GitHub link: License: Apache 2.0

The FsAutoComplete project (FSAC) provides a backend service for rich editing or intellisense features for editors. It can be hosted using the Language Server Protocol.

Currently it is used by:

It's based on:

Required software

FsAutoComplete runs on modern .NET (formerly .NET Core).

  • .NET SDK (Required: >= 6.0)

Communication protocol

FsAutoComplete supports LSP as a communication protocol.

Supported LSP endpoints

  • initialize
  • textDocument/didOpen
  • textDocument/didChange
  • textDocument/didSave
  • textDocument/hover
  • textDocument/completion & completionItem/resolve
  • textDocument/rename
  • textDocument/definition
  • textDocument/typeDefinition
  • textDocument/implementation
  • textDocument/codeAction:
    • Remove unused open
    • Resolve namespace/module
    • Replace unused symbol with _
    • Fix typo based on error message
    • Remove redundant qualifier
    • Add missing new keyword for IDisposable
    • Generate cases for all DU case in pattern matching
    • Generate empty interface implementation
    • Fixes suggested by FSharpLint
  • textDocument/codeLens & codeLens/resolve:
    • signature Code Lenses
    • reference number Code Lenses
  • textDocument/formatting - powered by fantomas
  • textDocument/references
  • textDocument/documentHighlight
  • textDocument/signatureHelp
  • textDocument/documentSymbol
  • workspace/didChangeWatchedFiles
  • workspace/didChangeConfiguration
  • workspace/symbol

Custom endpoints

Custom endpoints are using (for messages body) PlainNotification type and string format serialized with exactly same serialization format as old JSON protocol

  • fsharp/signature - accepts TextDocumentPositionParams, returns signature of symbol at given position as a formatted string
  • fsharp/signatureData - accepts TextDocumentPositionParams, returns signature of symbol at given position as DTO
  • fsharp/lineLens - accepts ProjectParms (Project filed contain F# file path), returns locations where LineLenses should be displayed
  • fsharp/compilerLocation - no input, returns paths to FCS, FSI and MsBuild
  • fsharp/compile - accepts ProjectParms, tries to compile project, returns list of errors and exit status code
  • fsharp/workspacePeek - accepts WorkspacePeekRequest, returns list of possible workspaces (resolved solution files, or list of projects if there are no solution files)
  • fsharp/workspaceLoad - accepts WorkspaceLoadParms, loads given list of projects in the background, partial result notified by fsharp/notifyWorkspace notification
  • fsharp/project - accepts ProjectParms, loads given project
  • fsharp/fsdn - accepts ProjectParms (Project filed contain query string), query FSDN and returns list of functions
  • fsharp/f1Help - accepts TextDocumentPositionParams, returns URL to MSDN documentation for symbol at given position
  • fsharp/documentation - accepts TextDocumentPositionParams, returns documentation data about symbol at given position, used for InfoPanel
  • fsharp/documentationSymbol - accepts DocumentationForSymbolReuqest, returns documentation data about given symbol from given assembly, used for InfoPanel

Supported LSP notifications

  • window/showMessage
  • window/logMessage
  • textDocument/publishDiagnostics

Custom notifications

  • fsharp/notifyWorkspace - notification for workspace/solution/project loading events
  • fsharp/notifyWorkspacePeek - notification for initial workspace peek

Additional startup options

  • --background-service-enabled - passing this flag enables background service feature, increasing FSAC responsiveness by moving some of the operations (especially background type checking) to other process. It results in increased memory usage. Used by default in Ionide.
  • --verbose - passing this flag enables additional logging being printed out in stderr
  • DOTNET_ROOT - setting this environment variable will set the dotnet SDK root, which is used when finding references for FSX scripts.

Initialization options

Options that should be send as initializationOptions as part of initialize request.

  • AutomaticWorkspaceInit - setting it to true will start Workspace Loading without need to run fsharp/workspacePeek and fsharp/workspaceLoad commands. It will always choose top workspace from the found list - all projects in workspace if 0 .sln files are found, .sln file if 1 .sln file was found, .sln file with most projects if multiple .sln files were found. It's designed to be used in clients that doesn't allow to create custom UI for selecting workspaces.


  • FSharp.keywordsAutocomplete - provides keywords in autocomplete list, recommended default value: true
  • FSharp.ExternalAutocomplete - provides autocomplete for symbols from not opened namespaces/modules, insert open on accept, recommended default value: false
  • FSharp.Linter - enables FSharpLint integration, provides additional warnings and code action fixes, recommended default value: true
  • FSharp.UnionCaseStubGeneration - enables code action to generate pattern matching cases, recommended default value: true
  • FSharp.UnionCaseStubGenerationBody - defines dummy body used by pattern matching generator, recommended default value: "failwith \"Not Implemented\""
  • FSharp.RecordStubGeneration - enables code action to generate record stub, recommended default value: true
  • FSharp.RecordStubGenerationBody - defines dummy body used by record stub generator, recommended default value: "failwith \"Not Implemented\""
  • FSharp.InterfaceStubGeneration - enables code action to generate interface stub, recommended default value: true
  • FSharp.InterfaceStubGenerationObjectIdentifier - defines object identifier used by interface stub generator,recommended default value: "this"
  • FSharp.InterfaceStubGenerationMethodBody - defines dummy body used by interface stub generator, recommended default value: "failwith \"Not Implemented\""
  • FSharp.UnusedOpensAnalyzer - enables unused open detections, recommended default value: true
  • FSharp.UnusedDeclarationsAnalyzer - enables unused symbol detection, recommended default value: true
  • FSharp.UseSdkScripts - enables the use of .NET (Core) SDKs for script file type-checking and evaluation, otherwise the .NET Framework reference lists will be used. Current default and recommended value: true
  • FSharp.SimplifyNameAnalyzer - enables simplify name analyzer and remove redundant qualifier quick fix, recommended default value: false
  • FSharp.ResolveNamespaces - enables resolve namespace quick fix (add open if symbol is from not yet opened module/namespace), recommended default value: true
  • FSharp.EnableReferenceCodeLens - enables reference count code lenses, recommended default value: true if --background-service-enabled is used by default, false otherwise
  • FSharp.dotNetRoot - sets the root path for finding dotnet SDK references. Primarily used for FSX Scripts. Default value: operating-system dependent. On windows, C:\Program Files\dotnet; on Unix, /usr/local/share/dotnet
  • FSharp.fsiExtraParameters - an array of additional runtime arguments that are passed to FSI. These are used when typechecking scripts to ensure that typechecking has the same context as your FSI instances. An example would be to set the following parameters to enable Preview features (like opening static classes) for typechecking.