diff --git a/.changeset/api-override-conflict-error-defaults.md b/.changeset/api-override-conflict-error-defaults.md
new file mode 100644
index 0000000000..703f76fb6e
--- /dev/null
+++ b/.changeset/api-override-conflict-error-defaults.md
@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-defaults': minor
+---
+
+**BREAKING**: The `API_FACTORY_CONFLICT` warning is now treated as an error and will prevent the app from starting.
diff --git a/.changeset/api-override-conflict-error.md b/.changeset/api-override-conflict-error.md
new file mode 100644
index 0000000000..ed88452ccf
--- /dev/null
+++ b/.changeset/api-override-conflict-error.md
@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-app-api': minor
+---
+
+**BREAKING**: Updated the behavior of the new API override logic to reject the override and block app startup instead of just logging a deprecation warning.