diff --git a/src/libstore/unix/build/local-derivation-goal.cc b/src/libstore/unix/build/local-derivation-goal.cc
index 805c3bbca..9ab0da32b 100644
--- a/src/libstore/unix/build/local-derivation-goal.cc
+++ b/src/libstore/unix/build/local-derivation-goal.cc
@@ -2656,6 +2656,12 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
                             worker.store.printStorePath(drvPath),
                             wanted.to_string(HashFormat::SRI, true),
                             got.to_string(HashFormat::SRI, true)));
+                    // FIXME: put this in BuildResult and log that as JSON.
+                    act->result(resHashMismatch,
+                        {worker.store.printStorePath(drvPath),
+                         wanted.to_string(HashFormat::SRI, true),
+                         got.to_string(HashFormat::SRI, true)
+                        });
                 }
                 if (!newInfo0.references.empty()) {
                     auto numViolations = newInfo.references.size();
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index e8112c6b0..21493b969 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -36,6 +36,7 @@ typedef enum {
     resSetExpected = 106,
     resPostBuildLogLine = 107,
     resFetchStatus = 108,
+    resHashMismatch = 109,
 } ResultType;
 
 typedef uint64_t ActivityId;