[BUGFIX] Fix query param redirects during active transitions

packages/ember/tests/routing/router_service_test/transitionTo_test.js

@@ -439,5 +439,86 @@ moduleFor(
         assert.equal(this.routerService.get('currentURL'), '/?url_sort=a');
       });
     }
+
+    async ['@test Redirecting to same route with query params from beforeModel works via transition'](
+      assert
+    ) {
+      assert.expect(2);
+
+      this.add(
+        'route:parent.child',
+        class extends Route {
+          @service
+          router;
+
+          beforeModel(transition) {
+            if (!transition.to.queryParams.bar) {
+              return this.router.transitionTo('parent.child', {
+                queryParams: { bar: '1' },
+              });
+            }
+          }
+        }
+      );
+
+      this.add(
+        'controller:parent.child',
+        Controller.extend({
+          queryParams: ['bar'],
+          bar: null,
+        })
+      );
+
+      await this.visit('/');
+      await this.routerService.transitionTo('parent.child').followRedirects();
+
+      assert.equal(
+        this.routerService.get('currentURL'),
+        '/child?bar=1',
+        'redirected to self with query param'
+      );
+
+      assert.equal(this.routerService.get('currentRouteName'), 'parent.child');
+    }
+
+    async ['@test Redirecting to same route with query params from beforeModel works via direct visit'](
+      assert
+    ) {
+      assert.expect(2);
+
+      this.add(
+        'route:parent.child',
+        class extends Route {
+          @service
+          router;
+
+          beforeModel(transition) {
+            if (!transition.to.queryParams.bar) {
+              return this.router.transitionTo('parent.child', {
+                queryParams: { bar: '1' },
+              });
+            }
+          }
+        }
+      );
+
+      this.add(
+        'controller:parent.child',
+        Controller.extend({
+          queryParams: ['bar'],
+          bar: null,
+        })
+      );
+
+      await this.visit('/child');
+
+      assert.equal(
+        this.routerService.get('currentURL'),
+        '/child?bar=1',
+        'redirected to self with query param'
+      );
+
+      assert.equal(this.routerService.get('currentRouteName'), 'parent.child');
+    }
   }
 );

packages/router_js/lib/router.ts

@@ -208,19 +208,27 @@ export default abstract class Router<R extends Route> {
     if (routeInfosEqual(newState.routeInfos, oldState!.routeInfos)) {
       // This is a no-op transition. See if query params changed.
       if (queryParamChangelist) {
-        let newTransition = this.queryParamsTransition(
-          queryParamChangelist,
-          wasTransitioning,
-          oldState!,
-          newState
-        );
-        newTransition.queryParamsOnly = true;
-        // SAFETY: The returned OpaqueTransition should actually be this.
-        return newTransition as InternalTransition<R>;
+        if (!wasTransitioning) {
+          // Not in an active transition — use the QP-only fast path
+          // which updates the URL without re-resolving route hooks.
+          let newTransition = this.queryParamsTransition(
+            queryParamChangelist,
+            wasTransitioning,
+            oldState!,
+            newState
+          );
+          newTransition.queryParamsOnly = true;
+          // SAFETY: The returned OpaqueTransition should actually be this.
+          return newTransition as InternalTransition<R>;
+        }
+        // When a query-param-only transition is started during an active
+        // transition (for example, from beforeModel/afterModel), avoid the
+        // QP-only fast path and intentionally fall through to the normal
+        // transition path so it can abort and replace the active transition.
+      } else {
+        // No-op. No need to create a new transition.
+        return this.activeTransition || new InternalTransition(this, undefined, undefined);
       }
-
-      // No-op. No need to create a new transition.
-      return this.activeTransition || new InternalTransition(this, undefined, undefined);
     }
 
     if (isIntermediate) {