diff --git a/packages/ai/src/api/anthropic-messages.ts b/packages/ai/src/api/anthropic-messages.ts
index 2221ce82..b2c6a220 100644
--- a/packages/ai/src/api/anthropic-messages.ts
+++ b/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") {
diff --git a/packages/ai/test/anthropic-empty-thinking-signature-compat.test.ts b/packages/ai/test/anthropic-empty-thinking-signature-compat.test.ts
index f5c88368..0a3720fb 100644
--- a/packages/ai/test/anthropic-empty-thinking-signature-compat.test.ts
+++ b/packages/ai/test/anthropic-empty-thinking-signature-compat.test.ts
@@ -32,10 +32,10 @@ function makeModel(allowEmptySignature?: boolean): Model<"anthropic-messages"> {
 	};
 }
 
-function makeContext(thinkingSignature: string): Context {
+function makeContext(thinkingSignature: string, thinking = "internal reasoning"): Context {
 	const assistant: AssistantMessage = {
 		role: "assistant",
-		content: [{ type: "thinking", thinking: "internal reasoning", thinkingSignature }],
+		content: [{ type: "thinking", thinking, thinkingSignature }],
 		provider: "xiaomi-token-plan-ams",
 		api: "anthropic-messages",
 		model: "mimo-v2.5-pro",
@@ -80,6 +80,12 @@ describe("Anthropic empty thinking signature compat", () => {
 		expect(assistant?.content).toEqual([{ type: "text", text: "internal reasoning" }]);
 	});
 
+	it("preserves empty thinking text when the signature is present", async () => {
+		const payload = await capturePayload(makeModel(), makeContext("signed-thinking", ""));
+		const assistant = payload.messages?.find((message) => message.role === "assistant");
+		expect(assistant?.content).toEqual([{ type: "thinking", thinking: "", signature: "signed-thinking" }]);
+	});
+
 	it("preserves empty-signature thinking when allowEmptySignature is enabled", async () => {
 		const payload = await capturePayload(makeModel(true), makeContext(" "));
 		const assistant = payload.messages?.find((message) => message.role === "assistant");