diff --git a/doc/manual/rl-next/new-assertions.md b/doc/manual/rl-next/new-assertions.md
new file mode 100644
index 000000000..efaaeb306
--- /dev/null
+++ b/doc/manual/rl-next/new-assertions.md
@@ -0,0 +1,23 @@
+---
+synopsis: Lix turns more internal bugs into crashes
+cls: [797, 626]
+---
+
+Lix now enables build options such as trapping on signed overflow and enabling
+libstdc++ assertions by default. These may find new bugs in Lix, which will
+present themselves as Lix processes aborting, potentially without an error
+message.
+
+If Lix processes abort on your machine, this is a bug. Please file a bug,
+ideally with the core dump (or information from it).
+
+On Linux, run `coredumpctl list`, find the crashed process's PID at
+the bottom of the list, then run `coredumpctl info THE-PID`. You can then paste
+the output into a bug report.
+
+On macOS, open the Console app from Applications/Utilities, select Crash
+Reports, select the crash report in question. Right click on it, select Open In
+Finder, then include that file in your bug report. [See the Apple
+documentation][apple-crashreport] for more details.
+
+[apple-crashreport]: https://developer.apple.com/documentation/xcode/acquiring-crash-reports-and-diagnostic-logs#Locate-crash-reports-and-memory-logs-on-the-device
diff --git a/local.mk b/local.mk
index a756c8272..50899cb03 100644
--- a/local.mk
+++ b/local.mk
@@ -4,7 +4,7 @@
 # This has an overhead of 0-4% on gcc and unmeasurably little on clang, in
 # Nix evaluation benchmarks.
 DEFAULT_SANITIZE_FLAGS = -fsanitize=signed-integer-overflow -fsanitize-undefined-trap-on-error
-GLOBAL_CXXFLAGS += -Wno-deprecated-declarations -Werror=switch $(DEFAULT_SANITIZE_FLAGS)
+GLOBAL_CXXFLAGS += -Wno-deprecated-declarations -Werror=switch $(DEFAULT_SANITIZE_FLAGS) -D_GLIBCXX_ASSERTIONS=1
 GLOBAL_LDFLAGS += $(DEFAULT_SANITIZE_FLAGS)
 # Allow switch-enum to be overridden for files that do not support it, usually because of dependency headers.
 ERROR_SWITCH_ENUM = -Werror=switch-enum
diff --git a/meson.build b/meson.build
index 2cedabad5..456be5aa2 100644
--- a/meson.build
+++ b/meson.build
@@ -344,6 +344,12 @@ add_project_arguments(
   '-Wimplicit-fallthrough',
   '-Werror=switch',
   '-Werror=switch-enum',
+  # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked
+  # at ~1% overhead in `nix search`.
+  #
+  # FIXME: remove when we get meson 1.4.0 which will default this to on for us:
+  # https://mesonbuild.com/Release-notes-for-1-4-0.html#ndebug-setting-now-controls-c-stdlib-assertions
+  '-D_GLIBCXX_ASSERTIONS=1',
   language : 'cpp',
 )
 
@@ -351,8 +357,7 @@ if cxx.get_id() in ['gcc', 'clang']
   # 2024-03-24: jade benchmarked the default sanitize reporting in clang and got
   # a regression of about 10% on hackage-packages.nix with clang. So we are trapping instead.
   #
-  # This has an overhead of 0-4% on gcc and unmeasurably little on clang, in
-  # Nix evaluation benchmarks.
+  # This has an unmeasurably low overhead in Nix evaluation benchmarks.
   #
   # N.B. Meson generates a completely nonsense warning here:
   # https://github.com/mesonbuild/meson/issues/9822