NB: I do have a package.json file in the root directory that has 'version' field, returned by steps.version.outputs.value

What is the value of the version field in your package.json? This error could occur if the value is not valid semver.

Also, that usage of the tag input looks suspicious. The meaning is npm's dist-tag, not the package version

Also also, is that publish step running inside a matrix? If so, that's a bad idea! Might just be a copy paste issue

@michaelnlindsay it looks like this is your package.json version:

{
  "version": "2.11.2a",
}

2.11.2a is not a valid semantic version, and all packages published to npm must follow semver. I have opened #161 to improve the error message to something more helpful:

InvalidPackageVersionError: Package version must be a valid semantic version, got "2.11.2.a"

Brilliant! Managed to land myself on the new error message too. Looks good, I understand what it's complaining about especially with inclusion of the version I supplied. It worked like a charm after I stopped trying to provide tag, any value give for that, even proper semvar, was throwing errors. Which is probably fine? Shouldn't need it anyway? Thanks for the swift response! Much appreciated :)

As mentioned above, tag should not be a version. The default value of tag is latest, as in:

npm install example@latest

The purpose of tag is to create "release channels" of a sort. For example, you may publish canary versions that shouldn't be installed by default, but you could use:

with:
  tag: canary

To publish versions that can be installed via:

npm install example@canary

Got it... I was obviously experimenting off the range, sorry about that. tag will be useful to me there then :D and canary is an excellent suggestion.