diff --git a/.github/workflows/build-and-test.yml b/.github/workflows/build-and-test.yml
index 6babaadcbe..dc40e35c26 100644
--- a/.github/workflows/build-and-test.yml
+++ b/.github/workflows/build-and-test.yml
@@ -23,7 +23,7 @@ jobs:
           - {runs_on: ubuntu-latest, name: "GCC 10 ASAN+UBSAN", container: "ghcr.io/quick-lint/quick-lint-js-github-gcc:v1", CC: gcc-10, CXX: g++-10, CFLAGS: "-fsanitize=address,undefined -fsanitize-address-use-after-scope -fno-sanitize-recover=address,undefined -fuse-ld=gold", CMAKE_BUILD_TYPE: "Debug", WARNINGS: "-Werror;{0}", CMAKE_FLAGS: "-G Ninja", ASAN_OPTIONS: "strict_string_checks=1:detect_stack_use_after_return=1:check_initialization_order=1:strict_init_order=1"}
           - {runs_on: ubuntu-latest, name: "GCC 10 Release", container: "ghcr.io/quick-lint/quick-lint-js-github-gcc:v1", CC: gcc-10, CXX: g++-10, CFLAGS: "", CMAKE_BUILD_TYPE: "Release", WARNINGS: "-Werror;{0}"}
           # TODO(strager): Enable Ninja for this job.
-          - {runs_on: ubuntu-latest, name: "GCC 10 dev", container: "ghcr.io/quick-lint/quick-lint-js-github-gcc:v1", CC: gcc-10, CXX: g++-10, CFLAGS: "", CMAKE_BUILD_TYPE: "Debug", WARNINGS: "-Werror;{0}", CMAKE_FLAGS: "-DQUICK_LINT_JS_FEATURE_DEBUG_SERVER=ON -DQUICK_LINT_JS_FEATURE_VECTOR_PROFILING=ON"}
+          - {runs_on: ubuntu-latest, name: "GCC 10 dev", container: "ghcr.io/quick-lint/quick-lint-js-github-gcc:v1", CC: gcc-10, CXX: g++-10, CFLAGS: "", CMAKE_BUILD_TYPE: "Debug", WARNINGS: "-Werror;{0}", CMAKE_FLAGS: "-DQUICK_LINT_JS_FEATURE_DEBUG_SERVER=ON -DQUICK_LINT_JS_FEATURE_EXTRA_DEBUG=ON -DQUICK_LINT_JS_FEATURE_VECTOR_PROFILING=ON"}
           # TODO(strager): Also test MinGW-based builds.
           - {runs_on: windows-latest, name: "MSVC", CFLAGS: "", CMAKE_BUILD_TYPE: "Debug", WARNINGS: ""}
           - {runs_on: windows-latest, name: "MSVC Release", CFLAGS: "", CMAKE_BUILD_TYPE: "Release", WARNINGS: ""}
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 711a69c949..c2053cf093 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -125,6 +125,11 @@ option(
   "Enable the HTTP monitoring server at run-time"
   FALSE
 )
+option(
+  QUICK_LINT_JS_FEATURE_EXTRA_DEBUG
+  "Enable extra slow debug checking at run-time"
+  FALSE
+)
 option(
   QUICK_LINT_JS_FEATURE_VECTOR_PROFILING
   "Enable the QLJS_DUMP_VECTORS option at run-time"
@@ -147,6 +152,9 @@ quick_lint_js_disable_unwanted_warnings()
 if (WIN32)
   add_definitions(-D_WIN32_WINNT=0x0602)
 endif ()
+if (QUICK_LINT_JS_FEATURE_EXTRA_DEBUG)
+  add_compile_definitions(QLJS_DEBUG)
+endif ()
 
 # HACK(strager): Work around issues with CI. We should consider using
 # find_package(Python3) instead.