What causes "Unsupported VCS dependency {vcs}" in python-poetry/poetry?

Convert the dependency to git (mirror the repo on a git host if needed).

python-poetry/poetry · error · ValueError

Unsupported VCS dependency {vcs}

Error message

Unsupported VCS dependency {vcs}

What it means

Raised by DirectOrigin.get_package_from_vcs at src/poetry/packages/direct_origin.py:118-119 when the vcs argument is not 'git'. Poetry currently supports only git for direct-origin VCS dependencies; any other value (hg, svn, bzr) is rejected. ValueError.

Source

Thrown at src/poetry/packages/direct_origin.py:119

        package = self.get_package_from_file(artifact)

        package._source_type = "url"
        package._source_url = url

        return package

    @staticmethod
    def get_package_from_vcs(
        vcs: str,
        url: str,
        branch: str | None = None,
        tag: str | None = None,
        rev: str | None = None,
        subdirectory: str | None = None,
        source_root: Path | None = None,
    ) -> Package:
        if vcs != "git":
            raise ValueError(f"Unsupported VCS dependency {vcs}")

        return _get_package_from_git(
            url=url,
            branch=branch,
            tag=tag,
            rev=rev,
            subdirectory=subdirectory,
            source_root=source_root,
        )

View on GitHub (pinned to 92b74dcfe3)

Solutions

Convert the dependency to git (mirror the repo on a git host if needed).
If the VCS repo must stay non-git, vendor the code as a directory or file dependency instead.
Verify the requirement string uses git+... and that the underlying tool is git.

Example fix

# before (pyproject.toml)
[tool.poetry.dependencies]
lib = { vcs = "hg", url = "https://.../lib" }

# after: mirror to git, then
lib = { git = "https://github.com/org/lib.git" }
# or vendor
lib = { path = "./vendor/lib" }

Defensive patterns

Strategy: validation

Validate before calling

SUPPORTED_VCS = {'git'}

def validate_vcs(vcs: str) -> None:
    if vcs not in SUPPORTED_VCS:
        raise ValueError(
            f'Unsupported VCS dependency {vcs!r}; only git is supported (mirror or vendor instead).'
        )

Type guard

def is_supported_vcs(vcs: str) -> bool:
    return vcs == 'git'

Try / catch

from poetry.packages.direct_origin import DirectOrigin

try:
    pkg = DirectOrigin.get_package_from_vcs(vcs=vcs, url=url)
except ValueError as e:
    if 'Unsupported VCS' in str(e):
        raise SystemExit(f'Mirror {url} to git or vendor it; {e}') from e
    raise

Prevention

Use git for VCS dependencies; mirror non-git repos to a git host.
If a non-git repo must be used, vendor it as a path dependency.

When it happens

Trigger: Calling get_package_from_vcs(vcs='hg', url=...) or declaring a dependency with a non-git VCS URL (e.g. hg+https://...). Reached during dependency resolution for VCS-type requirements.

Common situations: A project migrating from Pip/setuptools that used Mercurial or Subversion dependencies; a dependency spec copied from pip's VCS syntax that uses an unsupported VCS; typo'd scheme.

Related errors

AI-assisted analysis of python-poetry/poetry@92b74dcfe3 (2026-08-04). Data as JSON: /data/errors/b2453225defb6e81.json. Report an issue: GitHub.