Adds JSON mode support

examples/src/models/chat/integration_openai.ts

@@ -71,3 +71,29 @@ AIMessage {
   }
 }
 */
+
+// Coerce response type with JSON mode.
+// Requires "gpt-4-1106-preview" or later
+const jsonModeModel = new ChatOpenAI({
+  modelName: "gpt-4-1106-preview",
+  maxTokens: 128,
+}).bind({
+  response_format: {
+    type: "json",
+  },
+});
+
+// Must be invoked with a system message containing the string "JSON":
+// https://platform.openai.com/docs/guides/text-generation/json-mode
+const res = await jsonModeModel.invoke([
+  ["system", "Only return JSON"],
+  ["human", "Hi there!"],
+]);
+console.log(res);
+
+/*
+  AIMessage {
+    content: '```json\n{\n  "response": "Hello! How can I assist you today?"\n}\n```',
+    additional_kwargs: { function_call: undefined }
+  }
+*/

langchain/src/chat_models/openai.ts

@@ -155,6 +155,7 @@ export interface ChatOpenAICallOptions
     BaseFunctionCallOptions {
   tools?: StructuredTool[];
   promptIndex?: number;
+  response_format?: { type: "json" };
 }
 
 /**

langchain/src/chat_models/tests/chatopenai.int.test.ts

@@ -775,3 +775,17 @@ test("Test ChatOpenAI token usage reporting for streaming calls", async () => {
     expect(streamingTokenUsed).toEqual(nonStreamingTokenUsed);
   }
 });
+
+test("Test ChatOpenAI JSON mode", async () => {
+  const chat = new ChatOpenAI({
+    modelName: "gpt-4-1106-preview",
+    maxTokens: 128,
+  }).bind({
+    response_format: {
+      type: "json",
+    },
+  });
+  const message = new HumanMessage("Hello!");
+  const res = await chat.invoke([["system", "Only return JSON"], message]);
+  console.log(JSON.stringify(res));
+});