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json/add/core does not support Symbols for Hash keys today. This is a major issue for downstream systems like Sidekiq which rely on JSON for serialization because we can't transparently serialize the full range of method arguments like keywords.

I'd like to propose a way to support Symbol'd keys by "smuggling" the type in the String itself:

{
  :mike => 123
}

{
  "_s:mike": 123
}

This would allow Sidekiq users to use keyword arguments in their jobs without any issues. What do you think?

See also https://github.com/sidekiq/sidekiq/pull/6209

And I would be perfectly ok with this being opt-in: JSON.generate(..., type_smuggling: true)

I don't think the JSON gem should support this. It sounds like a reasonable thing to do for Sidekiq but is too specific for a general JSON gem (hard to draw a line for which specific classes/types this should/shouldn't support) and isn't part of the standard. The good news is that this seems best implemented in pure-Ruby wherever it lives!

One problem this has is that if Ruby's {:mike => 123} is represented as the JSON "_s:mike" how do you represent the Ruby {"_s:john" => 456}? Presumably with a different prefix but once that is done "standard" JSON isn't 1:1 loadable as "smuggled" JSON and you might as well have a heavier/specific format.

This would allow Sidekiq users to use keyword arguments in their jobs without any issues.

Keyword arguments probably also requires additional work/hacks/annotations, since in Ruby 3.0+ they'll need to be separated from your positional arguments (in which case you could just know the keys are Symbols) OR you'll have to round-trip the ruby2_keywords value of the hash, which is what ActiveJob does.