Add support for virtual projects (#6585)

## Summary

The basic idea here is: any project can either be a package, or not
("virtual").

If a project is virtual, we don't build or install it.

A project is virtual if either of the following are true:

- `tool.uv.virtual = true` is set.
- `[build-system]` is absent.

The concept of "virtual projects" only applies to workspace member right
now; it doesn't apply to `path` dependencies which are treated like
arbitrary Python source trees.

TODOs that should be resolved prior to merging:

- [ ] Documentation
- [ ] How do we reconcile this with "virtual workspace roots" which are
a little different -- they omit `[project]` entirely and don't even have
a name?
- [x] `uv init --virtual` should create a virtual project rather than a
virtual workspace.
- [x] Running `uv sync` in a virtual project after `uv init --virtual`
shows `Audited 0 packages in 0.01ms`, which is awkward. (See:
https://github.com/astral-sh/uv/pull/6588.)

Closes https://github.com/astral-sh/uv/issues/6511.

crates/uv-cli/src/lib.rs

@@ -2099,11 +2099,12 @@ pub struct InitArgs {
     #[arg(long)]
     pub name: Option<PackageName>,
 
-    /// Create a virtual workspace instead of a project.
+    /// Create a virtual project, rather than a package.
     ///
-    /// A virtual workspace does not define project dependencies and cannot be
+    /// A virtual project is a project that is not intended to be built as a Python package,
-    /// published. Instead, workspace members declare project dependencies.
+    /// such as a project that only contains scripts or other application code.
-    /// Development dependencies may still be declared.
+    ///
+    /// Virtual projects themselves are not installed into the Python environment.
     #[arg(long)]
     pub r#virtual: bool,
 

crates/uv-settings/src/settings.rs

@@ -70,6 +70,10 @@ pub struct Options {
     #[serde(default, skip_serializing)]
     #[cfg_attr(feature = "schemars", schemars(skip))]
     managed: serde::de::IgnoredAny,
+
+    #[serde(default, skip_serializing)]
+    #[cfg_attr(feature = "schemars", schemars(skip))]
+    r#package: serde::de::IgnoredAny,
 }
 
 impl Options {

crates/uv-workspace/src/pyproject.rs

@@ -33,6 +33,10 @@ pub struct PyProjectToml {
     /// The raw unserialized document.
     #[serde(skip)]
     pub raw: String,
+
+    /// Used to determine whether a `build-system` is present.
+    #[serde(default, skip_serializing)]
+    build_system: Option<serde::de::IgnoredAny>,
 }
 
 impl PyProjectToml {
@@ -41,6 +45,23 @@ impl PyProjectToml {
         let pyproject = toml::from_str(&raw)?;
         Ok(PyProjectToml { raw, ..pyproject })
     }
+
+    /// Returns `true` if the project should be considered a Python package, as opposed to a
+    /// non-package ("virtual") project.
+    pub fn is_package(&self) -> bool {
+        // If `tool.uv.package` is set, defer to that explicit setting.
+        if let Some(is_package) = self
+            .tool
+            .as_ref()
+            .and_then(|tool| tool.uv.as_ref())
+            .and_then(|uv| uv.package)
+        {
+            return is_package;
+        }
+
+        // Otherwise, a project is assumed to be a package if `build-system` is present.
+        self.build_system.is_some()
+    }
 }
 
 // Ignore raw document in comparison.
@@ -100,6 +121,24 @@ pub struct ToolUv {
         "#
     )]
     pub managed: Option<bool>,
+    /// Whether the project should be considered a Python package, or a non-package ("virtual")
+    /// project.
+    ///
+    /// Packages are built and installed into the virtual environment in editable mode and thus
+    /// require a build backend, while virtual projects are _not_ built or installed; instead, only
+    /// their dependencies are included in the virtual environment.
+    ///
+    /// Creating a package requires that a `build-system` is present in the `pyproject.toml`, and
+    /// that the project adheres to a structure that adheres to the build backend's expectations
+    /// (e.g., a `src` layout).
+    #[option(
+        default = r#"true"#,
+        value_type = "bool",
+        example = r#"
+            package = false
+        "#
+    )]
+    pub package: Option<bool>,
     /// The project's development dependencies. Development dependencies will be installed by
     /// default in `uv run` and `uv sync`, but will not appear in the project's published metadata.
     #[cfg_attr(

crates/uv-workspace/src/workspace.rs

@@ -1339,7 +1339,7 @@ impl VirtualProject {
         }
     }
 
-    /// Return the [`PackageName`] of the project, if it's not a virtual workspace.
+    /// Return the [`PackageName`] of the project, if it's not a virtual workspace root.
     pub fn project_name(&self) -> Option<&PackageName> {
         match self {
             VirtualProject::Project(project) => Some(project.project_name()),
@@ -1347,7 +1347,7 @@ impl VirtualProject {
         }
     }
 
-    /// Returns `true` if the project is a virtual workspace.
+    /// Returns `true` if the project is a virtual workspace root.
     pub fn is_virtual(&self) -> bool {
         matches!(self, VirtualProject::Virtual(_))
     }
@@ -1535,6 +1535,7 @@ mod tests {
                         "exclude": null
                       },
                       "managed": null,
+                      "package": null,
                       "dev-dependencies": null,
                       "environments": null,
                       "override-dependencies": null,
@@ -1607,6 +1608,7 @@ mod tests {
                         "exclude": null
                       },
                       "managed": null,
+                      "package": null,
                       "dev-dependencies": null,
                       "environments": null,
                       "override-dependencies": null,

crates/uv/src/commands/project/init.rs

@@ -15,7 +15,7 @@ use uv_python::{
 };
 use uv_resolver::RequiresPython;
 use uv_workspace::pyproject_mut::{DependencyTarget, PyProjectTomlMut};
-use uv_workspace::{check_nested_workspaces, DiscoveryOptions, Workspace, WorkspaceError};
+use uv_workspace::{DiscoveryOptions, Workspace, WorkspaceError};
 
 use crate::commands::project::find_requires_python;
 use crate::commands::reporters::PythonDownloadReporter;
@@ -69,24 +69,21 @@ pub(crate) async fn init(
         }
     };
 
-    if r#virtual {
+    init_project(
-        init_virtual_workspace(&path, no_workspace)?;
+        &path,
-    } else {
+        &name,
-        init_project(
+        r#virtual,
-            &path,
+        no_readme,
-            &name,
+        python,
-            no_readme,
+        no_workspace,
-            python,
+        python_preference,
-            no_workspace,
+        python_downloads,
-            python_preference,
+        connectivity,
-            python_downloads,
+        native_tls,
-            connectivity,
+        cache,
-            native_tls,
+        printer,
-            cache,
+    )
-            printer,
+    .await?;
-        )
-        .await?;
-    }
 
     // Create the `README.md` if it does not already exist.
     if !no_readme {
@@ -126,29 +123,12 @@ pub(crate) async fn init(
     Ok(ExitStatus::Success)
 }
 
-/// Initialize a virtual workspace at the given path.
-fn init_virtual_workspace(path: &Path, no_workspace: bool) -> Result<()> {
-    // Ensure that we aren't creating a nested workspace.
-    if !no_workspace {
-        check_nested_workspaces(path, &DiscoveryOptions::default());
-    }
-
-    // Create the `pyproject.toml`.
-    let pyproject = indoc::indoc! {r"
-        [tool.uv.workspace]
-        members = []
-    "};
-
-    fs_err::create_dir_all(path)?;
-    fs_err::write(path.join("pyproject.toml"), pyproject)?;
-
-    Ok(())
-}
-
 /// Initialize a project (and, implicitly, a workspace root) at the given path.
+#[allow(clippy::fn_params_excessive_bools)]
 async fn init_project(
     path: &Path,
     name: &PackageName,
+    r#virtual: bool,
     no_readme: bool,
     python: Option<String>,
     no_workspace: bool,
@@ -265,38 +245,56 @@ async fn init_project(
         RequiresPython::greater_than_equal_version(&interpreter.python_minor_version())
     };
 
-    // Create the `pyproject.toml`.
+    if r#virtual {
-    let pyproject = indoc::formatdoc! {r#"
+        // Create the `pyproject.toml`, but omit `[build-system]`.
-        [project]
+        let pyproject = indoc::formatdoc! {r#"
-        name = "{name}"
+            [project]
-        version = "0.1.0"
+            name = "{name}"
-        description = "Add your description here"{readme}
+            version = "0.1.0"
-        requires-python = "{requires_python}"
+            description = "Add your description here"{readme}
-        dependencies = []
+            requires-python = "{requires_python}"
+            dependencies = []
+            "#,
+            readme = if no_readme { "" } else { "\nreadme = \"README.md\"" },
+            requires_python = requires_python.specifiers(),
+        };
 
-        [build-system]
+        fs_err::create_dir_all(path)?;
-        requires = ["hatchling"]
+        fs_err::write(path.join("pyproject.toml"), pyproject)?;
-        build-backend = "hatchling.build"
+    } else {
-        "#,
+        // Create the `pyproject.toml`.
-        readme = if no_readme { "" } else { "\nreadme = \"README.md\"" },
+        let pyproject = indoc::formatdoc! {r#"
-        requires_python = requires_python.specifiers(),
+            [project]
-    };
+            name = "{name}"
+            version = "0.1.0"
+            description = "Add your description here"{readme}
+            requires-python = "{requires_python}"
+            dependencies = []
 
-    fs_err::create_dir_all(path)?;
+            [build-system]
-    fs_err::write(path.join("pyproject.toml"), pyproject)?;
+            requires = ["hatchling"]
+            build-backend = "hatchling.build"
+            "#,
+            readme = if no_readme { "" } else { "\nreadme = \"README.md\"" },
+            requires_python = requires_python.specifiers(),
+        };
 
-    // Create `src/{name}/__init__.py`, if it doesn't exist already.
+        fs_err::create_dir_all(path)?;
-    let src_dir = path.join("src").join(&*name.as_dist_info_name());
+        fs_err::write(path.join("pyproject.toml"), pyproject)?;
-    let init_py = src_dir.join("__init__.py");
+
-    if !init_py.try_exists()? {
+        // Create `src/{name}/__init__.py`, if it doesn't exist already.
-        fs_err::create_dir_all(&src_dir)?;
+        let src_dir = path.join("src").join(&*name.as_dist_info_name());
-        fs_err::write(
+        let init_py = src_dir.join("__init__.py");
-            init_py,
+        if !init_py.try_exists()? {
-            indoc::formatdoc! {r#"
+            fs_err::create_dir_all(&src_dir)?;
+            fs_err::write(
+                init_py,
+                indoc::formatdoc! {r#"
             def hello() -> str:
                 return "Hello from {name}!"
             "#},
-        )?;
+            )?;
+        }
     }
 
     if let Some(workspace) = workspace {

crates/uv/src/commands/project/sync.rs

@@ -1,5 +1,8 @@
 use anyhow::{Context, Result};
 use itertools::Itertools;
+use rustc_hash::FxHashSet;
+
+use distribution_types::Name;
 use pep508_rs::MarkerTree;
 use uv_auth::store_credentials_from_url;
 use uv_cache::Cache;
@@ -195,6 +198,9 @@ pub(super) async fn do_sync(
     // Read the lockfile.
     let resolution = lock.to_resolution(project, &markers, tags, extras, &dev)?;
 
+    // Always skip virtual projects, which shouldn't be built or installed.
+    let resolution = apply_no_virtual_project(resolution, project);
+
     // Filter resolution based on install-specific options.
     let resolution = install_options.filter_resolution(resolution, project);
 
@@ -289,3 +295,32 @@ pub(super) async fn do_sync(
 
     Ok(())
 }
+
+/// Filter out any virtual workspace members.
+fn apply_no_virtual_project(
+    resolution: distribution_types::Resolution,
+    project: &VirtualProject,
+) -> distribution_types::Resolution {
+    let VirtualProject::Project(project) = project else {
+        // If the project is _only_ a virtual workspace root, we don't need to filter it out.
+        return resolution;
+    };
+
+    let virtual_members = project
+        .workspace()
+        .packages()
+        .iter()
+        .filter_map(|(name, package)| {
+            // A project is a package if it's explicitly marked as such, _or_ if a build system is
+            // present.
+            if package.pyproject_toml().is_package() {
+                None
+            } else {
+                Some(name)
+            }
+        })
+        .collect::<FxHashSet<_>>();
+
+    // Remove any virtual members from the resolution.
+    resolution.filter(|dist| !virtual_members.contains(dist.name()))
+}

crates/uv/tests/common/mod.rs

@@ -1110,8 +1110,9 @@ pub fn make_project(dir: &Path, name: &str, body: &str) -> anyhow::Result<()> {
         {body}
 
         [build-system]
-        requires = ["flit_core>=3.8,<4"]
+        requires = ["setuptools>=42", "wheel"]
-        build-backend = "flit_core.buildapi"
+        build-backend = "setuptools.build_meta"
+
         "#
     };
     fs_err::create_dir_all(dir)?;

crates/uv/tests/edit.rs

@@ -20,9 +20,12 @@ fn add_registry() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["anyio==3.7.0"]), @r###"
@@ -50,11 +53,14 @@ fn add_registry() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = [
             "anyio==3.7.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -143,6 +149,10 @@ fn add_git() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.lock(), @r###"
@@ -212,6 +222,10 @@ fn add_git() -> Result<()> {
             "uv-public-pypackage",
         ]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         uv-public-pypackage = { git = "https://github.com/astral-test/uv-public-pypackage", tag = "0.0.1" }
         "###
@@ -313,6 +327,10 @@ fn add_git_private_source() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&[&format!("uv-private-pypackage @ git+https://{token}@github.com/astral-test/uv-private-pypackage")]), @r###"
@@ -343,6 +361,10 @@ fn add_git_private_source() -> Result<()> {
             "uv-private-pypackage",
         ]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         uv-private-pypackage = { git = "https://github.com/astral-test/uv-private-pypackage" }
         "###
@@ -409,6 +431,10 @@ fn add_git_private_raw() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&[&format!("uv-private-pypackage @ git+https://{token}@github.com/astral-test/uv-private-pypackage")]).arg("--raw-sources"), @r###"
@@ -443,6 +469,10 @@ fn add_git_private_raw() -> Result<()> {
         dependencies = [
             "uv-private-pypackage @ git+https://***@github.com/astral-test/uv-private-pypackage",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -504,6 +534,10 @@ fn add_git_error() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.lock(), @r###"
@@ -562,6 +596,10 @@ fn add_git_raw() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.lock(), @r###"
@@ -617,6 +655,10 @@ fn add_git_raw() -> Result<()> {
             "anyio==3.7.0",
             "uv-public-pypackage @ git+https://github.com/astral-test/uv-public-pypackage@0.0.1",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -714,6 +756,10 @@ fn add_git_implicit() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.lock(), @r###"
@@ -770,6 +816,10 @@ fn add_raw_error() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Provide a tag without a Git source.
@@ -800,9 +850,12 @@ fn add_unnamed() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["git+https://github.com/astral-test/uv-public-pypackage"]).arg("--tag=0.0.1"), @r###"
@@ -828,12 +881,15 @@ fn add_unnamed() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = [
             "uv-public-pypackage",
         ]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         uv-public-pypackage = { git = "https://github.com/astral-test/uv-public-pypackage", tag = "0.0.1" }
         "###
@@ -897,6 +953,10 @@ fn add_remove_dev() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["anyio==3.7.0"]).arg("--dev"), @r###"
@@ -927,6 +987,10 @@ fn add_remove_dev() -> Result<()> {
         requires-python = ">=3.12"
         dependencies = []
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv]
         dev-dependencies = [
             "anyio==3.7.0",
@@ -1049,6 +1113,10 @@ fn add_remove_dev() -> Result<()> {
         requires-python = ">=3.12"
         dependencies = []
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv]
         dev-dependencies = []
         "###
@@ -1101,6 +1169,10 @@ fn add_remove_optional() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["anyio==3.7.0"]).arg("--optional=io"), @r###"
@@ -1135,6 +1207,10 @@ fn add_remove_optional() -> Result<()> {
         io = [
             "anyio==3.7.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -1254,6 +1330,10 @@ fn add_remove_optional() -> Result<()> {
 
         [project.optional-dependencies]
         io = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -1310,6 +1390,10 @@ fn add_remove_workspace() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     let pyproject_toml = context.temp_dir.child("child2/pyproject.toml");
@@ -1319,6 +1403,10 @@ fn add_remove_workspace() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Adding a workspace package with a mismatched source should error.
@@ -1374,6 +1462,10 @@ fn add_remove_workspace() -> Result<()> {
             "child2",
         ]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         child2 = { workspace = true }
         "###
@@ -1457,6 +1549,10 @@ fn add_remove_workspace() -> Result<()> {
         requires-python = ">=3.12"
         dependencies = []
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         "###
         );
@@ -1525,6 +1621,10 @@ fn add_workspace_editable() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     let pyproject_toml = context.temp_dir.child("child2/pyproject.toml");
@@ -1534,6 +1634,10 @@ fn add_workspace_editable() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     let child1 = context.temp_dir.join("child1");
@@ -1568,6 +1672,10 @@ fn add_workspace_editable() -> Result<()> {
             "child2",
         ]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         child2 = { workspace = true }
         "###
@@ -1638,6 +1746,10 @@ fn add_path() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     let child = workspace.child("child");
@@ -1647,6 +1759,10 @@ fn add_path() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["./child"]).current_dir(workspace.path()), @r###"
@@ -1679,6 +1795,10 @@ fn add_path() -> Result<()> {
             "child",
         ]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         child = { path = "child" }
         "###
@@ -1743,9 +1863,11 @@ fn update() -> Result<()> {
         name = "project"
         version = "0.1.0"
         requires-python = ">=3.12"
-        dependencies = [
+        dependencies = ["requests==2.31.0"]
-            "requests==2.31.0"
+
-        ]
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.lock(), @r###"
@@ -1801,6 +1923,10 @@ fn update() -> Result<()> {
         dependencies = [
             "requests[security]==2.31.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -1836,6 +1962,10 @@ fn update() -> Result<()> {
             "requests[security]==2.31.0",
             "requests[socks,use-chardet-on-py3]>=2.31.0 ; python_full_version >= '3.8'",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -1873,6 +2003,10 @@ fn update() -> Result<()> {
             "requests[socks,use-chardet-on-py3]>=2.31.0 ; python_full_version >= '3.8'",
         ]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         requests = { git = "https://github.com/psf/requests", tag = "v2.32.3" }
         "###
@@ -2021,9 +2155,11 @@ fn add_update_marker() -> Result<()> {
         name = "project"
         version = "0.1.0"
         requires-python = ">=3.8"
-        dependencies = [
+        dependencies = ["requests>=2.30; python_version >= '3.11'"]
-            "requests>=2.30; python_version >= '3.11'"
+
-        ]
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     uv_snapshot!(context.filters(), context.lock(), @r###"
     success: true
@@ -2080,6 +2216,10 @@ fn add_update_marker() -> Result<()> {
             "requests>=2.30; python_version >= '3.11'",
             "requests>=2.0,<2.29 ; python_full_version < '3.11'",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2114,6 +2254,10 @@ fn add_update_marker() -> Result<()> {
             "requests>=2.30; python_version >= '3.11'",
             "requests>=2.0,<2.20 ; python_full_version < '3.11'",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2153,6 +2297,10 @@ fn add_update_marker() -> Result<()> {
             "requests>=2.0,<2.20 ; python_full_version < '3.11'",
             "requests>=2.31 ; python_full_version >= '3.12' and sys_platform == 'win32'",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2190,6 +2338,10 @@ fn add_update_marker() -> Result<()> {
             "requests>=2.31 ; python_full_version >= '3.12' and sys_platform == 'win32'",
             "requests>=2.10 ; sys_platform == 'win32'",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2226,6 +2378,10 @@ fn add_update_marker() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.8"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2247,6 +2403,10 @@ fn update_source_replace_url() -> Result<()> {
         dependencies = [
             "requests[security] @ https://files.pythonhosted.org/packages/f9/9b/335f9764261e915ed497fcdeb11df5dfd6f7bf257d4a6a2a686d80da4d54/requests-2.32.3-py3-none-any.whl"
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Change the source. The existing URL should be removed.
@@ -2282,6 +2442,10 @@ fn update_source_replace_url() -> Result<()> {
             "requests[security]",
         ]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         requests = { git = "https://github.com/psf/requests", tag = "v2.32.3" }
         "###
@@ -2302,9 +2466,11 @@ fn add_inexact() -> Result<()> {
         name = "project"
         version = "0.1.0"
         requires-python = ">=3.12"
-        dependencies = [
+        dependencies = ["anyio == 3.7.0"]
-            "anyio == 3.7.0",
+
-        ]
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.lock(), @r###"
@@ -2337,6 +2503,10 @@ fn add_inexact() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["iniconfig==2.0.0"]), @r###"
@@ -2367,6 +2537,10 @@ fn add_inexact() -> Result<()> {
         dependencies = [
             "iniconfig==2.0.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2445,6 +2619,10 @@ fn remove_registry() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.lock(), @r###"
@@ -2498,6 +2676,10 @@ fn remove_registry() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2545,11 +2727,12 @@ fn add_preserves_indentation_in_pyproject_toml() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
-        dependencies = [
+        dependencies = ["anyio==3.7.0"]
-          "anyio==3.7.0"
+
-        ]
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["requests==2.31.0"]), @r###"
@@ -2581,12 +2764,15 @@ fn add_preserves_indentation_in_pyproject_toml() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = [
-          "anyio==3.7.0",
+            "anyio==3.7.0",
-          "requests==2.31.0",
+            "requests==2.31.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2602,9 +2788,12 @@ fn add_puts_default_indentation_in_pyproject_toml_if_not_observed() -> Result<()
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["requests==2.31.0"]), @r###"
@@ -2636,12 +2825,15 @@ fn add_puts_default_indentation_in_pyproject_toml_if_not_observed() -> Result<()
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = [
             "anyio==3.7.0",
             "requests==2.31.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2658,9 +2850,12 @@ fn add_frozen() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["anyio==3.7.0"]).arg("--frozen"), @r###"
@@ -2681,11 +2876,14 @@ fn add_frozen() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = [
             "anyio==3.7.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2705,9 +2903,12 @@ fn add_no_sync() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["anyio==3.7.0"]).arg("--no-sync"), @r###"
@@ -2729,11 +2930,14 @@ fn add_no_sync() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = [
             "anyio==3.7.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2821,9 +3025,12 @@ fn add_error() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["xyz"]), @r###"
@@ -2863,6 +3070,10 @@ fn add_lower_bound() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Adding `anyio` should include a lower-bound.
@@ -2895,6 +3106,10 @@ fn add_lower_bound() -> Result<()> {
         dependencies = [
             "anyio>=4.3.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2914,6 +3129,10 @@ fn add_lower_bound_existing() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Adding `anyio` should _not_ set a lower-bound, since it's already present (even if
@@ -2947,6 +3166,10 @@ fn add_lower_bound_existing() -> Result<()> {
         dependencies = [
             "anyio",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -2966,6 +3189,10 @@ fn add_lower_bound_raw() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Adding `anyio` should _not_ set a lower-bound when using `--raw-sources`.
@@ -2998,6 +3225,10 @@ fn add_lower_bound_raw() -> Result<()> {
         dependencies = [
             "anyio",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -3017,6 +3248,10 @@ fn add_lower_bound_dev() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Adding `anyio` should include a lower-bound.
@@ -3048,6 +3283,10 @@ fn add_lower_bound_dev() -> Result<()> {
         requires-python = ">=3.12"
         dependencies = []
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv]
         dev-dependencies = [
             "anyio>=4.3.0",
@@ -3071,6 +3310,10 @@ fn add_lower_bound_optional() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Adding `anyio` should include a lower-bound.
@@ -3106,6 +3349,10 @@ fn add_lower_bound_optional() -> Result<()> {
         io = [
             "anyio>=4.3.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -3185,6 +3432,10 @@ fn add_lower_bound_local() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Adding `torch` should include a lower-bound, but no local segment.
@@ -3215,6 +3466,10 @@ fn add_lower_bound_local() -> Result<()> {
         dependencies = [
             "local-simple-a>=1.2.3",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -3359,9 +3614,12 @@ fn add_repeat() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     uv_snapshot!(context.filters(), context.add(&["anyio"]), @r###"
@@ -3389,11 +3647,14 @@ fn add_repeat() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = [
             "anyio>=4.3.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -3418,11 +3679,14 @@ fn add_repeat() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = [
             "anyio>=4.3.0",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });
@@ -3440,9 +3704,12 @@ fn add_requirements_file() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     let requirements_txt = context.temp_dir.child("requirements.txt");
@@ -3481,13 +3748,16 @@ fn add_requirements_file() -> Result<()> {
         [project]
         name = "project"
         version = "0.1.0"
-        # ...
         requires-python = ">=3.12"
         dependencies = [
             "flask==2.3.2",
             "anyio",
         ]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         anyio = { git = "https://github.com/agronholm/anyio.git", rev = "4.4.0" }
         "###
@@ -3985,6 +4255,10 @@ fn fail_to_add_revert_project() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Adding `pytorch==1.0.2` should produce an error
@@ -4032,6 +4306,10 @@ fn fail_to_add_revert_project() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "###
         );
     });

crates/uv/tests/init.rs

@@ -882,9 +882,9 @@ fn init_explicit_workspace() -> Result<()> {
     Ok(())
 }
 
-/// Run `uv init` from within a virtual workspace.
+/// Run `uv init --virtual` to create a virtual project.
 #[test]
-fn init_virtual_workspace() -> Result<()> {
+fn init_virtual_project() -> Result<()> {
     let context = TestContext::new("3.12");
 
     let child = context.temp_dir.child("foo");
@@ -907,8 +907,13 @@ fn init_virtual_workspace() -> Result<()> {
     }, {
         assert_snapshot!(
             pyproject, @r###"
-        [tool.uv.workspace]
+        [project]
-        members = []
+        name = "foo"
+        version = "0.1.0"
+        description = "Add your description here"
+        readme = "README.md"
+        requires-python = ">=3.12"
+        dependencies = []
         "###
         );
     });
@@ -923,6 +928,56 @@ fn init_virtual_workspace() -> Result<()> {
     Initialized project `bar` at `[TEMP_DIR]/foo/bar`
     "###);
 
+    let pyproject = fs_err::read_to_string(pyproject_toml)?;
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject, @r###"
+        [project]
+        name = "foo"
+        version = "0.1.0"
+        description = "Add your description here"
+        readme = "README.md"
+        requires-python = ">=3.12"
+        dependencies = []
+
+        [tool.uv.workspace]
+        members = ["bar"]
+        "###
+        );
+    });
+
+    Ok(())
+}
+
+/// Run `uv init` from within a virtual workspace.
+#[test]
+fn init_virtual_workspace() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let child = context.temp_dir.child("foo");
+    child.create_dir_all()?;
+
+    // Create a virtual workspace.
+    let pyproject_toml = child.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {
+        r"
+        [tool.uv.workspace]
+        members = []
+        ",
+    })?;
+
+    uv_snapshot!(context.filters(), context.init().current_dir(&child).arg("bar"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Adding `bar` as member of workspace `[TEMP_DIR]/foo`
+    Initialized project `bar` at `[TEMP_DIR]/foo/bar`
+    "###);
+
     let pyproject = fs_err::read_to_string(pyproject_toml)?;
     insta::with_settings!({
         filters => context.filters(),
@@ -957,7 +1012,7 @@ fn init_nested_virtual_workspace() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    warning: Nested workspaces are not supported, but outer workspace (`[TEMP_DIR]/`) includes `[TEMP_DIR]/foo`
+    Adding `foo` as member of workspace `[TEMP_DIR]/`
     Initialized workspace `foo` at `[TEMP_DIR]/foo`
     "###);
 
@@ -967,8 +1022,13 @@ fn init_nested_virtual_workspace() -> Result<()> {
     }, {
         assert_snapshot!(
             pyproject, @r###"
-        [tool.uv.workspace]
+        [project]
-        members = []
+        name = "foo"
+        version = "0.1.0"
+        description = "Add your description here"
+        readme = "README.md"
+        requires-python = ">=3.12"
+        dependencies = []
         "###
         );
     });
@@ -980,7 +1040,7 @@ fn init_nested_virtual_workspace() -> Result<()> {
         assert_snapshot!(
             workspace, @r###"
         [tool.uv.workspace]
-        members = []
+        members = ["foo"]
         "###
         );
     });

crates/uv/tests/run.rs

@@ -27,6 +27,10 @@ fn run_with_python_version() -> Result<()> {
           "anyio==3.6.0 ; python_version == '3.11'",
           "anyio==3.7.0 ; python_version == '3.12'",
         ]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
     let test_script = context.temp_dir.child("main.py");
@@ -146,6 +150,10 @@ fn run_args() -> Result<()> {
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -201,6 +209,10 @@ fn run_pep723_script() -> Result<()> {
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["anyio"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -363,6 +375,10 @@ fn run_pythonw_script() -> Result<()> {
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["anyio"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -473,6 +489,10 @@ fn run_managed_false() -> Result<()> {
         requires-python = ">=3.8"
         dependencies = ["anyio"]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv]
         managed = false
         "#
@@ -501,6 +521,10 @@ fn run_with() -> Result<()> {
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["anyio", "sniffio==1.3.1"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -597,6 +621,10 @@ fn run_with_editable() -> Result<()> {
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["anyio", "sniffio==1.3.1"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -660,6 +688,10 @@ fn run_with_editable() -> Result<()> {
         requires-python = ">=3.8"
         dependencies = ["anyio", "sniffio==1.3.1"]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         anyio = { path = "./src/anyio_local", editable = true }
         "#
@@ -719,6 +751,10 @@ fn run_locked() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -745,6 +781,10 @@ fn run_locked() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["iniconfig"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -797,6 +837,10 @@ fn run_frozen() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -820,6 +864,10 @@ fn run_frozen() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["iniconfig"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -853,6 +901,10 @@ fn run_empty_requirements_txt() -> Result<()> {
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["anyio", "sniffio==1.3.1"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -909,6 +961,10 @@ fn run_requirements_txt() -> Result<()> {
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["anyio", "sniffio==1.3.1"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -1024,6 +1080,10 @@ fn run_requirements_txt_arguments() -> Result<()> {
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["typing_extensions"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -1069,15 +1129,15 @@ fn run_editable() -> Result<()> {
 
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
     pyproject_toml.write_str(indoc! { r#"
-        [build-system]
-        requires = ["hatchling"]
-        build-backend = "hatchling.build"
-
         [project]
         name = "foo"
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -1136,6 +1196,10 @@ fn run_from_directory() -> Result<()> {
 
         [project.scripts]
         main = "main:main"
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
     let main_script = project_dir.child("main.py");
@@ -1180,6 +1244,10 @@ fn run_without_output() -> Result<()> {
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["anyio", "sniffio==1.3.1"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -1219,15 +1287,15 @@ fn run_isolated_python_version() -> Result<()> {
 
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
     pyproject_toml.write_str(indoc! { r#"
-        [build-system]
-        requires = ["hatchling"]
-        build-backend = "hatchling.build"
-
         [project]
         name = "foo"
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["anyio"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 
@@ -1318,15 +1386,15 @@ fn run_no_project() -> Result<()> {
 
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
     pyproject_toml.write_str(indoc! { r#"
-        [build-system]
-        requires = ["hatchling"]
-        build-backend = "hatchling.build"
-
         [project]
         name = "foo"
         version = "1.0.0"
         requires-python = ">=3.8"
         dependencies = ["anyio"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#
     })?;
 

crates/uv/tests/sync.rs

@@ -21,6 +21,10 @@ fn sync() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["iniconfig"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -55,6 +59,10 @@ fn locked() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -81,6 +89,10 @@ fn locked() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["iniconfig"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -115,6 +127,10 @@ fn frozen() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -138,6 +154,10 @@ fn frozen() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["iniconfig"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -212,6 +232,10 @@ fn package() -> Result<()> {
         requires-python = ">=3.12"
         dependencies = ["child", "anyio>3"]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         child = { workspace = true }
 
@@ -237,6 +261,10 @@ fn package() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["iniconfig>1"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -312,6 +340,10 @@ fn mixed_requires_python() -> Result<()> {
         name = "bird-feeder"
         version = "0.1.0"
         requires-python = ">=3.8"
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -381,6 +413,10 @@ fn virtual_workspace_dev_dependencies() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["iniconfig>1"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -441,6 +477,10 @@ fn sync_build_isolation() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["source-distribution @ https://files.pythonhosted.org/packages/10/1f/57aa4cce1b1abf6b433106676e15f9fa2c92ed2bd4cf77c3b50a9e9ac773/source_distribution-0.0.1.tar.gz"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -877,6 +917,10 @@ fn sync_environment() -> Result<()> {
         requires-python = ">=3.10"
         dependencies = ["iniconfig"]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv]
         environments = ["python_version < '3.11'"]
         "#,
@@ -916,6 +960,10 @@ fn read_metadata_statically_over_the_cache() -> Result<()> {
         requires-python = ">=3.12"
         # Python string sorting is the other way round.
         dependencies = ["anyio>=4,<5"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -945,6 +993,10 @@ fn no_install_project() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 
@@ -996,6 +1048,10 @@ fn no_install_workspace() -> Result<()> {
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0", "child"]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.workspace]
         members = ["child"]
 
@@ -1015,8 +1071,8 @@ fn no_install_workspace() -> Result<()> {
         dependencies = ["iniconfig>1"]
 
         [build-system]
-        requires = ["hatchling"]
+        requires = ["setuptools>=42", "wheel"]
-        build-backend = "hatchling.build"
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
     child
@@ -1073,6 +1129,10 @@ fn no_install_package() -> Result<()> {
         version = "0.1.0"
         requires-python = ">=3.12"
         dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
         "#,
     )?;
 

crates/uv/tests/workspace.rs

@@ -965,6 +965,10 @@ fn workspace_inherit_sources() -> Result<()> {
         dependencies = []
         requires-python = ">=3.12"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.workspace]
         members = ["packages/*"]
     "#})?;
@@ -977,6 +981,10 @@ fn workspace_inherit_sources() -> Result<()> {
         name = "leaf"
         version = "0.1.0"
         dependencies = ["library"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     leaf.child("src/__init__.py").touch()?;
 
@@ -987,6 +995,10 @@ fn workspace_inherit_sources() -> Result<()> {
         name = "library"
         version = "0.1.0"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     library.child("src/__init__.py").touch()?;
 
@@ -1013,6 +1025,10 @@ fn workspace_inherit_sources() -> Result<()> {
         version = "0.1.0"
         dependencies = ["library"]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         library = { path = "../../../library", editable = true }
     "#})?;
@@ -1036,6 +1052,10 @@ fn workspace_inherit_sources() -> Result<()> {
         name = "leaf"
         version = "0.1.0"
         dependencies = ["library"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
 
     // Update the root to include the source.
@@ -1046,6 +1066,10 @@ fn workspace_inherit_sources() -> Result<()> {
         dependencies = []
         requires-python = ">=3.12"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         library = { path = "../library", editable = true }
 
@@ -1117,6 +1141,10 @@ fn workspace_inherit_sources() -> Result<()> {
         dependencies = []
         requires-python = ">=3.12"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         library = { path = "../library", editable = true }
 
@@ -1131,9 +1159,12 @@ fn workspace_inherit_sources() -> Result<()> {
         version = "0.1.0"
         dependencies = ["library"]
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.sources]
         application = { path = "../application", editable = true }
-
     "#})?;
 
     // Resolving should succeed; the member should still use the root's source, despite defining
@@ -1167,6 +1198,10 @@ fn workspace_unsatisfiable_member_dependencies() -> Result<()> {
         dependencies = []
         requires-python = ">=3.12"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.workspace]
         members = ["packages/*"]
     "#})?;
@@ -1179,6 +1214,10 @@ fn workspace_unsatisfiable_member_dependencies() -> Result<()> {
         name = "leaf"
         version = "0.1.0"
         dependencies = ["httpx>9999"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     leaf.child("src/__init__.py").touch()?;
 
@@ -1215,6 +1254,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting() -> Result<()> {
         dependencies = []
         requires-python = ">=3.12"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.workspace]
         members = ["packages/*"]
     "#})?;
@@ -1227,6 +1270,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting() -> Result<()> {
         name = "foo"
         version = "0.1.0"
         dependencies = ["anyio==4.1.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     foo.child("src/__init__.py").touch()?;
     let bar = workspace.child("packages").child("bar");
@@ -1235,6 +1282,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting() -> Result<()> {
         name = "bar"
         version = "0.1.0"
         dependencies = ["anyio==4.2.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     bar.child("src/__init__.py").touch()?;
 
@@ -1271,6 +1322,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_threeway() -> Result<
         dependencies = []
         requires-python = ">=3.12"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.workspace]
         members = ["packages/*"]
     "#})?;
@@ -1283,6 +1338,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_threeway() -> Result<
         name = "red"
         version = "0.1.0"
         dependencies = ["anyio==4.1.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     red.child("src/__init__.py").touch()?;
     let knot = workspace.child("packages").child("knot");
@@ -1291,6 +1350,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_threeway() -> Result<
         name = "knot"
         version = "0.1.0"
         dependencies = ["anyio==4.2.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     knot.child("src/__init__.py").touch()?;
 
@@ -1302,6 +1365,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_threeway() -> Result<
         name = "bird"
         version = "0.1.0"
         dependencies = ["anyio==4.3.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     bird.child("src/__init__.py").touch()?;
 
@@ -1338,6 +1405,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_extra() -> Result<()>
         dependencies = []
         requires-python = ">=3.12"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.workspace]
         members = ["packages/*"]
     "#})?;
@@ -1350,6 +1421,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_extra() -> Result<()>
         name = "foo"
         version = "0.1.0"
         dependencies = ["anyio==4.1.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     foo.child("src/__init__.py").touch()?;
     let bar = workspace.child("packages").child("bar");
@@ -1360,6 +1435,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_extra() -> Result<()>
 
         [project.optional-dependencies]
         some_extra = ["anyio==4.2.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     bar.child("src/__init__.py").touch()?;
 
@@ -1396,6 +1475,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_dev() -> Result<()> {
         dependencies = []
         requires-python = ">=3.12"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.workspace]
         members = ["packages/*"]
     "#})?;
@@ -1408,6 +1491,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_dev() -> Result<()> {
         name = "foo"
         version = "0.1.0"
         dependencies = ["anyio==4.1.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     foo.child("src/__init__.py").touch()?;
     let bar = workspace.child("packages").child("bar");
@@ -1416,6 +1503,10 @@ fn workspace_unsatisfiable_member_dependencies_conflicting_dev() -> Result<()> {
         name = "bar"
         version = "0.1.0"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv]
         dev-dependencies = ["anyio==4.2.0"]
     "#})?;
@@ -1455,6 +1546,10 @@ fn workspace_member_name_shadows_dependencies() -> Result<()> {
         dependencies = []
         requires-python = ">=3.12"
 
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+
         [tool.uv.workspace]
         members = ["packages/*"]
     "#})?;
@@ -1467,6 +1562,10 @@ fn workspace_member_name_shadows_dependencies() -> Result<()> {
         name = "foo"
         version = "0.1.0"
         dependencies = ["anyio==4.1.0"]
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     foo.child("src/__init__.py").touch()?;
 
@@ -1477,6 +1576,10 @@ fn workspace_member_name_shadows_dependencies() -> Result<()> {
         name = "anyio"
         version = "0.1.0"
         dependencies = []
+
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
     "#})?;
     anyio.child("src/__init__.py").touch()?;
 

docs/reference/cli.md

@@ -455,9 +455,11 @@ uv init [OPTIONS] [PATH]
 
 </dd><dt><code>--version</code>, <code>-V</code></dt><dd><p>Display the uv version</p>
 
-</dd><dt><code>--virtual</code></dt><dd><p>Create a virtual workspace instead of a project.</p>
+</dd><dt><code>--virtual</code></dt><dd><p>Create a virtual project, rather than a package.</p>
 
-<p>A virtual workspace does not define project dependencies and cannot be published. Instead, workspace members declare project dependencies. Development dependencies may still be declared.</p>
+<p>A virtual project is a project that is not intended to be built as a Python package, such as a project that only contains scripts or other application code.</p>
+
+<p>Virtual projects themselves are not installed into the Python environment.</p>
 
 </dd></dl>
 

docs/reference/settings.md

@@ -890,6 +890,40 @@ requirements of any constituent packages.
 
 ---
 
+#### [`package`](#package) {: #package }
+
+Whether the project should be considered a Python package, or a non-package ("virtual")
+project.
+
+Packages are built and installed into the virtual environment in editable mode and thus
+require a build backend, while virtual projects are _not_ built or installed; instead, only
+their dependencies are included in the virtual environment.
+
+Creating a package requires that a `build-system` is present in the `pyproject.toml`, and
+that the project adheres to a structure that adheres to the build backend's expectations
+(e.g., a `src` layout).
+
+**Default value**: `true`
+
+**Type**: `bool`
+
+**Example usage**:
+
+=== "pyproject.toml"
+
+    ```toml
+    [tool.uv]
+    package = false
+    ```
+=== "uv.toml"
+
+    ```toml
+    
+    package = false
+    ```
+
+---
+
 #### [`prerelease`](#prerelease) {: #prerelease }
 
 The strategy to use when considering pre-release versions.

uv.schema.json

@@ -274,6 +274,13 @@
         "type": "string"
       }
     },
+    "package": {
+      "description": "Whether the project should be considered a Python package, or a non-package (\"virtual\") project.\n\nPackages are built and installed into the virtual environment in editable mode and thus require a build backend, while virtual projects are _not_ built or installed; instead, only their dependencies are included in the virtual environment.\n\nCreating a package requires that a `build-system` is present in the `pyproject.toml`, and that the project adheres to a structure that adheres to the build backend's expectations (e.g., a `src` layout).",
+      "type": [
+        "boolean",
+        "null"
+      ]
+    },
     "pip": {
       "anyOf": [
         {