Got one example working.

src/FsAutoComplete/CodeFixes/UpdateTypeAbbreviationInSignatureFile.fs

@@ -0,0 +1,89 @@
+module FsAutoComplete.CodeFix.UpdateTypeAbbreviationInSignatureFile
+
+open FSharp.Compiler.Symbols
+open FSharp.Compiler.Syntax
+open FSharp.Compiler.Text
+open FsToolkit.ErrorHandling
+open Ionide.LanguageServerProtocol.Types
+open FsAutoComplete.CodeFix.Types
+open FsAutoComplete
+open FsAutoComplete.LspHelpers
+
+// TODO: add proper title for code fix
+let title = "UpdateTypeAbbreviationInSignatureFile Codefix"
+
+let fix (getParseResultsForFile: GetParseResultsForFile) : CodeFix =
+  Run.ifDiagnosticByCode (Set.ofList [ "318" ]) (fun diagnostic codeActionParams ->
+    asyncResult {
+      let implFilePath = codeActionParams.TextDocument.GetFilePath()
+      let implFileName = Utils.normalizePath implFilePath
+
+      let! (implParseAndCheckResults: ParseAndCheckResults, _implLine: string, implSourceText: IFSACSourceText) =
+        getParseResultsForFile implFileName (protocolPosToPos diagnostic.Range.Start)
+
+      let mDiag =
+        protocolRangeToRange implParseAndCheckResults.GetParseResults.FileName diagnostic.Range
+
+      let implTypeName =
+        (mDiag.Start, implParseAndCheckResults.GetParseResults.ParseTree)
+        ||> ParsedInput.tryPick (fun _ node ->
+          match node with
+          | SyntaxNode.SynTypeDefn(SynTypeDefn(
+              typeInfo = SynComponentInfo(longId = [ typeIdent ])
+              typeRepr = SynTypeDefnRepr.Simple(simpleRepr = SynTypeDefnSimpleRepr.TypeAbbrev _; range = mBody))) when
+            Range.equals typeIdent.idRange mDiag
+            ->
+            Some(typeIdent, mBody)
+          | _ -> None)
+
+      match implTypeName with
+      | None -> return []
+      | Some(typeName, mImplBody) ->
+        match implParseAndCheckResults.TryGetSymbolUseFromIdent implSourceText typeName with
+        | None -> return []
+        | Some typeSymbolUse ->
+          match typeSymbolUse.Symbol with
+          | :? FSharpEntity as entity ->
+            match entity.SignatureLocation with
+            | None -> return []
+            | Some signatureLocation ->
+              if not (Utils.isSignatureFile signatureLocation.FileName) then
+                // A little weird
+                return []
+              else
+                let sigFilePath = $"%s{implFilePath}i"
+                let sigFileName = Utils.normalizePath sigFilePath
+
+                let sigTextDocumentIdentifier: TextDocumentIdentifier =
+                  { Uri = $"%s{codeActionParams.TextDocument.Uri}i" }
+
+                let! (sigParseAndCheckResults: ParseAndCheckResults, _sigLine: string, _sigSourceText: IFSACSourceText) =
+                  getParseResultsForFile sigFileName (Position.mkPos 1 0)
+
+                let mSigTypeAbbrev =
+                  (signatureLocation.Start, sigParseAndCheckResults.GetParseResults.ParseTree)
+                  ||> ParsedInput.tryPick (fun _path node ->
+                    match node with
+                    | SyntaxNode.SynTypeDefnSig(SynTypeDefnSig(
+                        typeInfo = SynComponentInfo(longId = [ typeIdent ])
+                        typeRepr = SynTypeDefnSigRepr.Simple(repr = SynTypeDefnSimpleRepr.TypeAbbrev _; range = m))) when
+                      Range.equals typeIdent.idRange signatureLocation
+                      ->
+                      Some m
+                    | _ -> None)
+
+                match mSigTypeAbbrev with
+                | None -> return []
+                | Some mSigTypeAbbrev ->
+                  let newText = implSourceText.GetSubTextFromRange mImplBody
+
+                  return
+                    [ { SourceDiagnostic = None
+                        Title = title
+                        File = sigTextDocumentIdentifier
+                        Edits =
+                          [| { Range = fcsRangeToLsp mSigTypeAbbrev
+                               NewText = newText } |]
+                        Kind = FixKind.Fix } ]
+          | _ -> return []
+    })

src/FsAutoComplete/CodeFixes/UpdateTypeAbbreviationInSignatureFile.fsi

@@ -0,0 +1,6 @@
+module FsAutoComplete.CodeFix.UpdateTypeAbbreviationInSignatureFile
+
+open FsAutoComplete.CodeFix.Types
+
+val title: string
+val fix: getParseResultsForFile: GetParseResultsForFile -> CodeFix

src/FsAutoComplete/LspServers/AdaptiveServerState.fs

@@ -1903,7 +1903,8 @@ type AdaptiveState(lspClient: FSharpLspClient, sourceTextFactory: ISourceTextFac
          AdjustConstant.fix tryGetParseAndCheckResultsForFile
          UpdateValueInSignatureFile.fix tryGetParseAndCheckResultsForFile
          RemoveUnnecessaryParentheses.fix forceFindSourceText
-         AddTypeAliasToSignatureFile.fix forceGetFSharpProjectOptions tryGetParseAndCheckResultsForFile |])
+         AddTypeAliasToSignatureFile.fix forceGetFSharpProjectOptions tryGetParseAndCheckResultsForFile
+         UpdateTypeAbbreviationInSignatureFile.fix tryGetParseAndCheckResultsForFile |])
 
   let forgetDocument (uri: DocumentUri) =
     async {

test/FsAutoComplete.Tests.Lsp/CodeFixTests/Tests.fs

@@ -3433,4 +3433,5 @@ let tests textFactory state =
       removePatternArgumentTests state
       UpdateValueInSignatureFileTests.tests state
       removeUnnecessaryParenthesesTests state
-      AddTypeAliasToSignatureFileTests.tests state ]
+      AddTypeAliasToSignatureFileTests.tests state
+      UpdateTypeAbbreviationInSignatureFileTests.tests state ]

test/FsAutoComplete.Tests.Lsp/CodeFixTests/UpdateTypeAbbreviationInSignatureFileTests.fs

@@ -0,0 +1,20 @@
+module private FsAutoComplete.Tests.CodeFixTests.UpdateTypeAbbreviationInSignatureFileTests
+
+open Expecto
+open Helpers
+open Utils.ServerTests
+open Utils.CursorbasedTests
+open FsAutoComplete.CodeFix
+
+let tests state =
+  serverTestList (nameof UpdateTypeAbbreviationInSignatureFile) state defaultConfigDto None (fun server ->
+    [ let selectCodeFix = CodeFix.withTitle UpdateTypeAbbreviationInSignatureFile.title
+
+      ftestCaseAsync "first unit test for UpdateTypeAbbreviationInSignatureFile"
+      <| CodeFix.check
+        server
+        "let a$0 b c = ()"
+        Diagnostics.acceptAll
+        selectCodeFix
+        "let Text replaced by UpdateTypeAbbreviationInSignatureFile b c = ()"
+    ])