@
+using System.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("Expressif.LanguageServer.Tests")]