diff --git a/core/src/main/java/ai/timefold/solver/core/impl/phase/custom/DefaultCustomPhase.java b/core/src/main/java/ai/timefold/solver/core/impl/phase/custom/DefaultCustomPhase.java
index 3496d7ac328..bc907dfb0dc 100644
--- a/core/src/main/java/ai/timefold/solver/core/impl/phase/custom/DefaultCustomPhase.java
+++ b/core/src/main/java/ai/timefold/solver/core/impl/phase/custom/DefaultCustomPhase.java
@@ -88,6 +88,10 @@ private void doStep(CustomStepScope<Solution_> stepScope, PhaseCommand<Solution_
                 () -> phaseTermination.isPhaseTerminated(stepScope.getPhaseScope()));
         customPhaseCommand.changeWorkingSolution(commandContext);
         calculateWorkingStepScore(stepScope, customPhaseCommand);
+        if (stepScope.getScore().isInvalid()) {
+            throw new IllegalStateException("The custom phase command (%s) resulted in an inconsistent solution."
+                    .formatted(customPhaseCommand));
+        }
         var solver = stepScope.getPhaseScope().getSolverScope().getSolver();
         solver.getBestSolutionRecaller().processWorkingSolutionDuringStep(stepScope);
     }
diff --git a/core/src/test/java/ai/timefold/solver/core/impl/solver/DefaultSolverTest.java b/core/src/test/java/ai/timefold/solver/core/impl/solver/DefaultSolverTest.java
index 79833139d36..83c4c1f9d7e 100644
--- a/core/src/test/java/ai/timefold/solver/core/impl/solver/DefaultSolverTest.java
+++ b/core/src/test/java/ai/timefold/solver/core/impl/solver/DefaultSolverTest.java
@@ -1555,6 +1555,55 @@ void solveWhenIgnoringInconsistentSolutionsThrowsIfInconsistentEntityPinned() {
                 .hasMessageContainingAll("Entity", "b2", "while involved in a dependency loop");
     }
 
+    @Test
+    void solveCustomPhaseReturnsInconsistent() {
+        // Solver config
+        var solverConfig = PlannerTestUtils.buildSolverConfig(
+                TestdataDependencyNoInconsistentFieldSolution.class, TestdataDependencyNoInconsistentFieldEntity.class,
+                TestdataDependencyNoInconsistentFieldValue.class)
+                .withEasyScoreCalculatorClass(null)
+                .withConstraintProviderClass(TestdataDependencyNoInconsistentFieldConstraintProvider.class)
+                .withPhases(new CustomPhaseConfig()
+                        .withCustomPhaseCommands(command -> {
+                            var solution = (TestdataDependencyNoInconsistentFieldSolution) command.getWorkingSolution();
+                            var sE1 = solution.getEntities().getFirst();
+                            var sB1 = solution.getValues().get(2);
+                            var sB2 = solution.getValues().get(3);
+                            var metaModel = command.getSolutionMetaModel()
+                                    .genuineEntity(TestdataDependencyNoInconsistentFieldEntity.class)
+                                    .listVariable("values", TestdataDependencyNoInconsistentFieldValue.class);
+                            command.execute(Moves.compose(
+                                    Moves.assign(metaModel, sB2, sE1, 0),
+                                    Moves.assign(metaModel, sB1, sE1, 1)));
+                        }));
+
+        var e1 = new TestdataDependencyNoInconsistentFieldEntity("a");
+        var e2 = new TestdataDependencyNoInconsistentFieldEntity("b");
+
+        var a1 = new TestdataDependencyNoInconsistentFieldValue("a1");
+        var a2 = new TestdataDependencyNoInconsistentFieldValue("a2");
+        var b1 = new TestdataDependencyNoInconsistentFieldValue("b1");
+        var b2 = new TestdataDependencyNoInconsistentFieldValue("b2");
+
+        a2.setDependencies(List.of(a1));
+        b2.setDependencies(List.of(b1));
+
+        e1.setValues(List.of(b2, b1));
+        e2.setValues(List.of(a1, a2));
+
+        var entities = List.of(e1, e2);
+        var values = List.of(a1, a2, b1, b2);
+
+        var problem = new TestdataDependencyNoInconsistentFieldSolution();
+
+        problem.setEntities(entities);
+        problem.setValues(values);
+
+        assertThatCode(() -> PlannerTestUtils.solve(solverConfig, problem, false))
+                .isInstanceOf(IllegalStateException.class)
+                .hasMessageContainingAll("The custom phase command", "resulted in an inconsistent solution.");
+    }
+
     @Test
     void solveIgnoreInconsistent() {
         // Solver config