diff --git a/packages/testing/src/consensus_testing/test_fixtures/state_transition.py b/packages/testing/src/consensus_testing/test_fixtures/state_transition.py
index c11c67fc4..898850adc 100644
--- a/packages/testing/src/consensus_testing/test_fixtures/state_transition.py
+++ b/packages/testing/src/consensus_testing/test_fixtures/state_transition.py
@@ -38,6 +38,17 @@ class StateTransitionFixture(BaseConsensusFixture):
     post_state_root: Bytes32 | None = None
     """Hash tree root of the full post-state, so two clients cannot pass with divergent state."""
 
+    skip_slot_processing: list[bool] | None = None
+    """
+    Per block, whether the runner must skip slot processing before processing that block.
+
+    Most tests advance slots for every block, and there the field is absent. It is emitted
+    only by the tests that deliberately leave the state behind the block's slot, because
+    that is the state they are testing the rejection of. A runner that calls the whole state
+    transition for such a block advances the slot first and never reaches the mismatch, so
+    without this the fixture cannot be reproduced from its own contents.
+    """
+
 
 class StateTransitionTest(BaseTestSpec):
     """
@@ -140,12 +151,18 @@ def generate(self) -> StateTransitionFixture:
         if self.post is not None and actual_post_state is not None:
             self.post.validate_against_state(actual_post_state, block_registry=block_registry)
 
+        # Emitted only when some block needs it, so the ordinary fixture is unchanged.
+        skip_slot_processing = [block_spec.skip_slot_processing for block_spec in self.blocks]
+        if not any(skip_slot_processing):
+            skip_slot_processing = None
+
         return StateTransitionFixture(
             pre=self.pre,
             blocks=filled_blocks,
             post=self.post,
             post_state_root=post_state_root,
             rejection_reason=rejection_reason,
+            skip_slot_processing=skip_slot_processing,
         )
 
     def _build_block_from_spec(
diff --git a/tests/testing/test_state_transition_fixture.py b/tests/testing/test_state_transition_fixture.py
new file mode 100644
index 000000000..cb90daedd
--- /dev/null
+++ b/tests/testing/test_state_transition_fixture.py
@@ -0,0 +1,62 @@
+"""The emitted state-transition fixture must be reproducible from its own contents."""
+
+from __future__ import annotations
+
+from consensus_testing import BlockSpec, ExpectedRejection, build_genesis_state
+from consensus_testing.test_fixtures.state_transition import StateTransitionTest
+from lean_spec.spec.forks import RejectionReason, Slot
+from lean_spec.spec.forks.lstar.spec import LstarSpec
+
+
+def test_an_ordinary_fixture_carries_no_slot_processing_instruction() -> None:
+    """A test that advances slots normally emits nothing extra, as every runner does that."""
+    fixture = StateTransitionTest(
+        pre=build_genesis_state(),
+        blocks=[BlockSpec(slot=Slot(1)), BlockSpec(slot=Slot(2))],
+    ).generate()
+
+    assert fixture.skip_slot_processing is None
+
+
+def test_a_skipped_slot_processing_block_says_so_in_the_fixture() -> None:
+    """
+    A block the spec processes without advancing slots must say so.
+
+    The rejection under test only happens while the state is behind the block's slot. A
+    runner that calls the whole state transition advances the slot first, never reaches the
+    mismatch, and reports the fixture as broken rather than reproducing it.
+    """
+    pre = LstarSpec().process_slots(build_genesis_state(), Slot(1))
+
+    fixture = StateTransitionTest(
+        pre=pre,
+        blocks=[BlockSpec(slot=Slot(2), skip_slot_processing=True)],
+        expected_rejection=ExpectedRejection(
+            reason=RejectionReason.BLOCK_SLOT_MISMATCH,
+            exact_message="Block slot mismatch",
+        ),
+    ).generate()
+
+    assert fixture.skip_slot_processing == [True]
+
+
+def test_the_instruction_is_per_block_and_in_order() -> None:
+    """
+    A run that mixes both kinds pins which block skips, not merely that one of them does.
+
+    A single flag, or a list built in the wrong order, would pass a test where every block
+    skips. This one would not.
+    """
+    pre = LstarSpec().process_slots(build_genesis_state(), Slot(1))
+
+    fixture = StateTransitionTest(
+        pre=pre,
+        blocks=[
+            # Processed against a state already at slot 1, so slots must not advance again.
+            BlockSpec(slot=Slot(1), skip_slot_processing=True),
+            # An ordinary block, which the runner advances to as usual.
+            BlockSpec(slot=Slot(2)),
+        ],
+    ).generate()
+
+    assert fixture.skip_slot_processing == [True, False]