feat: Add Perplexity support to universal chat model (langchain-ai#9028)

Co-authored-by: Hunter Lovell <40191806+hntrl@users.noreply.github.com>

Author: amaze-28

.changeset/green-squids-fetch.md

@@ -0,0 +1,5 @@
+---
+"langchain": patch
+---
+
+feat: Add Perplexity support to universal chat model

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

@@ -598,6 +598,17 @@ describe("Works with all model providers", () => {
     expect(deepSeekResult).toBeDefined();
     expect(deepSeekResult.content.length).toBeGreaterThan(0);
   });
+
+  it("Can invoke perplexity", async () => {
+    const perplexity = await initChatModel("sonar-pro", {
+      modelProvider: "perplexity",
+      temperature: 0,
+    });
+
+    const perplexityResult = await perplexity.invoke("what's your name");
+    expect(perplexityResult).toBeDefined();
+    expect(perplexityResult.content.length).toBeGreaterThan(0);
+  });
 });
 
 test("Is compatible with agents", async () => {

langchain/src/chat_models/universal.ts

@@ -51,6 +51,7 @@ const _SUPPORTED_PROVIDERS = [
   "cerebras",
   "deepseek",
   "xai",
+  "perplexity",
 ] as const;
 
 export type ChatModelProvider = (typeof _SUPPORTED_PROVIDERS)[number];
@@ -166,6 +167,19 @@ async function _initChatModelHelper(
         );
         return new ChatTogetherAI({ model, ...passedParams });
       }
+      case "perplexity": {
+        // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+        // @ts-ignore - Can not install as a proper dependency due to circular dependency
+        const { ChatPerplexity } = await import(
+          // We can not 'expect-error' because if you explicitly build `@langchain/community`
+          // this import will be able to be resolved, thus there will be no error. However
+          // this will never be the case in CI.
+          // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+          // @ts-ignore - Can not install as a proper dependency due to circular dependency
+          "@langchain/community/chat_models/perplexity"
+        );
+        return new ChatPerplexity({ model, ...passedParams });
+      }
       default: {
         const supported = _SUPPORTED_PROVIDERS.join(", ");
         throw new Error(
@@ -220,6 +234,8 @@ export function _inferModelProvider(modelName: string): string | undefined {
     return "bedrock";
   } else if (modelName.startsWith("mistral")) {
     return "mistralai";
+  } else if (modelName.startsWith("sonar") || modelName.startsWith("pplx")) {
+    return "perplexity";
   } else {
     return undefined;
   }
@@ -638,6 +654,7 @@ export async function initChatModel<
  *   - mistralai (@langchain/mistralai)
  *   - groq (@langchain/groq)
  *   - ollama (@langchain/ollama)
+ *   - perplexity (@langchain/community/chat_models/perplexity)
  *   - cerebras (@langchain/cerebras)
  *   - deepseek (@langchain/deepseek)
  *   - xai (@langchain/xai)