diff --git a/doc/manual/rl-next/post-build-hook-config.md b/doc/manual/rl-next/post-build-hook-config.md
new file mode 100644
index 000000000..f4663a67b
--- /dev/null
+++ b/doc/manual/rl-next/post-build-hook-config.md
@@ -0,0 +1,10 @@
+---
+synopsis: "`post-build-hook` only receives settings that are set"
+cls: [2800]
+issues: [fj#739]
+category: Fixes
+credits: jade
+---
+If one is using `post-build-hook` to upload paths to a cache, it used to be broken if CppNix was used inside the script, since CppNix would fail about unsupported configuration option values in some of Lix's defaults.
+This is because `post-build-hook` receives the settings of the nix daemon in the `NIX_CONFIG` environment variable.
+Now Lix only emits overridden settings to `post-build-hook` invocations, which fixes this issue in the majority of cases: where the configuration is not explicitly incompatible.
diff --git a/lix/libstore/build/derivation-goal.cc b/lix/libstore/build/derivation-goal.cc
index 8023281eb..2a1034748 100644
--- a/lix/libstore/build/derivation-goal.cc
+++ b/lix/libstore/build/derivation-goal.cc
@@ -977,7 +977,7 @@ void runPostBuildHook(
     auto drvPathPretty = store.printStorePath(drvPath);
     hookEnvironment.emplace("DRV_PATH", drvPathPretty);
     hookEnvironment.emplace("OUT_PATHS", chomp(concatStringsSep(" ", store.printStorePathSet(outputPaths))));
-    hookEnvironment.emplace("NIX_CONFIG", globalConfig.toKeyValue());
+    hookEnvironment.emplace("NIX_CONFIG", globalConfig.toKeyValue(true));
 
     struct LogSink : Sink {
         Activity & act;
diff --git a/lix/libutil/config.cc b/lix/libutil/config.cc
index 5cbd81541..4fbfdc8e8 100644
--- a/lix/libutil/config.cc
+++ b/lix/libutil/config.cc
@@ -187,15 +187,6 @@ nlohmann::json Config::toJSON()
     return res;
 }
 
-std::string Config::toKeyValue()
-{
-    std::string res;
-    for (const auto & s : _settings)
-        if (s.second.isAlias)
-            res += fmt("%s = %s\n", s.first, s.second.setting->to_string());
-    return res;
-}
-
 void Config::convertToArgs(Args & args, const std::string & category)
 {
     for (auto & s : _settings) {
@@ -534,11 +525,11 @@ nlohmann::json GlobalConfig::toJSON()
     return res;
 }
 
-std::string GlobalConfig::toKeyValue()
+std::string GlobalConfig::toKeyValue(bool overriddenOnly)
 {
     std::string res;
     std::map<std::string, Config::SettingInfo> settings;
-    globalConfig.getSettings(settings);
+    globalConfig.getSettings(settings, overriddenOnly);
     for (const auto & s : settings)
         res += fmt("%s = %s\n", s.first, s.second.value);
     return res;
diff --git a/lix/libutil/config.hh b/lix/libutil/config.hh
index 33a60afde..fe14fcb4d 100644
--- a/lix/libutil/config.hh
+++ b/lix/libutil/config.hh
@@ -95,12 +95,6 @@ public:
      */
     virtual nlohmann::json toJSON() = 0;
 
-    /**
-     * Outputs all settings in a key-value pair format suitable to be used as
-     * `nix.conf`
-     */
-    virtual std::string toKeyValue() = 0;
-
     /**
      * Converts settings to `Args` to be used on the command line interface
      * - args: args to write to
@@ -166,8 +160,6 @@ public:
 
     nlohmann::json toJSON() override;
 
-    std::string toKeyValue() override;
-
     void convertToArgs(Args & args, const std::string & category) override;
 };
 
@@ -371,7 +363,11 @@ struct GlobalConfig : public AbstractConfig
 
     nlohmann::json toJSON() override;
 
-    std::string toKeyValue() override;
+    /**
+     * Outputs all settings in a key-value pair format suitable to be used as
+     * `nix.conf`
+     */
+    std::string toKeyValue(bool overriddenOnly = false);
 
     void convertToArgs(Args & args, const std::string & category) override;
 
diff --git a/tests/functional/post-hook.sh b/tests/functional/post-hook.sh
index 752f8220c..4be7da417 100644
--- a/tests/functional/post-hook.sh
+++ b/tests/functional/post-hook.sh
@@ -29,3 +29,30 @@ clearStore
 nix copy --from "$REMOTE_STORE" --no-require-sigs -f dependencies.nix
 nix copy --from "$REMOTE_STORE" --no-require-sigs -f dependencies.nix input1_drv
 nix copy --from "$REMOTE_STORE" --no-require-sigs -f multiple-outputs.nix a^second
+
+clearStore
+
+# Should fail if the build hook fails
+cat > "$TEST_ROOT/fail.sh" <<-EOF
+#!${shell}
+false
+EOF
+chmod +x "$TEST_ROOT/fail.sh"
+expect 1 nix-build -o "$TEST_ROOT/result" dependencies.nix --post-build-hook "$TEST_ROOT/fail.sh"
+clearStore
+
+# Ensure that settings are passed into the post-build-hook, but only overridden
+# ones.
+rm -f "${TEST_ROOT}/nix-config"
+cat > "${TEST_ROOT}/settings.sh" <<-EOF
+#!${shell}
+echo "\$NIX_CONFIG" > "${TEST_ROOT}/nix-config"
+EOF
+chmod +x "${TEST_ROOT}/settings.sh"
+nix-build -o "${TEST_ROOT}/result" dependencies.nix --timeout 1337 --post-build-hook "${TEST_ROOT}/settings.sh"
+# Ensure pure-eval cannot become not a setting with the test passing.
+nix config show pure-eval
+# Defaulted setting does not appear.
+expect 1 grepQuiet pure-eval "${TEST_ROOT}/nix-config"
+# Overridden setting appears.
+grepQuiet "timeout = 1337" "${TEST_ROOT}/nix-config"