diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 45018c8f..6b52681d 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -210,8 +210,25 @@ endif()
 include(Catch)
 # Every test is sub-second in practice; cap each at 120s so a hung/deadlocked
 # test fails fast instead of stalling the whole CI job (observed: a single test
-# hanging blocked a Windows runner for over half an hour).
-catch_discover_tests(morph_tests DISCOVERY_MODE PRE_TEST PROPERTIES TIMEOUT 120)
+# hanging blocked a Windows runner for over half an hour). One named exception
+# below, registered separately: DISCOVERY_MODE PRE_TEST defers test discovery
+# to ctest invocation time, so a per-test TIMEOUT can't be set with
+# set_tests_properties() here (nothing named that test yet at configure time)
+# -- excluding it by tag and giving the excluded tag its own
+# catch_discover_tests() call is the mechanism that's actually available.
+catch_discover_tests(morph_tests DISCOVERY_MODE PRE_TEST TEST_SPEC "~[slow]" PROPERTIES TIMEOUT 120)
+
+# `[slow]` (issue #589): `equation()` walks a 70,000-node provenance chain
+# purely to prove the morph#574 iterative rewrite doesn't overflow the stack
+# -- it has no performance budget of its own, but the O(n^2) string-building
+# cost #582 measured (32.3s at this size under ASan+UBSan) is close enough to
+# 120s that TSan's heavier per-access instrumentation pushes it over: observed
+# at ">120.07s" on master's own CI (the sanitizer killed it at the deadline,
+# so the true completion time is unknown, just past it). Widened generously
+# rather than precisely measured, to leave headroom for sanitizer variance,
+# without raising the cap for every other test and weakening the "hang fails
+# fast" property the 120s default exists for.
+catch_discover_tests(morph_tests DISCOVERY_MODE PRE_TEST TEST_SPEC "[slow]" PROPERTIES TIMEOUT 600)
 
 # ── Two-binary journal-path skew test (issue #246) ───────────────────────────
 # The executable form of the journal's data-at-rest contract, which
diff --git a/tests/test_quantity.cpp b/tests/test_quantity.cpp
index 93295481..73a937ba 100644
--- a/tests/test_quantity.cpp
+++ b/tests/test_quantity.cpp
@@ -734,7 +734,7 @@ TEST_CASE("A 100000-node provenance chain is destroyed without overflowing the s
 }
 
 TEST_CASE("equation() walks a 70000-node provenance chain without overflowing the stack",
-          "[quantity][provenance][equation][morph574]") {
+          "[quantity][provenance][equation][morph574][slow]") {
     Euro const total = runningTotal(kDeepEquationNodes);
 
     auto const lines = total.equation();