Claude Function Calling with API Gateway: Build a Secure Serverless LLM Endpoint

Claude Function Calling is a mechanism that allows large language models, like Claude, to generate JSON payloads describing desired function calls with associated arguments. When Claude wants to execute code, it sends a webhook request to an HTTP endpoint you control, which then processes the JSON payload into actionable work such as database operations.

A webhook acts as a doorbell notifying your system of an incoming request. The payload contains the name of the function and its arguments, while the Lambda URL is the direct HTTPS endpoint AWS creates for a Lambda function. Claude expects a specific response schema: { "status" : "success", "result" : { "userId" : 1234 } }. If the payload is malformed or the endpoint rejects the request, Claude falls back to a generic response, undermining the purpose of function calling.

While a Lambda URL might seem like a convenient solution, it lacks critical security and versioning features. In contrast, API Gateway + Lambda offers a production-ready, type-safe bridge for Claude's function calls. API Gateway provides authentication options like IAM and JWT authorizers, built-in throttling, comprehensive observability tools, versioning capabilities through stages (dev, prod), and automatic CORS configuration. It also supports retries and dead-letter queues.

A common issue arises when using non-proxy integration in API Gateway, where the request body is flattened, stripping any nested objects. This can lead to Claude's arguments being lost, making debugging challenging. Enabling Lambda proxy integration ensures the entire JSON structure, including nested objects, is passed to the Lambda function.

TypeScript developers can leverage type safety to validate Claude's payload at both runtime and compile-time. By defining Zod schemas for the expected Claude payload and full request body, you can enforce correct data shapes and catch errors early. The handler function parses and validates Claude's JSON payload, throwing an error if validation fails, and then proceeds to write the validated data to DynamoDB.