oraios/serena · error · RuntimeError

Could not parse OCaml version from output: {result.stdout.st

Error message

Could not parse OCaml version from output: {result.stdout.strip()}
Please ensure OCaml is properly installed: opam exec -- ocaml -version

What it means

This RuntimeError is raised when 'opam exec -- ocaml -version' succeeds but its output does not match the expected OCaml version pattern, so no version triple could be parsed. _detect_ocaml_version validates the toolchain at startup and cannot proceed without knowing the version. The raw unparsed stdout is included to aid diagnosis.

Source

Thrown at src/solidlsp/language_servers/ocaml_lsp_server.py:89

            )
            version_match = re.search(r"(\d+)\.(\d+)\.(\d+)", result.stdout)
            if version_match:
                major = int(version_match.group(1))
                minor = int(version_match.group(2))
                patch = int(version_match.group(3))
                version_tuple = (major, minor, patch)
                version_str = f"{major}.{minor}.{patch}"
                log.info(f"OCaml version: {version_str}")

                if version_tuple == (5, 1, 0):
                    raise RuntimeError(
                        f"OCaml {version_str} is incompatible with ocaml-lsp-server.\n"
                        "Please use OCaml < 5.1 or >= 5.1.1.\n"
                        "Consider creating a new opam switch:\n"
                        "  opam switch create <name> ocaml-base-compiler.4.14.2"
                    )
                return version_tuple
            raise RuntimeError(
                f"Could not parse OCaml version from output: {result.stdout.strip()}\n"
                "Please ensure OCaml is properly installed: opam exec -- ocaml -version"
            )
        except subprocess.CalledProcessError as e:
            raise RuntimeError(
                f"Failed to detect OCaml version: {e.stderr}\n"
                "Please ensure OCaml is installed and opam is configured:\n"
                "  opam switch show\n"
                "  opam exec -- ocaml -version"
            ) from e
        except FileNotFoundError as e:
            raise RuntimeError(
                "OCaml not found. Please install OCaml via opam:\n"
                "  opam switch create <name> ocaml-base-compiler.4.14.2\n"
                "  eval $(opam env)"
            ) from e

    @staticmethod

View on GitHub (pinned to 7fcbca7e62)

Solutions

Run 'opam exec -- ocaml -version' manually and check that output contains a parseable version like 'The OCaml toplevel, version 5.1.1'.
Remove or fix any ocaml shim/wrapper script or PATH entry shadowing the real compiler.
Ensure the correct opam switch is active (eval $(opam env)) so the expected ocaml binary runs.
Suppress or redirect any extra stdout noise (warnings) coming from shell init or wrappers.

Example fix

// before: ~/bin/ocaml wrapper prints banner text then execs real ocaml
#!/bin/sh
echo "custom ocaml wrapper v2"
exec /usr/bin/ocaml "$@"
// after
#!/bin/sh
exec /home/user/.opam/default/bin/ocaml "$@"

Defensive patterns

Strategy: validation

Validate before calling

import re, subprocess

def ocaml_version_parses() -> tuple:
    r = subprocess.run(
        ["opam", "exec", "--", "ocaml", "-version"],
        capture_output=True, text=True, check=True,
    )
    m = re.search(r"version\s+(\d+)\.(\d+)\.(\d+)", r.stdout)
    if not m:
        raise RuntimeError(
            f"Unparseable 'ocaml -version' output: {r.stdout!r}. "
            "Check for ocaml shims on PATH or extra stdout noise."
        )
    return tuple(map(int, m.groups()))

Try / catch

try:
    server = OcamlLanguageServer(**kwargs)
except RuntimeError as e:
    if "Could not parse OCaml version" in str(e):
        logger.error("Fix the ocaml toolchain so 'opam exec -- ocaml -version' prints a standard version line. Raw output shown in the error.")
        raise
    raise

Prevention

Avoid wrapper/shim scripts named 'ocaml' that add stdout output; keep the real binary first on PATH.
Test the raw output of 'opam exec -- ocaml -version' after any toolchain change.
Silence shell-init warnings that could pollute subprocess stdout.
Use official compiler packages from opam rather than custom-patched builds.

When it happens

Trigger: Constructing the OCaml language server when the 'ocaml -version' output is nonstandard: a wrapper/shim printing extra text, a customized ocaml build whose banner differs (e.g. missing 'The OCaml' version line format), locale/alias interference, or output from an unexpected binary named ocaml earlier on PATH.

Common situations: Custom opam wraps or scripts shadowing ocaml; minimal/embedded OCaml builds with altered version strings; environment where 'opam exec -- ocaml -version' emits warnings to stdout that break the regex.

Related errors

AI-assisted analysis of oraios/serena@7fcbca7e62 (2026-08-29). Data as JSON: /api/errors/845614039b4a1c47. Report an issue: GitHub.