Claude's new Managed Agents API requires a different anthropic-beta header for streaming than for other calls. Use "anthropic-beta: managed-agents-2026-04-01" to create sessions and events, but "anthropic-beta: agent-api-2026-03-01" to stream results.
https://www.lotharschulz.info/2026/04/11/claude-managed-agents-quickstart-400-error/




