diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 794be1568..76338ad10 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -1161,7 +1161,7 @@ HookReply DerivationGoal::tryBuildHook()
                     throw;
                 }
             }();
-            if (handleJSONLogMessage(s, worker.act, worker.hook->activities, true))
+            if (handleJSONLogMessage(s, worker.act, worker.hook->activities, "the build hook", true))
                 ;
             else if (s.substr(0, 2) == "# ") {
                 reply = s.substr(2);
@@ -1346,9 +1346,9 @@ void DerivationGoal::handleChildOutput(Descriptor fd, std::string_view data)
     if (hook && fd == hook->fromHook.readSide.get()) {
         for (auto c : data)
             if (c == '\n') {
-                auto json = parseJSONMessage(currentHookLine);
+                auto json = parseJSONMessage(currentHookLine, "the derivation builder");
                 if (json) {
-                    auto s = handleJSONLogMessage(*json, worker.act, hook->activities, true);
+                    auto s = handleJSONLogMessage(*json, worker.act, hook->activities, "the derivation builder", true);
                     // ensure that logs from a builder using `ssh-ng://` as protocol
                     // are also available to `nix log`.
                     if (s && !isWrittenToLog && logSink) {
@@ -1390,7 +1390,7 @@ void DerivationGoal::handleEOF(Descriptor fd)
 
 void DerivationGoal::flushLine()
 {
-    if (handleJSONLogMessage(currentLogLine, *act, builderActivities, false))
+    if (handleJSONLogMessage(currentLogLine, *act, builderActivities, "the derivation builder", false))
         ;
 
     else {
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index 152d6c985..4200342a2 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -280,20 +280,22 @@ static Logger::Fields getFields(nlohmann::json & json)
     return fields;
 }
 
-std::optional<nlohmann::json> parseJSONMessage(const std::string & msg)
+std::optional<nlohmann::json> parseJSONMessage(const std::string & msg, std::string_view source)
 {
     if (!hasPrefix(msg, "@nix ")) return std::nullopt;
     try {
         return nlohmann::json::parse(std::string(msg, 5));
     } catch (std::exception & e) {
-        printError("bad JSON log message from builder: %s", e.what());
+        printError("bad JSON log message from %s: %s",
+            Uncolored(source),
+            e.what());
     }
     return std::nullopt;
 }
 
 bool handleJSONLogMessage(nlohmann::json & json,
     const Activity & act, std::map<ActivityId, Activity> & activities,
-    bool trusted)
+    std::string_view source, bool trusted)
 {
     try {
         std::string action = json["action"];
@@ -329,7 +331,8 @@ bool handleJSONLogMessage(nlohmann::json & json,
         return true;
     } catch (nlohmann::json::exception &e) {
         warn(
-            "warning: Unable to handle a JSON message from the builder: %s",
+            "warning: Unable to handle a JSON message from %s: %s",
+            Uncolored(source),
             e.what()
         );
         return false;
@@ -337,12 +340,12 @@ bool handleJSONLogMessage(nlohmann::json & json,
 }
 
 bool handleJSONLogMessage(const std::string & msg,
-    const Activity & act, std::map<ActivityId, Activity> & activities, bool trusted)
+    const Activity & act, std::map<ActivityId, Activity> & activities, std::string_view source, bool trusted)
 {
-    auto json = parseJSONMessage(msg);
+    auto json = parseJSONMessage(msg, source);
     if (!json) return false;
 
-    return handleJSONLogMessage(*json, act, activities, trusted);
+    return handleJSONLogMessage(*json, act, activities, source, trusted);
 }
 
 Activity::~Activity()
diff --git a/src/libutil/logging.hh b/src/libutil/logging.hh
index 250f92099..11e4033a5 100644
--- a/src/libutil/logging.hh
+++ b/src/libutil/logging.hh
@@ -185,14 +185,25 @@ Logger * makeSimpleLogger(bool printBuildLogs = true);
 
 Logger * makeJSONLogger(Logger & prevLogger);
 
-std::optional<nlohmann::json> parseJSONMessage(const std::string & msg);
+/**
+ * @param source A noun phrase describing the source of the message, e.g. "the builder".
+ */
+std::optional<nlohmann::json> parseJSONMessage(const std::string & msg, std::string_view source);
 
+/**
+ * @param source A noun phrase describing the source of the message, e.g. "the builder".
+ */
 bool handleJSONLogMessage(nlohmann::json & json,
     const Activity & act, std::map<ActivityId, Activity> & activities,
+    std::string_view source,
     bool trusted);
 
+/**
+ * @param source A noun phrase describing the source of the message, e.g. "the builder".
+ */
 bool handleJSONLogMessage(const std::string & msg,
     const Activity & act, std::map<ActivityId, Activity> & activities,
+    std::string_view source,
     bool trusted);
 
 /**