diff --git a/CHANGELOG.md b/CHANGELOG.md
index e914d94..8e70c5c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,13 @@ All notable changes to EigenScript are documented here.
   inside an `arena_mark`…`arena_reset` scope or a nested evaluation is a
   `value` error. Not yet: mailboxes/`task_send` (increment 2), virtual
   time, the pluggable seeded-strategy hook for the DST consumer.
+  Increment 1c hardening: adversarial valgrind/ASan planted-fault
+  verification proved the arena guard and the save-buffer teardown are
+  load-bearing, and surfaced+fixed a real bug — a task ending inside an
+  active arena scope arena-allocated its error dict (which crosses to the
+  joiner) and left the arena active for the next task; `sched_finish` now
+  forces the arena inactive. Determinism is a suite gate (two-run
+  byte-identical), and SPEC gains an executable Tasks section.
 - **E003 is scope-precise (#404, increment two)** — the undefined-name
   pass models the runtime's real scope rules instead of a whole-file
   binding set: function-locals (fresh-name `is` and `local`) are
diff --git a/docs/SPEC.md b/docs/SPEC.md
index f83b8a8..cde8de1 100644
--- a/docs/SPEC.md
+++ b/docs/SPEC.md
@@ -1126,6 +1126,47 @@ print of (thread_join of h)
 9
 ```
 
+## Cooperative tasks
+
+`task_spawn` creates a **cooperative task** on the single interpreter
+thread (unlike `spawn`, which uses an OS thread). Tasks are scheduled
+round-robin: a task runs until it calls `task_yield of null`, which
+hands control to the next ready task; it resumes where it left off.
+Because there is one thread and a fixed policy, the interleaving is
+**deterministic by construction** — the same program prints identically
+on every run, with no threads and no clock. Args and results cross the
+task boundary deep-copied (share-nothing, like channel sends).
+
+`task_join of id` blocks until task `id` finishes and returns its
+result (or re-raises its uncaught error as the same `{kind, message,
+line}` dict — see [Error handling](#error-handling)). `task_alive of
+id` is 1 until the task finishes.
+
+```eigenscript
+order is []
+define step(tag) as:
+    order is append of [order, f"{tag}-1"]
+    task_yield of null
+    order is append of [order, f"{tag}-2"]
+    return tag
+
+a is task_spawn of [step, "a"]
+b is task_spawn of [step, "b"]
+ra is task_join of a
+rb is task_join of b
+print of order
+print of f"{ra}{rb}"
+```
+```output
+["a-1", "b-1", "a-2", "b-2"]
+ab
+```
+
+When the main program returns, any tasks still running are torn down
+(the program ends). If every task is blocked with none runnable — for
+example two tasks each `task_join`-ing the other — that is a `deadlock`
+error, reported loudly rather than hanging.
+
 ## Buffers
 
 `buffer of count` allocates a flat array of `count` nums (all 0).
diff --git a/src/vm.c b/src/vm.c
index 7f279b1..6af3196 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -5261,6 +5261,15 @@ static Value *task_start(Task *t) {
  * blocked on it. `r` is the value vm_run returned (NULL on suspend — not this
  * path). g_has_error distinguishes a normal end from an uncaught error. */
 static void sched_finish(TaskScheduler *s, Task *t, Value *r) {
+    /* A task that ended (returned OR died) while its per-thread arena is still
+     * active — arena_mark with no matching arena_reset, e.g. the arena-suspend
+     * guard raised inside the scope — must not leave the arena active: (1) its
+     * error dict / result below outlive the task and cross to the joiner, so
+     * they must be heap, not arena (a later arena_reset would dangle them); and
+     * (2) the next task must start from a clean arena baseline. The suspend
+     * guard guarantees a task never *yields* with the arena active, so the only
+     * way it's active here is an ending task that leaked the scope. */
+    g_arena.active = 0;
     if (g_has_error) {
         t->has_error = 1;
         t->error_value = vm_take_error_value();  /* the {kind,message,line} dict */
diff --git a/tests/run_all_tests.sh b/tests/run_all_tests.sh
index 4c76549..a026484 100755
--- a/tests/run_all_tests.sh
+++ b/tests/run_all_tests.sh
@@ -2140,9 +2140,23 @@ check_eigs_suite "recv-blocked worker doesn't hang exit" test_spawn_channel_exit
 echo "[104] Worker Arena Return (no cross-thread UAF, #302)"
 check_eigs_suite "worker arena return deep-copied before detach" test_spawn_arena_return.eigs "All tests passed" 1
 
-# #408 cooperative task layer — increment 1a: spawn + alive + leak-clean
-# teardown of unrun tasks (the interleaving scheduler lands in 1b).
-check_eigs_suite "cooperative tasks: spawn + alive + teardown (#408 inc 1a)" test_tasks.eigs "All tests passed" 1
+# #408 cooperative task layer: spawn/alive/yield/join/deadlock + leak-clean
+# teardown of suspended/killed tasks (incl. heap-on-saved-stack + arena-dier).
+check_eigs_suite "cooperative tasks: yield/join/deadlock/teardown (#408)" test_tasks.eigs "All tests passed" 1
+
+# #408 determinism-by-construction: a task program with cooperative yields must
+# print byte-identically on two fresh processes (the signature property — the
+# interleaving is a pure function of program order, no tape).
+TOTAL=$((TOTAL + 1))
+DET1=$(./eigenscript ../examples/task_pipeline.eigs </dev/null 2>&1)
+DET2=$(./eigenscript ../examples/task_pipeline.eigs </dev/null 2>&1)
+if [ "$DET1" = "$DET2" ] && echo "$DET1" | grep -q "= 120"; then
+    echo "  PASS: cooperative tasks replay byte-identically (#408 determinism)"
+    PASS=$((PASS + 1))
+else
+    echo "  FAIL: task interleaving diverged between runs (#408 determinism)"
+    FAIL=$((FAIL + 1))
+fi
 
 # [105] Builtin contract fixes (#312 negative indices, #316 predicate
 # type-rejection, #317 min/max N-ary reduction) + #314: a directory as the
diff --git a/tests/test_tasks.eigs b/tests/test_tasks.eigs
index b122993..333125c 100644
--- a/tests/test_tasks.eigs
+++ b/tests/test_tasks.eigs
@@ -110,4 +110,33 @@ catch e:
     assert_eq of [e.kind, "value", "task_yield in an arena scope is a value-error"]
 assert_eq of [threw2, 1, "arena-scoped yield raises"]
 
+# --- teardown: a task KILLED while suspended (main ends first) with a heap
+#     value live on its saved value stack must be reclaimed clean. The list
+#     literal evaluates left-to-right, so the string is on the stack when the
+#     inner task_yield suspends. Covered by the ASan leak gate (tally 0). ---
+define holder() as:
+    local keep is ["heap-string-on-the-value-stack", task_yield of null]
+    return len of keep
+hk is task_spawn of holder
+# NOT joined — reaped by kill-outstanding while suspended holding the string.
+
+# --- a task that dies inside an arena scope, UNJOINED, must not leave the
+#     per-thread arena active (would arena-allocate its error dict and
+#     contaminate the next task). Covered by the valgrind arena-poison gate. ---
+define arena_dier() as:
+    arena_mark of null
+    task_yield of null    # guard raises here; the task dies inside the scope
+    return 0
+ad is task_spawn of arena_dier
+# NOT joined — sched_finish must force the arena inactive on this dead task.
+
+# A clean task spawned AFTER the arena-dier proves the arena baseline is clean:
+# its string allocations must be heap, not arena (no cross-task contamination).
+define after_arena() as:
+    task_yield of null
+    local s is "fresh-heap-string"
+    return len of s
+aa is task_spawn of after_arena
+assert_eq of [task_join of aa, 17, "task after an arena-dier gets a clean arena"]
+
 test_summary of null