[FEATURE]: add roslyn-language-server as opt-in C# LSP alternative to csharp-ls

[jmbryan4]

Feature request

Is your feature request related to a problem?

The current C# LSP support uses csharp-ls, a community-maintained implementation. Microsoft now publishes an official C# LSP based on Roslyn — roslyn-language-server — as a .NET global tool on NuGet. This is the same server that powers the C# extension for VS Code and C# Dev Kit.

Describe the solution you'd like

Add roslyn-language-server as a built-in LSP option alongside csharp-ls, selectable via the OPENCODE_EXPERIMENTAL_LSP_ROSLYN=1 env flag (same pattern as OPENCODE_EXPERIMENTAL_LSP_TY for pyright vs ty).

• New server ID: csharp-roslyn
• Default: disabled (existing csharp-ls behaviour unchanged)
• Opt-in: set OPENCODE_EXPERIMENTAL_LSP_ROSLYN=1 to switch — this disables csharp-ls and enables roslyn-language-server
• Auto-installs via dotnet tool install --global roslyn-language-server --prerelease into opencode's bin dir if not found on PATH
• Spawns with --stdio --autoLoadProjects

.NET global tool on NuGet: https://www.nuget.org/packages/roslyn-language-server/

• I have verified this feature I'm about to request hasn't been suggested before

PR #14463

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• The title must start with [FEATURE]: (e.g. [FEATURE]: add roslyn-language-server as opt-in C# LSP alternative to csharp-ls)
• The feature request template requires checking the verification checkbox: "I have verified this feature I'm about to request hasn't been suggested before."

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

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[github-actions]

Thanks for updating the issue! It now meets the contributing guidelines and looks good. We appreciate the detailed description of the problem and the proposed solution.

[github-actions]

Thanks for updating the title and filling in the description — those look great! However, this issue is still missing one required element from the feature request template:

• Verification checkbox: Please add and check the box confirming you've verified this feature hasn't been requested before: - [x] I have verified this feature I'm about to request hasn't been suggested before.

Please edit the issue body to include this. Once that's done, the issue will be fully compliant.

[github-actions]

Thanks for updating the title — it now correctly starts with [FEATURE]: and the description is clear and detailed.

However, the issue is still missing one required element from the feature request template:

• Verification checkbox: Please edit the issue body to add and check the box confirming you've verified this feature hasn't been requested before:
  - [x] I have verified this feature I'm about to request hasn't been suggested before.

Once that's added, the issue will be fully compliant.

[github-actions]

Thanks for updating the issue! It now follows the contributing guidelines and feature request template. The needs:compliance label has been removed.

[github-actions]

Thanks for updating the issue — it now fully meets the contributing guidelines. The needs:compliance label has been removed.

[sebastianstudniczek]

Great idea, roslyn is already a standard choice in neovim/zed so would be nice to be able to use it as well with opencode!