01 Hello World Chat Panel And Echo Backend

01 — Hello-world: chat panel → backend → echo

Type: HITL Label: ready-for-agent

What to build

The minimum end-to-end vertical slice that proves the deployment topology and auth wiring work. A logged-in user clicks a chat button in the SPA, types a message, and receives a hardcoded reply from the [[Chat Service]]. No Bedrock calls yet; no tools yet; no streaming yet. The point is to land the infra and the auth path.

Stand up:

  • A new [[Chat Service]] Lambda behind the existing router service so it inherits route-based authn/authz. The Lambda accepts a request body of { transcript, userMessage } and returns { assistantText: "<echo of input>" }.
  • A CDK app provisioning the Lambda, its Function URL (configured for response streaming, even though we are not streaming yet), CloudFront in front of the Function URL, and the IAM execution role.
  • A chat UI surface in the SPA: a floating button to open a panel, a message list, an input box, a send button. Plain styling is fine; this is plumbing.

The browser sends { transcript: [...prior turns], userMessage: "..." } and appends the assistant reply to its own in-memory transcript. No persistence.

This slice is marked HITL because the first infra deploy and the IAM / CloudFront / Function URL wiring deserve human review before merge.

Acceptance criteria

  • Logged-in user can open the chat panel from anywhere in the SPA.
  • User can type and send a message; the assistant’s hardcoded reply appears in the message list.
  • Multi-turn works: previous messages remain visible and the transcript is sent with each request.
  • Anonymous (logged-out) request to the chat endpoint is rejected by the router service.
  • CDK app deploys cleanly to a non-prod environment.
  • No access token, message content, or response content is logged to CloudWatch.

Blocked by

None — can start immediately.