Usage

Ask Codex to use the roslyn Model Context Protocol (MCP) server. It exposes one tool: lsp.

Example usage:

Use Roslyn to report diagnostics for src/Calculator.cs, find its implementations,
and preview a rename. Do not apply changes.

The bridge reads saved files and loads the project independently of your editor. Available fixes and refactorings depend on the installed Roslyn version and project.

Operations

Action	Inputs	Result
diagnostics	file path or glob	Errors, warnings and suggestions
definition, type_definition, implementation, references, hover	file, line, character	Locations or symbol information
symbols	file or query	Document or workspace symbols
rename	file, line, character, newName	Symbol rename preview
rename_file	file, newName	File rename preview, if supported
code_actions	file, line, character	Fixes and refactorings
status, capabilities, reload	None	Session state, supported features or a fresh session
request	method, parameters	Raw Language Server Protocol (LSP) result

Pass arguments inside request:

{
  "request": {
    "action": "definition",
    "file": "src/Calculator.cs",
    "line": 12,
    "character": 9
  }
}

Paths are relative to the workspace. Input coordinates start at 1, with characters counted as UTF-16 code units. Returned LSP locations and raw request coordinates start at 0.

For diagnostics, omit file to scan C# files or use a glob such as src/**/*.cs. limit defaults to 100 and accepts 1 to 1000. Narrow the query if results are truncated or too large.

Preview and apply

Renames return an edit preview and a proposalId. Apply that preview with the same action:

{
  "request": {
    "action": "rename",
    "proposalId": "id-from-preview",
    "apply": true
  }
}

For code actions, request the listing, then send its proposalId and zero-based actionIndex to resolve a preview. Apply the preview with its new proposalId. Set endLine and endCharacter when an action needs a selection. Some Roslyn commands reveal their edits only when executed.

Proposals expire after five minutes. If source files change, stale_edit requires a new preview. Reloading also discards proposals. Applied file creation, deletion and renaming reload Roslyn before the next request.

Limits

Edits must stay inside the workspace. Symbolic links below its root are rejected. Each file replacement is atomic, but a failed operation across several files can leave partial changes.

Diagnostics scans cover at most 1000 files. An edit can affect up to 256 files, with 8 MiB per file and 64 MiB in total. The source snapshot used to validate edits is limited to 256 MiB.

Raw requests reserve lifecycle and document synchronization methods for the bridge. Unknown methods and workspace/executeCommand require apply: true because they may write files. Use reload if results remain stale after an external change.