diff --git a/packages/openai-adapters/src/test/vercelStreamConverter.test.ts b/packages/openai-adapters/src/test/vercelStreamConverter.test.ts
index 839e3a8c78..0c3feed93d 100644
--- a/packages/openai-adapters/src/test/vercelStreamConverter.test.ts
+++ b/packages/openai-adapters/src/test/vercelStreamConverter.test.ts
@@ -34,7 +34,10 @@ describe("convertVercelStreamPart", () => {
     const result = convertVercelStreamPart(part, options);
 
     expect(result).not.toBeNull();
-    expect(result?.choices[0].delta.content).toBe("Let me think...");
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    expect((result?.choices[0].delta as any).reasoning_content).toBe(
+      "Let me think...",
+    );
   });
 
   test("returns null for tool-call (handled by tool-input-start/delta)", () => {
diff --git a/packages/openai-adapters/src/vercelStreamConverter.ts b/packages/openai-adapters/src/vercelStreamConverter.ts
index 53f4cc75a8..21930b5582 100644
--- a/packages/openai-adapters/src/vercelStreamConverter.ts
+++ b/packages/openai-adapters/src/vercelStreamConverter.ts
@@ -86,8 +86,11 @@ export function convertVercelStreamPart(
       });
 
     case "reasoning-delta":
-      return chatChunk({
-        content: part.text,
+      return chatChunkFromDelta({
+        delta: {
+          // `reasoning_content` is not yet typed in OpenAI's SDK types.
+          reasoning_content: part.text,
+        } as ChatCompletionChunk.Choice["delta"],
         model,
       });