diff --git a/tests/algorithm/test_gigpo_advantage.py b/tests/algorithm/test_gigpo_advantage.py
index 1f1e90dd279..d5ce7212635 100644
--- a/tests/algorithm/test_gigpo_advantage.py
+++ b/tests/algorithm/test_gigpo_advantage.py
@@ -108,6 +108,32 @@ def test_anchor_state_grouping(self):
         )
         self.assertGreater(metrics["gigpo/anchor_groups_size_gt1"], 0)
 
+    def test_anchor_states_are_not_shared_across_tasks(self):
+        """The same observation in two different tasks is not one anchor group."""
+        fn_full = GiGPOAdvantageFn(omega=1.0, gamma=1.0, fnorm="none")
+        fn_episode = GiGPOAdvantageFn(omega=0.0, gamma=1.0, fnorm="none")
+        shared_hash = "Observation: Nothing happens."
+        exps = [
+            # task 0: the run that saw the shared observation goes on to succeed
+            _make_exp(0, 0, 0, step_reward=1.0, env_state_hash=shared_hash),
+            _make_exp(0, 1, 0, step_reward=0.0, env_state_hash="task0_other"),
+            # task 1: the run that saw the same observation fails
+            _make_exp(1, 0, 0, step_reward=0.0, env_state_hash=shared_hash),
+            _make_exp(1, 1, 0, step_reward=1.0, env_state_hash="task1_other"),
+        ]
+        for e in exps:
+            e.reward = e.info["step_reward"]
+        p_full, metrics = fn_full.process(list(exps))
+        p_episode, _ = fn_episode.process(list(exps))
+
+        self.assertEqual(metrics["gigpo/anchor_groups_size_gt1"], 0)
+
+        def by_run(exp):
+            return (exp.eid.task, exp.eid.run)
+
+        for a, b in zip(sorted(p_full, key=by_run), sorted(p_episode, key=by_run)):
+            self.assertTrue(torch.allclose(a.advantages, b.advantages))
+
     def test_singleton_anchor_zero_step_adv(self):
         """Unique anchor hashes -> A^S = 0; only episode term remains."""
         fn_none = GiGPOAdvantageFn(omega=1.0, fnorm="none")
diff --git a/trinity/algorithm/advantage_fn/gigpo_advantage.py b/trinity/algorithm/advantage_fn/gigpo_advantage.py
index a08ef472025..3652fef03aa 100644
--- a/trinity/algorithm/advantage_fn/gigpo_advantage.py
+++ b/trinity/algorithm/advantage_fn/gigpo_advantage.py
@@ -167,8 +167,8 @@ def process(self, exps: List[Experience]) -> Tuple[List[Experience], Dict]:
         """Compute GiGPO advantages for a batch of multi-step experiences.
 
         Episode-level: group by task, compare trajectory returns R(tau) across
-        runs (GRPO-style). Step-level: group by ``env_state_hash`` across the
-        batch, compare discounted returns R_t; singleton anchors get A_S = 0.
+        runs (GRPO-style). Step-level: group by ``env_state_hash`` within each
+        task, compare discounted returns R_t; singleton anchors get A_S = 0.
 
         Args:
             exps: Multi-step experiences with ``eid.task``, ``eid.run``,
@@ -223,7 +223,9 @@ def process(self, exps: List[Experience]) -> Tuple[List[Experience], Dict]:
             if state_hash is None:
                 continue
             r_t = exp_to_discounted_return[id(exp)]
-            anchor_buckets.setdefault(str(state_hash), []).append((exp, r_t))
+            # Anchor states are matched within the task group (as in GiGPO), so identical observations
+            # from different tasks are not compared.
+            anchor_buckets.setdefault(f"{exp.eid.tid}/{state_hash}", []).append((exp, r_t))
 
         exp_to_a_s: Dict[int, float] = {id(exp): 0.0 for exp in exps}
         for members in anchor_buckets.values():