docs: document ssr.resolve.conditions for custom package conditions

docs/guide/common-errors.md

@@ -64,6 +64,51 @@ vitest --pool=forks
 ```
 :::
 
+## Custom package conditions are not resolved
+
+If you are using custom conditions in your `package.json` [exports](https://nodejs.org/api/packages.html#package-entry-points) or [subpath imports](https://nodejs.org/api/packages.html#subpath-imports), you may find that Vitest does not respect these conditions by default.
+
+For example, if you have the following in your `package.json`:
+
+```json
+{
+  "exports": {
+    ".": {
+      "custom": "./lib/custom.js",
+      "import": "./lib/index.js"
+    }
+  },
+  "imports": {
+    "#internal": {
+      "custom": "./src/internal.js",
+      "default": "./lib/internal.js"
+    }
+  }
+}
+```
+
+By default, Vitest will only use the `import` and `default` conditions. To make Vitest respect custom conditions, you need to configure [`ssr.resolve.conditions`](https://vite.dev/config/ssr-options#ssr-resolve-conditions) in your Vitest config:
+
+```ts [vitest.config.js]
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  ssr: {
+    resolve: {
+      conditions: ['custom', 'import', 'default'],
+    },
+  },
+})
+```
+
+::: tip Why `ssr.resolve.conditions` and not `resolve.conditions`?
+Vitest inherits Vite's configuration convention where:
+- [`resolve.conditions`](https://vite.dev/config/shared-options#resolve-conditions) is used for browser-like environments (jsdom, happy-dom, or custom environments with `transformMode: 'web'`)
+- [`ssr.resolve.conditions`](https://vite.dev/config/ssr-options#ssr-resolve-conditions) is used for Node-like environments (node, or custom environments with `transformMode: 'ssr'`)
+
+By default, Vitest uses the `node` environment which has `transformMode: 'ssr'`, so it uses `ssr.resolve.conditions` for module resolution. This applies to both package exports and subpath imports.
+:::
+
 ## Segfaults and native code errors
 
 Running [native NodeJS modules](https://nodejs.org/api/addons.html) in `pool: 'threads'` can run into cryptic errors coming from the native code.