Summary

System.CommandLine symbols (Command, Option, Argument) are intentionally not thread-safe per the maintainers (dotnet/command-line-api#1258). The Opt<TOption> static singleton helper in AspNetCore.CommandLine shared option instances across all App command trees, and its lazy ??= initialization is racy. When multiple App instances are constructed concurrently (as the parallel test classes in AspNetCore.CommandLine.Tests do), a command tree could end up holding a different option instance than the one its action later passes to parseResult.GetValue, which silently returns null. For schema export this meant the --output value was dropped and the schema was exported to the current directory with exit code 0, surfacing as an intermittent snapshot failure in SchemaExportCommandTests on CI.

Each command now creates its own option instances and captures them in its action closure, removing all shared symbol state between command trees. The Opt<T> helper is deleted.

The Nitro CLI has its own copy of the same pattern; that is left for a follow-up.

Test plan

• dotnet test on AspNetCore.CommandLine.Tests (net10.0): all 12 tests pass.
• Race mechanism was reproduced in isolation against System.CommandLine 2.0.5 (silently lost --output in 36/5000 simulated process starts with the singleton pattern); the per-instance design removes the shared state that race requires.