diff --git a/src/libstore/build/derivation-goal.cc b/src/libstore/build/derivation-goal.cc
index 17b6afbb2..f43a2a5b6 100644
--- a/src/libstore/build/derivation-goal.cc
+++ b/src/libstore/build/derivation-goal.cc
@@ -119,6 +119,7 @@ std::string DerivationGoal::key()
 void DerivationGoal::killChild()
 {
     hook.reset();
+    builderOutFD = nullptr;
 }
 
 
@@ -814,6 +815,7 @@ void replaceValidPath(const Path & storePath, const Path & tmpPath)
 
 int DerivationGoal::getChildStatus()
 {
+    builderOutFD = nullptr;
     return hook->pid.kill();
 }
 
@@ -822,6 +824,7 @@ void DerivationGoal::closeReadPipes()
 {
     hook->builderOut.readSide.reset();
     hook->fromHook.readSide.reset();
+    builderOutFD = nullptr;
 }
 
 
@@ -1209,6 +1212,7 @@ HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
     std::set<int> fds;
     fds.insert(hook->fromHook.readSide.get());
     fds.insert(hook->builderOut.readSide.get());
+    builderOutFD = &hook->builderOut.readSide;
     worker.childStarted(shared_from_this(), fds, false, false);
 
     return rpAccept;
@@ -1271,13 +1275,10 @@ void DerivationGoal::closeLogFile()
 }
 
 
-bool DerivationGoal::isReadDesc(int fd)
-{
-    return fd == hook->builderOut.readSide.get();
-}
-
 Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data)
 {
+    assert(builderOutFD);
+
     auto tooMuchLogs = [&] {
         killChild();
         return done(
@@ -1287,7 +1288,7 @@ Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data
     };
 
     // local & `ssh://`-builds are dealt with here.
-    auto isWrittenToLog = isReadDesc(fd);
+    auto isWrittenToLog = fd == builderOutFD->get();
     if (isWrittenToLog)
     {
         logSize += data.size();
diff --git a/src/libstore/build/derivation-goal.hh b/src/libstore/build/derivation-goal.hh
index 250b437da..da935ceb5 100644
--- a/src/libstore/build/derivation-goal.hh
+++ b/src/libstore/build/derivation-goal.hh
@@ -186,6 +186,13 @@ struct DerivationGoal : public Goal
      */
     std::unique_ptr<HookInstance> hook;
 
+    /**
+      * Builder output is pulled from this file descriptor when not null.
+      * Owned by the derivation goal or subclass, must not be reset until
+      * the build has finished and no more output must be processed by us
+      */
+    AutoCloseFD * builderOutFD = nullptr;
+
     /**
      * The sort of derivation we are building.
      */
@@ -290,8 +297,6 @@ struct DerivationGoal : public Goal
     virtual void cleanupPostOutputsRegisteredModeCheck();
     virtual void cleanupPostOutputsRegisteredModeNonCheck();
 
-    virtual bool isReadDesc(int fd);
-
     /**
      * Callback used by the worker to write to the log.
      */
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 841d1f2f6..116fa0ae5 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -279,8 +279,10 @@ void LocalDerivationGoal::closeReadPipes()
 {
     if (hook) {
         DerivationGoal::closeReadPipes();
-    } else
-        builderOut.close();
+    } else {
+        builderOutPTY.close();
+        builderOutFD = nullptr;
+    }
 }
 
 
@@ -671,12 +673,13 @@ void LocalDerivationGoal::startBuilder()
     Path logFile = openLogFile();
 
     /* Create a pseudoterminal to get the output of the builder. */
-    builderOut = AutoCloseFD{posix_openpt(O_RDWR | O_NOCTTY)};
-    if (!builderOut)
+    builderOutPTY = AutoCloseFD{posix_openpt(O_RDWR | O_NOCTTY)};
+    if (!builderOutPTY)
         throw SysError("opening pseudoterminal master");
+    builderOutFD = &builderOutPTY;
 
     // FIXME: not thread-safe, use ptsname_r
-    std::string slaveName = ptsname(builderOut.get());
+    std::string slaveName = ptsname(builderOutPTY.get());
 
     if (buildUser) {
         if (chmod(slaveName.c_str(), 0600))
@@ -687,12 +690,12 @@ void LocalDerivationGoal::startBuilder()
     }
 #if __APPLE__
     else {
-        if (grantpt(builderOut.get()))
+        if (grantpt(builderOutPTY.get()))
             throw SysError("granting access to pseudoterminal slave");
     }
 #endif
 
-    if (unlockpt(builderOut.get()))
+    if (unlockpt(builderOutPTY.get()))
         throw SysError("unlocking pseudoterminal");
 
     /* Open the slave side of the pseudoterminal and use it as stderr. */
@@ -723,14 +726,14 @@ void LocalDerivationGoal::startBuilder()
 
     /* parent */
     pid.setSeparatePG(true);
-    worker.childStarted(shared_from_this(), {builderOut.get()}, true, true);
+    worker.childStarted(shared_from_this(), {builderOutPTY.get()}, true, true);
 
     /* Check if setting up the build environment failed. */
     std::vector<std::string> msgs;
     while (true) {
         std::string msg = [&]() {
             try {
-                return readLine(builderOut.get());
+                return readLine(builderOutPTY.get());
             } catch (Error & e) {
                 auto status = pid.wait();
                 e.addTrace({}, "while waiting for the build environment for '%s' to initialize (%s, previous messages: %s)",
@@ -742,7 +745,7 @@ void LocalDerivationGoal::startBuilder()
         }();
         if (msg.substr(0, 1) == "\2") break;
         if (msg.substr(0, 1) == "\1") {
-            FdSource source(builderOut.get());
+            FdSource source(builderOutPTY.get());
             auto ex = readError(source);
             ex.addTrace({}, "while setting up the build environment");
             throw ex;
@@ -2563,13 +2566,6 @@ void LocalDerivationGoal::deleteTmpDir(bool force)
 }
 
 
-bool LocalDerivationGoal::isReadDesc(int fd)
-{
-    return (hook && DerivationGoal::isReadDesc(fd)) ||
-        (!hook && fd == builderOut.get());
-}
-
-
 StorePath LocalDerivationGoal::makeFallbackPath(OutputNameView outputName)
 {
     return worker.store.makeStorePath(
diff --git a/src/libstore/build/local-derivation-goal.hh b/src/libstore/build/local-derivation-goal.hh
index 7617760a5..128e6fd36 100644
--- a/src/libstore/build/local-derivation-goal.hh
+++ b/src/libstore/build/local-derivation-goal.hh
@@ -40,7 +40,7 @@ struct LocalDerivationGoal : public DerivationGoal
      * Master side of the pseudoterminal used for the builder's
      * standard output/error.
      */
-    AutoCloseFD builderOut;
+    AutoCloseFD builderOutPTY;
 
     /**
      * Pipe for synchronising updates to the builder namespaces.
@@ -285,8 +285,6 @@ struct LocalDerivationGoal : public DerivationGoal
     void cleanupPostOutputsRegisteredModeCheck() override;
     void cleanupPostOutputsRegisteredModeNonCheck() override;
 
-    bool isReadDesc(int fd) override;
-
     /**
      * Delete the temporary directory, if we have one.
      */