fix(cursor): gated turn thinking steps to same-model K3

- Applied the same same-model K3 replay guard to ConversationState.turns thinking steps so foreign or non-K3 reasoning never leaks into Cursor turn history.

Fixes #7184

packages/ai/src/providers/cursor.ts

@@ -4311,6 +4311,7 @@ function buildConversationTurns(
 	messages: Message[],
 	blobStore: Map<string, Uint8Array>,
 	activeUserMessageIndex = findLastUserMessageIndex(messages),
+	targetModelId?: string,
 ): Uint8Array[] {
 	const turns: Uint8Array[] = [];
 	const historyEnd = activeUserMessageIndex >= 0 ? activeUserMessageIndex : messages.length;
@@ -4365,7 +4366,10 @@ function buildConversationTurns(
 							},
 						});
 					} else if (item.type === "thinking") {
-						if (!item.thinking) continue;
+						// Same guard as root-prompt replay: only same-model Cursor K3
+						// thinking is replayed, so foreign/hidden reasoning never leaks
+						// into Cursor's turn history as native thinking.
+						if (!item.thinking || !canReplayCursorThinking(stepMsg, targetModelId)) continue;
 						step = create(ConversationStepSchema, {
 							message: {
 								case: "thinkingMessage",
@@ -4431,7 +4435,7 @@ export function buildCursorHistoryForTest(
 	).map(blobId => JSON.parse(new TextDecoder().decode(readCursorBlob(blobStore, blobId))));
 	const turnUserMessagesJson: JsonValue[] = [];
 	const turnStepMessagesJson: JsonValue[][] = [];
-	for (const turnBlobId of buildConversationTurns(messages, blobStore, activeUserMessageIndex)) {
+	for (const turnBlobId of buildConversationTurns(messages, blobStore, activeUserMessageIndex, targetModelId)) {
 		const turn = fromBinary(ConversationTurnStructureSchema, readCursorBlob(blobStore, turnBlobId));
 		if (turn.turn.case !== "agentConversationTurn") {
 			continue;
@@ -4535,7 +4539,12 @@ function buildGrpcRequest(
 
 	// Build conversation turns from prior messages, excluding only the active user message
 	// when the request is sending one. Resume actions must preserve trailing tool results.
-	const turns = buildConversationTurns(context.messages, blobStore, activeUserMessage ? activeUserMessageIndex : -1);
+	const turns = buildConversationTurns(
+		context.messages,
+		blobStore,
+		activeUserMessage ? activeUserMessageIndex : -1,
+		model.id,
+	);
 
 	// Build `rootPromptMessagesJson` from prior messages. Cursor's server uses this
 	// field (not `turns[]`) to construct the actual model prompt; if we only send the

packages/ai/test/cursor-exec-handlers.test.ts

@@ -747,13 +747,43 @@ describe("Cursor history encoding", () => {
 			{ role: "assistant", content: [{ type: "text", text: "Visible answer." }] },
 		]);
 		expect(history.turnStepMessagesJson).toEqual([
-			[
-				expect.objectContaining({ thinkingMessage: { text: "Internal reasoning." } }),
-				expect.objectContaining({ assistantMessage: { text: "Visible answer." } }),
-			],
+			[expect.objectContaining({ assistantMessage: { text: "Visible answer." } })],
 		]);
 	});
 
+	it("keeps foreign thinking out of turns when switching to a non-K3 Cursor model", () => {
+		const messages: Context["messages"] = [
+			{ role: "user", content: "Plan this change.", timestamp: 1 },
+			{
+				...cursorAssistant(
+					"claude-4.6-opus-high",
+					[
+						{ type: "thinking", thinking: "Foreign signed reasoning.", thinkingSignature: "signature" },
+						{ type: "text", text: "Here is the plan." },
+					],
+					2,
+				),
+				api: "anthropic-messages",
+				provider: "anthropic",
+			},
+			{ role: "user", content: "Continue.", timestamp: 3 },
+		];
+
+		const history = buildCursorHistoryForTest(messages, undefined, "cursor-composer-2.5");
+		expect(history.rootPromptMessagesJson).toEqual([
+			{ role: "user", content: [{ type: "text", text: "Plan this change." }] },
+			{ role: "assistant", content: [{ type: "text", text: "Here is the plan." }] },
+		]);
+		expect(history.turnStepMessagesJson).toEqual([
+			[expect.objectContaining({ assistantMessage: { text: "Here is the plan." } })],
+		]);
+		for (const steps of history.turnStepMessagesJson) {
+			for (const step of steps) {
+				expect(step).not.toHaveProperty("thinkingMessage");
+			}
+		}
+	});
+
 	it("preserves image-only user turns in root prompt history and conversation turns", () => {
 		const imageData = "aW1hZ2U=";
 		const history = buildCursorHistoryForTest([