Unhandled method window/workDoneProgress/create when using Claude Code

[jbrantly]

Hi! Claude Code recently got built-in LSP support and added this project as an official plugin. I was trying it out but immediately ran into this error:

2026-01-22T20:28:36.152Z [DEBUG] [LSP PROTOCOL plugin:csharp-lsp:csharp-ls] Received response 'initialize - (0)' in 793ms.
2026-01-22T20:28:36.152Z [DEBUG] [LSP PROTOCOL plugin:csharp-lsp:csharp-ls] Sending notification 'initialized'.
2026-01-22T20:28:36.153Z [DEBUG] LSP server plugin:csharp-lsp:csharp-ls initialized
2026-01-22T20:28:36.153Z [DEBUG] LSP server instance started: plugin:csharp-lsp:csharp-ls
2026-01-22T20:28:36.166Z [DEBUG] [LSP PROTOCOL plugin:csharp-lsp:csharp-ls] Received request 'client/registerCapability - (2)'.
2026-01-22T20:28:36.167Z [DEBUG] [LSP PROTOCOL plugin:csharp-lsp:csharp-ls] Sending response 'client/registerCapability - (2)'. Processing request took 0ms
2026-01-22T20:28:36.180Z [DEBUG] [LSP SERVER plugin:csharp-lsp:csharp-ls] 15:28:36.180 warn: Initialization[0]
      handleInitialized: dynamic cap registration has failed with System.AggregateException: One or more errors occurred. (Unhandled method client/registerCapability)
       ---> StreamJsonRpc.RemoteMethodNotFoundException: Unhandled method client/registerCapability
         at StreamJsonRpc.JsonRpc.InvokeCoreAsync[TResult](RequestId id, String targetName, IReadOnlyList`1 arguments, IReadOnlyList`1 positionalArgumentDeclaredTypes, IReadOnlyDictionary`2 namedArgumentDeclaredTypes, CancellationToken cancellationToken, Boolean isParameterObject)
         --- End of inner exception stack trace ---
2026-01-22T20:28:36.184Z [DEBUG] [LSP PROTOCOL plugin:csharp-lsp:csharp-ls] Received request 'workspace/configuration - (3)'.
2026-01-22T20:28:36.184Z [DEBUG] LSP: Received workspace/configuration request from plugin:csharp-lsp:csharp-ls
2026-01-22T20:28:36.185Z [DEBUG] [LSP PROTOCOL plugin:csharp-lsp:csharp-ls] Sending response 'workspace/configuration - (3)'. Processing request took 1ms
2026-01-22T20:28:36.423Z [DEBUG] [LSP PROTOCOL plugin:csharp-lsp:csharp-ls] Received request 'window/workDoneProgress/create - (4)'.
2026-01-22T20:28:36.423Z [DEBUG] [LSP PROTOCOL plugin:csharp-lsp:csharp-ls] Sending response 'window/workDoneProgress/create - (4)'. Processing request took 0ms
2026-01-22T20:28:36.426Z [DEBUG] [LSP SERVER plugin:csharp-lsp:csharp-ls] 15:28:36.425 fail: serverEventLoop[0]
      serverEventLoop: crashed with System.AggregateException: One or more errors occurred. (Unhandled method window/workDoneProgress/create)
       ---> StreamJsonRpc.RemoteMethodNotFoundException: Unhandled method window/workDoneProgress/create
         at StreamJsonRpc.JsonRpc.InvokeCoreAsync[TResult](RequestId id, String targetName, IReadOnlyList`1 arguments, IReadOnlyList`1 positionalArgumentDeclaredTypes, IReadOnlyDictionary`2 namedArgumentDeclaredTypes, CancellationToken cancellationToken, Boolean isParameterObject)
         --- End of inner exception stack trace ---
      System.AggregateException: One or more errors occurred. (Unhandled method window/workDoneProgress/create)
       ---> StreamJsonRpc.RemoteMethodNotFoundException: Unhandled method window/workDoneProgress/create
         at StreamJsonRpc.JsonRpc.InvokeCoreAsync[TResult](RequestId id, String targetName, IReadOnlyList`1 arguments, IReadOnlyList`1 positionalArgumentDeclaredTypes, IReadOnlyDictionary`2 namedArgumentDeclaredTypes, CancellationToken cancellationToken, Boolean isParameterObject)
         --- End of inner exception stack trace ---

After that there are a bunch of additional errors but they're all Unhandled method window/workDoneProgress/create.

I wanted to get a better idea if this was on the csharp-ls side or the Claude Code client side before opening a ticket, so I asked Claude to try and diagnose the issue. I am not a F# expert, but wanted to leave it here just in case (at the risk of posting AI slop 😅).

I also noticed this note:

csharp-language-server/src/CSharpLanguageServer/Lsp/Client.fs

Line 12 in f4c1c9f

// TODO: Send notifications / requests to client only if client support it

---

Diagnosis from Claude:

Root Cause: The csharp-language-server crashes on startup when the LSP client (Claude Code's LSP plugin) doesn't
implement the window/workDoneProgress/create method.

Sequence of Events:

1. During workspace initialization, workspaceWithSolutionsLoaded calls progressReporter.Begin() (Workspace.fs:430)
2. ProgressReporter.Begin() calls client.WindowWorkDoneProgressCreate (ProgressReporter.fs:16)
3. WindowWorkDoneProgressCreate calls sendServerRequest.Send "window/workDoneProgress/create" (Client.fs:45)
4. When the client doesn't implement this method, StreamJsonRpc throws a RemoteMethodNotFoundException
5. The exception propagates up through the server event loop and crashes it (ServerState.fs:590-592)

Key Insight: The ProgressReporter is designed to gracefully handle when WindowWorkDoneProgressCreate returns an Error
result (lines 18-19 in ProgressReporter.fs set canReport <- false). However, the RemoteMethodNotFoundException is
thrown as an exception rather than returned as an Error result, bypassing this graceful handling.

The Fix: Modify WindowWorkDoneProgressCreate in Client.fs to catch RemoteMethodNotFoundException (and potentially
other RPC exceptions) and return an Error result instead of letting the exception propagate.

[HowardvanRooijen]

Excellent - you filed this before I did! But, yes, experiencing exactly the same error and diagnosed the same root cause.

[HowardvanRooijen]

Right, I think I've managed to fix the issue.

Firstly, I forked the code and got it running in a dev container. For me the build works, but there's 1 test failure. I got Claude Code to fix the issue (sorry, I'm no F# expert) - and the fix for that is in this PR:

endjin#2

I then got CC to investigate, fix and write a test for the claude csharp-lsp plugin. The PR for that is available here:

endjin#1

I created a new branch and merged in both of these changes. Got it all built and all tests passing. Installed it as a .NET Global tool, installed the CC csharp-lsp plugin and managed to get it working against a sample project.

The PR for this is here, and I've included a copy of the output from Claude Code after it successfully activated and explore the LSP:

endjin#3

If this looks acceptable to @razzmatazz - I'll happily issue PR 3 against this repo.

[razzmatazz]

IMHO claude went sideways here. The correct fix is to respect the corresponding client capability under 'window' (on the phone, can't remember which in particular) and not send progress reports when that is not signalled to be supported by the client

[HowardvanRooijen]

OK! Take 2: https://github.com/endjin/csharp-language-server/pull/3/files

[razzmatazz]

much better

[HowardvanRooijen]

OK! thanks! I'll issue a proper PR.

[Agasper]

Workaround: .NET-based LSP adapter

While waiting for the fix, I created a transparent proxy adapter that intercepts and handles these problematic LSP methods:

CSharpLspAdapter

Quick install

dotnet tool install --global CSharpLspAdapter

Or just ask Claude Code to set it up:

Install C# LSP support following https://github.com/Agasper/CSharpLspAdapter

How it works

Claude Code  ←→  csharp-ls-adapter  ←→  csharp-ls
                      ↓
               Intercepts and handles:
               - workspace/configuration
               - client/registerCapability  
               - window/workDoneProgress/create

All LSP operations work: documentSymbol, hover, goToDefinition, findReferences, etc.

[razzmatazz]

Closing as 0.22.0 has been released.