diff --git a/fontc/build.rs b/fontc/build.rs
index d12d8308d..02e84f870 100644
--- a/fontc/build.rs
+++ b/fontc/build.rs
@@ -22,5 +22,8 @@ fn main() -> Result<(), Box<dyn Error>> {
         .add_instructions(&gitcl)?
         .add_instructions(&RustcBuilder::all_rustc()?)? // VERGEN_RUSTC_* for `--vv`
         .emit()?;
+    // vergen honours VERGEN_GIT_DESCRIBE as an override but doesn't ask cargo to
+    // watch it; without this a warm target dir keeps the previous value.
+    println!("cargo:rerun-if-env-changed=VERGEN_GIT_DESCRIBE");
     Ok(())
 }
diff --git a/fontc_crater/src/ci.rs b/fontc_crater/src/ci.rs
index 8944e6ca6..ba6acfd61 100644
--- a/fontc_crater/src/ci.rs
+++ b/fontc_crater/src/ci.rs
@@ -48,6 +48,9 @@ struct RunSummary {
     #[serde(alias = "input_file")]
     input_file_sha: String,
     stats: super::ttx_diff_runner::Summary,
+    /// How many targets skipped the comparison because fontc's output was unchanged.
+    #[serde(default)]
+    reused_cached_results: usize,
 }
 
 impl RunSummary {
@@ -170,6 +173,7 @@ fn run_crater_and_save_results(args: &CiArgs) -> Result<(), Error> {
         normalizer_path,
         source_cache: cache_dir,
         results_cache,
+        reused_cached_results: Default::default(),
     };
 
     let began = Utc::now();
@@ -182,7 +186,12 @@ fn run_crater_and_save_results(args: &CiArgs) -> Result<(), Error> {
     let elapsed = elapsed.to_std().unwrap_or_default();
     let elapsed = super::human_readable_duration(elapsed);
 
-    log::info!("completed {n_targets} targets in {elapsed}");
+    let reused_cached_results = context
+        .reused_cached_results
+        .load(std::sync::atomic::Ordering::Relaxed);
+    log::info!(
+        "completed {n_targets} targets in {elapsed} ({reused_cached_results}/{n_targets} results reused from cache)"
+    );
 
     let summary = super::ttx_diff_runner::Summary::new(&results);
     // if nothing has changed we still want to report it, but we don't need to
@@ -200,6 +209,7 @@ fn run_crater_and_save_results(args: &CiArgs) -> Result<(), Error> {
         results_file,
         input_file_sha,
         stats: summary,
+        reused_cached_results,
     };
 
     prev_runs.push(summary);
@@ -231,9 +241,8 @@ fn ttx_diff_has_changes(last_run_sha: &str) -> bool {
         .arg(last_run_sha)
         .output()
         .unwrap();
-    std::str::from_utf8(&output.stdout)
-        .unwrap()
-        .contains("ttx_diff/")
+    let diff = std::str::from_utf8(&output.stdout).unwrap();
+    diff.contains("ttx_diff/") || diff.contains("otl-normalizer/")
 }
 
 #[derive(Debug, Default)]
@@ -394,6 +403,9 @@ fn copy_file_into_dir(file_path: &Path, dir_path: &Path) -> PathBuf {
 fn compile_crate_or_die(name: &str) -> PathBuf {
     let status = Command::new("cargo")
         .args(["build", "-p", name, "--release"])
+        // prevent fontc from including a git sha in its version in the name table
+        // this lets us skip work when two fontc builds produce identical output
+        .env("VERGEN_GIT_DESCRIBE", "VERGEN_IDEMPOTENT_OUTPUT")
         .status()
         .expect("failed to run cargo build");
     if !status.success() {
diff --git a/fontc_crater/src/ttx_diff_runner.rs b/fontc_crater/src/ttx_diff_runner.rs
index fa8f3af7b..4db822601 100644
--- a/fontc_crater/src/ttx_diff_runner.rs
+++ b/fontc_crater/src/ttx_diff_runner.rs
@@ -2,6 +2,7 @@ use std::{
     collections::BTreeMap,
     path::{Path, PathBuf},
     process::Command,
+    sync::atomic::{AtomicUsize, Ordering},
 };
 
 use crate::{BuildType, Results, RunResult, Target, ci::ResultsCache};
@@ -19,6 +20,7 @@ pub(super) struct TtxContext {
     pub normalizer_path: PathBuf,
     pub source_cache: PathBuf,
     pub results_cache: ResultsCache,
+    pub reused_cached_results: AtomicUsize,
 }
 
 pub(super) fn run_ttx_diff(ctx: &TtxContext, target: &Target) -> RunResult<DiffOutput, DiffError> {
@@ -89,6 +91,7 @@ pub(super) fn run_ttx_diff(ctx: &TtxContext, target: &Target) -> RunResult<DiffO
         // fontc produced the same font as last time, so last time's result stands
         Some(UNCHANGED_EXIT_CODE) => match cached {
             Some(cached) => {
+                ctx.reused_cached_results.fetch_add(1, Ordering::Relaxed);
                 log::trace!("reused cached result for {target}");
                 return cached.into_result();
             }
diff --git a/resources/scripts/check_no_println.sh b/resources/scripts/check_no_println.sh
index baa1b8bb8..704579da7 100755
--- a/resources/scripts/check_no_println.sh
+++ b/resources/scripts/check_no_println.sh
@@ -93,6 +93,7 @@ allowlist+=("fea-rs/src/tests/parse.rs")
 allowlist+=("otl-normalizer")
 allowlist+=("glyphs-reader/build.rs")
 allowlist+=("fontdrasil/build.rs")
+allowlist+=("fontc/build.rs")
 allowlist+=("fontc_crater/src/main.rs")
 allowlist+=("fontc_crater/src/ttx_diff_runner.rs")
 allowlist+=("fontc_crater/src/ci.rs")