Keyword completion throws exception when typing in a new document

[RikkiGibson]

Repro:

1. Open a blank file in VS Code. Save it as a C# file or set language mode to C#.
2. Type 2-3 characters, like Con to use Console

Expected behavior: A filtered completion list is shown.

Actual behavior: An error popu appears, and a completion list is eventually shown which is missing many common items, which causes the editor to fight you really hard when typing normal C# code.

An exception is thrown in the language server, see stack:

2026-01-29 20:37:08.381 [error] [textDocument/completion] [LanguageServerExtensionManager] Microsoft.CodeAnalysis.CSharp.Completion.Providers.KeywordCompletionProvider threw an exception. System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values. (Parameter 'position')
   at Microsoft.CodeAnalysis.SyntaxNode.FindTokenCore(Int32 position, Boolean findInsideTrivia) in src\Compilers\Core\Portable\Syntax\SyntaxNode.cs:line 1458
   at Microsoft.CodeAnalysis.SyntaxNode.FindToken(Int32 position, Boolean findInsideTrivia) in src\Compilers\Core\Portable\Syntax\SyntaxNode.cs:line 1010
   at Microsoft.CodeAnalysis.Shared.Extensions.DocumentExtensions.ReuseExistingSpeculativeModelAsync(Document document, TextSpan span, CancellationToken cancellationToken) in src\Workspaces\SharedUtilitiesAndExtensions\Workspace\Core\Extensions\DocumentExtensions.cs:line 133
   at Microsoft.CodeAnalysis.Completion.Utilities.CreateSyntaxContextWithExistingSpeculativeModelAsync(Document document, Int32 position, CancellationToken cancellationToken) in src\Features\Core\Portable\Completion\Utilities.cs:line 55
   at Microsoft.CodeAnalysis.Threading.ProducerConsumer`1.<>c__DisplayClass11_0`3.<<RunParallelChannelAsync>b__2>d.MoveNext() in src\Dependencies\Threading\ProducerConsumer.cs:line 238
   at Microsoft.CodeAnalysis.Completion.Providers.AbstractKeywordCompletionProvider`1.RecommendCompletionItemsAsync(Document document, CompletionContext context, CancellationToken cancellationToken) in src\Features\Core\Portable\Completion\Providers\AbstractKeywordCompletionProvider.cs:line 48
   at System.Threading.Tasks.Parallel.<>c__57`1.<<ForEachAsync>b__57_0>d.MoveNext()
   at Microsoft.CodeAnalysis.Threading.ProducerConsumer`1.<>c__DisplayClass11_0`3.<<RunParallelChannelAsync>b__2>d.MoveNext() in src\Dependencies\Threading\ProducerConsumer.cs:line 238
--- End of stack trace from previous location ---
   at System.Threading.Tasks.Parallel.<>c__57`1.<<ForEachAsync>b__57_0>d.MoveNext()
   at System.Threading.Tasks.Parallel.<>c__57`1.<<ForEachAsync>b__57_0>d.MoveNext()
   at Microsoft.CodeAnalysis.Shared.Extensions.DocumentExtensions.GetUnionItemsFromDocumentAndLinkedDocumentsAsync[T](Document document, IEqualityComparer`1 comparer, Func`2 getItemsWorker) in src\Features\Core\Portable\Shared\Extensions\DocumentExtensions.cs:line 64
   at Microsoft.CodeAnalysis.Completion.Providers.AbstractKeywordCompletionProvider`1.ProvideCompletionsAsync(CompletionContext context) in src\Features\Core\Portable\Completion\Providers\AbstractKeywordCompletionProvider.cs:line 39
   at Microsoft.CodeAnalysis.Extensions.IExtensionManagerExtensions.PerformActionAsync(IExtensionManager extensionManager, Object extension, Func`1 function) in src\Workspaces\Core\Portable\ExtensionManager\IExtensionManagerExtensions.cs:line 58

It looks like this method is getting a syntax tree for an empty file (i.e. root.GetText().Length == 0), and then trying to get the token at position 1.

roslyn/src/Workspaces/SharedUtilitiesAndExtensions/Workspace/Core/Extensions/DocumentExtensions.cs

Lines 128 to 134 in 21f8f2f

	public static async ValueTask<SemanticModel> ReuseExistingSpeculativeModelAsync(this Document document, TextSpan span, CancellationToken cancellationToken)
	{
	Contract.ThrowIfFalse(document.SupportsSemanticModel);
	var root = await document.GetRequiredSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
	var token = root.FindToken(span.Start);
	var node = token.Parent!.AncestorsAndSelf().First(a => a.FullSpan.Contains(span));

[CyrusNajmabadi]

Definitely odd as the completion harness tests tries to get completion in cases like these just to validate end of file handling.

[RikkiGibson]

I was wondering if ReuseExistingSpeculativeModelAsync (referenced in issue description) needs to be updated to handle the case where span.Start is out of range of root.FullSpan.

Basically, whether calling this helper with an out-of-range span, indicates that something went wrong earlier on, or if it's something this method itself should be handling.

I also noticed that SemanticModelReuseWorkspaceService.ReuseExistingSpeculativeModelAsync, does not appear to handle the compilation unit case (i.e. top-level statements). I wondered if this could lead to completion perf degradation in top-level statements.

roslyn/src/Workspaces/SharedUtilitiesAndExtensions/Workspace/Core/SemanticModelReuse/SemanticModelWorkspaceServiceFactory.SemanticModelWorkspaceService.cs

Lines 100 to 102 in 700a414

	var bodyNode = reuseService.TryGetContainingMethodBodyForSpeculation(node);
	if (bodyNode == null)
	return await document.GetRequiredSemanticModelAsync(cancellationToken).ConfigureAwait(false);

[RikkiGibson]

It seems like just patching the one site in ReuseSpeculativeModelAsync which is calling FindToken, to first check root.FullSpan.IntersectsWith(position), just causes a different exception to appear later:

2026-01-30 19:29:14.156 [error] [textDocument/completion] [LanguageServerExtensionManager] Microsoft.CodeAnalysis.CSharp.Completion.Providers.KeywordCompletionProvider threw an exception. System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values. (Parameter 'position')
   at Microsoft.CodeAnalysis.SyntaxNode.FindTokenCore(Int32 position, Boolean findInsideTrivia) in src\Compilers\Core\Portable\Syntax\SyntaxNode.cs:line 1458
   at Microsoft.CodeAnalysis.SyntaxNode.FindToken(Int32 position, Boolean findInsideTrivia) in src\Compilers\Core\Portable\Syntax\SyntaxNode.cs:line 1010
   at Microsoft.CodeAnalysis.SyntaxNode.FindTokenCore(Int32 position, Func`2 stepInto) in src\Compilers\Core\Portable\Syntax\SyntaxNode.cs:line 1521
   at Microsoft.CodeAnalysis.SyntaxNode.FindToken(Int32 position, Func`2 findInsideTrivia) in src\Compilers\Core\Portable\Syntax\SyntaxNode.cs:line 1508
   at Microsoft.CodeAnalysis.SyntaxNode.FindTokenCore(Int32 position, Boolean findInsideTrivia) in src\Compilers\Core\Portable\Syntax\SyntaxNode.cs:line 1447
   at Microsoft.CodeAnalysis.SyntaxNode.FindToken(Int32 position, Boolean findInsideTrivia) in src\Compilers\Core\Portable\Syntax\SyntaxNode.cs:line 1010
   at Microsoft.CodeAnalysis.CSharp.Extensions.SyntaxTreeExtensions.IsEntirelyWithinStringLiteral(SyntaxTree syntaxTree, Int32 position, SyntaxToken& stringLiteral, CancellationToken cancellationToken) in src\Workspaces\SharedUtilitiesAndExtensions\Compiler\CSharp\Extensions\SyntaxTreeExtensions.cs:line 395
   at Microsoft.CodeAnalysis.CSharp.Extensions.SyntaxTreeExtensions.IsEntirelyWithinStringLiteral(SyntaxTree syntaxTree, Int32 position, CancellationToken cancellationToken) in src\Workspaces\SharedUtilitiesAndExtensions\Compiler\CSharp\Extensions\SyntaxTreeExtensions.cs:line 390
   at Microsoft.CodeAnalysis.CSharp.Extensions.SyntaxTreeExtensions.IsEntirelyWithinStringOrCharLiteral(SyntaxTree syntaxTree, Int32 position, CancellationToken cancellationToken) in src\Workspaces\SharedUtilitiesAndExtensions\Compiler\CSharp\Extensions\SyntaxTreeExtensions.cs:line 384
   at Microsoft.CodeAnalysis.CSharp.Extensions.SyntaxTreeExtensions.IsInNonUserCode(SyntaxTree syntaxTree, Int32 position, CancellationToken cancellationToken) in src\Workspaces\SharedUtilitiesAndExtensions\Workspace\CSharp\Extensions\SyntaxTreeExtensions.cs:line 24
   at Microsoft.CodeAnalysis.CSharp.Extensions.ContextQuery.CSharpSyntaxContext.CreateContextWorker(Document document, SemanticModel semanticModel, Int32 position, CancellationToken cancellationToken) in src\Workspaces\SharedUtilitiesAndExtensions\Workspace\CSharp\Extensions\ContextQuery\CSharpSyntaxContext.cs:line 189
   at Microsoft.CodeAnalysis.CSharp.Extensions.ContextQuery.CSharpSyntaxContext.CreateContext(Document document, SemanticModel semanticModel, Int32 position, CancellationToken cancellationToken) in src\Workspaces\SharedUtilitiesAndExtensions\Workspace\CSharp\Extensions\ContextQuery\CSharpSyntaxContext.cs:line 182
   at Microsoft.CodeAnalysis.CSharp.Extensions.ContextQuery.CSharpSyntaxContextService.CreateContext(Document document, SemanticModel semanticModel, Int32 position, CancellationToken cancellationToken) in src\Workspaces\SharedUtilitiesAndExtensions\Workspace\CSharp\Extensions\ContextQuery\CSharpSyntaxContextService.cs:line 19
   at Microsoft.CodeAnalysis.Completion.Utilities.CreateSyntaxContextWithExistingSpeculativeModelAsync(Document document, Int32 position, CancellationToken cancellationToken) in src\Features\Core\Portable\Completion\Utilities.cs:line 58
   at Microsoft.CodeAnalysis.Completion.Providers.AbstractKeywordCompletionProvider`1.RecommendCompletionItemsAsync(Document document, CompletionContext context, CancellationToken cancellationToken) in src\Features\Core\Portable\Completion\Providers\AbstractKeywordCompletionProvider.cs:line 48
   at Microsoft.CodeAnalysis.Shared.Extensions.DocumentExtensions.GetUnionItemsFromDocumentAndLinkedDocumentsAsync[T](Document document, IEqualityComparer`1 comparer, Func`2 getItemsWorker) in src\Features\Core\Portable\Shared\Extensions\DocumentExtensions.cs:line 64
   at Microsoft.CodeAnalysis.Completion.Providers.AbstractKeywordCompletionProvider`1.ProvideCompletionsAsync(CompletionContext context) in src\Features\Core\Portable\Completion\Providers\AbstractKeywordCompletionProvider.cs:line 39
   at Microsoft.CodeAnalysis.Extensions.IExtensionManagerExtensions.PerformActionAsync(IExtensionManager extensionManager, Object extension, Func`1 function) in src\Workspaces\Core\Portable\ExtensionManager\IExtensionManagerExtensions.cs:line 58

This seems like a sign that some deeper invariant has been violated earlier on.

[CyrusNajmabadi]

Yup. Definitely something not good. My general point though is that I'm surprised by this since the completion test harness was designed to test that exact cases :-)

[RikkiGibson]

I am seeing a lot of weird, bad completion behaviors in loose files, involving things like missing keywords. I am unsure if the root cause is the same.

bad-completion-demo.mp4