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+# Master Custom Tools: Build a Token Balance Checker
+
+**Author:** jadonamite
+**Topic:** Agent Extensibility & Viem
+**Level:** Intermediate
+**Prerequisites:** Node.js v18+, Previous AgentKit Setup
+
+Out of the box, AgentKit is powerful, but it doesn't know *everything*. To build a truly "degen" agent, you need to teach it new tricks.
+
+In this tutorial, we will build a **Custom Tool** that allows your agent to check the balance of *any* specific ERC-20 token contract (like a meme coin) that might not be in the default allowlist. We will use **Viem** to read directly from the blockchain and **LangChain** to wrap that logic into a tool your agent can understand.
+
+---
+
+## 1. Architecture
+
+* **The Logic:** A TypeScript function using `viem` to call `balanceOf` and `decimals` on a smart contract.
+* **The Wrapper:** The `@langchain/core/tools` SDK to define inputs (Zod schema) and outputs.
+* **The Integration:** Injecting this new tool into the `createReactAgent` loop.
+
+---
+
+## 2. Prerequisites
+
+You can continue from the previous `agent-cli` project. If starting fresh, ensure you have:
+
+* `npm install viem @langchain/core z.od`
+
+---
+
+## 3. Implementation
+
+### Step 1: Create the Tool File (`src/tools/token_balance.ts`)
+
+Create a new directory `src/tools` and a file `token_balance.ts`.
+
+This code does three things:
+
+1. Defines the input schema (what the AI needs to provide).
+2. Sets up a `viem` public client to read from Base Sepolia.
+3. Exports a `tool` object that the Agent can invoke.
+
+```typescript
+import { tool } from "@langchain/core/tools";
+import { createPublicClient, http, parseAbi, formatUnits } from "viem";
+import { baseSepolia } from "viem/chains";
+import { z } from "zod";
+
+// 1. Setup Viem Client (Read-Only)
+const publicClient = createPublicClient({
+  chain: baseSepolia,
+  transport: http(),
+});
+
+// 2. Define the exact ABI we need (ERC-20 standard)
+const ERC20_ABI = parseAbi([
+  "function balanceOf(address owner) view returns (uint256)",
+  "function decimals() view returns (uint8)",
+  "function symbol() view returns (string)"
+]);
+
+// 3. Define the Input Schema
+const BalanceInput = z.object({
+  tokenAddress: z.string().describe("The 0x contract address of the ERC-20 token"),
+  walletAddress: z.string().describe("The 0x wallet address to check the balance for"),
+});
+
+// 4. Create the Tool
+export const tokenBalanceTool = tool(
+  async ({ tokenAddress, walletAddress }) => {
+    try {
+      // Validate addresses (basic check)
+      if (!tokenAddress.startsWith("0x") || !walletAddress.startsWith("0x")) {
+        return "Error: Invalid address format. Addresses must start with 0x.";
+      }
+
+      const [balance, decimals, symbol] = await Promise.all([
+        publicClient.readContract({
+          address: tokenAddress as `0x${string}`,
+          abi: ERC20_ABI,
+          functionName: "balanceOf",
+          args: [walletAddress as `0x${string}`],
+        }),
+        publicClient.readContract({
+          address: tokenAddress as `0x${string}`,
+          abi: ERC20_ABI,
+          functionName: "decimals",
+        }),
+        publicClient.readContract({
+          address: tokenAddress as `0x${string}`,
+          abi: ERC20_ABI,
+          functionName: "symbol",
+        }),
+      ]);
+
+      const formatted = formatUnits(balance, decimals);
+      return `The balance of ${walletAddress} is ${formatted} ${symbol}`;
+      
+    } catch (error: any) {
+      return `Failed to fetch balance: ${error.message}`;
+    }
+  },
+  {
+    name: "get_custom_token_balance",
+    description: "Gets the balance of a specific ERC-20 token contract for a given wallet address.",
+    schema: BalanceInput,
+  }
+);
+
+```
+
+### Step 2: Register the Tool (`src/index.ts`)
+
+Now, we import this tool and add it to our agent's toolkit.
+
+Open your `src/index.ts` (from the previous tutorial) and modify the `tools` array.
+
+```typescript
+// ... existing imports
+import { tokenBalanceTool } from "./tools/token_balance"; // Import your new tool
+
+// ... inside your main() function ...
+
+  const agentKit = await AgentKit.from({
+    walletProvider,
+    actionProviders: [
+      wethActionProvider(),
+      walletActionProvider(),
+      erc20ActionProvider(),
+      cdpApiActionProvider({
+        apiKeyName: process.env.CDP_API_KEY_NAME,
+        apiKeyPrivateKey: process.env.CDP_API_KEY_PRIVATE_KEY?.replace(/\\n/g, "\n"),
+      }),
+    ],
+  });
+
+  // Combine AgentKit tools with your Custom Tool
+  const agentKitTools = await getLangChainTools(agentKit);
+  const tools = [...agentKitTools, tokenBalanceTool]; // <--- ADD IT HERE
+
+  const agent = createReactAgent({
+    llm,
+    tools, // The agent now has access to your custom tool
+    checkpointSaver: memory,
+    messageModifier: `
+      You are a helpful agent.
+      If asked to check a token balance, use the 'get_custom_token_balance' tool.
+      You can find token addresses using your search tools or ask the user for them.
+    `,
+  });
+
+// ... rest of the file
+
+```
+
+### Step 3: Test the Tool
+
+Run your agent:
+
+```bash
+npx tsx src/index.ts
+
+```
+
+**Conversation:**
+
+> **User:** "Check the balance of the USDC contract (0x036CbD53842c5426634e7929541eC2318f3dCF7e) for my wallet address."
+> **Agent:** *Thinking... Calls `get_custom_token_balance`...*
+> "The balance of 0x... is 100.5 USDC"
+
+---
+
+## 4. Common Pitfalls
+
+1. **Viem Type Errors:**
+* **Gotcha:** Viem is strict about `0x` prefixes.
+* **Fix:** Ensure you cast strings to ``0x${string}`` when passing them to `readContract`.
+
+
+2. **Schema Mismatches:**
+* **Gotcha:** The LLM might try to pass `token_address` (snake_case) instead of `tokenAddress` (camelCase).
+* **Fix:** Zod handles validation, but using clear descriptions in `z.string().describe(...)` helps the LLM get it right.
+
+
+3. **RPC Limits:**
+* **Gotcha:** Making too many `readContract` calls in parallel can hit rate limits on the public RPC.
+* **Fix:** Use `multicall` (Viem feature) if you plan to check 10+ tokens at once.
+
+
+
+---
+
+[Coinbase AgentKit: Build Your First AI Agent in 15 Minutes](https://www.google.com/search?q=https://www.youtube.com/watch%3Fv%3DkYv_6EaL4kY)
+
+This video is relevant because it provides a visual walkthrough of setting up AgentKit and introduces the concepts of tools and actions, which reinforces the custom tool implementation discussed in the tutorial.