diff --git a/.github/workflows/cross-platform-tests.yml b/.github/workflows/cross-platform-tests.yml
index b3bed7bc..a8c42104 100644
--- a/.github/workflows/cross-platform-tests.yml
+++ b/.github/workflows/cross-platform-tests.yml
@@ -56,9 +56,14 @@ jobs:
       #
       # Deliberately without -Dcountly.ui.pageLoadTests: that enables the one class showing a
       # transparent stage, which segfaults the JVM everywhere it has been tried.
+      #
+      # testUiWithLocalCore (see sdk-java-ui/build.gradle) runs the UI on this checkout's core. The
+      # release of a ui- tag still tests it against the released core it names.
       - name: Run the JavaFX UI tests
         if: always()
         shell: bash
+        env:
+          ORG_GRADLE_PROJECT_testUiWithLocalCore: 'true'
         run: |
           if [ "$RUNNER_OS" = "Linux" ]; then
             sudo apt-get update -qq && sudo apt-get install -y -qq xvfb
diff --git a/sdk-java-ui/CHANGELOG.md b/sdk-java-ui/CHANGELOG.md
index 26be802d..cdfbb57d 100644
--- a/sdk-java-ui/CHANGELOG.md
+++ b/sdk-java-ui/CHANGELOG.md
@@ -1,5 +1,6 @@
 ## XX.XX.XX
-* ! Minor breaking change ! `ly.count.sdk:java-ui` now has its own version numbers and is released only when it changes, so its version no longer follows the core SDK `ly.count.sdk:java`. It names the core version it needs, currently 26.8.0; if your project also declares `ly.count.sdk:java`, use that version or a newer one.
+* ! Minor breaking change ! `ly.count.sdk:java-ui` now has its own version numbers and is released only when it changes, so its version no longer follows the core SDK `ly.count.sdk:java`.
+  It names the core version it needs, currently 26.8.0; if your project also declares `ly.count.sdk:java`, use that version or a newer one.
 
 ## 26.8.0
 * First release, together with the Java SDK 26.8.0. See the Java SDK's [CHANGELOG](../CHANGELOG.md) for what it displays.
diff --git a/sdk-java-ui/build.gradle b/sdk-java-ui/build.gradle
index 613e3463..3a690707 100644
--- a/sdk-java-ui/build.gradle
+++ b/sdk-java-ui/build.gradle
@@ -71,6 +71,18 @@ dependencies {
   testImplementation 'org.mockito:mockito-core:4.11.0'
 }
 
+// -PtestUiWithLocalCore=true builds and tests this module's source against the sdk-java project
+// instead of the released core above. This module uses the core's internal classes, so a core
+// change that breaks it fails CI before the core is released. It does not test the released
+// java-ui jar on a newer core.
+if (findProperty('testUiWithLocalCore') == 'true') {
+  configurations.configureEach {
+    resolutionStrategy.dependencySubstitution {
+      substitute module('ly.count.sdk:java') using project(':sdk-java')
+    }
+  }
+}
+
 // Coverage. Reports land in build/reports/jacoco/test/ (HTML to read, XML for tooling).
 // "./gradlew coverage" from the repo root runs this and prints a summary.
 apply plugin: 'jacoco'