feat(langchain): add X402SearchTool for API discovery

[x402-index]

Summary

Adds X402SearchTool — a native LangChain tool that gives xpaysh agents natural language search across 14,000+ indexed API services.

Why

This kit already enables agents to pay for APIs autonomously. This tool adds the missing piece: finding which API to call in the first place.

The full autonomous loop: discover -> pay -> call. No human in the loop at any step.

What

• New X402SearchTool in packages/langchain/src/tools/x402search.ts
• Subclasses existing X402Tool — zero new dependencies, same payment pattern
• Exported from packages/langchain/src/tools.ts
• Cost: 0.01 USDC per query on Base mainnet, paid automatically

Usage

import { X402SearchTool } from '@xpaysh/agent-kit-langchain';

const searchTool = new X402SearchTool({
privateKey: process.env.PRIVATE_KEY,
spendingLimits: { perCallLimit: 0.01 }
});

Best queries

• crypto: 112 results
• token price: 88 results
• crypto market data: 10 results
• btc price: 8 results

Links

• Live API: https://x402search.xyz
• Related: feat(python): add x402search ActionProvider coinbase/agentkit#1029

[x402-index]

Updates in this revision

Fixed three issues in X402SearchTool:

1. Wrong base class — changed from Tool to StructuredTool. The original implementation caused a type mismatch: LangChain's Tool expects a plain string input, but the tool needed structured { query: string } input. StructuredTool is the correct base class for this.

2. Query params never sent — X402AgentClient.makePayedCall() has no params field in ApiCallOptions. For GET requests, data is set to undefined. The original defaultParams: { limit: 5 } were silently dropped and ?q=... never reached the server. Fixed by building the full URL with encodeURIComponent(query) appended directly.

3. Wrong chainId — X402AgentClient defaults to 84532 (Base Sepolia). x402search.xyz operates on Base mainnet (8453). Payment attempts would fail silently on the wrong chain. Fixed by hardcoding chainId: 8453 in the constructor.

All three issues would have caused silent failures — the tool would have been importable but non-functional. The fix is contained to packages/langchain/src/tools/x402search.ts only.

[x402-index]

Hi — wanted to make the review as easy as possible:

What this does: Adds X402SearchTool as a native LangChain tool so agents can discover x402-payable APIs by capability in a single call. No new dependencies. No breaking changes. Follows the existing X402Tool pattern exactly.

Two bugs fixed in this revision:

1. Query params now correctly appended to the URL — original defaultParams were silently dropped on GET requests because makePayedCall sets data: undefined for GET, so ?q=... never reached the server
2. chainId hardcoded to 8453 (Base mainnet) — original defaulted to 84532 (Base Sepolia), causing silent payment failures

Working example:

import { X402SearchTool } from '@xpaysh/agent-kit-langchain';

const tool = new X402SearchTool({
  privateKey: process.env.WALLET_PRIVATE_KEY,
  chainId: 8453,
});
// Tool name: search_apis
// Cost: $0.01 USDC per query via x402 on Base mainnet — paid automatically
// Returns ranked list of x402-payable endpoints with name, description, url

Happy to address any feedback before review.