diff --git a/fontc_crater/src/ci.rs b/fontc_crater/src/ci.rs
index 4c8dca91..8fd36a2c 100644
--- a/fontc_crater/src/ci.rs
+++ b/fontc_crater/src/ci.rs
@@ -35,6 +35,8 @@ struct RunSummary {
     began: DateTime<Utc>,
     finished: DateTime<Utc>,
     fontc_rev: String,
+    #[serde(default)]
+    pip_freeze_sha: String,
     results_file: PathBuf,
     // the name of the file listing targets used by this run.
     // it is intended that when this list is updated, the filename is changed.
@@ -81,11 +83,13 @@ fn run_crater_and_save_results(args: &CiArgs) -> Result<(), Error> {
     let mut prev_runs: Vec<RunSummary> = load_json_if_exists_else_default(&summary_file)?;
     // todo: fontc_repo should be checked out by us, and have a known path
     let fontc_rev = super::get_git_rev(None).unwrap();
+    let pip_freeze_sha = super::pip_freeze_sha();
     if let Some(last_run) = prev_runs.last() {
         if last_run.fontc_rev == fontc_rev
             && Some(last_run.input_file.as_os_str()) == args.to_run.file_name()
+            && pip_freeze_sha == last_run.pip_freeze_sha
         {
-            log::info!("fontc rev & inputs is unchanged from last run, skipping");
+            log::info!("no changes since last run, skipping");
             return Ok(());
         }
     }
@@ -121,6 +125,7 @@ fn run_crater_and_save_results(args: &CiArgs) -> Result<(), Error> {
         began,
         finished,
         fontc_rev,
+        pip_freeze_sha,
         results_file: out_file.into(),
         input_file,
         stats: summary,
diff --git a/fontc_crater/src/main.rs b/fontc_crater/src/main.rs
index 7665316d..90b1e422 100644
--- a/fontc_crater/src/main.rs
+++ b/fontc_crater/src/main.rs
@@ -4,6 +4,7 @@ use std::{
     collections::BTreeMap,
     fmt::Display,
     path::{Path, PathBuf},
+    process::{Command, Stdio},
     str::FromStr,
     sync::atomic::{AtomicUsize, Ordering},
 };
@@ -135,6 +136,22 @@ fn get_git_rev(repo_path: Option<&Path>) -> Option<String> {
     )
 }
 
+fn pip_freeze_sha() -> String {
+    let pipfreeze = Command::new("pip")
+        .arg("freeze")
+        .stdout(Stdio::piped())
+        .spawn()
+        .unwrap();
+    let sha1sum = Command::new("shasum")
+        .stdin(Stdio::from(pipfreeze.stdout.unwrap()))
+        .output()
+        .expect("shasum should be preinstalled everywhere");
+    assert!(sha1sum.status.success());
+    std::str::from_utf8(sha1sum.stdout.trim_ascii())
+        .expect("shasum output always ascii")
+        .to_owned()
+}
+
 impl Target {
     pub(crate) fn id(&self) -> TargetId {
         TargetId {