diff --git a/crates/puffin-build/src/lib.rs b/crates/puffin-build/src/lib.rs
index bcc88a299..7926ebd6b 100644
--- a/crates/puffin-build/src/lib.rs
+++ b/crates/puffin-build/src/lib.rs
@@ -35,7 +35,7 @@ static MISSING_HEADER_RE: Lazy<Regex> = Lazy::new(|| {
     Regex::new(
         r".*\.(c|c..|h|h..):\d+:\d+: fatal error: (?<header>.*\.(h|h..)): No such file or directory"
     )
-    .unwrap()
+        .unwrap()
 });
 
 #[derive(Error, Debug)]
@@ -133,6 +133,7 @@ pub struct PyProjectToml {
 }
 
 /// `[build-backend]` from pyproject.toml
+#[derive(Debug)]
 struct Pep517Backend {
     /// The build backend string such as `setuptools.build_meta:__legacy__` or `maturin` from
     /// `build-backend.backend` in pyproject.toml
@@ -252,14 +253,15 @@ impl SourceBuild {
 
         let venv = gourgeist::create_venv(&temp_dir.path().join(".venv"), interpreter)?;
 
-        // There are packages such as DTLSSocket 0.1.16 that say
+        // There are packages such as DTLSSocket 0.1.16 that say:
         // ```toml
         // [build-system]
         // requires = ["Cython<3", "setuptools", "wheel"]
         // ```
-        // In this case we need to install requires PEP 517 style but then call setup.py in the
-        // legacy way
-        let requirements = if let Some(build_system) = &build_system {
+        // In this case, we need to install requires PEP 517 style but then call setup.py in the
+        // legacy way.
+        let requirements = if let Some(build_system) = build_system.as_ref() {
+            // .filter(|build_system| build_system.build_backend.is_some()) {
             let resolved_requirements = build_context
                 .resolve(&build_system.requires)
                 .await
@@ -324,13 +326,11 @@ impl SourceBuild {
                 source_dist,
             )
             .await?;
-        } else {
-            if !source_tree.join("setup.py").is_file() {
-                return Err(Error::InvalidSourceDist(
-                    "The archive contains neither a pyproject.toml or a setup.py at the top level"
-                        .to_string(),
-                ));
-            }
+        } else if !source_tree.join("setup.py").is_file() {
+            return Err(Error::InvalidSourceDist(
+                "The archive contains neither a `pyproject.toml` nor a `setup.py` file at the top level"
+                    .to_string(),
+            ));
         }
 
         Ok(Self {
@@ -381,7 +381,7 @@ impl SourceBuild {
             return Err(Error::from_command_output(
                 "Build backend failed to determine metadata through `prepare_metadata_for_build_wheel`".to_string(),
                 &output,
-            &self.package_id));
+                &self.package_id));
         }
         let message = output
             .stdout
@@ -445,7 +445,7 @@ impl SourceBuild {
                         "Expected exactly wheel in `dist/` after invoking setup.py, found {dist_dir:?}"
                     ),
                     &output,
-                &self.package_id));
+                    &self.package_id));
             };
             // TODO(konstin): Faster copy such as reflink? Or maybe don't really let the user pick the target dir
             let wheel = wheel_dir.join(dist_wheel.file_name());
@@ -598,8 +598,9 @@ async fn create_pep517_build_environment(
     }
     Ok(())
 }
+
 /// Returns the directory with the `pyproject.toml`/`setup.py`
-#[instrument(skip_all, fields(sdist = ?sdist.file_name().unwrap_or(sdist.as_os_str())))]
+#[instrument(skip_all, fields(sdist = ? sdist.file_name().unwrap_or(sdist.as_os_str())))]
 fn extract_archive(sdist: &Path, extracted: &PathBuf) -> Result<PathBuf, Error> {
     if sdist
         .extension()
diff --git a/crates/puffin-interpreter/src/interpreter.rs b/crates/puffin-interpreter/src/interpreter.rs
index 332e7fe13..017316331 100644
--- a/crates/puffin-interpreter/src/interpreter.rs
+++ b/crates/puffin-interpreter/src/interpreter.rs
@@ -103,7 +103,7 @@ impl Interpreter {
     }
 }
 
-#[derive(Deserialize, Serialize, Clone)]
+#[derive(Debug, Deserialize, Serialize, Clone)]
 pub(crate) struct InterpreterQueryResult {
     pub(crate) markers: MarkerEnvironment,
     pub(crate) base_exec_prefix: PathBuf,
@@ -166,24 +166,31 @@ impl InterpreterQueryResult {
         let cache_dir = cache.bucket(CacheBucket::Interpreter);
         let cache_path = cache_dir.join(format!("{}.json", digest(&executable_bytes)));
 
+        let modified = fs_err::metadata(executable)?
+            // Note: This is infallible on windows and unix (i.e., all platforms we support).
+            .modified()?
+            .duration_since(UNIX_EPOCH)
+            .map_err(|err| Error::SystemTime(executable.to_path_buf(), err))?;
+
         // Read from the cache.
         if let Ok(data) = fs::read(&cache_path) {
             if let Ok(cached) = serde_json::from_slice::<CachedByTimestamp<Self>>(&data) {
-                debug!("Using cached markers for {}", executable.display());
-                return Ok(cached.data);
+                if cached.timestamp == modified.as_millis() {
+                    debug!("Using cached markers for: {}", executable.display());
+                    return Ok(cached.data);
+                }
+
+                debug!(
+                    "Ignoring stale cached markers for: {}",
+                    executable.display()
+                );
             }
         }
 
         // Otherwise, run the Python script.
-        debug!("Detecting markers for {}", executable.display());
+        debug!("Detecting markers for: {}", executable.display());
         let info = Self::query(executable)?;
 
-        let modified = fs_err::metadata(executable)?
-            // Note: This is infallible on windows and unix (i.e. all platforms we support)
-            .modified()?
-            .duration_since(UNIX_EPOCH)
-            .map_err(|err| Error::SystemTime(executable.to_path_buf(), err))?;
-
         // If `executable` is a pyenv shim, a bash script that redirects to the activated
         // python executable at another path, we're not allowed to cache the interpreter info
         if executable == info.sys_executable {