diff --git a/FIX.org b/FIX.org
index 1a0f890..0621920 100644
--- a/FIX.org
+++ b/FIX.org
@@ -5430,3 +5430,41 @@ with a *new* def rather than one the fixture declares — see below.
   rule for diagnostics, but it is [session.ml]'s stale-caller tripwire, which
   is documented as unreachable and is not this lane's to loosen. A brand-new
   def is unaffected, which is what the test uses.
+
+* The heavy sweep, 2026-09-21 — and the leak question bytes-copy cannot answer
+@x86, @sanitize and @valgrind, batched over the four lanes since the last
+sweep, all three green at the end. What they found, and what they could not:
+
+One x86 failure, two rows of it: bytes-view-write and dev-segv, both from the
+bytes/bytes-view lane. Not a miscompile — both store through a bytes-view of a
+string literal, which is .rodata, and the survey compares at its own -O2 where
+LLVM deletes the undefined store and exits 0 while the hand-written backend
+executes it and takes 139. At -O0 the two agree exactly, and test_acceptance's
+dies_segv rows already pin that on both backends. Excluded by name in
+survey.sh, with the reason.
+
+One real bug, pre-existing and found by hand rather than by an alias:
+--dev --sanitize did not compile at all. See the commit; the short of it is
+that clang 20's ASan module pass segfaults on an llvm.global_ctors naming a
+declaration, so the weak __asan_init yield that keeps the dev build's SIGSEGV
+handler out of ASan's way had never once run.
+
+** What neither corpus can answer about the new (bytes s)
+bytes-copy.flan is in both sweeps now, and it is worth writing down what that
+does not buy. Its first two cases leak 24 bytes through flan_bytes_dup —
+measured, with valgrind --leak-check=full — and both sweeps miss it on
+purpose: @sanitize runs with detect_leaks=0 and @valgrind with
+--leak-check=no, each for the reason its own file gives, which is that
+allocate-once-never-free is this runtime's design and a leak check produces a
+suppression list rather than information.
+
+So the row proves the copy is in bounds and its bytes are written. It does not
+prove anything about who frees it, and nobody should read a green sweep as
+saying the new allocating bytes has an owner. The leak question is worth
+asking on purpose one day, across the whole corpus and not one program, and
+that is a session of its own.
+
+** Certified against 190fdad
+The four-green result is measured against that base. dev-loop has moved since
+— runtime/flan_rt.c, vendor/agent/flan_agent.c and lib/dev.ml among others —
+and those belong to the next batch, not to this one.
diff --git a/lib/emit.ml b/lib/emit.ml
index fc1c06d..2f48dac 100644
--- a/lib/emit.ml
+++ b/lib/emit.ml
@@ -4404,18 +4404,42 @@ let program ?(checks = true) ?(dev = false) ?(debug = false) ?(pnames = [])
        handler has to be installed before any program code can fault. Only in
        a dev build — the constructor is emitted here and nowhere else — so a
        release build dies exactly the way it always did. *)
-    Buffer.add_string m.out
-      "@llvm.global_ctors = appending global [2 x { i32, ptr, ptr }] \
-       [{ i32, ptr, ptr } { i32 65535, ptr @flan_dev_reg_enable, ptr null }, \
-        { i32, ptr, ptr } { i32 65535, ptr @flan_dev_crash_enable, ptr null }]\n";
+    (* Both of them behind one constructor defined here, rather than named
+       directly in the table, and that shape is load-bearing rather than
+       tidiness. [llvm.global_ctors] naming a *declaration* — which both of
+       these are, they are C in the runtime — crashes clang 20's
+       AddressSanitizer module pass outright, so `--dev --sanitize` could not
+       compile any program at all; the whole combination was unreachable, and
+       that includes the [__asan_init] yield in [flan_dev_crash_enable] that
+       is the one thing keeping the crash handler out of ASan's way. Reduced
+       to five lines of IR here; it is an upstream crash and nobody has filed
+       it. A local definition in the table is what clang itself emits for its
+       own constructors, and it costs a call before main.
+       The ordering the wrapper now fixes was unspecified before — two entries
+       at one priority — and arming the registry before the handler is the
+       order that was wanted anyway. Its name is [Mangle.dev_ctor] and the
+       leading dot there is load-bearing too; that comment says why. *)
     (* Declared here rather than in the preamble, which is the one place a
-       declare is worth gating: this lane then adds no dev-only text at all to
-       a release module, and the only line it does add there —
+       declare is worth gating: the crash-handler lane then adds no dev-only
+       text at all to a release module, and the only line it does add there —
        [flan_bytes_dup] — is a function release builds really call, since
        (bytes s) allocates in every build. The neighbouring
-       [flan_dev_reg_enable] stays in the preamble ungated; it predates this
-       and moving it is not this lane's to make. *)
+       [flan_dev_reg_enable] stays in the preamble ungated; it predates that
+       lane and moving it was not its to make. *)
     Buffer.add_string m.out "declare void @flan_dev_crash_enable()\n";
+    let ctor = "@" ^ quoted Mangle.dev_ctor in
+    Buffer.add_string m.out
+      (Printf.sprintf
+         "define internal void %s() {\n\
+         \  call void @flan_dev_reg_enable()\n\
+         \  call void @flan_dev_crash_enable()\n\
+         \  ret void\n\
+          }\n"
+         ctor);
+    Buffer.add_string m.out
+      (Printf.sprintf
+         "@llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] \
+          [{ i32, ptr, ptr } { i32 65535, ptr %s, ptr null }]\n" ctor);
     Buffer.add_char m.out '\n'
   end;
   List.iter (emit_global m ~hidden) p.Tast.globals;
diff --git a/lib/mangle.ml b/lib/mangle.ml
index 81390aa..834f1a9 100644
--- a/lib/mangle.ml
+++ b/lib/mangle.ml
@@ -30,6 +30,16 @@ let prefix = "flan."
    and [.init-data] start with a dot no reader token can produce. *)
 let sym n = prefix ^ n
 
+(* The single constructor a dev build emits, which calls the runtime's two
+   enable entry points — the allocation registry and the crash handler — in
+   that order. It goes through [sym] with a leading dot for exactly the reason
+   that comment gives, and the reason is not decoration: the first spelling of
+   this was a plain [flan.dev.ctor], which is what [(defn dev.ctor [] i32 7)]
+   mangles to. That program compiles and runs as a release build and fails a
+   dev build with a redefinition clang refuses. A dot is what no reader token
+   can produce, so there is no Flan name that reaches this one. *)
+let dev_ctor = prefix ^ ".dev-ctor"
+
 (* A dev build's indirection cell: a mutable global holding the address of the
    function that is currently this name's body. *)
 let cell n = prefix ^ "cell." ^ n
diff --git a/spike/x86/survey.sh b/spike/x86/survey.sh
index 8a245f5..63e15cf 100755
--- a/spike/x86/survey.sh
+++ b/spike/x86/survey.sh
@@ -92,6 +92,31 @@ forever="dev-loop dev-watch dev-chatty agent-auto"
 # in the epilogue that every exit already went through. The five are in the
 # sweep now and they are five of the MATCHes.
 
+# The ones whose whole point is a fault, and which therefore cannot be compared
+# at this sweep's optimisation level. Both write through a bytes-view of a
+# string literal, which is a store into .rodata: measured here, LLVM exits 0
+# having printed the unmodified literal and this backend exits 139, because the
+# store is undefined and the optimiser deleted it on one side and there is no
+# optimiser on the other. That is not a lowering disagreement. At -O0 the two
+# agree exactly -- 139, no output, both backends -- and test_acceptance.ml's
+# dies_segv rows pin precisely that, on both backends, which is the coverage
+# this sweep would otherwise be duplicating at the one level where, as that
+# file's own comment puts it, there is nothing left to pin but the UB.
+#
+# Excluded by name rather than by building these two at -O0 here, and the
+# reason is the coverage and not the counts: a per-name -O0 list would move the
+# counts exactly as much as this does, so that is no argument at all. The
+# argument is that dies_segv already builds both of them at -O0, on both
+# backends, and asserts the exit status and the empty output -- everything this
+# sweep would check, in the file where the ruling is written down.
+#
+# One more thing about dev-segv, which is a reason to keep it out of the
+# comparison rather than a reason for this list: it calls agent/start, so it
+# leaves a socket in /tmp on both runs, and under SURVEY_FLAGS=--dev it parks
+# in the break loop instead of dying -- which is a forever-list problem, met
+# here by a program that was never going to be compared anyway.
+faults="bytes-view-write dev-segv"
+
 TIMEOUT=${TIMEOUT:-20}
 
 # Extra flags, given to *both* sides. SURVEY_FLAGS=--dev is the one that has a
@@ -115,6 +140,7 @@ for src in "$corpus"/test/programs/*.flan "$corpus"/spike/x86/*.flan \
     [ $want = 1 ] || continue
   fi
   case " $forever " in *" $name "*) skip+=("$name:runs-forever"); continue;; esac
+  case " $faults " in *" $name "*) skip+=("$name:faults-by-design"); continue;; esac
 
   # LLVM first. A program that does not compile at all, or has no main, is not
   # this backend's business -- the frontend refused it either way.
diff --git a/test/test_sanitize.ml b/test/test_sanitize.ml
index b43cd72..2a4e481 100644
--- a/test/test_sanitize.ml
+++ b/test/test_sanitize.ml
@@ -118,6 +118,14 @@ let corpus =
        an output comparison does not. *)
     "programs/two-numbers.flan", [];
     "programs/bytes2.flan", [];
+    (* (bytes s) allocates a copy now rather than reinterpreting the string,
+       which is the one change in that lane this tool can see: the copy is a
+       block from an allocator, it is written through immediately, and the
+       last case takes its block from an arena that is then freed and
+       destroyed. A copy one byte short, or a write landing after the block,
+       is a heap overflow here and a correct-looking program everywhere
+       else. *)
+    "programs/bytes-copy.flan", [];
     "programs/cleanup.flan", [];
     "programs/conditions.flan", [];
     "programs/debug.flan", [];
diff --git a/test/test_valgrind.ml b/test/test_valgrind.ml
index 007c79c..724b879 100644
--- a/test/test_valgrind.ml
+++ b/test/test_valgrind.ml
@@ -198,6 +198,11 @@ let corpus =
     "programs/arena-value.flan", [];
     "programs/bounds.flan", [ "0" ];
     "programs/bytes2.flan", [];
+    (* The same program [test_sanitize.ml] added, and here for the half that
+       one cannot answer: a copy whose tail was never written reads as a
+       perfectly addressable block to ASan and as an uninitialised value to
+       memcheck. *)
+    "programs/bytes-copy.flan", [];
     "programs/cleanup.flan", [];
     "programs/conditions.flan", [];
     "programs/debug.flan", [];