When you execute a program "foo" with spawnProcess, it will find "foo.exe" if it is a file in the PATH environment variable.  However, it will not find programs with the ".bat" extension.

When searching for a program in PATH on windows, all files with an extension from the PATHEXT environment variable should be checked, i.e.

 PATHEXT=.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC

I've created a small program to demonstrate/reproduce this issue:

import std.file, std.process, std.stdio;

void main(string[] args)
{
    if (args.length == 1)
        runTest();
    else
        writeln("running from an exe!");
}
void runTest()
{
    const runDir = "C:\\temp\\testworkdir";
    const pathDir = "C:\\temp\\testpathdir";
    if (!exists(runDir)) mkdir(runDir);
    if (!exists(pathDir)) mkdir(pathDir);
    chdir(runDir);

    const thisExe = thisExePath();
    writefln("thisExePath is '%s'", thisExe);
    const tempExe = pathDir ~ "\\anexeprogram.exe";
    std.file.copy(thisExe, tempExe);
    
    environment["PATH"] = environment["PATH"] ~ ";" ~ pathDir;
    writefln("Added '%s' to path", pathDir);
    run(["anexeprogram", "recursive"]); // works

    const tempBat = pathDir ~ "\\abatprogram.bat";
    {
        auto batFile = File(tempBat, "w");
        batFile.writeln("@echo running from a bat file");
    }
    run(["abatprogram.bat"]); // works
    run(["abatprogram"]); // FAILS
    writefln("Success!");
}

void run(string[] args)
{
    writefln("Running %s", args);
    auto p = spawnProcess(args);
    assert(0 == wait(p));
}