Compatibility

monoruby aims at CRuby 4.0 compatibility, and measures it against ruby/spec — the executable specification suite CRuby itself is tested with. The suite is re-run on every push to master that touches the interpreter, and the results are published as live dashboards.

Dashboards

Each dashboard carries a per-category table, a history chart, and the raw latest.json / data/history.csv behind it. The core dashboard also publishes per-branch pages for branches under active spec work.

Where things stand

As of commit a13bfe0 (2026-08-31):

Additionally, as of July 2026 monoruby passed 100% of the command-line specs and 99.6% of the language specs.

The core figure is the result of a sustained compliance push through 2026 — the first published measurement, on 2026-04-27, was 59.5%:

The largest core categories are in good shape — array 100% over 2898 examples, file 99.6%, kernel 99.5%, module 98.9%, io 96.2%, and string 95.4% over 3905. What remains is concentrated rather than spread out: tracepoint (TracePoint is deliberately not implemented — the JIT’s speculation assumes its absence, see Invariants compiled code speculates on), objectspace at 39.3%, then time, marshal and encoding in the 89–95% band.

The library group is the frontier. matrix and net-http are essentially complete (100% and 99.5%), while the biggest gaps are socket (40.2% over 1129 examples), stringio (67.5%), net-ftp (54.9%) and bigdecimal (68.6%). Libraries needing facilities monoruby does not have — openssl, coverage, mkmf, irb — sit at 0%.

How the numbers are produced

Both dashboards come from the same workflow shape (.github/workflows/spec-core.yml and spec-library.yml):

• Every *_spec.rb under the group is run through mspec, category by category, in batches of 10 files.
• Each batch runs under a hard 60-second deadline with timeout -k 5. The -k matters: monoruby installs its own SIGTERM handler which is deferred to a VM poll point, so a process stuck in a blocking read never dies to a plain TERM.
• A killed batch prints no summary line, which would lose the tally for all ten files, so the batch is re-run one file at a time — completing files are counted after all, and the hanging file is pinned down and recorded in data/timeouts.csv. Both groups currently record zero timeouts.
• --excl-tag fails excludes examples tagged in this repository’s spec/tags/. That list is deliberately tiny — 6 examples in 4 files at present, covering Refinement#import_methods from a C extension, $LOAD_PATH.resolve_feature_path for a .so, the /o Regexp modifier, and three refinement-driven pattern-matching cases. The published pass rate is therefore very close to an unfiltered one.

The audit that cut the skip list down from coarse file-level exclusions to that handful is written up in ruby/spec hang countermeasures. Green threads removed the last structural hangs: blocking IO now parks the calling thread on the scheduler’s fd poller instead of blocking the process, so specs like core/io/copy_stream_spec.rb and core/io/select_spec.rb run to completion.

Running the specs yourself

ruby/spec and mspec are cloned alongside the monoruby checkout, not inside it. See Development and Build Options → ruby/spec for the layout and the commands.

Deliberate differences from CRuby

A few behaviours differ by design rather than by omission:

• TracePoint is not implemented. Compiled code speculates on its absence.
• Backtraces are formatted lazily. Raise, unwind and catch record what is needed; the backtrace string is not built until something asks for it. See Exception handling.
• The garbage collector is non-moving, single-threaded and stop-the-world (generational since June 2026). Compiled code relies on objects not moving.
• Threads are M:1 green threads with time-slice preemption, not OS threads. See Thread / Fiber / non-blocking IO / preemption.
• C extensions cannot be loaded. Libraries that CRuby backs with a C extension — json, date, digest, stringio, zlib, openssl and others — are either reimplemented in Rust/Ruby and shipped in the vendored tree, or unavailable. C extension support is a design study, not a shipped feature.