VSTest drops Newtonsoft.Json in .NET 11 Preview 4 and what breaks if you relied on it transitively

The .NET team announced on April 29 that VSTest, the engine behind dotnet test and Visual Studio’s Test Explorer, is finally cutting its dependency on Newtonsoft.Json. The change lands in .NET 11 Preview 4 (planned May 12, 2026) and Visual Studio 18.8 Insiders 1 (planned June 9, 2026). On .NET, VSTest switches its internal serializer to System.Text.Json. On .NET Framework, where System.Text.Json is too heavy a payload, it uses a small library called JSONite. The work is tracked in microsoft/vstest#15540 and the SDK breaking change in dotnet/docs#53174.

Most projects do not need to do anything

If your test project already declares Newtonsoft.Json with a normal PackageReference, nothing changes. The package keeps working, and any code that uses JObject, JToken, or the JsonConvert static keeps compiling. The single public type VSTest used to expose, Newtonsoft.Json.Linq.JToken, lived on one spot of the VSTest communication protocol, and the team’s own assessment is that essentially no real-world consumers depend on that surface.

Where it actually breaks

The interesting failure mode is the project that never asked for Newtonsoft.Json and got it anyway, because VSTest dragged the assembly along. Once Preview 4 cuts the transitive flow, that copy disappears at runtime and you see a FileNotFoundException for Newtonsoft.Json during the test run. The fix is one line in the .csproj:

<ItemGroup>
  <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
</ItemGroup>

The second flavour is projects that explicitly excluded the runtime asset of a transitive Newtonsoft.Json, usually to keep deployment payloads small:

<PackageReference Include="Newtonsoft.Json" Version="13.0.3">
  <ExcludeAssets>runtime</ExcludeAssets>
</PackageReference>

That used to work because VSTest itself shipped the runtime DLL. After Preview 4 it stops working for the same reason: nobody is bringing the binary along anymore. Drop the ExcludeAssets element or move the package to a project that does ship its runtime.

Why bother

Carrying Newtonsoft.Json inside the test platform was an old compatibility wart. It pinned a 13.x major into every test session, surfaced occasional binding-redirect drama on .NET Framework, and forced teams that intentionally banned Newtonsoft.Json from their app to still tolerate it under tests. Using System.Text.Json on .NET shrinks the test host’s footprint and lines test execution up with the rest of the modern SDK (related: System.Text.Json in .NET 11 Preview 3). For .NET Framework, JSONite keeps the same protocol on a tiny dedicated parser instead of a shared library that has bitten teams before.

If you want to know early whether you are in the broken bucket, point your CI at the preview package Microsoft.TestPlatform 1.0.0-alpha-stj-26213-07 and run your existing test suite. A green build now means a green build on May 12.