Add OpenAI functions agent prompt customization example (langchain-ai…

…#2015)

* Cleanup

* Adds OpenAI functions custom prompt example

* Fix link

docs/snippets/modules/agents/agent_types/openai_functions_agent.mdx

@@ -8,3 +8,11 @@ Must be used with an [OpenAI Functions](https://platform.openai.com/docs/guides/
 This agent also supports `StructuredTool`s with more complex input schemas.
 
 <CodeBlock language="typescript">{Example}</CodeBlock>
+
+## Prompt customization
+
+You can pass in a custom string to be used as the system message of the prompt as follows:
+
+import CustomPromptExample from "@examples/agents/openai_custom_prompt.ts";
+
+<CodeBlock language="typescript">{CustomPromptExample}</CodeBlock>

examples/src/agents/openai_custom_prompt.ts

@@ -0,0 +1,22 @@
+import { initializeAgentExecutorWithOptions } from "langchain/agents";
+import { ChatOpenAI } from "langchain/chat_models/openai";
+import { SerpAPI } from "langchain/tools";
+import { Calculator } from "langchain/tools/calculator";
+
+const tools = [new Calculator(), new SerpAPI()];
+const chat = new ChatOpenAI({ modelName: "gpt-4", temperature: 0 });
+const prefix =
+  "You are a helpful AI assistant. However, all final response to the user must be in pirate dialect.";
+
+const executor = await initializeAgentExecutorWithOptions(tools, chat, {
+  agentType: "openai-functions",
+  verbose: true,
+  agentArgs: {
+    prefix,
+  },
+});
+
+const result = await executor.run("What is the weather in New York?");
+console.log(result);
+
+// Arr matey, in New York, it be feelin' like 75 degrees, with a gentle breeze blowin' from the northwest at 3 knots. The air be 77% full o' water, and the clouds be coverin' 35% of the sky. There be no rain in sight, yarr!