dotnet-lambda package fails with AWS SDK v4: Missing type IUnmarshaller<T1, T2> from AWSSDK.Core

[ncipollina]

Describe the bug

When using AWS SDK for .NET v4 (e.g. AWSSDK.Core version 4.0.0), the dotnet-lambda package command fails with the following error:

/usr/local/share/dotnet/sdk/9.0.203/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.CrossGen.targets(470,5): error : Error: Failed to load type 'Amazon.Runtime.Internal.Transform.IUnmarshaller`2' from assembly 'AWSSDK.Core, Version=4.0.0.0'

This appears to be due to a breaking change in AWSSDK.Core, where IUnmarshaller<T1, T2> has been removed or refactored in version 4.x. The Lambda packaging tool still expects it to exist.

Regression Issue

• Select this option if this issue appears to be a regression.

Expected Behavior

dotnet-lambda package should successfully build and package the Lambda function using AWS SDK v4.

Current Behavior

The command fails with:

Failed to load type 'Amazon.Runtime.Internal.Transform.IUnmarshaller`2'

Reproduction Steps

1. Create a .NET 8 Lambda project.
2. Upgrade AWS SDK packages (e.g., AWSSDK.Core, AWSSDK.Lambda, etc.) to version 4.0.0.
3. Run dotnet tool run dotnet-lambda package.

Possible Solution

Please update Amazon.Lambda.Tools to be compatible with AWS SDK v4.

Additional Information/Context

No response

Targeted .NET platform

NET SDK: 9.0.203

CLI extension version

5.12.4

Environment details (OS name and version, etc.)

OS: macOS / Ubuntu runner in GitHub Actions

[normj]

The lambda tooling hasn't been updated to V4 yet or had a recent release. I assume what is happening is when dotnet lambda package is running it is triggering a compilation error on your lambda package. What happens if you run dotnet publish command in the Lambda project directory?

My guess is somehow your Lambda function is having a mix of V4 and V3 packages getting included in it.

[ncipollina]

Looks like you are correct. There are some implictly included packages that are 3.7 version packages.

[normj]

Closing as the issue sounds like it isn't related to the lambda tooling.

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