Avoid removing seed packages for uv venv --seed environments

crates/uv-build/src/lib.rs

@@ -290,6 +290,7 @@ impl SourceBuild {
                 false,
                 false,
                 false,
+                false,
             )?
         };
 

crates/uv-installer/src/plan.rs

@@ -412,9 +412,9 @@ impl<'a> Planner<'a> {
 
         // Remove any unnecessary packages.
         if site_packages.any() {
-            // If uv created the virtual environment, then remove all packages, regardless of
-            // whether they're considered "seed" packages.
-            let seed_packages = !venv.cfg().is_ok_and(|cfg| cfg.is_uv());
+            // Retain seed packages unless: (1) the virtual environment was created by uv and
+            // (2) the `--seed` argument was not passed to `uv venv`.
+            let seed_packages = !venv.cfg().is_ok_and(|cfg| cfg.is_uv() && !cfg.is_seed());
             for dist_info in site_packages {
                 if seed_packages
                     && matches!(

crates/uv-python/src/virtualenv.rs

@@ -23,13 +23,16 @@ pub struct VirtualEnvironment {
 
 /// A parsed `pyvenv.cfg`
 #[derive(Debug, Clone)]
+#[allow(clippy::struct_excessive_bools)]
 pub struct PyVenvConfiguration {
-    /// If the virtualenv package was used to create the virtual environment.
+    /// Was the virtual environment created with the `virtualenv` package?
     pub(crate) virtualenv: bool,
-    /// If the uv package was used to create the virtual environment.
+    /// Was the virtual environment created with the `uv` package?
     pub(crate) uv: bool,
     /// Is the virtual environment relocatable?
     pub(crate) relocatable: bool,
+    /// Was the virtual environment populated with seed packages?
+    pub(crate) seed: bool,
 }
 
 #[derive(Debug, Error)]
@@ -139,6 +142,7 @@ impl PyVenvConfiguration {
         let mut virtualenv = false;
         let mut uv = false;
         let mut relocatable = false;
+        let mut seed = false;
 
         // Per https://snarky.ca/how-virtual-environments-work/, the `pyvenv.cfg` file is not a
         // valid INI file, and is instead expected to be parsed by partitioning each line on the
@@ -159,6 +163,9 @@ impl PyVenvConfiguration {
                 "relocatable" => {
                     relocatable = value.trim().to_lowercase() == "true";
                 }
+                "seed" => {
+                    seed = value.trim().to_lowercase() == "true";
+                }
                 _ => {}
             }
         }
@@ -167,6 +174,7 @@ impl PyVenvConfiguration {
             virtualenv,
             uv,
             relocatable,
+            seed,
         })
     }
 
@@ -184,4 +192,9 @@ impl PyVenvConfiguration {
     pub fn is_relocatable(&self) -> bool {
         self.relocatable
     }
+
+    /// Returns true if the virtual environment was populated with seed packages.
+    pub fn is_seed(&self) -> bool {
+        self.seed
+    }
 }

crates/uv-tool/src/lib.rs

@@ -258,6 +258,7 @@ impl InstalledTools {
             false,
             false,
             false,
+            false,
         )?;
 
         Ok(venv)

crates/uv-virtualenv/src/lib.rs

@@ -46,13 +46,15 @@ impl Prompt {
 }
 
 /// Create a virtualenv.
+#[allow(clippy::fn_params_excessive_bools)]
 pub fn create_venv(
     location: &Path,
     interpreter: Interpreter,
     prompt: Prompt,
     system_site_packages: bool,
     allow_existing: bool,
     relocatable: bool,
+    seed: bool,
 ) -> Result<PythonEnvironment, Error> {
     // Create the virtualenv at the given location.
     let virtualenv = virtualenv::create(
@@ -62,6 +64,7 @@ pub fn create_venv(
         system_site_packages,
         allow_existing,
         relocatable,
+        seed,
     )?;
 
     // Create the corresponding `PythonEnvironment`.

crates/uv-virtualenv/src/virtualenv.rs

@@ -43,13 +43,15 @@ fn write_cfg(f: &mut impl Write, data: &[(String, String)]) -> io::Result<()> {
 }
 
 /// Create a [`VirtualEnvironment`] at the given location.
+#[allow(clippy::fn_params_excessive_bools)]
 pub(crate) fn create(
     location: &Path,
     interpreter: &Interpreter,
     prompt: Prompt,
     system_site_packages: bool,
     allow_existing: bool,
     relocatable: bool,
+    seed: bool,
 ) -> Result<VirtualEnvironment, Error> {
     // Determine the base Python executable; that is, the Python executable that should be
     // considered the "base" for the virtual environment. This is typically the Python executable
@@ -350,16 +352,16 @@ pub(crate) fn create(
                 "false".to_string()
             },
         ),
-        (
-            "relocatable".to_string(),
-            if relocatable {
-                "true".to_string()
-            } else {
-                "false".to_string()
-            },
-        ),
     ];
 
+    if relocatable {
+        pyvenv_cfg_data.push(("relocatable".to_string(), "true".to_string()));
+    }
+
+    if seed {
+        pyvenv_cfg_data.push(("seed".to_string(), "true".to_string()));
+    }
+
     if let Some(prompt) = prompt {
         pyvenv_cfg_data.push(("prompt".to_string(), prompt));
     }

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

@@ -104,6 +104,7 @@ impl CachedEnvironment {
             false,
             false,
             true,
+            false,
         )?;
 
         sync_environment(

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

@@ -531,6 +531,7 @@ pub(crate) async fn get_or_init_environment(
                 false,
                 false,
                 false,
+                false,
             )?)
         }
     }

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

@@ -247,6 +247,7 @@ pub(crate) async fn run(
                 false,
                 false,
                 false,
+                false,
             )?;
 
             Some(environment.into_interpreter())
@@ -432,6 +433,7 @@ pub(crate) async fn run(
                     false,
                     false,
                     false,
+                    false,
                 )?
             } else {
                 // If we're not isolating the environment, reuse the base environment for the
@@ -554,6 +556,7 @@ pub(crate) async fn run(
                     false,
                     false,
                     false,
+                    false,
                 )?;
                 venv.into_interpreter()
             } else {
@@ -602,6 +605,7 @@ pub(crate) async fn run(
                     false,
                     false,
                     false,
+                    false,
                 )?
             }
             Some(spec) => {

crates/uv/src/commands/venv.rs

@@ -255,6 +255,7 @@ async fn venv_impl(
         system_site_packages,
         allow_existing,
         relocatable,
+        seed,
     )
     .map_err(VenvError::Creation)?;
 

crates/uv/tests/pip_sync.rs

@@ -5536,3 +5536,76 @@ fn compatible_build_constraint() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn sync_seed() -> Result<()> {
+    let context = TestContext::new("3.8");
+
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("requests==1.2")?;
+
+    // Add `pip` to the environment.
+    uv_snapshot!(context.filters(), context.pip_install()
+        .arg("pip"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + pip==24.0
+    "###
+    );
+
+    // Syncing should remove the seed packages.
+    uv_snapshot!(context.filters(), context.pip_sync()
+        .arg("requirements.txt"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    Prepared 1 package in [TIME]
+    Uninstalled 1 package in [TIME]
+    Installed 1 package in [TIME]
+     - pip==24.0
+     + requests==1.2.0
+    "###
+    );
+
+    // Re-create the environment with seed packages.
+    uv_snapshot!(context.filters(), context.venv()
+        .arg("--seed"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.8.[X] interpreter at: [PYTHON-3.8]
+    Creating virtualenv with seed packages at: .venv
+     + pip==24.0
+     + setuptools==69.2.0
+     + wheel==0.43.0
+    Activate with: source .venv/bin/activate
+    "###
+    );
+
+    // Syncing should retain the seed packages.
+    uv_snapshot!(context.filters(), context.pip_sync()
+        .arg("requirements.txt"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + requests==1.2.0
+    "###
+    );
+
+    Ok(())
+}

crates/uv/tests/venv.rs

@@ -4,6 +4,7 @@ use anyhow::Result;
 use assert_cmd::prelude::*;
 use assert_fs::prelude::*;
 use indoc::indoc;
+use predicates::prelude::*;
 use uv_python::{PYTHON_VERSIONS_FILENAME, PYTHON_VERSION_FILENAME};
 
 use crate::common::{uv_snapshot, TestContext};
@@ -913,7 +914,7 @@ fn verify_pyvenv_cfg() {
     pyvenv_cfg.assert(predicates::str::contains(search_string));
 
     // Not relocatable by default.
-    pyvenv_cfg.assert(predicates::str::contains("relocatable = false"));
+    pyvenv_cfg.assert(predicates::str::contains("relocatable").not());
 }
 
 #[test]