astral-sh/uv · error · anyhow::Error

pip-compile's `--resolver=legacy` is unsupported (uv always

Error message

pip-compile's `--resolver=legacy` is unsupported (uv always backtracks)

What it means

uv's pip-compile compatibility layer rejects flags whose behavior it cannot reproduce. pip-tools' `--resolver legacy` selects the old (pre-2020) legacy resolver; uv implements only the backtracking resolver and always uses it, so `PipCompileCompatArgs::validate()` returns an error for `Resolver::Legacy`. Passing `--resolver backtracking` only warns that it has no effect, because that matches uv's behavior.

Source

Thrown at crates/uv-cli/src/compat.rs:97

        if self.reuse_hashes {
            return Err(anyhow!(
                "pip-compile's `--reuse-hashes` is unsupported (uv doesn't reuse hashes)"
            ));
        }

        if self.no_reuse_hashes {
            warn_user!("pip-compile's `--no-reuse-hashes` has no effect (uv doesn't reuse hashes)");
        }

        if let Some(resolver) = self.resolver {
            match resolver {
                Resolver::Backtracking => {
                    warn_user!(
                        "pip-compile's `--resolver=backtracking` has no effect (uv always backtracks)"
                    );
                }
                Resolver::Legacy => {
                    return Err(anyhow!(
                        "pip-compile's `--resolver=legacy` is unsupported (uv always backtracks)"
                    ));
                }
            }
        }

        if self.max_rounds.is_some() {
            return Err(anyhow!(
                "pip-compile's `--max-rounds` is unsupported (uv always resolves until convergence)"
            ));
        }

        if self.client_cert.is_some() {
            return Err(anyhow!(
                "pip-compile's `--client-cert` is unsupported (uv doesn't support dedicated client certificates)"
            ));
        }

View on GitHub (pinned to f1a42680ff)

Solutions

1. Drop `--resolver legacy`; uv's backtracking resolver is the only mode.
2. If resolution differences are the concern, pin inputs (constraints, `--resolution lowest-direct`, lockfile) instead of the resolver mode.
3. Remove the equally-dead `--resolver backtracking` form too (it only warns) to keep scripts clean.

Example fix

# before
uv pip compile --resolver legacy requirements.in

# after
uv pip compile requirements.in

def sanitize_pip_compile_args(argv: list[str]) -> list[str]:
    out = []
    for arg in argv:
        if arg in ("--resolver", "--resolver=legacy"):
            raise ValueError("--resolver legacy has no uv equivalent; uv always backtracks")
        if arg == "legacy":  # value of --resolver
            continue
        out.append(arg)
    return out

When it happens

Trigger: `uv pip compile --resolver legacy requirements.in`, or a pip-compile invocation copied into a Makefile/CI job and rerun with `uv pip compile` unchanged.

Common situations: Legacy pip-compile invocations pinned to the old resolver for reproducibility; migration to uv without pruning flags; scripts shared across teams where one member still uses pip-tools and another uv.

Related errors

• pip-compile's `--max-rounds` is unsupported (uv always resol
• pip-compile's `--reuse-hashes` is unsupported (uv doesn't re
• pip-compile's `--client-cert` is unsupported (uv doesn't sup
• pip-compile's `--emit-trusted-host` is unsupported
• pip-compile's `--config` is unsupported (uv does not use a c

AI-assisted analysis of astral-sh/uv@f1a42680ff (2026-08-16). Data as JSON: /api/errors/1ffb4f5dc77d2a23. Report an issue: GitHub.