AGUI threadId is regenerated on every message, breaking session persistence

[robechon]

Description

When running agentcore dev in browser mode (default, without -b), the HTTP proxy
layer generates a new threadId for every AGUI request. This prevents agents from
maintaining multi-turn session state (e.g., via AgentCore Memory STM).

The root cause is in src/cli/operations/dev/web-ui/handlers/invocations.ts,
function handleAguiInvocation:

  const aguiBody = JSON.stringify({
      threadId: randomUUID(),  // ← new UUID on every message
      runId: randomUUID(),
      ...
  });

The sessionId parameter is already available in scope, stable per chat session,
and even returned as an x-session-id response header — but it's never used as the
threadId.

Suggested fix:

  - threadId: randomUUID(),
  + threadId: sessionId ?? randomUUID()

Steps to Reproduce

1. Create an AGUI agent with "protocol": "AGUI" in agentcore.json
2. Run agentcore dev (browser mode, no -b flag)
3. Send two or more messages in the chat UI
4. Log input_data.thread_id in the agent's request handler

Expected Behavior

The threadId remains stable across messages within the same conversation, allowing
agents to maintain session state.

Actual Behavior

A new random threadId is sent with every message:

  [2026-07-01T10:32:56] thread_id=d5fa1ec3-87c9-4ca3-84ab-498c15608fbe
  [2026-07-01T10:33:05] thread_id=a4fe3ccf-c0cb-4954-9a03-2344efd277ee

Note: TUI mode (agentcore dev -b) does not have this issue — it uses
aguiThreadIdRef in useDevServer.ts which correctly maintains a stable threadId.
The bug is specific to the browser mode HTTP proxy.

CLI Version

0.21.1

Operating System

Linux

Additional Context

No response