Move MCP client creation (clientFromConnection, createVirtualClientFrom, createModelProviderFromClient) from the synchronous request handler into the stream execute callback. This ensures the SSE response headers are flushed to the client immediately, before the potentially slow MCP connection setup begins.

Previously, the handler awaited all MCP client connections before returning createUIMessageStreamResponse, causing Cloudflare to timeout (524) when client initialization exceeded 100 seconds.

Agent-specific system instructions are now lazily enriched inside execute once the passthrough client is available, identified by the stable message id decopilot-system.

Made-with: Cursor

What is this contribution about?

Describe your changes and why they're needed.

Screenshots/Demonstration

Add screenshots or a Loom video if your changes affect the UI.

How to Test

Provide step-by-step instructions for reviewers to test your changes:

1. Step one
2. Step two
3. Expected outcome

Migration Notes

If this PR requires database migrations, configuration changes, or other setup steps, document them here. Remove this section if not applicable.

Review Checklist

• PR title is clear and descriptive
• Changes are tested and working
• Documentation is updated (if needed)
• No breaking changes

Summary by cubic

Prevents Cloudflare 524 timeouts on the Decopilot stream endpoint by moving MCP client setup into the SSE execute callback so headers flush immediately. Agent instructions are now injected lazily once the passthrough client is ready.

• Bug Fixes
  • Initialize model, passthrough, and strategy MCP clients inside execute to avoid blocking the response.
  • Preload a base system prompt and enrich it later via the decopilot-system message with agent instructions.
  • Keep tool wiring intact; gateway modes expose only strategy + built-in tools to the model.
  • Abort handling closes clients and fails the thread to prevent leaks.

^{Written for commit c84fcf2. Summary will update on new commits.}