diff --git a/packages/llm/src/protocols/openai-chat.ts b/packages/llm/src/protocols/openai-chat.ts
index 470a1473c40b..4b00d05ab738 100644
--- a/packages/llm/src/protocols/openai-chat.ts
+++ b/packages/llm/src/protocols/openai-chat.ts
@@ -185,8 +185,13 @@ const lowerToolCall = (part: ToolCallPart): OpenAIChatAssistantToolCall => ({
   },
 })
 
-const openAICompatibleReasoningContent = (native: unknown) =>
-  isRecord(native) && typeof native.reasoning_content === "string" ? native.reasoning_content : undefined
+const openAICompatibleReasoningContent = (native: unknown) => {
+  if (!isRecord(native)) return undefined
+  if (!isRecord(native["providerOptions"])) return undefined
+  if (!isRecord(native["providerOptions"]["openaiCompatible"])) return undefined
+  const reasoning = native["providerOptions"]["openaiCompatible"]["reasoning_content"]
+  return typeof reasoning === "string" ? reasoning : undefined
+}
 
 const lowerUserMessage = Effect.fn("OpenAIChat.lowerUserMessage")(function* (message: OpenAIChatRequestMessage) {
   const content: TextPart[] = []
@@ -219,7 +224,7 @@ const lowerAssistantMessage = Effect.fn("OpenAIChat.lowerAssistantMessage")(func
     role: "assistant" as const,
     content: content.length === 0 ? null : ProviderShared.joinText(content),
     tool_calls: toolCalls.length === 0 ? undefined : toolCalls,
-    reasoning_content: openAICompatibleReasoningContent(message.native?.openaiCompatible),
+    reasoning_content: openAICompatibleReasoningContent(message.native),
   }
 })
 
diff --git a/packages/llm/test/provider/openai-chat.test.ts b/packages/llm/test/provider/openai-chat.test.ts
index 4303a69ffa5c..98a62a58956a 100644
--- a/packages/llm/test/provider/openai-chat.test.ts
+++ b/packages/llm/test/provider/openai-chat.test.ts
@@ -179,6 +179,38 @@ describe("OpenAI Chat route", () => {
     }),
   )
 
+  it.effect("prepares assistant tool-call message with reasoning_content from native providerOptions", () =>
+    Effect.gen(function* () {
+      const prepared = yield* LLMClient.prepare<OpenAIChat.OpenAIChatBody>(
+        LLM.request({
+          id: "req_reasoning_tool_call",
+          model,
+          messages: [
+            Message.user("What is the weather?"),
+            new Message({
+              role: "assistant",
+              content: [ToolCallPart.make({ id: "call_1", name: "lookup", input: { query: "weather" } })],
+              native: { providerOptions: { openaiCompatible: { reasoning_content: "thinking about weather" } } },
+            }),
+          ],
+        }),
+      )
+
+      expect(prepared.body.messages[1]).toEqual({
+        role: "assistant",
+        content: null,
+        tool_calls: [
+          {
+            id: "call_1",
+            type: "function",
+            function: { name: "lookup", arguments: encodeJson({ query: "weather" }) },
+          },
+        ],
+        reasoning_content: "thinking about weather",
+      })
+    }),
+  )
+
   it.effect("rejects unsupported user media content", () =>
     Effect.gen(function* () {
       const error = yield* LLMClient.prepare(