diff --git a/cylc/flow/prerequisite.py b/cylc/flow/prerequisite.py
index a2d13fbe4c9..b4abfe737dc 100644
--- a/cylc/flow/prerequisite.py
+++ b/cylc/flow/prerequisite.py
@@ -18,7 +18,7 @@
 
 import math
 import re
-from typing import Iterable, List, TYPE_CHECKING
+from typing import Iterable, Set, TYPE_CHECKING
 
 from cylc.flow.cycling.loader import get_point
 from cylc.flow.exceptions import TriggerExpressionError
@@ -202,25 +202,25 @@ def _conditional_is_satisfied(self):
                 '"%s":\n%s' % (self.get_raw_conditional_expression(), err_msg))
         return res
 
-    def satisfy_me(self, outputs: Iterable['Tokens']) -> List['Tokens']:
+    def satisfy_me(self, outputs: Iterable['Tokens']) -> Set[str]:
         """Attempt to satisfy me with given outputs.
 
         Updates cache with the result.
-        Return which outputs were not used.
+        Return outputs that match.
 
         """
-        not_used = []
+        valid = set()
         for output in outputs:
             prereq = output.to_prereq_tuple()
             if prereq not in self.satisfied:
-                not_used.append(output)
                 continue
+            valid.add(output.relative_id_with_selectors)
             self.satisfied[prereq] = self.DEP_STATE_SATISFIED
             if self.conditional_expression is None:
                 self._all_satisfied = all(self.satisfied.values())
             else:
                 self._all_satisfied = self._conditional_is_satisfied()
-        return not_used
+        return valid
 
     def api_dump(self):
         """Return list of populated Protobuf data objects."""
diff --git a/cylc/flow/task_proxy.py b/cylc/flow/task_proxy.py
index c5be53cab41..6a47f3a6d59 100644
--- a/cylc/flow/task_proxy.py
+++ b/cylc/flow/task_proxy.py
@@ -505,17 +505,17 @@ def state_reset(
         return False
 
     def satisfy_me(self, outputs: Iterable[str]) -> None:
-        """Attempt to satisfy my prerequisites with given outputs.
+        """Try to satisfy my prerequisites with given outputs.
 
         The output strings are of the form "cycle/task:message"
+        Log a warning for outputs that I don't depend on.
 
         """
         tokens = [Tokens(p, relative=True) for p in outputs]
-        not_used = self.state.satisfy_me(tokens)
-        for output in not_used:
+        used = self.state.satisfy_me(tokens)
+        for output in set(outputs) - used:
             LOG.warning(
-                f"{self.identity} does not depend on"
-                f" {output.relative_id_with_selectors}"
+                f"{self.identity} does not depend on {output}"
             )
 
     def clock_expire(self) -> bool:
diff --git a/cylc/flow/task_state.py b/cylc/flow/task_state.py
index 3f1fea4198a..26b651fe613 100644
--- a/cylc/flow/task_state.py
+++ b/cylc/flow/task_state.py
@@ -17,7 +17,7 @@
 """Task state related logic."""
 
 
-from typing import List, Iterable, TYPE_CHECKING
+from typing import List, Iterable, Set, TYPE_CHECKING
 from cylc.flow.prerequisite import Prerequisite
 from cylc.flow.task_outputs import TaskOutputs
 from cylc.flow.wallclock import get_current_time_string
@@ -313,18 +313,20 @@ def __call__(
     def satisfy_me(
         self,
         outputs: Iterable['Tokens']
-    ) -> List['Tokens']:
-        """Attempt to get my prerequisites satisfied."""
-        not_used: List['Tokens'] = []
-        for prereqs in [self.prerequisites, self.suicide_prerequisites]:
-            for prereq in prereqs:
-                nope = prereq.satisfy_me(outputs)
-                if nope:
-                    not_used += nope
-                    continue
-                self._is_satisfied = None
-                self._suicide_is_satisfied = None
-        return not_used
+    ) -> Set[str]:
+        """Try to satisfy my prerequisites with given outputs.
+
+        Return which outputs I actually depend on.
+        """
+        valid: Set[str] = set()
+        for prereq in (*self.prerequisites, *self.suicide_prerequisites):
+            yep = prereq.satisfy_me(outputs)
+            if yep:
+                valid = valid.union(yep)
+                continue
+            self._is_satisfied = None
+            self._suicide_is_satisfied = None
+        return valid
 
     def xtriggers_all_satisfied(self):
         """Return True if all xtriggers are satisfied."""
diff --git a/tests/integration/test_task_pool.py b/tests/integration/test_task_pool.py
index a453fd4a82e..3a6942750bd 100644
--- a/tests/integration/test_task_pool.py
+++ b/tests/integration/test_task_pool.py
@@ -21,7 +21,6 @@
     Callable,
     Iterable,
     List,
-    Optional,
     Tuple,
     Union
 )
@@ -1463,3 +1462,56 @@ async def test_set_outputs_future(
         # try to set an invalid output
         schd.pool.set(["1/b"], ["shrub"], None, ['all'])
         assert log_filter(log, contains="output 1/b:shrub not found")
+
+
+async def test_prereq_satisfaction(
+    flow,
+    scheduler,
+    start,
+    log_filter,
+):
+    """Check manual setting of future task outputs.
+
+    """
+    id_ = flow(
+        {
+            'scheduler': {
+                'allow implicit tasks': 'True',
+            },
+            'scheduling': {
+                'graph': {
+                    'R1': "a:x & a:y => b"
+                }
+            },
+            'runtime': {
+                'a': {
+                    'outputs': {
+                        'x': 'x',
+                        'y': 'y'
+                    }
+                }
+            }
+        }
+    )
+    schd = scheduler(id_)
+    async with start(schd) as log:
+        # it should start up with just 1/a
+        assert pool_get_task_ids(schd.pool) == ["1/a"]
+        # spawn b
+        schd.pool.set(["1/a"], ["x"], None, ['all'])
+        assert (
+            pool_get_task_ids(schd.pool) == ["1/a", "1/b"]
+        )
+
+        b = schd.pool.get_task(IntegerPoint("1"), "b")
+
+        assert not b.is_waiting_prereqs_done()
+
+        # set valid and invalid prerequisites, check log.
+        b.satisfy_me(["1/a:x", "1/a:y", "1/a:z", "1/a:w"])
+        assert log_filter(log, contains="1/b does not depend on 1/a:z")
+        assert log_filter(log, contains="1/b does not depend on 1/a:w")
+        assert not log_filter(log, contains="1/b does not depend on 1/a:x")
+        assert not log_filter(log, contains="1/b does not depend on 1/a:y")
+
+        assert b.is_waiting_prereqs_done()