diff --git a/Cargo.lock b/Cargo.lock
index c71838587..4a3fafc82 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2765,6 +2765,7 @@ dependencies = [
  "test-case",
  "thiserror",
  "tracing",
+ "url",
 ]
 
 [[package]]
diff --git a/crates/distribution-types/src/lib.rs b/crates/distribution-types/src/lib.rs
index 99ef2ddf2..db5c59c4a 100644
--- a/crates/distribution-types/src/lib.rs
+++ b/crates/distribution-types/src/lib.rs
@@ -5,7 +5,7 @@ use url::Url;
 
 use pep440_rs::Version;
 use puffin_normalize::PackageName;
-use pypi_types::File;
+use pypi_types::{File, IndexUrl};
 
 pub use crate::any::*;
 pub use crate::cached::*;
@@ -62,6 +62,7 @@ pub struct RegistryBuiltDist {
     pub name: PackageName,
     pub version: Version,
     pub file: File,
+    pub index: IndexUrl,
 }
 
 /// A built distribution (wheel) that exists at an arbitrary URL.
@@ -77,6 +78,7 @@ pub struct RegistrySourceDist {
     pub name: PackageName,
     pub version: Version,
     pub file: File,
+    pub index: IndexUrl,
 }
 
 /// A source distribution that exists at an arbitrary URL.
@@ -95,7 +97,7 @@ pub struct GitSourceDist {
 
 impl Dist {
     /// Create a [`Dist`] for a registry-based distribution.
-    pub fn from_registry(name: PackageName, version: Version, file: File) -> Self {
+    pub fn from_registry(name: PackageName, version: Version, file: File, index: IndexUrl) -> Self {
         if Path::new(&file.filename)
             .extension()
             .is_some_and(|ext| ext.eq_ignore_ascii_case("whl"))
@@ -104,12 +106,14 @@ impl Dist {
                 name,
                 version,
                 file,
+                index,
             }))
         } else {
             Self::Source(SourceDist::Registry(RegistrySourceDist {
                 name,
                 version,
                 file,
+                index,
             }))
         }
     }
diff --git a/crates/puffin-client/src/client.rs b/crates/puffin-client/src/client.rs
index ecf5f20eb..fa368926d 100644
--- a/crates/puffin-client/src/client.rs
+++ b/crates/puffin-client/src/client.rs
@@ -19,7 +19,7 @@ use url::Url;
 use distribution_filename::WheelFilename;
 use install_wheel_rs::find_dist_info;
 use puffin_normalize::PackageName;
-use pypi_types::{File, Metadata21, SimpleJson};
+use pypi_types::{File, IndexUrl, Metadata21, SimpleJson};
 
 use crate::cached_client::CachedClient;
 use crate::error::Error;
@@ -30,8 +30,8 @@ use crate::remote_metadata::{
 /// A builder for an [`RegistryClient`].
 #[derive(Debug, Clone)]
 pub struct RegistryClientBuilder {
-    index: Url,
-    extra_index: Vec<Url>,
+    index: IndexUrl,
+    extra_index: Vec<IndexUrl>,
     no_index: bool,
     proxy: Url,
     retries: u32,
@@ -41,7 +41,7 @@ pub struct RegistryClientBuilder {
 impl RegistryClientBuilder {
     pub fn new(cache: impl Into<PathBuf>) -> Self {
         Self {
-            index: Url::parse("https://pypi.org/simple").unwrap(),
+            index: IndexUrl::from(Url::parse("https://pypi.org/simple").unwrap()),
             extra_index: vec![],
             no_index: false,
             proxy: Url::parse("https://pypi-metadata.ruff.rs").unwrap(),
@@ -54,13 +54,13 @@ impl RegistryClientBuilder {
 impl RegistryClientBuilder {
     #[must_use]
     pub fn index(mut self, index: Url) -> Self {
-        self.index = index;
+        self.index = IndexUrl::from(index);
         self
     }
 
     #[must_use]
     pub fn extra_index(mut self, extra_index: Vec<Url>) -> Self {
-        self.extra_index = extra_index;
+        self.extra_index = extra_index.into_iter().map(IndexUrl::from).collect();
         self
     }
 
@@ -135,8 +135,8 @@ impl RegistryClientBuilder {
 // TODO(konstin): Clean up the clients once we moved everything to common caching
 #[derive(Debug, Clone)]
 pub struct RegistryClient {
-    pub(crate) index: Url,
-    pub(crate) extra_index: Vec<Url>,
+    pub(crate) index: IndexUrl,
+    pub(crate) extra_index: Vec<IndexUrl>,
     /// Ignore the package index, instead relying on local archives and caches.
     pub(crate) no_index: bool,
     pub(crate) client: ClientWithMiddleware,
@@ -149,14 +149,14 @@ pub struct RegistryClient {
 
 impl RegistryClient {
     /// Fetch a package from the `PyPI` simple API.
-    pub async fn simple(&self, package_name: PackageName) -> Result<SimpleJson, Error> {
+    pub async fn simple(&self, package_name: PackageName) -> Result<(IndexUrl, SimpleJson), Error> {
         if self.no_index {
             return Err(Error::NoIndex(package_name.as_ref().to_string()));
         }
 
         for index in std::iter::once(&self.index).chain(self.extra_index.iter()) {
             // Format the URL for PyPI.
-            let mut url = index.clone();
+            let mut url: Url = index.clone().into();
             url.path_segments_mut().unwrap().push(package_name.as_ref());
             url.path_segments_mut().unwrap().push("");
             url.set_query(Some("format=application/vnd.pypi.simple.v1+json"));
@@ -170,8 +170,9 @@ impl RegistryClient {
             // Fetch from the index.
             match self.simple_impl(&url).await {
                 Ok(text) => {
-                    return serde_json::from_str(&text)
-                        .map_err(move |e| Error::from_json_err(e, url));
+                    let data = serde_json::from_str(&text)
+                        .map_err(move |e| Error::from_json_err(e, url))?;
+                    return Ok((index.clone(), data));
                 }
                 Err(err) => {
                     if err.status() == Some(StatusCode::NOT_FOUND) {
diff --git a/crates/puffin-resolver/src/finder.rs b/crates/puffin-resolver/src/finder.rs
index cb2850d4e..2eab301f4 100644
--- a/crates/puffin-resolver/src/finder.rs
+++ b/crates/puffin-resolver/src/finder.rs
@@ -17,7 +17,7 @@ use platform_tags::{TagPriority, Tags};
 use puffin_client::RegistryClient;
 use puffin_interpreter::InterpreterInfo;
 use puffin_normalize::PackageName;
-use pypi_types::{File, SimpleJson};
+use pypi_types::{File, IndexUrl, SimpleJson};
 
 use crate::error::ResolveError;
 use crate::resolution::Resolution;
@@ -68,7 +68,9 @@ impl<'a> DistFinder<'a> {
                 Request::Package(requirement) => self
                     .client
                     .simple(requirement.name.clone())
-                    .map_ok(move |metadata| Response::Package(requirement, metadata)),
+                    .map_ok(move |(index, metadata)| {
+                        Response::Package(requirement, index, metadata)
+                    }),
             })
             .buffer_unordered(32)
             .ready_chunks(32);
@@ -104,9 +106,10 @@ impl<'a> DistFinder<'a> {
             for result in chunk {
                 let result: Response = result?;
                 match result {
-                    Response::Package(requirement, metadata) => {
+                    Response::Package(requirement, index, metadata) => {
                         // Pick a version that satisfies the requirement.
-                        let Some(distribution) = self.select(&requirement, metadata.files) else {
+                        let Some(distribution) = self.select(&requirement, &index, metadata.files)
+                        else {
                             return Err(ResolveError::NotFound(requirement));
                         };
 
@@ -134,7 +137,12 @@ impl<'a> DistFinder<'a> {
     }
 
     /// select a version that satisfies the requirement, preferring wheels to source distributions.
-    fn select(&self, requirement: &Requirement, files: Vec<File>) -> Option<Dist> {
+    fn select(
+        &self,
+        requirement: &Requirement,
+        index: &IndexUrl,
+        files: Vec<File>,
+    ) -> Option<Dist> {
         let mut best_version: Option<Version> = None;
         let mut best_wheel: Option<(Dist, TagPriority)> = None;
         let mut best_sdist: Option<Dist> = None;
@@ -173,7 +181,7 @@ impl<'a> DistFinder<'a> {
                             .map_or(true, |(.., existing)| priority > *existing)
                         {
                             best_wheel = Some((
-                                Dist::from_registry(wheel.name, wheel.version, file),
+                                Dist::from_registry(wheel.name, wheel.version, file, index.clone()),
                                 priority,
                             ));
                         }
@@ -197,7 +205,12 @@ impl<'a> DistFinder<'a> {
 
                     if requirement.is_satisfied_by(&sdist.version) {
                         best_version = Some(sdist.version.clone());
-                        best_sdist = Some(Dist::from_registry(sdist.name, sdist.version, file));
+                        best_sdist = Some(Dist::from_registry(
+                            sdist.name,
+                            sdist.version,
+                            file,
+                            index.clone(),
+                        ));
                     }
                 }
             }
@@ -216,7 +229,7 @@ enum Request {
 #[derive(Debug)]
 enum Response {
     /// The returned metadata for a package.
-    Package(Requirement, SimpleJson),
+    Package(Requirement, IndexUrl, SimpleJson),
 }
 
 pub trait Reporter: Send + Sync {
diff --git a/crates/puffin-resolver/src/resolution.rs b/crates/puffin-resolver/src/resolution.rs
index 376593cc6..7fac38dc2 100644
--- a/crates/puffin-resolver/src/resolution.rs
+++ b/crates/puffin-resolver/src/resolution.rs
@@ -14,7 +14,7 @@ use distribution_types::{BuiltDist, Dist, Metadata, SourceDist};
 use pep440_rs::{Version, VersionSpecifier, VersionSpecifiers};
 use pep508_rs::{Requirement, VersionOrUrl};
 use puffin_normalize::PackageName;
-use pypi_types::File;
+use pypi_types::{File, IndexUrl};
 
 use crate::pubgrub::{PubGrubPackage, PubGrubPriority, PubGrubVersion};
 
@@ -58,7 +58,7 @@ impl Graph {
     /// Create a new graph from the resolved `PubGrub` state.
     pub fn from_state(
         selection: &SelectedDependencies<PubGrubPackage, PubGrubVersion>,
-        pins: &FxHashMap<PackageName, FxHashMap<Version, File>>,
+        pins: &FxHashMap<PackageName, FxHashMap<Version, (IndexUrl, File)>>,
         redirects: &WaitMap<Url, Url>,
         state: &State<PubGrubPackage, Range<PubGrubVersion>, PubGrubPriority>,
     ) -> Self {
@@ -73,12 +73,13 @@ impl Graph {
             match package {
                 PubGrubPackage::Package(package_name, None, None) => {
                     let version = Version::from(version.clone());
-                    let file = pins
+                    let (index, file) = pins
                         .get(package_name)
                         .and_then(|versions| versions.get(&version))
                         .unwrap()
                         .clone();
-                    let pinned_package = Dist::from_registry(package_name.clone(), version, file);
+                    let pinned_package =
+                        Dist::from_registry(package_name.clone(), version, file, index);
 
                     let index = graph.add_node(pinned_package);
                     inverse.insert(package_name, index);
diff --git a/crates/puffin-resolver/src/resolver.rs b/crates/puffin-resolver/src/resolver.rs
index 8e993ad3d..df5addeb7 100644
--- a/crates/puffin-resolver/src/resolver.rs
+++ b/crates/puffin-resolver/src/resolver.rs
@@ -27,7 +27,7 @@ use puffin_cache::CanonicalUrl;
 use puffin_client::RegistryClient;
 use puffin_normalize::{ExtraName, PackageName};
 use puffin_traits::BuildContext;
-use pypi_types::{File, Metadata21, SimpleJson};
+use pypi_types::{File, IndexUrl, Metadata21, SimpleJson};
 
 use crate::candidate_selector::CandidateSelector;
 use crate::distribution::{BuiltDistFetcher, SourceDistFetcher, SourceDistributionReporter};
@@ -330,7 +330,7 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
             let Some(entry) = self.index.packages.get(package_name) else {
                 continue;
             };
-            let version_map = entry.value();
+            let (index, version_map) = entry.value();
 
             // Try to find a compatible version. If there aren't any compatible versions,
             // short-circuit and return `None`.
@@ -345,6 +345,7 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
                     candidate.package_name,
                     candidate.version.into(),
                     candidate.file.into(),
+                    index.clone(),
                 );
                 request_sink.unbounded_send(Request::Dist(distribution))?;
             }
@@ -358,7 +359,7 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
         &self,
         package: &PubGrubPackage,
         range: &Range<PubGrubVersion>,
-        pins: &mut FxHashMap<PackageName, FxHashMap<pep440_rs::Version, File>>,
+        pins: &mut FxHashMap<PackageName, FxHashMap<pep440_rs::Version, (IndexUrl, File)>>,
         in_flight: &mut InFlight,
         request_sink: &futures::channel::mpsc::UnboundedSender<Request>,
     ) -> Result<Option<PubGrubVersion>, ResolveError> {
@@ -405,7 +406,7 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
             PubGrubPackage::Package(package_name, _extra, None) => {
                 // Wait for the metadata to be available.
                 let entry = self.index.packages.wait(package_name).await.unwrap();
-                let version_map = entry.value();
+                let (index, version_map) = entry.value();
 
                 debug!("Searching for a compatible version of {package_name} ({range})");
 
@@ -428,7 +429,7 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
                     .or_default()
                     .insert(
                         candidate.version.clone().into(),
-                        candidate.file.clone().into(),
+                        (index.clone(), candidate.file.clone().into()),
                     );
 
                 let version = candidate.version.clone();
@@ -439,6 +440,7 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
                         candidate.package_name,
                         candidate.version.into(),
                         candidate.file.into(),
+                        index.clone(),
                     );
                     request_sink.unbounded_send(Request::Dist(distribution))?;
                 }
@@ -453,7 +455,7 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
         &self,
         package: &PubGrubPackage,
         version: &PubGrubVersion,
-        pins: &mut FxHashMap<PackageName, FxHashMap<pep440_rs::Version, File>>,
+        pins: &mut FxHashMap<PackageName, FxHashMap<pep440_rs::Version, (IndexUrl, File)>>,
         priorities: &mut PubGrubPriorities,
         in_flight: &mut InFlight,
         request_sink: &futures::channel::mpsc::UnboundedSender<Request>,
@@ -498,7 +500,7 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
                         .unwrap(),
                     None => {
                         let versions = pins.get(package_name).unwrap();
-                        let file = versions.get(version.into()).unwrap();
+                        let (_index, file) = versions.get(version.into()).unwrap();
                         self.index
                             .distributions
                             .wait(&file.distribution_id())
@@ -550,7 +552,7 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
 
         while let Some(response) = response_stream.next().await {
             match response? {
-                Response::Package(package_name, metadata) => {
+                Response::Package(package_name, index, metadata) => {
                     trace!("Received package metadata for: {package_name}");
                     let version_map = VersionMap::from_metadata(
                         metadata,
@@ -559,7 +561,9 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
                         self.build_context.interpreter_info().version(),
                         self.exclude_newer.as_ref(),
                     );
-                    self.index.packages.insert(package_name, version_map);
+                    self.index
+                        .packages
+                        .insert(package_name, (index, version_map));
                 }
                 Response::Dist(Dist::Built(distribution), metadata, ..) => {
                     trace!("Received built distribution metadata for: {distribution}");
@@ -596,7 +600,9 @@ impl<'a, Context: BuildContext + Sync> Resolver<'a, Context> {
             Request::Package(package_name) => {
                 self.client
                     .simple(package_name.clone())
-                    .map_ok(move |metadata| Response::Package(package_name, metadata))
+                    .map_ok(move |(index, metadata)| {
+                        Response::Package(package_name, index, metadata)
+                    })
                     .map_err(ResolveError::Client)
                     .await
             }
@@ -803,7 +809,7 @@ enum Request {
 #[allow(clippy::large_enum_variant)]
 enum Response {
     /// The returned metadata for a package hosted on a registry.
-    Package(PackageName, SimpleJson),
+    Package(PackageName, IndexUrl, SimpleJson),
     /// The returned metadata for a distribution.
     Dist(Dist, Metadata21, Option<Url>),
 }
@@ -839,8 +845,9 @@ impl InFlight {
 
 /// In-memory index of package metadata.
 struct Index {
-    /// A map from package name to the metadata for that package.
-    packages: WaitMap<PackageName, VersionMap>,
+    /// A map from package name to the metadata for that package and the index where the metadata
+    /// came from.
+    packages: WaitMap<PackageName, (IndexUrl, VersionMap)>,
 
     /// A map from distribution SHA to metadata for that distribution.
     distributions: WaitMap<String, Metadata21>,
diff --git a/crates/pypi-types/Cargo.toml b/crates/pypi-types/Cargo.toml
index 2fa065cbe..36dc5cec2 100644
--- a/crates/pypi-types/Cargo.toml
+++ b/crates/pypi-types/Cargo.toml
@@ -22,6 +22,7 @@ rfc2047-decoder = { workspace = true }
 serde = { workspace = true }
 thiserror = { workspace = true }
 tracing = { workspace = true }
+url = { workspace = true }
 
 [dev-dependencies]
 indoc = { version = "2.0.4" }
diff --git a/crates/pypi-types/src/index_url.rs b/crates/pypi-types/src/index_url.rs
new file mode 100644
index 000000000..fa5c3f217
--- /dev/null
+++ b/crates/pypi-types/src/index_url.rs
@@ -0,0 +1,17 @@
+use url::Url;
+
+/// The url of an index, newtype'd to avoid mixing it with file urls
+#[derive(Debug, Clone, Hash, Eq, PartialEq)]
+pub struct IndexUrl(Url);
+
+impl From<Url> for IndexUrl {
+    fn from(url: Url) -> Self {
+        Self(url)
+    }
+}
+
+impl From<IndexUrl> for Url {
+    fn from(index: IndexUrl) -> Self {
+        index.0
+    }
+}
diff --git a/crates/pypi-types/src/lib.rs b/crates/pypi-types/src/lib.rs
index e5b7ffd1a..e8c5623c0 100644
--- a/crates/pypi-types/src/lib.rs
+++ b/crates/pypi-types/src/lib.rs
@@ -1,9 +1,11 @@
 pub use direct_url::{ArchiveInfo, DirectUrl, VcsInfo, VcsKind};
+pub use index_url::IndexUrl;
 pub use lenient_requirement::LenientVersionSpecifiers;
 pub use metadata::{Error, Metadata21};
 pub use simple_json::{File, SimpleJson, Yanked};
 
 mod direct_url;
+mod index_url;
 mod lenient_requirement;
 mod metadata;
 mod simple_json;