modify-agent

name	modify-agent
description	Modify TypeScript LangChain agent configuration and behavior. Use when: (1) User wants to change agent settings, (2) Add/remove tools, (3) Update system prompt, (4) Change model parameters.

Modify Agent

Key Files

Customize These Files

File	Purpose	When to Edit
src/agent.ts	Agent logic, tools, prompt	Change agent behavior
src/tools.ts	Tool definitions	Add/remove tools
src/mcp-servers.ts	MCP server connections	Add Databricks resources
app.yaml	Runtime configuration	Env vars, resources
databricks.yml	Bundle resources	Permissions, targets
.env	Local environment	Local development

Framework Files (leave alone)

File	Purpose
src/framework/server.ts	Express server, request routing
src/framework/tracing.ts	MLflow/OTel tracing setup
src/framework/routes/invocations.ts	Responses API SSE streaming

Tests

Directory	Contents
tests/	✏️ Agent unit & integration tests — add yours here
tests/e2e/	✏️ End-to-end tests against deployed app
tests/framework/	Framework tests — no need to modify
tests/e2e/framework/	Framework e2e tests — no need to modify

Common Modifications

1. Change Model

In .env (local):

DATABRICKS_MODEL=databricks-gpt-5-2

In app.yaml (deployed):

env:
  - name: DATABRICKS_MODEL
    value: "databricks-gpt-5-2"

Available models:

• databricks-claude-sonnet-4-5
• databricks-gpt-5-2
• databricks-meta-llama-3-3-70b-instruct
• Your custom endpoint name

2. Update System Prompt

Edit src/agent.ts:

const DEFAULT_SYSTEM_PROMPT = `You are a helpful AI assistant specialized in [YOUR DOMAIN].

Your key capabilities:
- [Capability 1]
- [Capability 2]

When answering:
- [Instruction 1]
- [Instruction 2]

Be concise but thorough.`;

Or pass custom prompt when creating agent:

const agent = await createAgent({
  systemPrompt: "Your custom instructions here...",
});

3. Adjust Model Parameters

Temperature (0.0 = deterministic, 1.0 = creative):

.env:

TEMPERATURE=0.7

app.yaml:

env:
  - name: TEMPERATURE
    value: "0.7"

Max Tokens:

.env:

MAX_TOKENS=4000

app.yaml:

env:
  - name: MAX_TOKENS
    value: "4000"

Use Responses API (for citations, reasoning):

.env:

USE_RESPONSES_API=true

4. Add New Tools

Basic Function Tool

Edit src/tools.ts:

import { tool } from "@langchain/core/tools";
import { z } from "zod";

export const myCustomTool = tool(
  async ({ param1, param2 }) => {
    // Tool logic here
    return `Result: ${param1} and ${param2}`;
  },
  {
    name: "my_custom_tool",
    description: "Description of what this tool does",
    schema: z.object({
      param1: z.string().describe("Description of param1"),
      param2: z.number().describe("Description of param2"),
    }),
  }
);

Add to tool list:

export function getBasicTools() {
  return [
    weatherTool,
    calculatorTool,
    timeTool,
    myCustomTool,  // Add here
  ];
}

MCP Tool Integration

For adding MCP tools (SQL, Vector Search, Genie, UC Functions), see the add-tools skill.

MCP tools are configured in src/mcp-servers.ts with required permissions in databricks.yml.

5. Remove Tools

Edit src/tools.ts:

export function getBasicTools() {
  return [
    weatherTool,
    // calculatorTool,  // Commented out to disable
    timeTool,
  ];
}

Or filter tools:

export function getBasicTools() {
  const allTools = [weatherTool, calculatorTool, timeTool];
  return allTools.filter(t => t.name !== "calculator");
}

6. Customize Agent Behavior

The agent uses standard LangGraph createReactAgent API in src/agent.ts:

import { createReactAgent } from "@langchain/langgraph/prebuilt";

export async function createAgent(config: AgentConfig = {}) {
  // Create chat model
  const model = new ChatDatabricks({
    model: modelName,
    useResponsesApi,
    temperature,
    maxTokens,
  });

  // Load tools (basic + MCP if configured)
  const tools = await getAllTools(mcpServers);

  // Create agent using standard LangGraph API
  const agent = createReactAgent({
    llm: model,
    tools,
  });

  return new StandardAgent(agent, systemPrompt);
}

The LangGraph agent automatically handles:

• Tool calling and execution
• Multi-turn reasoning with state management
• Error handling and retries
• Streaming support out of the box

7. Add API Endpoints

Edit src/framework/server.ts:

// New endpoint example
app.post("/api/evaluate", async (req: Request, res: Response) => {
  const { input, expected } = req.body;

  const response = await invokeAgent(agent, input);

  // Custom evaluation logic
  const score = calculateScore(response.output, expected);

  res.json({
    input,
    output: response.output,
    expected,
    score,
  });
});

8. Modify MLflow Tracing

Edit src/framework/tracing.ts or initialize with custom config in src/framework/server.ts:

const tracing = initializeMLflowTracing({
  serviceName: "my-custom-service",
  experimentId: process.env.MLFLOW_EXPERIMENT_ID,
  useBatchProcessor: false,  // Use simple processor for debugging
});

9. Change Port

.env:

PORT=3001

app.yaml:

env:
  - name: PORT
    value: "3001"

10. Add Streaming Configuration

Edit src/server.ts to customize streaming behavior:

if (stream) {
  res.setHeader("Content-Type", "text/event-stream");
  res.setHeader("Cache-Control", "no-cache");
  res.setHeader("Connection", "keep-alive");
  res.setHeader("X-Accel-Buffering", "no");  // Disable buffering

  // Custom streaming logic
  try {
    for await (const chunk of streamAgent(agent, userInput, chatHistory)) {
      // Add custom formatting
      const formatted = {
        chunk,
        timestamp: Date.now(),
      };
      res.write(`data: ${JSON.stringify(formatted)}\n\n`);
    }
    res.write(`data: ${JSON.stringify({ done: true })}\n\n`);
    res.end();
  } catch (error) {
    // Handle errors
  }
}

Testing Changes

After modifying agent:

# Test locally
npm run dev

# Run tests
npm test

# Build to check for TypeScript errors
npm run build

Deploying Changes

See the deploy skill for complete deployment instructions.

Advanced Modifications

For advanced LangChain patterns (custom chains, stateful agents, RAG), see:

• LangChain.js Documentation
• LangGraph Documentation

Add RAG with Vector Search

Use DatabricksVectorSearch from @databricks/langchainjs. See LangChain Vector Store docs.

TypeScript Best Practices

Type Safety

Define interfaces for agent inputs/outputs:

interface AgentInput {
  messages: AgentMessage[];
  config?: AgentConfig;
}

interface AgentOutput {
  message: AgentMessage;
  intermediateSteps?: ToolStep[];
  metadata?: Record<string, any>;
}

Module Organization

Keep modules focused:

• src/agent.ts: Agent logic only
• src/tools.ts: Tool definitions only
• src/framework/server.ts: API routes only
• src/framework/tracing.ts: Tracing setup only

Async/Await

Always handle promises properly:

// Good
try {
  const result = await agent.invoke(input);
  return result;
} catch (error) {
  console.error("Agent error:", error);
  throw error;
}

// Bad
agent.invoke(input).then(result => {
  // ...
});

Debugging

Enable Debug Logging

The agent already includes comprehensive logging in src/agent.ts:

// Tool execution logging (already included)
console.log(`✅ Agent initialized with ${tools.length} tool(s)`);
console.log(`   Tools: ${tools.map((t) => t.name).join(", ")}`);

// Add more logging in streamEvents() method
if (event.event === "on_tool_start") {
  console.log(`[Tool] Calling ${event.name} with:`, event.data?.input);
}

Add Debug Logs

console.log("Agent input:", input);
console.log("Tool calls:", response.intermediateSteps);
console.log("Final output:", response.output);

Use TypeScript Compiler

Check for type errors:

npx tsc --noEmit

Related Skills

• quickstart: Initial setup
• run-locally: Local testing
• deploy: Deploy changes to Databricks