diff --git a/subprojects/hydra-queue-runner/src/state/build.rs b/subprojects/hydra-queue-runner/src/state/build.rs
index f0a688a61..e6fac08a5 100644
--- a/subprojects/hydra-queue-runner/src/state/build.rs
+++ b/subprojects/hydra-queue-runner/src/state/build.rs
@@ -356,7 +356,7 @@ impl BuildTimings {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct BuildOutput {
     pub failed: bool,
     pub timings: BuildTimings,
diff --git a/subprojects/hydra-queue-runner/src/state/mod.rs b/subprojects/hydra-queue-runner/src/state/mod.rs
index ad8819725..2753f75db 100644
--- a/subprojects/hydra-queue-runner/src/state/mod.rs
+++ b/subprojects/hydra-queue-runner/src/state/mod.rs
@@ -189,6 +189,14 @@ pub struct State {
     /// restart.
     pub resolved_drv_map: parking_lot::RwLock<HashMap<StorePath, StorePath>>,
 
+    /// Cache mapping a CA derivation path to the `BuildOutput` it produced.
+    /// CA floating outputs have no statically-known path, so an already-built
+    /// CA derivation can only be detected via the in-memory `Steps` map, which
+    /// drops entries once the owning build finishes. A later build resolving to
+    /// the same derivation would then rebuild it (and lose its output). This
+    /// cache lets such builds be marked cached instead.
+    pub resolved_output_cache: parking_lot::RwLock<HashMap<StorePath, BuildOutput>>,
+
     pub fod_checker: Option<Arc<FodChecker>>,
 
     pub started_at: jiff::Timestamp,
@@ -292,6 +300,7 @@ impl State {
             db,
             machines: Machines::new(),
             resolved_drv_map: parking_lot::RwLock::new(HashMap::new()),
+            resolved_output_cache: parking_lot::RwLock::new(HashMap::new()),
             log_dir,
             builds: Builds::new(),
             jobsets: Jobsets::new(),
@@ -655,6 +664,26 @@ impl State {
                 // Otherwise, `succeed_step` would prematurely mark the build as
                 // finished when an intermediate resolved step completes.
                 let is_toplevel = build.toplevel.load().as_deref() == Some(&*step_info.step);
+
+                // If this resolved derivation was already built, mark the
+                // build cached instead of rebuilding. Only safe at toplevel: an
+                // intermediate resolved step does not finish the build itself.
+                if is_toplevel {
+                    let cached_output = self
+                        .resolved_output_cache
+                        .read()
+                        .get(&resolved_path)
+                        .cloned();
+                    if let Some(output) = cached_output {
+                        tracing::info!(
+                            "resolved derivation {resolved_path} already built, marking build {} as cached",
+                            build.id
+                        );
+                        self.mark_build_cached(build.clone(), output).await?;
+                        return Ok(RealiseStepResult::Resolved);
+                    }
+                }
+
                 let referring_build = if is_toplevel {
                     Some(build.clone())
                 } else {
@@ -2594,8 +2623,47 @@ impl State {
     }
 
     #[tracing::instrument(skip(self, build), fields(build_id=build.id), err)]
+    /// Mark a build succeeded and cached using a known build output, without
+    /// re-deriving it from the store.
+    async fn mark_build_cached(
+        &self,
+        build: Arc<Build>,
+        output: BuildOutput,
+    ) -> Result<(), StateError> {
+        {
+            let mut db = self.db.get().await?;
+            let mut tx = db.begin_transaction().await?;
+            let now = jiff::Timestamp::now().as_second();
+            tx.mark_succeeded_build(
+                get_mark_build_sccuess_data(&build, &output),
+                true,
+                i32::try_from(now)?,
+                i32::try_from(now)?,
+                self.pool.store_dir(),
+            )
+            .await?;
+            self.metrics.nr_builds_done.inc();
+            tx.notify_build_finished(build.id, &[]).await?;
+            tx.commit().await?;
+        }
+        build.set_finished_in_db(true);
+        self.builds.remove_by_id(build.id);
+        Ok(())
+    }
+
     async fn handle_cached_build(&self, build: Arc<Build>) -> Result<(), StateError> {
-        let res = self.get_build_output_cached(&build.drv_path).await?;
+        // CA floating outputs have no statically-known path, so
+        // `get_build_output_cached` cannot recover them from the derivation
+        // alone and would yield an empty output. Prefer a cached output.
+        let cached = self
+            .resolved_output_cache
+            .read()
+            .get(&build.drv_path)
+            .cloned();
+        let res = match cached {
+            Some(output) => output,
+            None => self.get_build_output_cached(&build.drv_path).await?,
+        };
 
         {
             let mut db = self.db.get().await?;
diff --git a/subprojects/hydra-tests/content-addressed/early-cutoff.t b/subprojects/hydra-tests/content-addressed/early-cutoff.t
index d7fb119cb..0e9f5819c 100644
--- a/subprojects/hydra-tests/content-addressed/early-cutoff.t
+++ b/subprojects/hydra-tests/content-addressed/early-cutoff.t
@@ -13,55 +13,65 @@ use Test2::V0;
 
 my $db = $ctx{context}->db();
 
-my $project = $db->resultset('Projects')->create({name => "tests", displayname => "", owner => "root"});
+# Loop to reproduce the flaky "One downstream build should be cached" race.
+my $iterations = $ENV{HYDRA_EARLY_CUTOFF_ITERATIONS} // 10;
 
-my $jobset = createBaseJobset($db, "content-addressed-early-cutoff", "content-addressed-early-cutoff.nix", $ctx{jobsdir});
+for my $iter (1 .. $iterations) {
+    my $project = $db->resultset('Projects')->create({
+        name => "tests-$iter",
+        displayname => "",
+        owner => "root",
+    });
 
-ok(evalSucceeds($ctx{context}, $jobset), "Evaluating early cutoff jobs should exit with return code 0");
-is(nrQueuedBuildsForJobset($jobset), 4, "Should queue 4 early cutoff builds");
+    my $jobset = createBaseJobset(
+        $db,
+        "content-addressed-early-cutoff-$iter",
+        "content-addressed-early-cutoff.nix",
+        $ctx{jobsdir},
+    );
 
-my @builds = queuedBuildsForJobset($jobset);
-ok(runBuilds($ctx{context}, @builds), "Building all early cutoff jobs should exit with code 0");
+    ok(evalSucceeds($ctx{context}, $jobset), "[$iter] Evaluating early cutoff jobs should exit with return code 0");
+    is(nrQueuedBuildsForJobset($jobset), 4, "[$iter] Should queue 4 early cutoff builds");
 
-for my $build (@builds) {
-    my $newbuild = $db->resultset('Builds')->find($build->id);
-    is($newbuild->finished, 1, "Build '".$build->job."' should be finished.");
-    is($newbuild->buildstatus, 0, "Build '".$build->job."' should have buildstatus 0.");
-}
+    my @builds = queuedBuildsForJobset($jobset);
+    ok(runBuilds($ctx{context}, @builds), "[$iter] Building all early cutoff jobs should exit with code 0");
+
+    for my $build (@builds) {
+        my $newbuild = $db->resultset('Builds')->find($build->id);
+        is($newbuild->finished, 1, "[$iter] Build '".$build->job."' should be finished.");
+        is($newbuild->buildstatus, 0, "[$iter] Build '".$build->job."' should have buildstatus 0.");
+    }
+
+    # earlyCutoffUpstream1 and earlyCutoffUpstream2 differ but produce the same
+    # content-addressed output. Building earlyCutoffDownstream1 should let
+    # earlyCutoffDownstream2 be cached, since its resolved input is identical.
+    my $upstream1 = $db->resultset('Builds')->find({ jobset_id => $jobset->id, job => "earlyCutoffUpstream1" });
+    my $upstream2 = $db->resultset('Builds')->find({ jobset_id => $jobset->id, job => "earlyCutoffUpstream2" });
+
+    my $upstream1_out = $upstream1->buildoutputs->find({ name => "out" });
+    my $upstream2_out = $upstream2->buildoutputs->find({ name => "out" });
+    is($upstream1_out->path, $upstream2_out->path,
+        "[$iter] Both upstream builds should resolve to the same content-addressed output path");
 
-# Early cutoff: earlyCutoffUpstream1 and earlyCutoffUpstream2 have
-# different derivations but produce the same content-addressed output.
-# After building earlyCutoffDownstream1, earlyCutoffDownstream2 should
-# be cached because its resolved input is identical.
-my $upstream1 = $db->resultset('Builds')->find({
-    jobset_id => $jobset->id,
-    job => "earlyCutoffUpstream1",
-});
-my $upstream2 = $db->resultset('Builds')->find({
-    jobset_id => $jobset->id,
-    job => "earlyCutoffUpstream2",
-});
-
-my $upstream1_out = $upstream1->buildoutputs->find({ name => "out" });
-my $upstream2_out = $upstream2->buildoutputs->find({ name => "out" });
-is($upstream1_out->path, $upstream2_out->path,
-    "Both upstream builds should resolve to the same content-addressed output path");
-
-my $downstream1 = $db->resultset('Builds')->find({
-    jobset_id => $jobset->id,
-    job => "earlyCutoffDownstream1",
-});
-my $downstream2 = $db->resultset('Builds')->find({
-    jobset_id => $jobset->id,
-    job => "earlyCutoffDownstream2",
-});
-
-my $downstream1_out = $downstream1->buildoutputs->find({ name => "out" });
-my $downstream2_out = $downstream2->buildoutputs->find({ name => "out" });
-is($downstream1_out->path, $downstream2_out->path,
-    "Both downstream builds should create the same content-addressed output path");
-
-ok($downstream1->iscachedbuild || $downstream2->iscachedbuild,
-    "One downstream build should be cached");
+    my $downstream1 = $db->resultset('Builds')->find({ jobset_id => $jobset->id, job => "earlyCutoffDownstream1" });
+    my $downstream2 = $db->resultset('Builds')->find({ jobset_id => $jobset->id, job => "earlyCutoffDownstream2" });
+
+    my $downstream1_out = $downstream1->buildoutputs->find({ name => "out" });
+    my $downstream2_out = $downstream2->buildoutputs->find({ name => "out" });
+    is($downstream1_out->path, $downstream2_out->path,
+        "[$iter] Both downstream builds should create the same content-addressed output path");
+
+    my $passed = ok($downstream1->iscachedbuild || $downstream2->iscachedbuild,
+        "[$iter] One downstream build should be cached");
+
+    unless ($passed) {
+        for my $b ($upstream1, $upstream2, $downstream1, $downstream2) {
+            my $out = $b->buildoutputs->find({ name => "out" });
+            diag(sprintf("build id=%d job=%s out=%s iscached=%s start=%s stop=%s",
+                $b->id, $b->job, ($out ? $out->path : "<undef>"),
+                $b->iscachedbuild, $b->starttime // "?", $b->stoptime // "?"));
+        }
+    }
+}
 
 done_testing;