diff --git a/solitaire_engine/src/lib.rs b/solitaire_engine/src/lib.rs
index 753cce5..2ab2b93 100644
--- a/solitaire_engine/src/lib.rs
+++ b/solitaire_engine/src/lib.rs
@@ -43,6 +43,7 @@ pub mod replay_overlay;
 pub mod replay_playback;
 pub mod resources;
 pub mod safe_area;
+mod schedule_checks;
 pub mod selection_plugin;
 pub mod settings_plugin;
 pub mod splash_plugin;
diff --git a/solitaire_engine/src/schedule_checks.rs b/solitaire_engine/src/schedule_checks.rs
new file mode 100644
index 0000000..2a783f8
--- /dev/null
+++ b/solitaire_engine/src/schedule_checks.rs
@@ -0,0 +1,105 @@
+//! Schedule hygiene checks (issue #143).
+//!
+//! Bevy runs systems with conflicting data access in nondeterministic order
+//! unless an ordering edge exists. Nothing enforced this historically, and
+//! the headless gameplay cluster carries a large legacy backlog of such
+//! pairs — most benign (event/resource writers that never observably race),
+//! but unproven. Rather than a permanently red hard-error test or no test
+//! at all, this is a RATCHET: the count may only go down. New conflicting
+//! pairs fail CI immediately; the backlog can be triaged incrementally
+//! (add `.before`/`.after` where order matters, `.ambiguous_with` where it
+//! provably doesn't, then lower `AMBIGUITY_BASELINE`).
+//!
+//! To see the offending pair names, add the `bevy_debug` / debug feature to
+//! the bevy dev-dependency (system names are stripped otherwise) and set
+//! `ambiguity_detection: LogLevel::Error` manually.
+//!
+//! Scope note: `CoreGamePlugin` (the full composition) performs real
+//! storage/platform I/O in `build` and cannot run in a unit test; the
+//! cluster below is the same one the engine's headless behaviour tests use.
+
+#[cfg(test)]
+mod tests {
+    use bevy::ecs::schedule::{LogLevel, ScheduleBuildSettings};
+    use bevy::prelude::*;
+
+    use crate::auto_complete_plugin::AutoCompletePlugin;
+    use crate::card_plugin::CardPlugin;
+    use crate::game_plugin::GamePlugin;
+    use crate::hud_plugin::HudPlugin;
+    use crate::settings_plugin::SettingsPlugin;
+    use crate::table_plugin::TablePlugin;
+    use crate::ui_focus::UiFocusPlugin;
+    use crate::ui_modal::UiModalPlugin;
+
+    /// Legacy ambiguity backlog measured 2026-07-06 (issue #143). This
+    /// number may only decrease. If your change trips this assertion you
+    /// have added a pair of systems with conflicting data access and no
+    /// ordering edge — add `.before`/`.after` (order matters) or
+    /// `.ambiguous_with` (provably order-independent) at the registration
+    /// site. When triage lowers the real count, lower this constant too.
+    const AMBIGUITY_BASELINE: usize = 302;
+
+    fn cluster_app() -> App {
+        let mut app = App::new();
+        app.add_plugins(MinimalPlugins)
+            .add_plugins(GamePlugin)
+            .add_plugins(TablePlugin)
+            .add_plugins(CardPlugin)
+            .add_plugins(HudPlugin)
+            .add_plugins(AutoCompletePlugin)
+            .add_plugins(UiModalPlugin)
+            .add_plugins(UiFocusPlugin)
+            .add_plugins(SettingsPlugin::headless());
+        app.init_resource::<ButtonInput<KeyCode>>();
+        app
+    }
+
+    #[test]
+    fn update_schedule_ambiguities_do_not_grow() {
+        let mut app = cluster_app();
+        app.edit_schedule(Update, |s| {
+            s.set_build_settings(ScheduleBuildSettings {
+                ambiguity_detection: LogLevel::Error,
+                ..Default::default()
+            });
+        });
+
+        // With LogLevel::Error the first schedule build panics when any
+        // ambiguity exists, and the panic message begins with the pair
+        // count. Catch it and ratchet on that count. (Parsing a panic
+        // message is brittle by design — if a Bevy upgrade rewords it,
+        // this test fails loudly and the parse below needs one-line
+        // maintenance, which is preferable to losing the ratchet.)
+        let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+            app.update();
+        }));
+
+        let count = match result {
+            Ok(()) => 0,
+            Err(payload) => {
+                let msg = payload
+                    .downcast_ref::<String>()
+                    .cloned()
+                    .or_else(|| payload.downcast_ref::<&str>().map(|s| (*s).to_string()))
+                    .unwrap_or_default();
+                let parsed = msg
+                    .split(" pairs of systems")
+                    .next()
+                    .and_then(|prefix| prefix.split_whitespace().last())
+                    .and_then(|n| n.parse::<usize>().ok());
+                parsed.unwrap_or_else(|| {
+                    panic!(
+                        "ambiguity panic message no longer parseable (Bevy upgrade?): {msg}"
+                    )
+                })
+            }
+        };
+
+        assert!(
+            count <= AMBIGUITY_BASELINE,
+            "system-order ambiguities grew: {count} > baseline {AMBIGUITY_BASELINE}. \
+             Add .before/.after or .ambiguous_with at the new registration site.",
+        );
+    }
+}