diff --git a/.devcontainer/devcontainer.json b/.devcontainer/devcontainer.json
index ac63d29017f..2600981e3c5 100644
--- a/.devcontainer/devcontainer.json
+++ b/.devcontainer/devcontainer.json
@@ -1,12 +1,12 @@
 // For format details, see https://aka.ms/vscode-remote/devcontainer.json or this file's README at:
 {
     "name": "F#",
-    "image": "mcr.microsoft.com/dotnet/sdk:9.0.100-preview.7",
+    "image": "mcr.microsoft.com/dotnet/sdk:9.0.100-rc.1",
     "features": {
-        "ghcr.io/devcontainers/features/common-utils:2.4.6": {},
+        "ghcr.io/devcontainers/features/common-utils:2.5.1": {},
         "ghcr.io/devcontainers/features/git:1.3.2": {},
         "ghcr.io/devcontainers/features/github-cli:1.0.13": {},
-        "ghcr.io/devcontainers/features/dotnet:2.1.2": {}
+        "ghcr.io/devcontainers/features/dotnet:2.1.3": {}
     },
     "hostRequirements": {
         "cpus": 2,
diff --git a/Directory.Build.props b/Directory.Build.props
index 573faa9eb9e..719838f0a0d 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -46,7 +46,10 @@
 
   <PropertyGroup Condition="'$(BUILDING_USING_DOTNET)' == 'true'">
     <DisableAutoSetFscCompilerPath>false</DisableAutoSetFscCompilerPath>
-    <FSHARPCORE_USE_PACKAGE Condition="'$(FSHARPCORE_USE_PACKAGE)' == ''">true</FSHARPCORE_USE_PACKAGE>
+
+    <!-- TODO(vlza): This probably should be `true` once fslib with nullness ships, since shipped library is preferred by default when building this solution. -->
+    <FSHARPCORE_USE_PACKAGE Condition="'$(FSHARPCORE_USE_PACKAGE)' == ''">false</FSHARPCORE_USE_PACKAGE>
+
     <DISABLE_ARCADE Condition="'$(DISABLE_ARCADE)' == ''">true</DISABLE_ARCADE>
     <ArtifactsDir>$(MSBuildThisFileDirectory)artifacts/</ArtifactsDir>
     <OutputPath>$(ArtifactsDir)/bin/$(MSBuildProjectName)/$(Configuration)/</OutputPath>
@@ -57,6 +60,7 @@
     <FsYaccPath>$(ArtifactsDir)/bin/fsyacc/$(Configuration)/$(FSharpNetCoreProductDefaultTargetFramework)/$(RuntimeIdentifier)/fsyacc.dll</FsYaccPath>
     <DefineConstants>NO_CHECKNULLS;BUILDING_WITH_LKG;NO_NULLCHECKING_LIB_SUPPORT;$(DefineConstants)</DefineConstants>
   </PropertyGroup>
+
   <Import Project="$(MSBuildThisFileDirectory)/eng/Versions.props" Condition="'$(DISABLE_ARCADE)' == 'true'" />
 
   <Import Project="FSharpBuild.Directory.Build.props" Condition=" '$(FSharpTestCompilerVersion)' == '' " />
diff --git a/Directory.Build.targets b/Directory.Build.targets
index 8aed019a00f..52109a82ee6 100644
--- a/Directory.Build.targets
+++ b/Directory.Build.targets
@@ -52,4 +52,29 @@
       StopOnFirstFailure="True" />
   </Target>
 
+  <!-- BEGIN workaround for https://github.com/dotnet/sdk/issues/43339; remove after updated to VS 17.12, future 17.11 patch or when arcade ships the the same workaround -->
+  <Target Name="WorkaroundDotnetSdk43339" BeforeTargets="ResolvePackageAssets" Condition=" '$(MSBuildRuntimeType)' == 'Full' and $([MSBuild]::VersionLessThan($(MSBuildVersion), 17.12.0))">
+    <PrimeSystemTextJson804 />
+  </Target>
+  <UsingTask
+    TaskName="PrimeSystemTextJson804"
+    TaskFactory="RoslynCodeTaskFactory"
+    AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll" >
+    <Task>
+      <Code Type="Fragment" Language="cs">
+<![CDATA[
+try
+{
+    System.Reflection.Assembly.LoadFrom(@"$(MicrosoftNETBuildTasksDirectoryRoot)\..\..\..\DotnetTools\dotnet-format\BuildHost-net472\System.Text.Json.dll");
+}
+catch
+{
+    // Best effort: if something moves in the SDK don't break the build.
+}
+]]>
+      </Code>
+    </Task>
+  </UsingTask>
+  <!-- END workaround for https://github.com/dotnet/sdk/issues/43339 -->
+
 </Project>
diff --git a/FSharp.Profiles.props b/FSharp.Profiles.props
index 811ee0d0e40..1d9eafa8c2b 100644
--- a/FSharp.Profiles.props
+++ b/FSharp.Profiles.props
@@ -22,6 +22,10 @@
     <OtherFlags>$(OtherFlags) /langversion:preview</OtherFlags>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(BUILDING_USING_DOTNET)' == 'true'"> 
+      <CheckNulls>false</CheckNulls>
+  </PropertyGroup>
+
   <PropertyGroup Condition="'$(CheckNulls)' == 'true'">
     <Nullable>enable</Nullable>
   </PropertyGroup>
diff --git a/global.json b/global.json
index f559dbefc56..bce954435c7 100644
--- a/global.json
+++ b/global.json
@@ -1,10 +1,10 @@
 {
   "sdk": {
-    "version": "9.0.100-preview.7.24407.12",
+    "version": "9.0.100-rc.1.24452.12",
     "allowPrerelease": true
   },
   "tools": {
-    "dotnet": "9.0.100-preview.7.24407.12",
+    "dotnet": "9.0.100-rc.1.24452.12",
     "vs": {
       "version": "17.8",
       "components": [
diff --git a/tests/AheadOfTime/Trimming/check.ps1 b/tests/AheadOfTime/Trimming/check.ps1
index 0f37c11cf31..f375accd624 100644
--- a/tests/AheadOfTime/Trimming/check.ps1
+++ b/tests/AheadOfTime/Trimming/check.ps1
@@ -43,7 +43,7 @@ function CheckTrim($root, $tfm, $outputfile, $expected_len) {
 # error NETSDK1124: Trimming assemblies requires .NET Core 3.0 or higher.
 
 # Check net7.0 trimmed assemblies
-CheckTrim -root "SelfContained_Trimming_Test" -tfm "net9.0" -outputfile "FSharp.Core.dll" -expected_len 287232
+CheckTrim -root "SelfContained_Trimming_Test" -tfm "net9.0" -outputfile "FSharp.Core.dll" -expected_len 297984
 
 # Check net8.0 trimmed assemblies
-CheckTrim -root "StaticLinkedFSharpCore_Trimming_Test" -tfm "net9.0" -outputfile "StaticLinkedFSharpCore_Trimming_Test.dll" -expected_len 8821248
+CheckTrim -root "StaticLinkedFSharpCore_Trimming_Test" -tfm "net9.0" -outputfile "StaticLinkedFSharpCore_Trimming_Test.dll" -expected_len 9148416
diff --git a/tests/FSharp.Compiler.ComponentTests/Interop/ByrefTests.fs b/tests/FSharp.Compiler.ComponentTests/Interop/ByrefTests.fs
index db89445122b..e5a278a4a85 100644
--- a/tests/FSharp.Compiler.ComponentTests/Interop/ByrefTests.fs
+++ b/tests/FSharp.Compiler.ComponentTests/Interop/ByrefTests.fs
@@ -115,7 +115,7 @@ open System.Collections.Generic
 let myDict = ["x",1;"xyz",2] |> dict |> Dictionary 
 
 let checkIfPresent (input:ReadOnlySpan<char>) = 
-    let altLookup = myDict.GetAlternateLookup<string,int,ReadOnlySpan<char>>()
+    let altLookup = myDict.GetAlternateLookup<ReadOnlySpan<char>>()
     let present = altLookup.ContainsKey(input)
     for c in input do
         printf "%c" c
@@ -134,7 +134,7 @@ let main _args =
         |> shouldSucceed
         |> verifyOutputContains [|": false";"x: true";"xyz: true"|]
         |> verifyIL 
-            ["call       valuetype [System.Collections]System.Collections.Generic.Dictionary`2/AlternateLookup`1<!!0,!!1,!!2> [System.Collections]System.Collections.Generic.CollectionExtensions::GetAlternateLookup<string,int32,valuetype [runtime]System.ReadOnlySpan`1<char>>(class [System.Collections]System.Collections.Generic.Dictionary`2<!!0,!!1>)"]
+            ["callvirt   instance valuetype [System.Collections]System.Collections.Generic.Dictionary`2/AlternateLookup`1<!0,!1,!!0> class [System.Collections]System.Collections.Generic.Dictionary`2<string,int32>::GetAlternateLookup<valuetype [runtime]System.ReadOnlySpan`1<char>>()"]
 
     [<FactForNETCOREAPP>]
     let ``Ref structs in generics - GetAlternateLookup`` () =
@@ -144,7 +144,7 @@ open System.Collections.Generic
 
 let main _args = 
     let myDict = ["x",1;"y",2] |> dict |> Dictionary 
-    let altLookup = myDict.GetAlternateLookup<string,int,ReadOnlySpan<char>>()
+    let altLookup = myDict.GetAlternateLookup<ReadOnlySpan<char>>()
     altLookup.ContainsKey(ReadOnlySpan([|'x'|]))
         """
         |> withLangVersionPreview