MSBuild "error MSB4126: invalid solution configuration" in CI

You passed a Configuration or Platform to the build that the solution does not define. MSB4126 means the combination (for example Release|x64) is not in the solution's configuration list.

What this error means

msbuild or dotnet build fails with "error MSB4126: The specified solution configuration "Release|x64" is invalid. Please specify a valid solution configuration."

error MSB4126: The specified solution configuration "Release|x64" is invalid.
Please specify a valid solution configuration using the Configuration and Platform properties.

Common causes

A Platform the solution does not define

The solution only defines Any CPU, but the build passes x64, so the configuration pair does not exist.

A misspelled Configuration name

A Configuration value that does not match any solution configuration (a typo or a renamed build) is rejected.

How to fix it

Pass a configuration the solution defines

1. List the solution configurations (in the .sln or your IDE).
2. Use a Configuration/Platform pair that exists, for example Release with Any CPU.
3. Re-run the build.

dotnet build MySolution.sln -c Release -p:Platform="Any CPU"

Add the missing solution configuration

If the platform is intentional, add the configuration/platform mapping to the solution so it resolves.

# Open the solution and add the Release|x64 mapping
# under SolutionConfigurationPlatforms, then commit the .sln

How to prevent it

• Reference only configurations defined in the solution.
• Keep platform names consistent (Any CPU vs x64).
• Validate -c/-p:Platform against the solution before CI.

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