diff --git a/packages/agent/src/types.ts b/packages/agent/src/types.ts
index ecdb2057..c291dae2 100644
--- a/packages/agent/src/types.ts
+++ b/packages/agent/src/types.ts
@@ -99,7 +99,7 @@ export interface AgentLoopConfig extends SimpleStreamOptions {
 
 /**
  * Thinking/reasoning level for models that support it.
- * Note: "xhigh" is only supported by OpenAI gpt-5.1-codex-max, gpt-5.2, and gpt-5.2-codex models.
+ * Note: "xhigh" is only supported by OpenAI gpt-5.1-codex-max, gpt-5.2, gpt-5.2-codex, gpt-5.3, and gpt-5.3-codex models.
  */
 export type ThinkingLevel = "off" | "minimal" | "low" | "medium" | "high" | "xhigh";
 
diff --git a/packages/ai/src/models.ts b/packages/ai/src/models.ts
index a9e987b9..2c96062f 100644
--- a/packages/ai/src/models.ts
+++ b/packages/ai/src/models.ts
@@ -50,7 +50,7 @@ export function calculateCost<TApi extends Api>(model: Model<TApi>, usage: Usage
  * Currently only certain OpenAI Codex models support this.
  */
 export function supportsXhigh<TApi extends Api>(model: Model<TApi>): boolean {
-	return model.id.includes("gpt-5.2");
+	return model.id.includes("gpt-5.2") || model.id.includes("gpt-5.3");
 }
 
 /**