diff --git a/packages/frontend-plugin-api/src/wiring/createExtension.test.ts b/packages/frontend-plugin-api/src/wiring/createExtension.test.ts
index 8da3504434..7f27df4947 100644
--- a/packages/frontend-plugin-api/src/wiring/createExtension.test.ts
+++ b/packages/frontend-plugin-api/src/wiring/createExtension.test.ts
@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+import { coreExtensionData } from './coreExtensionData';
 import { createExtension } from './createExtension';
 import { createExtensionDataRef } from './createExtensionDataRef';
 import { createExtensionInput } from './createExtensionInput';
@@ -563,7 +564,7 @@ describe('createExtension', () => {
   });
 
   describe('overrides', () => {
-    it('should allow overriding of the default factory', () => {
+    it('should allow overriding of config and merging', () => {
       const testExtension = createExtension({
         namespace: 'test',
         attachTo: { id: 'root', input: 'blob' },
@@ -591,5 +592,54 @@ describe('createExtension', () => {
 
       expect(true).toBe(true);
     });
+
+    it('should allow overriding of outputs', () => {
+      const testExtension = createExtension({
+        namespace: 'test',
+        attachTo: { id: 'root', input: 'blob' },
+        output: [stringDataRef],
+        inputs: {
+          test: createExtensionInput([stringDataRef], { singleton: true }),
+        },
+        config: {
+          schema: {
+            foo: z => z.string().optional(),
+          },
+        },
+        factory({ inputs }) {
+          return [stringDataRef(inputs.test.get(stringDataRef))];
+        },
+      });
+
+      const override = testExtension.override({
+        output: [numberDataRef],
+        factory(_, { inputs }) {
+          return [
+            numberDataRef(inputs.test.get(stringDataRef).length),
+            stringDataRef('default'),
+          ];
+        },
+      });
+
+      // @ts-expect-error - should fail as the string output from previous is not provided
+      const override2 = testExtension.override({
+        output: [numberDataRef],
+        factory(_, { inputs }) {
+          return [numberDataRef(inputs.test.get(stringDataRef).length)];
+        },
+      });
+
+      // @ts-expect-error - this should fail because string output should be merged?
+      const override3 = testExtension.override({
+        output: [numberDataRef],
+        factory(_, { inputs }) {
+          return [stringDataRef(inputs.test.get(stringDataRef))];
+        },
+      });
+
+      unused(override, override2, override3);
+
+      expect(true).toBe(true);
+    });
   });
 });
diff --git a/packages/frontend-plugin-api/src/wiring/createExtension.ts b/packages/frontend-plugin-api/src/wiring/createExtension.ts
index f1db16b4d2..3ec674f75f 100644
--- a/packages/frontend-plugin-api/src/wiring/createExtension.ts
+++ b/packages/frontend-plugin-api/src/wiring/createExtension.ts
@@ -246,11 +246,12 @@ export type OverrideExtensionOptions<
   TConfig,
   TConfigInput,
   TConfigSchema extends { [key: string]: (zImpl: typeof z) => z.ZodType },
-  UFactoryOutput extends ExtensionDataValue<any, any>,
+  UOriginalFactoryOutput extends ExtensionDataValue<any, any>,
   TConfigSchemaOverrides extends {
     [key: string]: (zImpl: typeof z) => z.ZodType;
   },
-  UOutputOverrides extends AnyExtensionDataRef = UOutput,
+  UOutputOverrides extends AnyExtensionDataRef,
+  UFactoryOverrideOutput extends ExtensionDataValue<any, any>,
 > = {
   kind?: string;
   namespace?: string;
@@ -262,7 +263,10 @@ export type OverrideExtensionOptions<
   /** @deprecated - use `config.schema` instead */
   configSchema?: PortableSchema<TConfig, TConfigInput>;
   config?: {
-    schema: TConfigSchemaOverrides;
+    schema: TConfigSchemaOverrides & {
+      [KName in keyof TConfig]?: `Error: Config key '${KName &
+        string}' is already defined in parent schema`;
+    };
   };
   factory?(
     originalFactory: (context?: {
@@ -270,7 +274,7 @@ export type OverrideExtensionOptions<
         [key in keyof TConfigSchema]: z.infer<ReturnType<TConfigSchema[key]>>;
       };
       inputs?: Expand<ResolvedExtensionInputs<TInputs>>;
-    }) => Iterable<ExtensionDataRefToValue<UOutput>>,
+    }) => Iterable<UOriginalFactoryOutput>,
     context: {
       node: AppNode;
       config: TConfig &
@@ -287,8 +291,9 @@ export type OverrideExtensionOptions<
             });
       inputs: Expand<ResolvedExtensionInputs<TInputs>>;
     },
-  ): Iterable<UFactoryOutput>;
-} & VerifyExtensionFactoryOutput<UOutput & UOutputOverrides, UFactoryOutput>;
+  ): Iterable<UFactoryOverrideOutput>;
+  // todo(blam): need to verify that the outputs are meged properly.
+} & VerifyExtensionFactoryOutput<UOutput, UFactoryOverrideOutput>;
 
 /** @public */
 export type OverridableExtension<
@@ -308,6 +313,8 @@ export type OverridableExtension<
     TConfigSchemaOverrides extends {
       [key in string]: (zImpl: typeof z) => z.ZodType;
     },
+    UOutputOverrides extends AnyExtensionDataRef,
+    UFactoryOverrideOutput extends ExtensionDataValue<any, any>,
   >(
     options: OverrideExtensionOptions<
       UOutput,
@@ -316,7 +323,9 @@ export type OverridableExtension<
       TConfigInput,
       TConfigSchema,
       UFactoryOutput,
-      TConfigSchemaOverrides
+      TConfigSchemaOverrides,
+      UOutputOverrides,
+      UFactoryOverrideOutput
     >,
   ): ExtensionDefinition<TConfig, TConfigInput>;
 };