diff --git a/.github/workflows/build-template.yml b/.github/workflows/build-template.yml
index 4579e6fa07..729d92643a 100644
--- a/.github/workflows/build-template.yml
+++ b/.github/workflows/build-template.yml
@@ -46,6 +46,8 @@ jobs:
       CCACHE_MAXSIZE: 400M
       # squelch error message about missing nixpkgs channel
       NIX_BUILD_SHELL: bash
+      # TODO
+      ASAN_OPTIONS: detect_leaks=0
       LSAN_OPTIONS: max_leaks=10
       # somehow MinGW clang64 (or cmake?) defaults to `g++` even though it doesn't exist
       CXX: c++
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b74f235084..7b611a8f50 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -108,7 +108,8 @@ jobs:
 
       # 0: PRs without special label
       # 1: PRs with `merge-ci` label, merge queue checks, master commits
-      # 2: PRs with `release-ci` label, releases (incl. nightlies)
+      # 2: nightlies
+      # 2: PRs with `release-ci` label, full releases
       - name: Set check level
         id: set-level
         # We do not use github.event.pull_request.labels.*.name here because
@@ -118,14 +119,16 @@ jobs:
           check_level=0
           fast=false
 
-          if [[ -n "${{ steps.set-nightly.outputs.nightly }}" || -n "${{ steps.set-release.outputs.RELEASE_TAG }}" || -n "${{ steps.set-release-custom.outputs.RELEASE_TAG }}" ]]; then
+          if [[ -n "${{ steps.set-release.outputs.RELEASE_TAG }}" || -n "${{ steps.set-release-custom.outputs.RELEASE_TAG }}" ]]; then
+            check_level=3
+          elif [[ -n "${{ steps.set-nightly.outputs.nightly }}" ]]; then
             check_level=2
           elif [[ "${{ github.event_name }}" != "pull_request" ]]; then
             check_level=1
           else
             labels="$(gh api repos/${{ github.repository_owner }}/${{ github.event.repository.name }}/pulls/${{ github.event.pull_request.number }} --jq '.labels')"
             if echo "$labels" | grep -q "release-ci"; then
-              check_level=2
+              check_level=3
             elif echo "$labels" | grep -q "merge-ci"; then
               check_level=1
             fi
@@ -210,17 +213,18 @@ jobs:
                 "test": true,
                 "CMAKE_PRESET": "reldebug",
               },
-              // TODO: suddenly started failing in CI
-              /*{
+              {
                 "name": "Linux fsanitize",
                 "os": "ubuntu-latest",
                 "enabled": level >= 2,
+                // do not have nightly release wait for this
+                "secondary": level <= 2,
                 "test": true,
                 // turn off custom allocator & symbolic functions to make LSAN do its magic
                 "CMAKE_PRESET": "sanitize",
-                // exclude seriously slow/problematic tests (laketests crash)
-                "CTEST_OPTIONS": "-E 'interactivetest|leanpkgtest|laketest|benchtest'"
-              },*/
+                // exclude seriously slow/problematic tests (laketests crash, async_base_functions timeouts)
+                "CTEST_OPTIONS": "-E '(interactive|pkg|lake|bench)/|treemap|StackOverflow|async_base_functions'"
+              },
               {
                 "name": "macOS",
                 "os": "macos-15-intel",
@@ -252,7 +256,7 @@ jobs:
               },
               {
                 "name": "Windows",
-                "os": large && (fast || level == 2) ? "namespace-profile-windows-amd64-4x16" : "windows-2022",
+                "os": large && (fast || level >= 2) ? "namespace-profile-windows-amd64-4x16" : "windows-2022",
                 "release": true,
                 "enabled": level >= 2,
                 "test": true,
diff --git a/CMakePresets.json b/CMakePresets.json
index 5ea57b5215..a917f36613 100644
--- a/CMakePresets.json
+++ b/CMakePresets.json
@@ -41,7 +41,7 @@
         "SMALL_ALLOCATOR": "OFF",
         "USE_MIMALLOC": "OFF",
         "BSYMBOLIC": "OFF",
-        "LEAN_TEST_VARS": "MAIN_STACK_SIZE=16000"
+        "LEAN_TEST_VARS": "MAIN_STACK_SIZE=16000 ASAN_OPTIONS=detect_leaks=0"
       },
       "generator": "Unix Makefiles",
       "binaryDir": "${sourceDir}/build/sanitize"
diff --git a/src/runtime/compact.cpp b/src/runtime/compact.cpp
index 7864162751..4c8fa8358f 100644
--- a/src/runtime/compact.cpp
+++ b/src/runtime/compact.cpp
@@ -397,7 +397,7 @@ compacted_region::~compacted_region() {
 
 inline object * compacted_region::fix_object_ptr(object * o) {
     if (lean_is_scalar(o)) return o;
-    return reinterpret_cast<object*>(static_cast<char*>(m_begin) + (reinterpret_cast<size_t>(o) - reinterpret_cast<size_t>(m_base_addr)));
+    return reinterpret_cast<object*>(static_cast<char*>(m_begin) - reinterpret_cast<char*>(m_base_addr) + reinterpret_cast<ptrdiff_t>(o));
 }
 
 inline void compacted_region::move(size_t d) {
diff --git a/src/runtime/io.cpp b/src/runtime/io.cpp
index 18c897a1c5..9f5e237143 100644
--- a/src/runtime/io.cpp
+++ b/src/runtime/io.cpp
@@ -1499,8 +1499,8 @@ extern "C" LEAN_EXPORT uint8_t lean_st_ref_ptr_eq(b_obj_arg ref1, b_obj_arg ref2
     return lean_to_ref(ref1) == lean_to_ref(ref2);
 }
 
-/* {α : Type} (act : BaseIO α) : α */
-static obj_res lean_io_as_task_fn(obj_arg act) {
+/* {α : Type} (act : BaseIO α) (_ : IO.RealWorld) : α */
+static obj_res lean_io_as_task_fn(obj_arg act, obj_arg) {
     object_ref r(apply_1(act, io_mk_world()));
     return object_ref(r.raw(), true).steal();
 }