Is this real?
No. There's no such thing as a wire stretcher.
Somebody with a few more years in than you sent you to the van for a tool that doesn't exist. That same thing happened to them, and to the person who did it to them. You're part of a long, proud, slightly annoying tradition now.
Walk back, laugh about it, and then go learn how to measure the run properly the first time. (Pull a little extra. Service loops are cheap. Re-pulls aren't.)
Next year, it's your turn.
Other things you'll be asked to fetch
A left-handed screwdriverScrewdrivers don't care which hand you are.
A box of ohmsResistance is measured, not shipped.
A bucket of steamFor "bending conduit." Bring back an empty bucket and a straight face.
Striped paintSold alongside the tartan and polka-dot varieties.
A sky hookThe load has to hang from something, and it isn't the sky.
Wireless cableEvery AV tech's favorite paradox.