That does apparently fail in quite a quiet way, but in fact there are some setup issues.

Here is a working example with a few small changes:

• renamed the subcommand from 333 to abc to make a bit clearer
• pass in test arguments for both subcommand and positional
• added position to subcommand setup: abc <eventID>

const { argv } = yargs(["abc", "333"]) // subcommand + positional to test parsing
  .scriptName("pirate-parser")
  .usage("$0 [WCA event ID")
  .command("abc <eventID>", "welcome ter yargs!", (yargs) => {
    yargs.positional("eventID", {
      type: "string",
      describe: "WCA event ID",
    });
    return yargs;
  })
  .strictOptions();

% node index.js 
{
  _: [ 'abc' ],
  '$0': 'pirate-parser',
  eventID: '333',
  'event-i-d': '333'
}

That does apparently fail in quite a quiet way, but in fact there are some setup issues.

The CLI I'm implementing doesn't have subcommands at the moment (which is quite common for commandline applications). What should I be doing in that case?

Oh, I see! I seem to have ended up with some nested command code from debugging. Lemme see if I can take it out.

Hmm, this seems to have the same issue:

import yargs from "yargs";

const { argv } = yargs(["333"])
  .scriptName("pirate-parser")
  .usage("$0 [WCA event ID")
  .positional("eventID", {
    type: "string",
    describe: "WCA event ID",
  })
  .strictOptions();
console.log(argv._[0] === "333"); // should be true?
console.log(argv._[0] === 333); // should be false?
console.log(argv.eventID); // or this should be defined?

There is a trick. .positional() is not available on the root command, and instead you can configure it using .usage() and define a "default command": https://yargs.js.org/docs/#api-reference-usage-desc-builder-handler

The positional gets stored using its name.

import yargs from "yargs";

const { argv } = yargs(["333"])
  .scriptName("pirate-parser")
  .usage("$0 [eventID]", "welcome ter yargs", (yargs) => {
    yargs.positional("eventID", {
      type: "string",
      describe: "WCA event ID",
    })
  })
  .strictOptions();
console.log(argv);

% node top.mjs 
{ _: [], '$0': 'pirate-parser', eventID: '333', 'event-i-d': '333' }

Ah, I see, thanks. I've used

https://yargs.js.org/docs/#api-reference-positionalkey-opt states that ".positional() … is not available on the top-level yargs instance."

If I now understand that correctly, wouldn't it make more sense for the code to error when called like that?

I'm a big fan of "if it compiles it works", and the current behaviour here is pretty unintuitive to me.