[Eduard van Valkenburg (eavanvalkenburg)]

Motivation and Context

SessionsPythonPlugin.UploadFileAsync accepts a model-supplied remote file name. Unlike the Python implementation, the .NET path did not reject standalone . or .. path segments before placing that name in the multipart upload request. It also needed platform-independent handling for Windows separators and rooted, drive-qualified, or UNC names.

Rejecting those forms keeps uploads within the intended session data path while preserving ordinary file names and relative nested paths.

The .NET build matrix also began failing NuGet audit after a new advisory was published for the centrally pinned MessagePack 3.1.7 dependency. The first patched 3.x release is 3.1.9.

Description

• Add a small remote file-name validator and run it before local file access or network I/O.
• Treat both / and \ as path separators and reject dot segments, rooted names, drive prefixes, and UNC paths.
• Document the remote path constraint in the kernel function parameter description.
• Add targeted regression cases for unsafe path forms, rejection without an HTTP request, and valid nested or dot-containing names.
• Update the central MessagePack pin from 3.1.7 to 3.1.9 so the required .NET build matrix passes NuGet audit.

Validation:

• $env:GITHUB_ACTIONS='true'; dotnet build dotnet\samples\Demos\AgentFrameworkWithAspire\ChatWithAgent.AppHost\ChatWithAgent.AppHost.csproj -c Release --warnaserror --nologo — passed with 0 warnings and 0 errors.
• dotnet test dotnet\src\Plugins\Plugins.UnitTests\Plugins.UnitTests.csproj --filter FullyQualifiedName~SessionsPythonPluginTests --no-restore --nologo — passed, 53/53 tests.
• git diff --check — passed.
• The repository-wide test suite and pre-submission formatting script were not run.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Copilot]

🟢 Approval recommended

The narrowly scoped validation matches the stated requirements and has targeted regression coverage, with no unresolved findings.

No open issues 🎉

What changed in this PR

This PR adds remote upload filename validation to the .NET SessionsPythonPlugin, rejecting standalone . and .. path segments while preserving ordinary filenames and nested paths.

Changes:

• Validate remote filenames before local file access or network I/O.
• Document the restriction in API documentation and kernel parameter metadata.
• Add regression tests for rejected paths, rejection without an HTTP request, and valid nested or dot-containing names.

File	Description
dotnet/​src/​Plugins/​Plugins.UnitTests/​Core/​SessionsPythonPluginTests.cs	Tests rejected dot segments and accepted filenames.
dotnet/​src/​Plugins/​Plugins.Core/​CodeInterpreter/​SessionsPythonPlugin.cs	Adds early remote filename validation and documents the constraint.

_{Review effort: Balanced}