diff --git a/CHANGELOG.md b/CHANGELOG.md
index f470d52d18..ce9f0a218c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Changelog
 
+## [Unreleased]
+
+### Changed
+* Update FCS to 'Prototype signature files for lex files', commit 5b74995780a0e9d24a5db025e68b31e6d910f71f
+
 ## [6.0.3] - 2023-05-14
 
 ### Fixed
diff --git a/Directory.Build.props b/Directory.Build.props
index eb91c9c62f..fca503b1ae 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -39,7 +39,7 @@ Some common use cases include:
     
     <!-- Versions -->
     <PropertyGroup>
-        <FCSCommitHash>e267bb9f8d590feed1b94b469d78cfce61afecad</FCSCommitHash>
+        <FCSCommitHash>5b74995780a0e9d24a5db025e68b31e6d910f71f</FCSCommitHash>
         <StreamJsonRpcVersion>2.8.28</StreamJsonRpcVersion>
         <FSharpCoreVersion>6.0.1</FSharpCoreVersion>
     </PropertyGroup>
diff --git a/src/Fantomas.FCS/Fantomas.FCS.fsproj b/src/Fantomas.FCS/Fantomas.FCS.fsproj
index 4005c8a660..75131bca54 100644
--- a/src/Fantomas.FCS/Fantomas.FCS.fsproj
+++ b/src/Fantomas.FCS/Fantomas.FCS.fsproj
@@ -174,7 +174,7 @@
       <Link>Facilities\prim-parsing.fs</Link>
     </Compile>
     <FsLex Include="..\..\.deps\$(FCSCommitHash)\src\Compiler\AbstractIL\illex.fsl">
-      <OtherFlags>--unicode --lexlib Internal.Utilities.Text.Lexing</OtherFlags>
+      <OtherFlags>--module FSharp.Compiler.AbstractIL.AsciiLexer --internal --open Internal.Utilities.Text.Lexing --open FSharp.Compiler.AbstractIL.AsciiParser --unicode --lexlib Internal.Utilities.Text.Lexing</OtherFlags>
       <Link>AbstractIL\illex.fsl</Link>
     </FsLex>
     <None Include="..\..\.deps\$(FCSCommitHash)\src\Compiler\AbstractIL\illex.fsl">
@@ -211,6 +211,9 @@
     <Compile Include="$(FsYaccOutputFolder)ilpars.fs">
       <Link>AbstractIL\FsYaccOut\ilpars.fs</Link>
     </Compile>
+    <Compile Include="$(FsLexOutputFolder)illex.fsi">
+      <Link>AbstractIL\FsLexOut\illex.fsi</Link>
+    </Compile>
     <Compile Include="$(FsLexOutputFolder)illex.fs">
       <Link>AbstractIL\FsLexOut\illex.fs</Link>
     </Compile>
@@ -221,7 +224,7 @@
       <Link>SyntaxTree\PrettyNaming.fs</Link>
     </Compile>
     <FsLex Include="..\..\.deps\$(FCSCommitHash)\src\Compiler\pplex.fsl">
-      <OtherFlags>--unicode --lexlib Internal.Utilities.Text.Lexing</OtherFlags>
+      <OtherFlags>--module FSharp.Compiler.PPLexer --internal --open FSharp.Compiler.Lexhelp --open Internal.Utilities.Text.Lexing --open FSharp.Compiler.PPParser --unicode --lexlib Internal.Utilities.Text.Lexing</OtherFlags>
       <Link>SyntaxTree\pplex.fsl</Link>
     </FsLex>
     <FsYacc Include="..\..\.deps\$(FCSCommitHash)\src\Compiler\pppars.fsy">
@@ -229,7 +232,7 @@
       <Link>SyntaxTree\pppars.fsy</Link>
     </FsYacc>
     <FsLex Include="..\..\.deps\$(FCSCommitHash)\src\Compiler\lex.fsl">
-      <OtherFlags>--unicode --lexlib Internal.Utilities.Text.Lexing</OtherFlags>
+      <OtherFlags>--module FSharp.Compiler.Lexer --open FSharp.Compiler.Lexhelp --open Internal.Utilities.Text.Lexing --open FSharp.Compiler.Parser --open FSharp.Compiler.Text --open FSharp.Compiler.ParseHelpers --internal --unicode --lexlib Internal.Utilities.Text.Lexing</OtherFlags>
       <Link>SyntaxTree\lex.fsl</Link>
     </FsLex>
     <FsYacc Include="..\..\.deps\$(FCSCommitHash)\src\Compiler\pars.fsy">
@@ -302,9 +305,15 @@
     <Compile Include="..\..\.deps\$(FCSCommitHash)\src\Compiler\SyntaxTree\LexHelpers.fs">
       <Link>SyntaxTree\LexHelpers.fs</Link>
     </Compile>
+    <Compile Include="$(FsYaccOutputFolder)pplex.fsi">
+      <Link>SyntaxTree\FsLexOutput\pplex.fsi</Link>
+    </Compile>
     <Compile Include="$(FsYaccOutputFolder)pplex.fs">
       <Link>SyntaxTree\FsLexOutput\pplex.fs</Link>
     </Compile>
+    <Compile Include="$(FsYaccOutputFolder)\lex.fsi">
+      <Link>SyntaxTree\FsLexOutput\lex.fsi</Link>
+    </Compile>
     <Compile Include="$(FsYaccOutputFolder)\lex.fs">
       <Link>SyntaxTree\FsLexOutput\lex.fs</Link>
     </Compile>
@@ -322,7 +331,7 @@
     <PackageReference Include="System.Diagnostics.DiagnosticSource" Version="7.0.0" />
     <PackageReference Include="System.Memory" Version="4.5.5" />
     <PackageReference Include="System.Runtime" Version="4.3.1" />
-    <PackageReference Include="FsLexYacc" Version="11.1.0" PrivateAssets="all" />
+    <PackageReference Include="FsLexYacc" Version="11.2.0" PrivateAssets="all" />
   </ItemGroup>
 
   <Target Name="AcquireCompilerFiles" Condition="!Exists('../../.deps/$(FCSCommitHash)')" BeforeTargets="CollectPackageReferences">
diff --git a/src/Fantomas.FCS/packages.lock.json b/src/Fantomas.FCS/packages.lock.json
index 9b730c85df..26e990f7a2 100644
--- a/src/Fantomas.FCS/packages.lock.json
+++ b/src/Fantomas.FCS/packages.lock.json
@@ -22,12 +22,12 @@
       },
       "FsLexYacc": {
         "type": "Direct",
-        "requested": "[11.1.0, )",
-        "resolved": "11.1.0",
-        "contentHash": "JoGbIVZVuJs4CsCED0Wi7Winhf6isw0qrblm4hNAdpaZX3Sp5EmPZa07Nhv6SVmoDNPP/HfjCBVtFbaxyzOD2w==",
+        "requested": "[11.2.0, )",
+        "resolved": "11.2.0",
+        "contentHash": "YfCWpEr1z34mtfBLINYLndmsUTLgADnwY1j1WibS1EbuGVVX9r6pkdqmN8/vWqBi5qvY/s7pBjdq51Ks1Dk5MQ==",
         "dependencies": {
           "FSharp.Core": "4.6.2",
-          "FsLexYacc.Runtime": "11.1.0"
+          "FsLexYacc.Runtime": "11.2.0"
         }
       },
       "Ionide.KeepAChangelog.Tasks": {
@@ -78,8 +78,8 @@
       },
       "FsLexYacc.Runtime": {
         "type": "Transitive",
-        "resolved": "11.1.0",
-        "contentHash": "NtCnm45VhoJKyz2Ie8OzSSUgrUYQ0TKp2LbgEBeEOr7j6+/0om3lf7NmmoDZDjuNx/KRXb95ZMDjGZRZD92ZfA==",
+        "resolved": "11.2.0",
+        "contentHash": "psv1xPIm2NOEdd1RfaBSnN7y6M7XRS69QYB2kP2hD8LKaepGIGM5qAC7xHNpqO1v7QfyMFmuZ5/KqyWhfAfKJg==",
         "dependencies": {
           "FSharp.Core": "4.6.2"
         }