docs: add input empty example

wan9chi · wan9chi · commit b10a9572365b · 2026-07-02T00:02:26.000+08:00
diff --git a/docs/guide/automatic-tracking.md b/docs/guide/automatic-tracking.md
@@ -87,7 +87,18 @@ tasks: {
 }
 ```
 
-Set `input: []` when no files should affect the cache fingerprint. Set `output: []` when no files should be restored on a cache hit.
+Set `input: []` rarely. Use it when no files should affect the cache fingerprint. For example, a download task can be cached when the same URL always serves the same file. Only changes to the command, such as changing the URL, invalidate the cache:
+
+```ts [vite.config.ts]
+tasks: {
+  downloadSchema: {
+    command: 'curl -O https://example.com/schema.json',
+    input: [],
+  },
+}
+```
+
+Set `output: []` when no files should be restored on a cache hit.
 
 ## Cooperative Tracking
 
