Let a derived tool inherit useShellExecute - #799
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Every optional flag on AddToolBasedOn is a nullable that falls back to the tool being derived from - except useShellExecute, which defaulted to true. So its `?? existing.UseShellExecute` was unreachable, and a caller who said nothing about it did not inherit: they got true. Five definitions set it false, and they are the ones where it matters: the bundled viewer, VS Code, Cursor, MsWordDiff and MsExcelDiff. Deriving from any of them silently changed how the tool launches, and took the inherited CreateNoWindow with it - which on the viewer is the console flash the definition exists to prevent. The test asserts the defaults rather than deriving a tool, and that is deliberate. AddToolBasedOn resolves through ToolLookup, so it only works for a tool actually installed on the machine, and the viewer resolves on neither this machine nor CI - I wrote the behavioural version first and it passed against the bug, because it was taking its own early return. Asserting that every bool? flag defaults to null pins the real contract and fails without the fix.
This was referenced Aug 26, 2026
This was referenced Aug 27, 2026
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Every optional flag on AddToolBasedOn is a nullable that falls back to the tool
being derived from - except useShellExecute, which defaulted to true. So its
?? existing.UseShellExecutewas unreachable, and a caller who said nothingabout it did not inherit: they got true.
Five definitions set it false, and they are the ones where it matters: the
bundled viewer, VS Code, Cursor, MsWordDiff and MsExcelDiff. Deriving from any
of them silently changed how the tool launches, and took the inherited
CreateNoWindow with it - which on the viewer is the console flash the definition
exists to prevent.
The test asserts the defaults rather than deriving a tool, and that is
deliberate. AddToolBasedOn resolves through ToolLookup, so it only works for a
tool actually installed on the machine, and the viewer resolves on neither this
machine nor CI - I wrote the behavioural version first and it passed against the
bug, because it was taking its own early return. Asserting that every bool? flag
defaults to null pins the real contract and fails without the fix.