Document custom SQLite artifact pairs and connection probes

client-sdks/advanced/sqlite-extensions.mdx

@@ -35,10 +35,27 @@ To link additional extensions, you could add similar build script additions to c
 
 Similar to Dart on the web, loading extensions with `@powersync/web` is only possible with a custom `wa-sqlite` WebAssembly build. PowerSync [forks wa-sqlite](https://github.com/powersync-ja/wa-sqlite) for this reason, loading custom extension requires patching build definitions in that repository and adding an override from your npm package manager to patch `@journeyapps/wa-sqlite`.
 
+When you build a custom module, pin the SQLite, wa-sqlite, PowerSync core and Emscripten versions. Compile the extension into the module and register it before opening databases, for example with `sqlite3_auto_extension()`. Keep the PowerSync core extension in the build.
+
+Deploy the generated JavaScript and WebAssembly files together. PowerSync's prebuilt database worker already contains wa-sqlite JavaScript, so a package override alone does not update that worker. Rebuild your database worker against the custom package and configure the SDK to use that worker. Use versioned asset filenames so a cached worker cannot load a module from a different build.
+
+Before running queries that need the extension, probe the actual PowerSync connection. For SQLite's decimal extension, this query checks scalar and aggregate functions:
+
+```sql
+SELECT decimal_cmp('007.5', '7.25') AS comparison,
+       decimal_sub('1000.00', '0.01') AS difference,
+       decimal_sum(amount) AS total
+FROM (SELECT '9007199254740993.01' AS amount UNION ALL SELECT '-0.01');
+```
+
+The expected result is `1`, `'999.99'` and `'9007199254740993.00'`. Run the probe for each database instance, including connections opened in workers or background tasks. If you support older builds without the extension, select a compatible query before preparing SQL that calls the missing functions.
+
 ### React Native
 
 The recommended OP-SQLite library has builtin support for custom extensions, and [excellent documentation](https://op-engineering.github.io/op-sqlite/docs/api#loading-extensions) on how to build and bundle them with your app.
 
+For a statically linked extension, initialize it on each SQLite connection or register it with `sqlite3_auto_extension()` before creating connections. If you use SQLCipher, preserve its existing initialization hook. Adding native code requires a new native app build; a JavaScript-only update cannot add the extension to an installed binary.
+
 ### Node.js
 
 Loading extensions is possible with `better-sqlite3` and the [`loadExtension`](https://github.com/WiseLibs/better-sqlite3/blob/HEAD/docs/api.md#loadextensionpath-entrypoint---this) method on databases.