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Deploying an Agent to AWS Lambda

May 5, 2026, 02:00 Europe/Berlin

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We start from the FastAPI service we deployed to Railway in the previous workshop, strip out FastAPI, and swap it for a custom AWS Lambda runtime. The runtime handles both the static frontend and the streaming agent API. We deploy one container image as a Lambda Function URL with SSE streaming.

Most of the code-writing work is delegated to a coding agent (Codex). The exact prompts I used are quoted verbatim.

This was a freestyle session, so it also surfaces a fair amount of meta-discussion: how to work with agents, when to trust them, and when to slow down and read the code.

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The shift versus the previous workshop

The diagram below shows where Lambda fits in the request path:

flowchart LR UI["Frontend UI<br/>vanilla JS, SSE"] LAMBDA["Lambda Function URL"] RUNTIME["Custom Lambda runtime<br/>backend/lambda_runtime.py"] AGENT["Agent loop"] SEARCH["FAQ search tool<br/>minsearch"] OPENAI["OpenAI Responses API"] UI -->|GET / and assets| LAMBDA UI -->|POST /ask, /ask/stream| LAMBDA LAMBDA --> RUNTIME RUNTIME --> AGENT AGENT -->|tool call| SEARCH AGENT -->|model call| OPENAI RUNTIME -->|JSON or streamed SSE| LAMBDA

The agent loop, the search tool, the renderer abstraction, and the frontend are unchanged from the previous workshop.

What changes is the web layer and the deployment pipeline:

  • FastAPI is gone. A custom Lambda runtime (backend/lambda_runtime.py) handles routing, static file serving, and SSE streaming directly against the Lambda Runtime API.
  • The Dockerfile is rebased on public.ecr.aws/lambda/python:3.14 instead of python:3.14-slim.
  • Deployment is done with ./deploy.sh, which builds a container image, pushes it to ECR, and deploys a CloudFormation stack that creates the Lambda function and a Function URL with RESPONSE_STREAM invoke mode.
  • Railway and the GitHub Actions promotion workflow are gone.

The benefit of moving to Lambda is the same in plain words: with Railway or Render you pay for a server that has to be up all the time. With a Lambda Function URL you only pay per invocation, which fits tools and small agents that are used occasionally.

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