fix(ai): stop defaulting max token request caps

closes #4675

packages/ai/src/providers/anthropic.ts

@@ -751,8 +751,10 @@ export const streamSimpleAnthropic: StreamFunction<"anthropic-messages", SimpleS
 		} satisfies AnthropicOptions);
 	}
 
+	// Undefined means the caller did not request an output cap; let the helper use the model cap.
+	// Do not coerce to 0 here, or the thinking budget would become the entire max_tokens value.
 	const adjusted = adjustMaxTokensForThinking(
-		base.maxTokens || 0,
+		base.maxTokens,
 		model.maxTokens,
 		options.reasoning,
 		options.thinkingBudgets,
@@ -891,7 +893,7 @@ function buildParams(
 	const params: MessageCreateParamsStreaming = {
 		model: model.id,
 		messages: convertMessages(context.messages, model, isOAuthToken, cacheControl),
-		max_tokens: options?.maxTokens || (model.maxTokens / 3) | 0,
+		max_tokens: options?.maxTokens ?? model.maxTokens,
 		stream: true,
 	};