google-genai[minor]: Add support for json output

libs/langchain-google-genai/src/chat_models.ts

@@ -75,6 +75,8 @@ export interface GoogleGenerativeAIChatInput
   extends BaseChatModelParams,
     Pick<GoogleGenerativeAIChatCallOptions, "streamUsage"> {
   /**
+   * @deprecated Use "model" instead.
+   *
    * Model Name to use
    *
    * Alias for `model`
@@ -167,6 +169,13 @@ export interface GoogleGenerativeAIChatInput
 
   /** Whether to stream the results or not */
   streaming?: boolean;
+
+  /**
+   * Whether or not to force the model to respond with JSON.
+   * Available for `gemini-1.5` models and later.
+   * @default false
+   */
+  json?: boolean;
 }
 
 /**
@@ -321,6 +330,7 @@ export class ChatGoogleGenerativeAI
           temperature: this.temperature,
           topP: this.topP,
           topK: this.topK,
+          ...(fields?.json ? { responseMimeType: "application/json" } : {}),
         },
       },
       {

libs/langchain-google-genai/src/tests/chat_models.int.test.ts

@@ -519,3 +519,22 @@ test("Invoke token count usage_metadata", async () => {
     res.usage_metadata.input_tokens + res.usage_metadata.output_tokens
   );
 });
+
+test("Invoke with JSON mode", async () => {
+  const model = new ChatGoogleGenerativeAI({
+    model: "gemini-1.5-flash",
+    temperature: 0,
+    maxOutputTokens: 10,
+    json: true,
+  });
+  const res = await model.invoke("Why is the sky blue? Be concise.");
+  expect(res?.usage_metadata).toBeDefined();
+  if (!res?.usage_metadata) {
+    return;
+  }
+  expect(res.usage_metadata.input_tokens).toBeGreaterThan(1);
+  expect(res.usage_metadata.output_tokens).toBeGreaterThan(1);
+  expect(res.usage_metadata.total_tokens).toBe(
+    res.usage_metadata.input_tokens + res.usage_metadata.output_tokens
+  );
+});