Package `{}` is missing a `{}`. For example, to build with `

Error message

Package `{}` is missing a `{}`. For example, to build with `{}`, add the following to `{}`:
```toml
[build-system]
requires = ["uv_build>={min_version},<{max_version}"]
build-backend = "uv_build"
```

What it means

`uv build --package <name>` found the member but its pyproject.toml fails is_package(true): there is no usable `[build-system]` table (with require_build_system semantics), so uv does not know how to build it. The message embeds a ready-to-paste `[build-system]` snippet pinned to the running uv's version range, using the uv_build backend.

Source

Thrown at crates/uv/src/commands/build_frontend.rs:378

            ));
        }

        let workspace = match workspace {
            Ok(ref workspace) => workspace,
            Err(err) => {
                return Err(err).context("`--package` was provided, but no workspace was found");
            }
        };

        let package = workspace
            .packages()
            .get(package)
            .ok_or_else(|| anyhow::anyhow!("Package `{package}` not found in workspace"))?;

        if !package.pyproject_toml().is_package(true) {
            let name = &package.project().name;
            let pyproject_toml = package.root().join("pyproject.toml");
            return Err(anyhow::anyhow!(
                "Package `{}` is missing a `{}`. For example, to build with `{}`, add the following to `{}`:\n```toml\n[build-system]\nrequires = [\"uv_build>={min_version},<{max_version}\"]\nbuild-backend = \"uv_build\"\n```",
                name.cyan(),
                "build-system".green(),
                "uv_build".cyan(),
                pyproject_toml.user_display().cyan()
            ));
        }

        vec![AnnotatedSource::from(Source::Directory(Cow::Borrowed(
            package.root(),
        )))]
    } else if all_packages {
        if matches!(src, Source::File(_)) {
            return Err(anyhow::anyhow!(
                "Cannot specify `--all-packages` when building from a file"
            ));
        }

View on GitHub (pinned to f1a42680ff)

Solutions

Add the exact `[build-system]` table shown in the error to the member's pyproject.toml (adjust the backend if you use setuptools/hatchling/flit instead of uv_build).
If the member is intentionally non-buildable (virtual), don't target it with --package; build a real package instead.
After editing, re-run `uv build --package <name>` from the workspace root.

Example fix

# before: crates/mylib/pyproject.toml has only [project]
# after: append
[build-system]
requires = ["uv_build>=0.8,<0.9"]
build-backend = "uv_build"

Defensive patterns

Strategy: validation

Validate before calling

# Rust: pre-check buildability before selecting a package
let pkg = workspace.packages().get(&name)
    .ok_or_else(|| anyhow::anyhow!("{name} not in workspace"))?;
if !pkg.pyproject_toml().is_package(true) {
    anyhow::bail!("{name} lacks [build-system]; add it before building");
}

Type guard

fn is_buildable(pkg: &WorkspacePackage) -> bool {
    pkg.pyproject_toml().is_package(true)
}

Prevention

Give every buildable member an explicit [build-system] table — uv never assumes a default backend.
In repo CI, lint that each workspace member intended for builds has build-system.requires and build-backend.

When it happens

Trigger: `uv build --package lib-x` where lib-x is a virtual/library member with only `[project]` and no `[build-system]`; member configured with a build backend uv considers non-buildable for this path; freshly split-out package whose pyproject.toml was copied without the build-system section.

Common situations: Workspaces that mix virtual helper packages and buildable ones; migrating packages from setuptools where the section was omitted (implicit backend) — uv requires it to be explicit for --package builds.

Related errors

AI-assisted analysis of astral-sh/uv@f1a42680ff (2026-08-16). Data as JSON: /api/errors/75aa5d88a9a4b14a. Report an issue: GitHub.