diff --git a/core/llm/openaiTypeConverters.test.ts b/core/llm/openaiTypeConverters.test.ts
index b0c63862f4d..4568e8a9f2c 100644
--- a/core/llm/openaiTypeConverters.test.ts
+++ b/core/llm/openaiTypeConverters.test.ts
@@ -22,6 +22,11 @@ type MessageItem = {
   content: unknown;
 };
 
+type ReasoningItem = {
+  type: "reasoning";
+  id: string;
+};
+
 // Helper functions for filtering results
 function getFunctionCalls(items: ResponseInputItem[]): FunctionCallItem[] {
   return items.filter(
@@ -47,6 +52,12 @@ function getMessagesByRole(
   }) as unknown as MessageItem[];
 }
 
+function getReasoningItems(items: ResponseInputItem[]): ReasoningItem[] {
+  return items.filter(
+    (item) => (item as { type?: string }).type === "reasoning",
+  ) as unknown as ReasoningItem[];
+}
+
 describe("openaiTypeConverters", () => {
   describe("toResponsesInput", () => {
     describe("tool calls handling - OpenAI Responses API", () => {
@@ -496,6 +507,44 @@ describe("openaiTypeConverters", () => {
         expect(userMessages.length).toBe(2);
       });
 
+      it("should skip trailing thinking messages without a following assistant item", () => {
+        const messages: ChatMessage[] = [
+          {
+            role: "thinking",
+            content: "",
+            reasoning_details: [{ type: "reasoning_id", id: "rs_trailing" }],
+          } as ChatMessage,
+        ];
+
+        const result = toResponsesInput(messages);
+
+        const reasoningItems = getReasoningItems(result);
+        expect(reasoningItems.length).toBe(0);
+      });
+
+      it("should preserve thinking messages when followed by assistant output", () => {
+        const messages: ChatMessage[] = [
+          {
+            role: "thinking",
+            content: "",
+            reasoning_details: [{ type: "reasoning_id", id: "rs_kept" }],
+          } as ChatMessage,
+          {
+            role: "assistant",
+            content: "Done.",
+          } as ChatMessage,
+        ];
+
+        const result = toResponsesInput(messages);
+
+        const reasoningItems = getReasoningItems(result);
+        const assistantMessages = getMessagesByRole(result, "assistant");
+
+        expect(reasoningItems.length).toBe(1);
+        expect(reasoningItems[0].id).toBe("rs_kept");
+        expect(assistantMessages.length).toBe(1);
+      });
+
       it("should handle system message (converted to developer role)", () => {
         const messages: ChatMessage[] = [
           { role: "system", content: "You are a helpful assistant" },
diff --git a/core/llm/openaiTypeConverters.ts b/core/llm/openaiTypeConverters.ts
index 9280a51c706..86f8e8fbc44 100644
--- a/core/llm/openaiTypeConverters.ts
+++ b/core/llm/openaiTypeConverters.ts
@@ -868,6 +868,18 @@ function convertThinkingMessageToReasoningItem(
   return reasoningItem;
 }
 
+function hasFollowingAssistantMessage(
+  messages: ChatMessage[],
+  startIndex: number,
+): boolean {
+  for (let i = startIndex + 1; i < messages.length; i++) {
+    const nextRole = messages[i]?.role;
+    if (nextRole === "thinking") continue;
+    return nextRole === "assistant";
+  }
+  return false;
+}
+
 export function toResponsesInput(messages: ChatMessage[]): ResponseInput {
   const input: ResponseInput = [];
 
@@ -967,6 +979,10 @@ export function toResponsesInput(messages: ChatMessage[]): ResponseInput {
         break;
       }
       case "thinking": {
+        if (!hasFollowingAssistantMessage(messages, i)) {
+          break;
+        }
+
         const reasoningItem = convertThinkingMessageToReasoningItem(
           msg as ThinkingChatMessage,
         );