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Platform Newsletter: February to May 2026

Welcome to the eighteenth edition of the OCaml Platform newsletter!

In this February to May 2026 edition, we are excited to bring you the latest on the OCaml Platform, continuing our tradition of highlighting recent developments as seen in previous editions. To understand the direction we're headed, especially regarding development workflows and user experience improvements, check out our roadmap.

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Highlights:

  • OCaml 5.5.0 reaches beta (Apr 20): a relocatable compiler plus modular explicits, polymorphic function parameters, generalised local bindings, and garbage-collector improvements; an rc1 followed, and the final OCaml 5.5.0 was released on June 19, 2026, just after this newsletter's February–May window.
  • Security: OCaml 5.4.1 / 4.14.3 (Feb 17) harden Marshal against malicious input (OSEC-2026-01) and opam 2.5.1 (Apr 16) blocks .install path escapes (OSEC-2026-03). Review your code and upgrade if you deserialise untrusted data, or upgrade if you maintain a distribution. You can also check the published security issues and upgrade if you use an affected package, and subscribe to security announcements.
  • Dune 3.22–3.23: sandboxing is now on by default for tests, user rules, and inline test-runner generation; 3.22 adds build tracing and OxCaml parameterised-library support; 3.23 stops auto-promoting generated opam files (run dune promote).
  • Editor tools (Apr 10): OCaml-LSP 1.26.0 + Merlin 5.7.0-504 add a code-extraction refactoring, type-aware navigation, and range formatting.
  • OCamlFormat 0.29.0 (Mar 17): OCaml 5.5 syntax and a new default ocaml-version=5.4.
  • opam-publish 3.0.0 (Feb 20): a breaking cmdliner 2.0 upgrade (no more prefix-matching), plus default-branch and fork-name auto-detection.
  • Other releases: Odoc 3.2.0, ppxlib 0.38.0, utop 2.17.0, and MDX 2.5.2 shipped OCaml 5.5 support; Odoc 3.2.1 fixed OCaml 5.5 documentation regressions; dune-release 2.2.1 adds a --prerelease flag.

Stable Releases:

Unstable Releases:

(OCaml 5.5.0~alpha2 was tagged but never officially released, due to an interaction between the relocatable compiler and bootstrapping.)

OCaml Compiler

Maintenance Releases: 5.4.1 and 4.14.3

OCaml 5.4.1 and 4.14.3 (February 17, 2026) are maintenance releases of the 5.4 and 4.14 stable branches. The headline is OSEC-2026-01: the intern.c Marshal implementation has been hardened against malicious inputs. Anyone deserialising untrusted data should review their code and upgrade. The 4.14.3 release also fixes a long-standing miscompilation of unsafe int32/int64/nativeint array accesses present since 4.04, a Lazy.force race, and a demarshal-exception memory-corruption issue; the 5.4.1 release adds OCaml 5-specific fixes including TSan fixes, a macOS -pack fix, and a string-operation miscompilation fix. See the release notes for the full list.

OCaml 5.5.0 Progresses to Beta

The OCaml 5.5.0 release cycle moved through two announced alphas, an unreleased alpha2 tag, and into a first beta during this period. 5.5.0 ships a particularly notable batch of language extensions, and is the first to ship a relocatable compiler.

Headline features in 5.5.0:

  • Relocatable compiler (deployment-side): the compiler and its associated tools can be moved/copied between directories after installation without breaking functionality. This makes installed compiler trees easier to reuse or redistribute. (See the relocatable-OCaml announcement referenced in the previous newsletter.)
  • Modular explicits (language): function arguments can now carry a module signature dependency ((module M : S) -> t[M]), enabling module-dependent function arguments without full functor syntax.
  • Polymorphic function parameters (language): functions can now take arguments with explicit polymorphic types (e.g. (getter : 'a. 'a t -> 'a)), enabling clean ST-monad-style and rank-2 patterns previously requiring records.
  • Generalised local bindings (language): let module, let exception, let open, and let type are now supported in expressions in more contexts.
  • Garbage-collector improvements (runtime): a batch of work from an ongoing GC-pacing overhaul has landed in 5.5.0, improving how the major collector accounts for and schedules its work, with the broader effort tracked in #14324 and continuing toward 5.6.
  • Standard library expansions: notably String.split_*, String.replace_*, String.includes, Option.product, List.split_map, Lazy.Mutexed.

Feature spotlight: modular explicits. Modular explicits are the most significant of the new language features. A function can now take a module as an explicit argument and let its later parameter and return types depend on it. Before 5.5 you could already express this, but you had to take the module as a first-class-module argument and thread its element type through a locally abstract type by hand. Here is how the list-sorting example (sort using whichever Set implementation is passed in) looked on 4.14:

let sort (type a) (module MSet : Set.S with type elt = a) li =
  MSet.elements (List.fold_right MSet.add li MSet.empty)
(* val sort : (module Set.S with type elt = 'a) -> 'a list -> 'a list *)

Note the explicit (type a) and with type elt = a, binding the module's element type so the return type can refer to it. It is manageable here, but becomes tedious once several type parameters need binding. With modular explicits in 5.5, that boilerplate disappears:

let sort (module MSet : Set.S) li =
  MSet.elements (List.fold_right MSet.add li MSet.empty)
(* val sort : (module MSet : Set.S) -> MSet.elt list -> MSet.elt list *)

module IntSet = Set.Make (Int)
let sorted = sort (module IntSet) [3; 1; 2]   (* [1; 2; 3] *)

The MSet.elt in the inferred type refers back to the module argument, so the return type depends on the module passed in. Application is more restricted than for ordinary first-class-module functions: the argument must be a literal module expression like (module IntSet), not a value computed at runtime. This restriction is what keeps the typing sound. The manual documents the feature (#14048). The related polymorphic function parameters feature (let f (g : 'a. 'a t -> 'a) = ...) lands in the same release.

Release timeline:

  • 5.5.0~alpha1 (Feb 27) - first alpha, opening the bug-hunting window.
  • 5.5.0~alpha2 - tagged but unreleased: an unforeseen interaction between the relocatable compiler and bootstrapping was discovered before the release was finalised.
  • 5.5.0~alpha3 (Mar 26) - fixes the bootstrap issue, alongside a type-soundness restoration, a register-allocator miscompilation fix, and an s390x heap-corruption fix.
  • 5.5.0~beta1 (Apr 20) - most developer tools are now available (at least in preview form). Compared to the last alpha, beta1 fixes two runtime bugs (ephemerons; the bytecode interpreter), two type-system bugs (classes; module-dependent functions), and three warning/error-message bugs. The release announcement explicitly invites users to test their libraries and programs against 5.5.0 in preparation for the final release; an rc1 followed, and the final OCaml 5.5.0 was released on June 19, 2026 (just after this newsletter's February–May window).

The release readiness meta-issue tracks ongoing work on the surrounding ecosystem.

Build System

Dune

Six Dune releases shipped during this period: a 3.21 patch, the 3.22.x series, and the 3.23.x series.

Dune 3.21.1 (Feb 11) - a small patch: a Melange -p <PKG> package-mask fix (relevant to scoped release/CI builds), and dune promote no longer starts the RPC server.

Dune 3.22.0 (Mar 19) - a bigger release. Highlights:

  • Build tracing: new functionality to inspect and diagnose the build process, plus a public dune-action-trace library so custom actions can emit trace events and a dune trace cat subcommand to read the output.
  • odoc documentation in Markdown: a new @doc-markdown build alias generates odoc output as Markdown.
  • OxCaml parameterised libraries: full support for OxCaml's parameterised libraries.
  • Tests sandboxed by default: (test) and (tests) stanzas (along with Melange rules, mdx, and ocamllex/ocamlyacc) are now sandboxed by default. This may affect many projects with tests or generated-code rules and may surface latent filesystem or path assumptions on upgrade.
  • dune runtest <file> runs individual tests (#13064), closing a request open since 2018: runtest is no longer all-or-nothing for (tests) and inline tests.

Dune 3.22.1 (Apr 6) and 3.22.2 (Apr 14) are regression fixes: a dune test crash in workspaces without a context named "default", and a --diff-command change that again passes non-existent files to the diff command rather than /dev/null.

Dune 3.23.0 (May 5) - a feature release. Highlights:

  • Generated opam files are no longer auto-promoted (#14108): a breaking change. Dune no longer writes generated .opam files back into the source tree on build; you now update them explicitly with dune promote. Worth flagging for release scripts and any CI that checks committed opam files are in sync.
  • c_library_flags in foreign_stubs (#13484): foreign_stubs now accepts c_library_flags, for finer control over how C stubs are linked.
  • Sandboxing extended: user rules and inline test-runner generation are now sandboxed by default, continuing the sandboxing-by-default work begun in 3.22.
  • Promotion and diffing improvements: numerous refinements, including sandboxed diff promotions and directory-level diffing.
  • Minimum OCaml to build Dune is now 4.14: this concerns only building Dune itself from source on older compilers, not projects that use Dune.

Dune 3.23.1 (May 19) - a patch release that narrows two of 3.23.0's opam-file-generation changes (a menhir lower bound is now added only to a dependency the user already declared, and version-bound de-duplication is gated on (lang dune 3.23)), restores the secondary-compiler fallback for packages capped below OCaml 4.14, and fixes the NetBSD bootstrap.

Dune Maintained by: Rudi Grinberg (@rgrinberg, Jane Street), Nicolás Ojeda Bär (@nojb, LexiFi), Marek Kubica (@Leonidas-from-XIV, Tarides), Ali Caglayan (@Alizter, Tarides), Stephen Sherratt (@gridbugs, Tarides), Antonio Nuno Monteiro (@anmonteiro), Sudha Parimala (@Sudha247, Tarides), Ambre Suhamy (@ElectreAAS, Tarides), Puneeth Chaganti (@punchagan, Tarides)

Package Management

opam

opam 2.5.1 (April 16, 2026) is primarily a security release.

  • Security fix (OSEC-2026-03): .install fields containing destination paths that try to escape their scope are now rejected (reported by @andrew). Distribution maintainers are advised to upgrade or backport.
  • A depexts-to-nix-build string-injection fix on os-family=nixos (thanks to @RyanGibb).
  • Restored distribution detection on Gentoo, with support for single-quoted values in /etc/os-release.
  • A fix for rare potential GC corruptions (thanks to @avsm).

opam Maintained by: Raja Boujbel (@rjbou, OCamlPro), Kate Deplaix (@kit-ty-kate, Ahrefs), David Allsopp (@dra27, Jane Street)

opam-publish

opam-publish 3.0.0 (February 20, 2026) is a major release with breaking changes and quality-of-life improvements:

  • Breaking: the deprecated --split option has been removed; following the upgrade to cmdliner 2.0, option names can no longer be abbreviated to an unambiguous prefix (for example, --dry is no longer accepted for --dry-run). The github-unix dependency has also been dropped.
  • Default-branch auto-detection: opam-publish no longer hardcodes master; it queries upstream for the default branch. This silently fixes failures against opam-repository forks that have migrated to main, which is relevant to release scripts and CI.
  • Fork-name auto-detection: supports users whose fork of opam-repository isn't named opam-repository.

Maintained by: Raja Boujbel (@rjbou, OCamlPro), Kate Deplaix (@kit-ty-kate, Ahrefs)

dune-release

dune-release 2.2.1 (March 26, 2026) adds a CLI flag to mark a GitHub release as a prerelease, exposing a GitHub REST API capability on the dune-release command line.

Maintained by: Tarides

Editor Tools

Roadmap: Edit / (W19) Navigate Code

Merlin and OCaml-LSP

The headline this cycle is a new code-extraction refactoring for Merlin and OCaml-LSP, joined by type-aware navigation and range formatting. A coordinated dual release of OCaml-LSP 1.26.0 and Merlin 5.7.0-504 was announced on April 10, 2026, preceded by two OCaml 5.5 preview builds and followed by a 5.4-series patch.

Two OCaml 5.5 preview builds came first: Merlin 5.7-505~preview (Feb 27) and OCaml-LSP 1.26.0-5.5~preview (Mar 19). Between them they carry the locate-types groundwork for type-aware navigation, a new destruct custom request, configuration of Merlin via build systems other than dune (through the OCAMLLSP_PROJECT_BUILD_SYSTEM and OCAMLLSP_PROJECT_ROOT environment variables), ocamlformat-formatted signature help, and several signature-help and autocompletion fixes.

Merlin 5.7.0-504 + OCaml-LSP 1.26.0 (Apr 10) - the paired stable release, with three new user-visible capabilities exposed through standard LSP support or OCaml-LSP custom requests:

  • Refactor: extract region: a new refactor-extract-region command (with a matching ocamllsp/refactorExtract request) extracts a selected region into a fresh let-binding (experimental). It is a new region-extraction refactoring alongside the existing rename, type-annotation, and destruct support.
  • Type-aware navigation: a new ocamllsp/locateTypes custom request lets editors jump to the definitions of types appearing inside a hovered type.
  • Range formatting: OCaml-LSP now supports textDocument/rangeFormatting, i.e. format-selection.

The release also improves type-enclosing on class- and object-related items and fixes a cluster of signature-help bugs. The Backstage post Improved Signature Help in Merlin walks through the signature-help changes from a user's perspective.

Merlin 5.7.1-504 (Apr 30) - a 5.4-series patch that fixes a typer cache-invalidation bug (Merlin now picks up newly created .cmi files, for instance when Dune builds a new module) and shrinks on-disk index files.

Code Quality and Documentation

OCamlFormat

OCamlFormat 0.29.0 (March 17, 2026) is a substantial release. Highlights:

  • Support for OCaml 5.5 syntax.
  • The vendored Odoc parser is updated to 3.0: the indentation of OCaml code-blocks is reduced by two (to avoid changing generated documentation), and indentation within code-blocks is now significant in Odoc.
  • The default ocaml-version is now 5.4, so the effect keyword is recognised without extra configuration. Caveat: codebases that use effect as an identifier must now set ocaml-version=5.2.
  • A new letop-punning option (preserve by default) controls whether bindings like let+ x = x in ... are punned to let+ x in ....

Tip: Ignore formatting commits in git blame:

Put all the formatting changes in a single dedicated commit and add the full commit hash into a file named .git-blame-ignore-revs.

Then, use it like this: git blame --ignore-revs-file=.git-blame-ignore-revs <file> or add it to the local configuration: git config blame.ignoreRevsFile .git-blame-ignore-revs. GitHub does this automatically in its web UI.

If you do this in a GitHub Pull Request, make sure not to use "Squash and merge" or "Rebase and merge" as these will change the commit hash in the main branch. See the documentation from GitHub.

Odoc

Odoc 3.2.0 (May 1, 2026) adds OCaml 5.5.0 and OxCaml support. It also brings persistent LaTeX macros in the HTML/KaTeX backend (a macro defined with \gdef in one math block can be reused in later ones), and lets markdown-generate accept multiple .odocl files in a single invocation.

Odoc 3.2.1 (May 12, 2026) is a patch release fixing documentation-build regressions under OCaml 5.5.0 (including broken docs for packages depending on base and merlin-lib), plus a regression introduced in 3.2.0.

MDX

MDX 2.5.2 (March 23, 2026) is a small compatibility release that adjusts MDX's toplevel support to work with OCaml 5.5, where the Env_functor_arg constructor in compiler-libs was renamed to Env_not_aliasable.

ppxlib

ppxlib 0.38.0 (March 20, 2026) adds OCaml 5.5 support, and changes the approach to getting there. It is the first ppxlib release to use the team's new strategy for supporting future compilers: instead of bumping ppxlib's internal AST (the change behind the ecosystem breakage around the 5.2 bump and ppxlib 0.36.0), support for every compiler >= 5.3 now encodes new language constructs into the existing AST. The payoff is that adding a new compiler no longer forces a breaking ppxlib release across the PPX ecosystem. New Ast_builder and Ast_pattern helpers let PPX authors produce and match encoded labeled tuples and effect patterns, while bivariant type parameters are also supported; these are documented in a new compatibility manual section. One behaviour change to note: duplicate attributes now raise instead of looping silently, which may surface latent bugs in existing PPXs.

utop

utop 2.17.0 (March 26, 2026) brings:

  • Support for OCaml 5.5.
  • Decoupling of utop.el from tuareg-mode: tuareg is now loaded lazily, and other OCaml major modes for Emacs (such as neocaml) can integrate with utop via the new utop-mode-compat-alist. Existing tuareg, caml, typerex, and reason users are unaffected.

WebAssembly

A new Backstage OCaml post, Wasm_of_ocaml: What Changed Since 6.1 (Apr 16), surveys the WebAssembly backend across js_of_ocaml 6.1–6.3. wasm_of_ocaml compiles OCaml bytecode to WebAssembly, targeting WasmGC so that OCaml values are managed by the host garbage collector. Highlights since 6.1:

  • The compiler now writes .wasm binary modules directly, instead of emitting WAT and converting it via Binaryen (still a required system dependency); WAT output remains available for debugging.
  • Improved Wasm code generation across 6.1–6.3.
  • The three larger features tracked in the post have now landed: dynlink/toplevel support (merged Apr 8), native effects via stack switching (May 28), and WASI support (Jun 12). Check js_of_ocaml release notes before relying on them in a released package.

As always, we encourage feedback and contributions from the community as we continue to improve the OCaml Platform ecosystem.