Does tailess replace clsx and tailwind-merge?

No, it uses both ideas. cn() is the same twMerge(clsx(...)) pairing your codebase probably already has, so tailess drops into one. Only tailwind-merge is a real dependency.

Should I use ss or cn?

ss does everything cn does, so either works. The habit worth having is cn while a className is only plain strings, and ss the moment a breakpoint or a state shows up, at which point everything including conditions moves inside the one call.

Do I have to migrate to nested groups?

No. Nesting is there for compound variants like dark:hover: and for grouping a breakpoint’s own states; a flat object stays perfectly idiomatic. Both shapes are supported.

Does tailess work without the build plugin?

The unprefixed base classes and match() do, because those are literals Tailwind finds by itself. Anything with a variant prefix needs the plugin, because Tailwind never runs your code and so never sees the finished class.

Is there a runtime cost in production?

Only the string building. The integration check and every warning are development-only and drop out of a production bundle.

Does tailess support Tailwind CSS v3?

No. The @source inline(...) directive introduced in Tailwind CSS v4 is what makes the bridge between runtime-built classes and generated CSS possible.

Does tailess work with a custom @theme?

Yes. Candidates go through Tailwind’s own pipeline, so your theme values resolve exactly as they do for classes written by hand.

FAQ

Whether tailess replaces clsx and tailwind-merge, ss or cn, optional nesting, working without the plugin, Tailwind v3, @theme and runtime cost.

Helpers

Does this replace clsx and tailwind-merge?: No — it uses both ideas. cn is the same twMerge(clsx(...)) pairing your codebase probably already has, so tailess drops into one — though only tailwind-merge is a real dependency.
ss or cn?: ss does everything cn does, so either works. The habit worth having is cn while a className is only plain strings, and ss the moment a breakpoint or a state shows up — at which point everything, conditions included, moves inside the one call.
Do I have to migrate to nested groups?: No. Nesting is there for compound variants like dark:hover:and for grouping a breakpoint's own states; a flat object stays perfectly idiomatic. The ss page has the rules for both shapes.

Flat and nested

// the same thing — nesting is optionalss({ md: "p-6" });ss({ md: { base: "p-6" } });

The plugin and runtime cost

Does it work without the plugin?: The unprefixed base classes and match() do, because those are literals Tailwind finds by itself. Anything with a variant prefix needs the plugin — see how it works.
Is there a runtime cost in production?: Only the string building. The integration check and every warning are dev-only and drop out of a production bundle. Performance and size has the measured per-call numbers.

Tailwind support

Tailwind v3?: No. The @source inline(...) directive in v4 is what makes the bridge possible.
Does it work with a custom @theme?: Yes. Candidates go through Tailwind's own pipeline, so your theme values resolve exactly as they do for classes written by hand.