diff --git a/src/client/app/data.ts b/src/client/app/data.ts
index abf379b7..b7770ab0 100644
--- a/src/client/app/data.ts
+++ b/src/client/app/data.ts
@@ -77,7 +77,7 @@ function debugConfigLayers(path: string, layers: SiteData[]): SiteData[] {
     const summary = layers.map((c, i, arr) => {
       const n = i + 1
       if (n === arr.length) return `${n}. .vitepress/config (root)`
-      return `${n}. ${(c as any)?.VP_SOURCE ?? '(Unknown Source)'}`
+      return `${n}. ${(c as any)?.['[VP_SOURCE]'] ?? '(Unknown Source)'}`
     })
     console.debug(
       [summaryTitle, ''.padEnd(summaryTitle.length, '='), ...summary].join('\n')
diff --git a/src/node/siteConfig.ts b/src/node/siteConfig.ts
index e09352ee..bffa8015 100644
--- a/src/node/siteConfig.ts
+++ b/src/node/siteConfig.ts
@@ -14,7 +14,10 @@ import type {
   SSGContext,
   SiteData
 } from './shared'
-import type { AdditionalConfig } from '../../types/shared'
+import type {
+  AdditionalConfigDict,
+  AdditionalConfigLoader
+} from '../../types/shared'
 
 export type RawConfigExports<ThemeConfig = any> =
   | Awaitable<UserConfig<ThemeConfig>>
@@ -194,7 +197,9 @@ export interface UserConfig<ThemeConfig = any>
    * Multi-layer configuration overloading.
    * Auto-resolves to docs/.../config.(ts|js|json) when unspecified.
    */
-  additionalConfig?: AdditionalConfig
+  additionalConfig?:
+    | AdditionalConfigDict<ThemeConfig>
+    | AdditionalConfigLoader<ThemeConfig>
 }
 
 export interface SiteConfig<ThemeConfig = any>
diff --git a/types/shared.d.ts b/types/shared.d.ts
index 62a618c2..b0f9bbd5 100644
--- a/types/shared.d.ts
+++ b/types/shared.d.ts
@@ -5,6 +5,14 @@ export type { DefaultTheme } from './default-theme.js'
 
 export type Awaitable<T> = T | PromiseLike<T>
 
+// Beware that this may cause performance issues or infinite loops
+// Use only when absolutely necessary
+export type DeepPartial<T> = T extends object
+  ? {
+      [K in keyof T]?: DeepPartial<T[K]>
+    }
+  : T
+
 export interface PageData {
   relativePath: string
   /**
@@ -134,7 +142,9 @@ export interface SiteData<ThemeConfig = any> {
   router: {
     prefetchLinks: boolean
   }
-  additionalConfig?: AdditionalConfig<ThemeConfig>
+  additionalConfig?:
+    | AdditionalConfigDict<ThemeConfig>
+    | AdditionalConfigLoader<ThemeConfig>
 }
 
 export type HeadConfig =
@@ -162,26 +172,18 @@ export interface LocaleSpecificConfig<ThemeConfig = any> {
   themeConfig?: ThemeConfig
 }
 
-export interface AdditionalConfigEntry<ThemeConfig = any>
-  extends LocaleSpecificConfig<ThemeConfig> {
-  /**
-   * Source of current config entry, only available in development mode
-   */
-  src?: string
-}
+export type AdditionalConfig<ThemeConfig = any> = DeepPartial<
+  LocaleSpecificConfig<ThemeConfig>
+>
 
 export type AdditionalConfigDict<ThemeConfig = any> = Record<
   string,
-  AdditionalConfigEntry<ThemeConfig>
+  AdditionalConfig<ThemeConfig>
 >
 
 export type AdditionalConfigLoader<ThemeConfig = any> = (
   path: string
-) => AdditionalConfigEntry<ThemeConfig>[]
-
-export type AdditionalConfig<ThemeConfig = any> =
-  | AdditionalConfigDict<ThemeConfig>
-  | AdditionalConfigLoader<ThemeConfig>
+) => AdditionalConfig<ThemeConfig>[]
 
 export type LocaleConfig<ThemeConfig = any> = Record<
   string,