diff --git a/specification/draft/apps.mdx b/specification/draft/apps.mdx
index cee96f05..0970d1ac 100644
--- a/specification/draft/apps.mdx
+++ b/specification/draft/apps.mdx
@@ -399,7 +399,7 @@ interface HostContext {
     tool: Tool,
   };
   /** Current color theme preference */
-  theme?: "light" | "dark" | "system";
+  theme?: "light" | "dark";
   /** How the UI is currently displayed */
   displayMode?: "inline" | "fullscreen" | "pip";
   /** Display modes the host supports */
diff --git a/src/types.ts b/src/types.ts
index c7864933..d0153c09 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -501,7 +501,7 @@ export const McpUiHostContextSchema: z.ZodType<McpUiHostContext> = z.object({
       tool: ToolSchema,
     })
     .optional(),
-  theme: z.enum(["light", "dark", "system"]).optional(),
+  theme: z.enum(["light", "dark"]).optional(),
   displayMode: z.enum(["inline", "fullscreen", "pip"]).optional(),
   availableDisplayModes: z.array(z.string()).optional(),
   viewport: z