diff --git a/packages/misskey-js/etc/misskey-js.api.md b/packages/misskey-js/etc/misskey-js.api.md
index 3592153f9..1dba2a70d 100644
--- a/packages/misskey-js/etc/misskey-js.api.md
+++ b/packages/misskey-js/etc/misskey-js.api.md
@@ -2165,7 +2165,7 @@ type IResponse = operations['i']['responses']['200']['content']['application/jso
 type IRevokeTokenRequest = operations['i/revoke-token']['requestBody']['content']['application/json'];
 
 // @public (undocumented)
-function isAPIError(reason: any): reason is APIError;
+function isAPIError(reason: Record<PropertyKey, unknown>): reason is APIError;
 
 // @public (undocumented)
 type ISigninHistoryRequest = operations['i/signin-history']['requestBody']['content']['application/json'];
diff --git a/packages/misskey-js/src/api.ts b/packages/misskey-js/src/api.ts
index 2e85fc293..134ead0d7 100644
--- a/packages/misskey-js/src/api.ts
+++ b/packages/misskey-js/src/api.ts
@@ -17,7 +17,7 @@ export type APIError = {
 	info: Record<string, any>;
 };
 
-export function isAPIError(reason: any): reason is APIError {
+export function isAPIError(reason: Record<PropertyKey, unknown>): reason is APIError {
 	return reason[MK_API_ERROR] === true;
 }
 
diff --git a/packages/misskey-js/src/api.types.ts b/packages/misskey-js/src/api.types.ts
index 632008192..af0bade5b 100644
--- a/packages/misskey-js/src/api.types.ts
+++ b/packages/misskey-js/src/api.types.ts
@@ -15,10 +15,10 @@ type Overwrite<T, U extends { [Key in keyof T]?: unknown }> = Omit<
 	keyof U
 > & U;
 
-type SwitchCase = {
+type SwitchCase<Condition = unknown, Result = unknown> = {
 	$switch: {
-		$cases: [any, any][],
-		$default: any;
+		$cases: [Condition, Result][],
+		$default: Result;
 	};
 };