diff --git a/crates/uv-installer/src/site_packages.rs b/crates/uv-installer/src/site_packages.rs
index 93a2c4d217c..6262d0482de 100644
--- a/crates/uv-installer/src/site_packages.rs
+++ b/crates/uv-installer/src/site_packages.rs
@@ -1,12 +1,14 @@
 use std::borrow::Cow;
 use std::iter::Flatten;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
+use std::str::FromStr;
 
 use anyhow::{Context, Result};
 use fs_err as fs;
 use rustc_hash::{FxBuildHasher, FxHashMap, FxHashSet};
 
 use uv_configuration::{ExcludeDependency, Excludes, Override, Overrides};
+use uv_distribution_filename::EggInfoFilename;
 use uv_distribution_types::{
     ConfigSettings, DependencyMetadata, Diagnostic, ExtraBuildRequires, ExtraBuildVariables,
     InstalledDist, InstalledDistKind, Name, NameRequirementSpecification, PackageConfigSettings,
@@ -49,8 +51,25 @@ impl SitePackages {
         Self::from_interpreter(environment.interpreter())
     }
 
+    /// Build an index of the requested installed packages from the given Python environment.
+    pub fn from_environment_for_packages<'a>(
+        environment: &PythonEnvironment,
+        package_names: impl IntoIterator<Item = &'a PackageName>,
+    ) -> Result<Self> {
+        let package_names = package_names.into_iter().collect::<FxHashSet<_>>();
+        Self::from_interpreter_with_filter(environment.interpreter(), Some(&package_names))
+    }
+
     /// Build an index of installed packages from the given Python executable.
     pub fn from_interpreter(interpreter: &Interpreter) -> Result<Self> {
+        Self::from_interpreter_with_filter(interpreter, None)
+    }
+
+    /// Build an index of installed packages from the given Python executable.
+    fn from_interpreter_with_filter(
+        interpreter: &Interpreter,
+        package_names: Option<&FxHashSet<&PackageName>>,
+    ) -> Result<Self> {
         let mut distributions: Vec<Option<InstalledDist>> = Vec::new();
         let mut by_name = FxHashMap::default();
         let mut by_url = FxHashMap::default();
@@ -77,6 +96,13 @@ impl SitePackages {
 
             // Index all installed packages by name.
             for path in site_packages {
+                if let Some(package_names) = package_names
+                    && let Some(package_name) = installed_dist_name(&path)
+                    && !package_names.contains(&package_name)
+                {
+                    continue;
+                }
+
                 let dist_info = match InstalledDist::try_from_path(&path) {
                     Ok(Some(dist_info)) => dist_info,
                     Ok(None) => continue,
@@ -99,6 +125,12 @@ impl SitePackages {
                     }
                 };
 
+                if let Some(package_names) = package_names
+                    && !package_names.contains(dist_info.name())
+                {
+                    continue;
+                }
+
                 let idx = distributions.len();
 
                 // Index the distribution by name.
@@ -596,6 +628,29 @@ pub enum SatisfiesResult {
     Unsatisfied(String),
 }
 
+/// Infer the package name from an installed distribution path without reading its metadata.
+///
+/// Returns `None` when the name cannot safely be derived from the filename alone.
+fn installed_dist_name(path: &Path) -> Option<PackageName> {
+    let extension = path.extension()?.to_str()?;
+    let file_stem = path.file_stem()?.to_str()?;
+
+    match extension {
+        "dist-info" => {
+            let (name, version) = file_stem.split_once('-')?;
+            Version::from_str(version).ok()?;
+            PackageName::from_str(name).ok()
+        }
+        "egg-info" => {
+            let filename = EggInfoFilename::parse(file_stem).ok()?;
+            filename.version?;
+            Some(filename.name)
+        }
+        // Legacy editables require reading metadata to determine their package name.
+        _ => None,
+    }
+}
+
 impl IntoIterator for SitePackages {
     type Item = InstalledDist;
     type IntoIter = Flatten<std::vec::IntoIter<Option<InstalledDist>>>;
diff --git a/crates/uv-types/src/traits.rs b/crates/uv-types/src/traits.rs
index aa08aa1e37e..040703633ea 100644
--- a/crates/uv-types/src/traits.rs
+++ b/crates/uv-types/src/traits.rs
@@ -226,12 +226,23 @@ pub trait SourceBuildTrait {
     ) -> impl Future<Output = Result<String, AnyErrorBuild>> + 'a;
 }
 
-/// A wrapper for [`uv_installer::SitePackages`]
+/// Provides access to installed distributions during resolution.
 pub trait InstalledPackagesProvider: Clone + Send + Sync + 'static {
     fn iter(&self) -> impl Iterator<Item = &InstalledDist>;
     fn get_packages(&self, name: &PackageName) -> Vec<&InstalledDist>;
 }
 
+impl<Provider: InstalledPackagesProvider> InstalledPackagesProvider for Option<Provider> {
+    fn iter(&self) -> impl Iterator<Item = &InstalledDist> {
+        self.as_ref().into_iter().flat_map(Provider::iter)
+    }
+
+    fn get_packages(&self, name: &PackageName) -> Vec<&InstalledDist> {
+        self.as_ref()
+            .map_or_else(Vec::new, |provider| provider.get_packages(name))
+    }
+}
+
 /// An [`InstalledPackagesProvider`] with no packages in it.
 #[derive(Clone)]
 pub struct EmptyInstalledPackages;
diff --git a/crates/uv/src/commands/pip/install.rs b/crates/uv/src/commands/pip/install.rs
index 35be805e040..213ef0b54ab 100644
--- a/crates/uv/src/commands/pip/install.rs
+++ b/crates/uv/src/commands/pip/install.rs
@@ -19,7 +19,7 @@ use uv_configuration::{KeyringProviderType, TargetTriple};
 use uv_dispatch::{BuildDispatch, SharedState};
 use uv_distribution::LoweredExtraBuildDependencies;
 use uv_distribution_types::{
-    ConfigSettings, DependencyMetadata, ExtraBuildVariables, Index, IndexLocations,
+    ConfigSettings, DependencyMetadata, ExtraBuildVariables, Index, IndexLocations, Name,
     NameRequirementSpecification, Origin, PackageConfigSettings, Requirement, Resolution,
 };
 use uv_fs::Simplified;
@@ -308,8 +308,19 @@ pub(crate) async fn pip_install(
         interpreter,
     )?;
 
-    // Determine the set of installed packages.
-    let site_packages = SitePackages::from_environment(&environment)?;
+    // With sufficient modifications, installation only needs installed distributions selected by
+    // the resolution. A `pylock.toml` resolution never consults the environment, while reinstalling
+    // every package excludes all installed distributions from candidate selection, including for
+    // transitive dependencies. Delay the environment scan in either case, then restrict it to the
+    // resolved package names.
+    let defer_site_packages = matches!(modifications, Modifications::Sufficient)
+        && (pylock.is_some() || matches!(&reinstall, Reinstall::All));
+
+    let site_packages = if defer_site_packages {
+        None
+    } else {
+        Some(SitePackages::from_environment(&environment)?)
+    };
 
     // Check if the current environment satisfies the requirements.
     // Ideally, the resolver would be fast enough to let us remove this check. But right now, for large environments,
@@ -320,6 +331,7 @@ pub(crate) async fn pip_install(
         && groups.is_empty()
         && pylock.is_none()
         && matches!(modifications, Modifications::Sufficient)
+        && let Some(site_packages) = &site_packages
     {
         match site_packages.satisfies_spec(
             &requirements,
@@ -596,6 +608,15 @@ pub(crate) async fn pip_install(
     // If necessary, convert editable distributions to non-editable.
     let resolution = apply_editable_mode(resolution, editable);
 
+    let site_packages = match site_packages {
+        // Only resolved packages can be modified when using sufficient installation semantics.
+        None => SitePackages::from_environment_for_packages(
+            &environment,
+            resolution.distributions().map(Name::name),
+        )?,
+        Some(site_packages) => site_packages,
+    };
+
     // Constrain any build requirements marked as `match-runtime = true`.
     let extra_build_requires = extra_build_requires.match_runtime(&resolution)?;
 
diff --git a/crates/uv/tests/pip_install/pip_install.rs b/crates/uv/tests/pip_install/pip_install.rs
index 4b8eac2f0b6..83c06ed0ab4 100644
--- a/crates/uv/tests/pip_install/pip_install.rs
+++ b/crates/uv/tests/pip_install/pip_install.rs
@@ -4044,6 +4044,12 @@ fn install_no_downgrade() -> Result<()> {
     "
     );
 
+    // An unrelated distribution should not be inspected when all resolved packages will be
+    // reinstalled.
+    let unrelated = context.site_packages().join("unrelated-1.0.0.dist-info");
+    fs::create_dir_all(&unrelated)?;
+    fs::write(unrelated.join("direct_url.json"), "invalid")?;
+
     // Install `anyio` with `--reinstall`, which should downgrade `idna`.
     uv_snapshot!(context.filters(), context.pip_install()
         .arg("--reinstall")
@@ -4917,6 +4923,7 @@ fn reinstall_duplicate() -> Result<()> {
     // Run `pip install`.
     uv_snapshot!(context1.pip_install()
         .arg("pip")
+        .arg("--no-deps")
         .arg("--reinstall"),
         @"
     success: true
@@ -5597,6 +5604,24 @@ fn path_name_version_change() {
     "
     );
 
+    // Reinstalling a direct wheel without dependencies should reinstall the requested package.
+    uv_snapshot!(context.filters(), context.pip_install()
+        .arg(context.workspace_root.join("test/links/ok-1.0.0-py3-none-any.whl"))
+        .arg("--no-deps")
+        .arg("--reinstall"), @"
+    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]
+     ~ ok==1.0.0 (from file://[WORKSPACE]/test/links/ok-1.0.0-py3-none-any.whl)
+    "
+    );
+
     // Installing a new path should succeed
     uv_snapshot!(context.filters(), context.pip_install()
         .arg(context.workspace_root.join("test/links/ok-2.0.0-py3-none-any.whl")), @"
@@ -12625,6 +12650,11 @@ fn pep_751_install_registry_wheel() -> Result<()> {
     "
     );
 
+    // An unrelated distribution should not be inspected when installing from a `pylock.toml`.
+    let unrelated = context.site_packages().join("unrelated-1.0.0.dist-info");
+    fs::create_dir_all(&unrelated)?;
+    fs::write(unrelated.join("direct_url.json"), "invalid")?;
+
     uv_snapshot!(context.filters(), context.pip_install()
         .arg("--preview")
         .arg("-r")