Ruby 4.0.0, released on December 25, 2025, adds two experimental features: ZJIT, a new just-in-time compiler, and Ruby::Box, which isolates Ruby definitions within a single process. ZJIT is faster than the interpreter but not yet as fast as YJIT; Ruby advises against deploying it in production for now. Ruby::Box can help keep definitions such as monkey patches separate, but it is not a replacement for process or container security boundaries.
What is new in Ruby 4.0.0?
The release’s headline changes are aimed at two different concerns: execution speed and controlling how code affects other code in the same Ruby process.
- ZJIT is a new method-based JIT compiler intended as the next generation of YJIT. It is experimental in Ruby 4.0.0.
- Ruby::Box provides experimental isolation for definitions loaded into a box. Enable the feature with
RUBY_BOX=1. - Ractors also receive updates: Ruby 4.0 adds
Ractor::PortandRactor.shareable_proc, alongside other Ractor improvements.
The Ruby project’s Ruby 4.0.0 release announcement reports 3,889 files changed, 230,769 insertions, and 297,003 deletions since Ruby 3.4.0. Those totals indicate the scope of changes, not a measure of application compatibility or performance.
What Ruby::Box isolates—and what it does not
Ruby describes Ruby::Box as an experimental way to separate definitions. With RUBY_BOX=1 enabled, the Ruby::Box class can create boxes whose loaded definitions are isolated from those in other boxes. The release announcement says, “Definitions loaded in a box are isolated in the box.”
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That scope includes monkey patches, global and class-variable changes, class and module definitions, and loaded Ruby or native libraries. This is useful when code that shares a process would otherwise affect another test or application version—for example, when testing code with a patched dependency alongside the unpatched version.
Where Ruby::Box may be useful
- Isolated tests: Keep definitions used by one test context from leaking into another.
- Parallel blue-green web-app boxes: Run different application versions in separate boxes within one Ruby process, a use case identified by Ruby.
- Dependency-update comparisons: Compare responses from code using different dependency definitions, another use case identified by Ruby.
Ruby::Box versus processes and containers
| Question | Ruby::Box | Separate process or container |
|---|---|---|
| Isolation scope | Isolates Ruby definitions inside one Ruby process, according to the Ruby 4.0.0 release announcement. | Provides an operating-system-level process boundary; container boundaries depend on their configuration. |
| Monkey patches and loaded libraries | Ruby says definitions, including changes and loaded libraries, can be isolated between boxes. | Separate processes have separate runtimes and loaded definitions. |
| Startup and memory cost | Not stated in the Ruby 4.0.0 release announcement. | Not stated there; actual costs depend on the application and deployment. |
| Security boundary | Definition isolation is not evidence of an operating-system security boundary. Do not treat a box as containment for untrusted code. | Processes and containers can provide stronger separation, but their security depends on operating-system and deployment configuration. |
| Best-supported use cases | Ruby documents isolated tests, parallel blue-green web-app boxes, and dependency-update response comparisons. | Use when the needed separation is at the process or deployment level; Ruby’s release announcement does not compare these approaches. |
Ruby::Box is therefore best understood as a way to manage definition collisions, not as a lightweight substitute for all the resource, failure, or security separation provided by processes. The release announcement establishes isolation of definitions; it does not establish that boxes contain arbitrary native-code behavior or isolate operating-system resources.
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What ZJIT does, and whether it is faster than YJIT
ZJIT is a method-based just-in-time compiler that compiles Ruby code while a program runs. Ruby calls it the next generation of YJIT. The key qualification is the release’s own performance statement: “ZJIT is faster than the interpreter, but not yet as fast as YJIT.” Ruby encourages experimentation but advises holding off on production deployment.
| Runtime option | Performance and maturity in Ruby 4.0.0 | Memory and setup |
|---|---|---|
| Interpreter | Baseline execution without JIT compilation. | ZJIT uses more memory than the interpreter because generated machine code and compiler state remain in memory. |
| YJIT | Ruby 4.0.0’s release announcement says ZJIT is not yet as fast as YJIT. A benchmark percentage is not stated in that announcement. | Not stated in the cited Ruby 4.0.0 release material. |
| ZJIT | Faster than the interpreter, but not yet as fast as YJIT; experimental in Ruby 4.0.0 and not recommended for production deployment by Ruby. | Requires a Ruby build with ZJIT support. The Ruby 4.0 manual lists a default executable-memory limit of 128 MiB and a default call threshold of 30. |
The documented default call threshold means a method must reach the configured call count before it is considered for JIT compilation. It is not a promise of a particular warm-up time or end-to-end speedup. Ruby has not published a comparison percentage in the cited release material, so a numeric claim about how much faster or slower ZJIT is than YJIT would be unsupported.
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How to enable ZJIT in Ruby 4.0
There are two runtime entry points, but both require a Ruby executable built with ZJIT support. If your Ruby installation lacks that support, passing the runtime flag or calling the API will not add it; install or build a compatible Ruby first.
- Check the build. Ruby must have been compiled with ZJIT support. Building Ruby with ZJIT requires Rust 1.85.0 or newer, according to the Ruby project’s Ruby 4.0 JIT documentation.
- Enable it for a command. Start Ruby with
--zjit, for exampleruby --zjit app.rb. - Or enable it at runtime. In a supported Ruby process, call
RubyVM::ZJIT.enable. - Measure before adjusting. Compare representative application behavior with and without ZJIT, including warm-up and memory use. Avoid inferring production readiness from a small script or a single result.
Platforms and memory
The Ruby 4.0 manual lists ZJIT support for macOS, Linux, and BSD on x86-64 and arm64/aarch64. It uses more memory than the interpreter because compiled machine code and compiler state remain in memory. The documented default --zjit-mem-size is 128 MiB; the default call threshold is 30. These are defaults, not performance guarantees or universal memory requirements.
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Build modes and tuning
The manual describes release, stats, dev_nodebug, and dev build modes. The development mode performs extensive IR validation, which slows compilation and warm-up; it is intended for development rather than representative production performance measurements. Ruby also documents options for executable-memory size, the call threshold, profile count, statistics, and tracing. Consult the Ruby 4.0 JIT manual for the exact option names and behavior for your build.
Is Ruby 4.0.0 safe for production?
The release’s experimental cautions apply to the new features, not automatically to every Ruby 4.0 application. Ruby explicitly advises against production deployment of ZJIT in 4.0.0 because it is still experimental and has not yet matched YJIT. That is the clearest answer for teams considering the new compiler: test it outside production, but do not treat it as a production-ready YJIT replacement.
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Ruby::Box is also experimental. Its documented definition-isolation use cases may be valuable for controlled testing or application-version comparisons, but the release does not establish it as a security boundary or guarantee suitability for every multi-tenant workload. Assess it against the isolation your application actually requires.
For the rest of a Ruby 4.0 upgrade, validate the application and dependencies in a representative environment before rollout. The fact that a feature is new in the release does not establish whether a particular application, native extension, or deployment workflow is compatible.
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