diff --git a/crates/solverforge-config/WIREFRAME.md b/crates/solverforge-config/WIREFRAME.md
index 361e1269..32ce35f3 100644
--- a/crates/solverforge-config/WIREFRAME.md
+++ b/crates/solverforge-config/WIREFRAME.md
@@ -633,7 +633,8 @@ Enum: `Sequential`, `RoundRobin`, `RotatingRoundRobin`, `Random`, and
 ### `UnionWeighting`
 
 Enum: `Equal` (default), `Fixed`, and `CandidateCount`. `Fixed` consumes the
-explicit `weights` vector; `CandidateCount` derives scheduling weight from the
+explicit `weights` vector; a zero weight disables its child (never opened, not
+counted in the union size), and at least one weight must be positive; `CandidateCount` derives scheduling weight from the
 children's declared candidate counts.
 
 ### `CartesianProductConfig`
diff --git a/crates/solverforge-config/src/move_selector/composite.rs b/crates/solverforge-config/src/move_selector/composite.rs
index c1fba9ad..1dc4fb91 100644
--- a/crates/solverforge-config/src/move_selector/composite.rs
+++ b/crates/solverforge-config/src/move_selector/composite.rs
@@ -54,8 +54,9 @@ pub struct UnionMoveSelectorConfig {
     #[serde(default)]
     pub weighting: UnionWeighting,
     /// One weight per child for `weighting = "fixed"`; a zero weight disables
-    /// that child. Weights other than `1` need `random` or `stratified_random`
-    /// order. Default: empty.
+    /// that child, which is then never opened nor counted in the union's
+    /// size. At least one weight must be positive. Weights other than `1`
+    /// need `random` or `stratified_random` order. Default: empty.
     #[serde(default)]
     pub weights: Vec<u64>,
     /// Child selectors.
diff --git a/crates/solverforge-solver/src/builder/selector/types/composite/state.rs b/crates/solverforge-solver/src/builder/selector/types/composite/state.rs
index abb36c9e..ae395e84 100644
--- a/crates/solverforge-solver/src/builder/selector/types/composite/state.rs
+++ b/crates/solverforge-solver/src/builder/selector/types/composite/state.rs
@@ -172,8 +172,11 @@ where
                     context,
                     resolve_union_weights(*weighting, weights, &sizes),
                 );
-                for &child in children {
-                    self.open_node(child, resources, director, child_context);
+                // A disabled child is never scheduled, so it is never opened.
+                for (index, &child) in children.iter().enumerate() {
+                    if union_child_enabled(*weighting, weights, index) {
+                        self.open_node(child, resources, director, child_context);
+                    }
                 }
                 CursorNode::Union(union)
             }
@@ -226,9 +229,16 @@ where
             } => self
                 .selector_size(*selector, director)
                 .min(*selected_count_limit),
-            SelectorCompositionNode::Union { children, .. } => children
+            SelectorCompositionNode::Union {
+                weighting,
+                weights,
+                children,
+                ..
+            } => children
                 .iter()
-                .map(|&child| self.selector_size(child, director))
+                .enumerate()
+                .filter(|&(index, _)| union_child_enabled(*weighting, weights, index))
+                .map(|(_, &child)| self.selector_size(child, director))
                 .sum(),
             SelectorCompositionNode::Cartesian { left, right, .. } => self
                 .selector_size(*left, director)
@@ -406,3 +416,13 @@ where
         self.selector_index_at(self.selector.root(), id)
     }
 }
+
+/// Whether a union child takes part: every child does, except one a fixed
+/// weighting gives weight `0`, which is disabled.
+fn union_child_enabled(
+    weighting: solverforge_config::UnionWeighting,
+    weights: &[u64],
+    index: usize,
+) -> bool {
+    !(weighting == solverforge_config::UnionWeighting::Fixed && weights[index] == 0)
+}
diff --git a/crates/solverforge-solver/src/builder/selector/types/composite/tests.rs b/crates/solverforge-solver/src/builder/selector/types/composite/tests.rs
index 3834a7cf..6e076792 100644
--- a/crates/solverforge-solver/src/builder/selector/types/composite/tests.rs
+++ b/crates/solverforge-solver/src/builder/selector/types/composite/tests.rs
@@ -483,3 +483,6 @@ fn arena_rejects_shared_mutable_child_nodes() {
     });
     let _ = SelectorCompositionState::<_, _, _, _, ()>::new(graph);
 }
+
+#[path = "tests/union_weights.rs"]
+mod union_weights;
diff --git a/crates/solverforge-solver/src/builder/selector/types/composite/tests/union_weights.rs b/crates/solverforge-solver/src/builder/selector/types/composite/tests/union_weights.rs
new file mode 100644
index 00000000..7bdfc9c5
--- /dev/null
+++ b/crates/solverforge-solver/src/builder/selector/types/composite/tests/union_weights.rs
@@ -0,0 +1,93 @@
+// A zero fixed union weight disables its child.
+
+use super::*;
+
+fn leaf(graph: &mut Graph, entity: usize, len: usize) -> CompositionNodeId {
+    graph.push(SelectorCompositionNode::Flat(CustomFlat {
+        entity,
+        values: (1..=len).map(|value| value as i64).collect(),
+    }))
+}
+
+fn observe_graph(graph: &Graph, context: MoveStreamContext) -> Vec<(usize, i64)> {
+    let mut director = create_nqueens_director(&[0, 0, 0]);
+    let state = new_stream_state(graph);
+    let mut cursor = open_cursor_with_owned_stream_state(graph, state, &mut (), &director, context);
+    observe(&mut cursor, &mut director)
+        .into_iter()
+        .map(|(_, entity, row)| (entity, row))
+        .collect()
+}
+
+#[test]
+fn a_disabled_child_produces_no_candidates() {
+    for seed in [0, 1, 17, 999] {
+        let context = MoveStreamContext::new(7, seed, None);
+        let mut graph = Graph::new();
+        let children = vec![
+            leaf(&mut graph, 0, 2),
+            leaf(&mut graph, 1, 3),
+            leaf(&mut graph, 2, 4),
+        ];
+        graph.push(SelectorCompositionNode::Union {
+            selection_order: UnionSelectionOrder::StratifiedRandom,
+            weighting: UnionWeighting::Fixed,
+            weights: vec![2, 0, 1],
+            children,
+        });
+        let seen = observe_graph(&graph, context);
+        assert_eq!(seen.len(), 2 + 4, "{seen:?}");
+        assert!(seen.iter().all(|(entity, _)| *entity != 1), "{seen:?}");
+    }
+}
+
+#[test]
+fn a_disabled_child_is_not_counted_in_the_union_size() {
+    // The outer union weighs its children by candidate count. Its inner
+    // union disables a three-candidate child, so it must weigh like a union
+    // of the two-candidate child alone.
+    for seed in [0, 1, 17, 999, 4242] {
+        let context = MoveStreamContext::new(3, seed, None);
+
+        let mut with_disabled = Graph::new();
+        let inner_children = vec![
+            leaf(&mut with_disabled, 0, 2),
+            leaf(&mut with_disabled, 1, 3),
+        ];
+        let inner = with_disabled.push(SelectorCompositionNode::Union {
+            selection_order: UnionSelectionOrder::StratifiedRandom,
+            weighting: UnionWeighting::Fixed,
+            weights: vec![1, 0],
+            children: inner_children,
+        });
+        let other = leaf(&mut with_disabled, 2, 8);
+        with_disabled.push(SelectorCompositionNode::Union {
+            selection_order: UnionSelectionOrder::StratifiedRandom,
+            weighting: UnionWeighting::CandidateCount,
+            weights: vec![],
+            children: vec![inner, other],
+        });
+
+        let mut without = Graph::new();
+        let only = leaf(&mut without, 0, 2);
+        let inner = without.push(SelectorCompositionNode::Union {
+            selection_order: UnionSelectionOrder::StratifiedRandom,
+            weighting: UnionWeighting::Fixed,
+            weights: vec![1],
+            children: vec![only],
+        });
+        let other = leaf(&mut without, 2, 8);
+        without.push(SelectorCompositionNode::Union {
+            selection_order: UnionSelectionOrder::StratifiedRandom,
+            weighting: UnionWeighting::CandidateCount,
+            weights: vec![],
+            children: vec![inner, other],
+        });
+
+        assert_eq!(
+            observe_graph(&with_disabled, context),
+            observe_graph(&without, context),
+            "seed {seed}"
+        );
+    }
+}
diff --git a/crates/solverforge-solver/src/heuristic/selector/decorator/vec_union.rs b/crates/solverforge-solver/src/heuristic/selector/decorator/vec_union.rs
index cafd4200..aa225df2 100644
--- a/crates/solverforge-solver/src/heuristic/selector/decorator/vec_union.rs
+++ b/crates/solverforge-solver/src/heuristic/selector/decorator/vec_union.rs
@@ -44,8 +44,8 @@ pub(crate) fn resolve_union_weights(
                 "fixed union weight count must match child count"
             );
             assert!(
-                fixed.iter().all(|weight| *weight > 0),
-                "fixed union weights must all be positive"
+                fixed.iter().any(|weight| *weight > 0),
+                "fixed union weights need at least one positive weight"
             );
             fixed.to_vec()
         }
diff --git a/crates/solverforge-solver/src/runtime/compiler/selector_tree.rs b/crates/solverforge-solver/src/runtime/compiler/selector_tree.rs
index 64f892fa..d0a1b5d7 100644
--- a/crates/solverforge-solver/src/runtime/compiler/selector_tree.rs
+++ b/crates/solverforge-solver/src/runtime/compiler/selector_tree.rs
@@ -286,8 +286,10 @@ where
                 {
                     Some("fixed union weight count must match selector count")
                 }
-                solverforge_config::UnionWeighting::Fixed if union.weights.contains(&0) => {
-                    Some("fixed union weights must all be positive")
+                solverforge_config::UnionWeighting::Fixed
+                    if union.weights.iter().all(|weight| *weight == 0) =>
+                {
+                    Some("fixed union weights need at least one positive weight; zero disables a child")
                 }
                 solverforge_config::UnionWeighting::CandidateCount if !union.weights.is_empty() => {
                     Some("candidate_count union weighting does not accept explicit weights")
diff --git a/crates/solverforge-solver/src/runtime/compiler/tests/invalid_config.rs b/crates/solverforge-solver/src/runtime/compiler/tests/invalid_config.rs
index 205ec92e..a6efe39c 100644
--- a/crates/solverforge-solver/src/runtime/compiler/tests/invalid_config.rs
+++ b/crates/solverforge-solver/src/runtime/compiler/tests/invalid_config.rs
@@ -142,3 +142,32 @@ fn unparsable_best_score_limits_fail_compilation_with_the_key_path() {
     };
     validate_best_score_limits::<HardSoftScore>(&valid).expect("valid limits compile");
 }
+
+fn fixed_union(weights: Vec<u64>) -> MoveSelectorConfig {
+    use solverforge_config::{UnionMoveSelectorConfig, UnionSelectionOrder, UnionWeighting};
+    MoveSelectorConfig::UnionMoveSelector(UnionMoveSelectorConfig {
+        selection_order: UnionSelectionOrder::StratifiedRandom,
+        weighting: UnionWeighting::Fixed,
+        weights,
+        selectors: vec![
+            MoveSelectorConfig::ListChangeMoveSelector(ListChangeMoveConfig::default()),
+            MoveSelectorConfig::ListRuinMoveSelector(ListRuinMoveSelectorConfig::default()),
+        ],
+    })
+}
+
+#[test]
+fn a_zero_fixed_union_weight_disables_its_child() {
+    compile(PrecedenceShape::Absent, fixed_union(vec![1, 0]))
+        .expect("a zero weight disables the child instead of failing");
+}
+
+#[test]
+fn fixed_union_weights_that_are_all_zero_are_rejected() {
+    let error = compile(PrecedenceShape::Absent, fixed_union(vec![0, 0]))
+        .expect_err("a union without a live child is rejected");
+    assert!(
+        error.to_string().contains("at least one positive weight"),
+        "{error}"
+    );
+}