diff --git a/.github/workflows/reusable-check.yml b/.github/workflows/reusable-check.yml
index 082fddf9a6..d5a5861687 100644
--- a/.github/workflows/reusable-check.yml
+++ b/.github/workflows/reusable-check.yml
@@ -309,7 +309,9 @@ jobs:
           # CCUD tags every shard with the same `CI job=test-shards`, which is the largest
           # single CI cost bucket and therefore the one worth splitting. Shard identity is
           # otherwise only recoverable by parsing the requested task list.
-          ./gradlew ${{ matrix.shard.tasks }} $kover_tasks -Pci=true -Pkotlin.daemon.useFallbackStrategy=false --continue \
+          # -PwarningsAsErrors: together the shards compile every main and test source set, so a new Kotlin
+          # compiler warning fails here.
+          ./gradlew ${{ matrix.shard.tasks }} $kover_tasks -Pci=true -Pkotlin.daemon.useFallbackStrategy=false -PwarningsAsErrors=true --continue \
             "-Dscan.value.CI shard=${{ matrix.shard.name }}"
 
       # A test fork that dies in native code (exit 134) names no test, and the JVM's crash report is the
diff --git a/.skills/testing-ci/SKILL.md b/.skills/testing-ci/SKILL.md
index 285e541672..3821e02c2b 100644
--- a/.skills/testing-ci/SKILL.md
+++ b/.skills/testing-ci/SKILL.md
@@ -140,6 +140,7 @@ The tiers are named here and the workflows carry the label versions.
 
 ### CI Conventions
 - **KMP Smoke Compile:** `./gradlew kmpSmokeCompile` is a lifecycle task (registered in `RootConventionPlugin`) that depends on `compileKotlinJvm` + `compileKotlinIosSimulatorArm64` for every KMP module in the hand-maintained `ALL_MODULES_FULL` list, plus `compileAndroidDeviceTest` for `:core:database` and `:core:model`. `scripts/check-module-list.py` fails the PR when that list drifts from `settings.gradle.kts`. CI runs it in `shard-core`.
+- **Kotlin warnings fail the test shards:** they pass `-PwarningsAsErrors=true`, which sets `allWarningsAsErrors` on every Kotlin compilation (`KotlinAndroid.kt`, plus `desktopApp` and `schema-strings`). The shards don't run the `compile*MainKotlinMetadata` tasks, so a warning only those report doesn't fail CI (today they warn about duplicate KLIB names). Reproduce locally with the same flag on the compile or test tasks you touched.
 - **`maxParallelForks` CI logic:** `ProjectExtensions.kt` reads the `ci` Gradle property (`providers.gradleProperty("ci")`) and uses full available processors in CI (4 forks on std runners) vs. half locally. All CI invocations pass `-Pci=true`.
 - **Detekt report formats:** Detekt.kt checks `project.findProperty("ci") == "true"` and disables html, txt, md reports in CI; only xml + sarif are retained for GitHub annotations.
 - **Robolectric SDK caching:** The `gradle-setup` composite action caches `~/.m2/repository/org/robolectric` to prevent flaky `SocketException` on SDK downloads. Cache key is `robolectric-{os}-{arch}-{hash of gradle/libs.versions.toml}`, restoring from the `robolectric-{os}-{arch}-` prefix, so a catalog change that bumps Robolectric rolls the key without a hand edit.