fix: stabilize TypeScript LSP startup

Seed new TypeScript LSP profiles with --stdio, validate that shared profiles keep the required flag, auto-add it for existing profiles in the sidecar runtime, and include recent stderr when a language server dies before completing a request. Add shared and sidecar regression coverage.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/src/Eryx.AgentHost/Services/LspToolSession.cs

@@ -25,6 +25,7 @@ internal sealed class LspToolSession : IAsyncDisposable
     private readonly SemaphoreSlim _writeLock = new(1, 1);
     private readonly SemaphoreSlim _documentLock = new(1, 1);
     private readonly HashSet<string> _openedDocumentUris = new(StringComparer.OrdinalIgnoreCase);
+    private readonly ConcurrentQueue<string> _stderrLines = new();
     private readonly Task _stdoutReaderTask;
     private readonly Task _stderrReaderTask;
     private int _nextRequestId;
@@ -97,7 +98,7 @@ internal sealed class LspToolSession : IAsyncDisposable
             CreateNoWindow = true,
         };
 
-        foreach (string arg in profile.Args)
+        foreach (string arg in ResolveProcessArguments(profile))
         {
             startInfo.ArgumentList.Add(arg);
         }
@@ -125,6 +126,19 @@ internal sealed class LspToolSession : IAsyncDisposable
         return session;
     }
 
+    internal static IReadOnlyList<string> ResolveProcessArguments(RunTurnLspProfileConfigDto profile)
+    {
+        List<string> args = profile.Args.ToList();
+
+        if (UsesTypeScriptLanguageServer(profile.Command)
+            && !args.Any(arg => string.Equals(arg, "--stdio", StringComparison.OrdinalIgnoreCase)))
+        {
+            args.Add("--stdio");
+        }
+
+        return args;
+    }
+
     public async ValueTask DisposeAsync()
     {
         _cts.Cancel();
@@ -483,8 +497,7 @@ internal sealed class LspToolSession : IAsyncDisposable
         }
         finally
         {
-            FailPendingRequests(new InvalidOperationException(
-                $"LSP profile \"{_profile.Name}\" stopped before a pending request completed."));
+            FailPendingRequests(CreatePendingRequestInterruptedException());
         }
     }
 
@@ -502,6 +515,7 @@ internal sealed class LspToolSession : IAsyncDisposable
 
                 if (!string.IsNullOrWhiteSpace(line))
                 {
+                    RecordStderrLine(line);
                     Console.Error.WriteLine($"[eryx lsp:{_profile.Id}] {line}");
                 }
             }
@@ -602,6 +616,28 @@ internal sealed class LspToolSession : IAsyncDisposable
         _pending.Clear();
     }
 
+    private void RecordStderrLine(string line)
+    {
+        _stderrLines.Enqueue(line);
+        while (_stderrLines.Count > 8 && _stderrLines.TryDequeue(out _))
+        {
+        }
+    }
+
+    private InvalidOperationException CreatePendingRequestInterruptedException()
+    {
+        string message = $"LSP profile \"{_profile.Name}\" stopped before a pending request completed.";
+        string[] stderrLines = _stderrLines.ToArray();
+
+        if (stderrLines.Length == 0)
+        {
+            return new InvalidOperationException(message);
+        }
+
+        string detail = string.Join(" ", stderrLines.TakeLast(3));
+        return new InvalidOperationException($"{message} Last stderr: {detail}");
+    }
+
     private string ResolveProjectPath(string relativePath)
     {
         if (string.IsNullOrWhiteSpace(relativePath))
@@ -667,6 +703,14 @@ internal sealed class LspToolSession : IAsyncDisposable
         return Math.Max(value - 1, 0);
     }
 
+    private static bool UsesTypeScriptLanguageServer(string command)
+    {
+        string executableName = Path.GetFileName(command.Trim()).ToLowerInvariant();
+        return executableName is "typescript-language-server"
+            or "typescript-language-server.cmd"
+            or "typescript-language-server.exe";
+    }
+
     private static string ToFileUri(string path)
     {
         return new Uri(path).AbsoluteUri;

sidecar/tests/Eryx.AgentHost.Tests/LspToolSessionTests.cs

@@ -1,4 +1,5 @@
 using System.Text.Json;
+using Eryx.AgentHost.Contracts;
 using Eryx.AgentHost.Services;
 
 namespace Eryx.AgentHost.Tests;
@@ -24,4 +25,32 @@ public sealed class LspToolSessionTests
 
         Assert.Contains("relativePath", json);
     }
+
+    [Fact]
+    public void ResolveProcessArguments_AddsStdioForTypeScriptLanguageServer()
+    {
+        RunTurnLspProfileConfigDto profile = new()
+        {
+            Command = "typescript-language-server",
+            Args = [],
+        };
+
+        IReadOnlyList<string> args = LspToolSession.ResolveProcessArguments(profile);
+
+        Assert.Equal(["--stdio"], args);
+    }
+
+    [Fact]
+    public void ResolveProcessArguments_DoesNotDuplicateStdioWhenAlreadyPresent()
+    {
+        RunTurnLspProfileConfigDto profile = new()
+        {
+            Command = "typescript-language-server",
+            Args = ["--stdio"],
+        };
+
+        IReadOnlyList<string> args = LspToolSession.ResolveProcessArguments(profile);
+
+        Assert.Equal(["--stdio"], args);
+    }
 }

src/renderer/App.tsx

@@ -72,7 +72,7 @@ function createDraftLspProfile(): LspProfileDefinition {
     id: createId('lsp'),
     name: 'New LSP Profile',
     command: '',
-    args: [],
+    args: ['--stdio'],
     languageId: 'typescript',
     fileExtensions: ['.ts', '.tsx'],
     createdAt: timestamp,

src/shared/domain/tooling.ts

@@ -126,6 +126,11 @@ export function validateLspProfileDefinition(profile: LspProfileDefinition): str
     return `LSP profile "${profile.name}" needs at least one file extension.`;
   }
 
+  if (requiresTypeScriptLanguageServerStdio(profile.command)
+    && !normalizeStringArray(profile.args).some((arg) => arg.toLowerCase() === '--stdio')) {
+    return `LSP profile "${profile.name}" needs the "--stdio" argument.`;
+  }
+
   return undefined;
 }
 
@@ -168,6 +173,18 @@ function normalizeFileExtensions(fileExtensions: string[]): string[] {
   return normalizeStringArray(fileExtensions).map((value) => (value.startsWith('.') ? value : `.${value}`));
 }
 
+function requiresTypeScriptLanguageServerStdio(command: string): boolean {
+  const executableName = command
+    .trim()
+    .split(/[\\/]/)
+    .at(-1)
+    ?.toLowerCase();
+
+  return executableName === 'typescript-language-server'
+    || executableName === 'typescript-language-server.cmd'
+    || executableName === 'typescript-language-server.exe';
+}
+
 function normalizeStringArray(values?: ReadonlyArray<string>): string[] {
   if (!values) {
     return [];

tests/shared/tooling.test.ts

@@ -135,7 +135,23 @@ describe('tooling settings helpers', () => {
       validateLspProfileDefinition({
         ...profile,
         command: 'typescript-language-server',
+        args: ['--stdio'],
       }),
     ).toBe('LSP profile "TypeScript" needs at least one file extension.');
   });
+
+  test('requires the stdio flag for the TypeScript language server profile', () => {
+    expect(
+      validateLspProfileDefinition({
+        id: 'lsp-ts',
+        name: 'Typescript LSP',
+        command: 'typescript-language-server',
+        args: [],
+        languageId: 'typescript',
+        fileExtensions: ['.ts', '.tsx'],
+        createdAt: TIMESTAMP,
+        updatedAt: TIMESTAMP,
+      }),
+    ).toBe('LSP profile "Typescript LSP" needs the "--stdio" argument.');
+  });
 });