fix: send anthropic thinking blocks also when thinking text is empty (#6457)

fixes: #6376

packages/ai/src/api/anthropic-messages.ts

@@ -1081,11 +1081,13 @@ function convertMessages(
 						});
 						continue;
 					}
-					if (block.thinking.trim().length === 0) continue;
+					const thinkingSignature = block.thinkingSignature;
+					const hasThinkingSignature = !!thinkingSignature && thinkingSignature.trim().length > 0;
+					if (block.thinking.trim().length === 0 && !hasThinkingSignature) continue;
 					// If thinking signature is missing/empty (e.g., from aborted stream),
 					// convert to plain text for Anthropic. Some compatible providers emit
 					// and accept empty signatures, so let marked models preserve the block.
-					if (!block.thinkingSignature || block.thinkingSignature.trim().length === 0) {
+					if (!hasThinkingSignature) {
 						blocks.push(
 							allowEmptySignature
 								? {
@@ -1102,7 +1104,7 @@ function convertMessages(
 						blocks.push({
 							type: "thinking",
 							thinking: sanitizeSurrogates(block.thinking),
-							signature: block.thinkingSignature,
+							signature: thinkingSignature,
 						});
 					}
 				} else if (block.type === "toolCall") {