The Sonnet 5 launch grabbed all the headlines — but in the same breath, Anthropic expanded the developer platform. If you build agents on Claude, Managed Agents got a handful of changes that matter more day-to-day than they might sound at first.
What’s new
Event deltas: Session event streams now support deltas. With the event_deltas[] query parameter, the event_start and event_delta events give you a preview of an agent message’s text as it’s generated — before the full agent.message event arrives. For live UIs, that’s a noticeable difference.
Backward pagination: Listing sessions now returns a prev_page cursor alongside next_page. Small, but anyone who’s navigated long session lists knows why it was missing.
Session overrides: When creating a session, you can now override the agent’s configuration for that specific session. With type: 'agent_with_overrides', you swap out the model, system prompt, tools, MCP servers, or skills — while the agent itself stays unchanged. That makes experimentation much easier.
Finer credential control: Vaults now have an injection_location setting for environment-variable credentials. It controls whether the value is injected, at egress, into the outbound request’s headers, body, or both.
Webhooks across the lifecycle: Webhooks now cover the agent, deployment, and deployment run. You can react to a newly published agent version, a paused deployment, or a failed scheduled run — without polling.
My take
This isn’t a glamorous update, and that’s exactly why I find it interesting. Model launches are the fireworks — but you build on the API, and these details decide whether building agents feels smooth or like constantly fighting the platform.
Session overrides and event deltas are the highlights for me. Overrides, because you can finally test variants without spinning up a new agent for every attempt. Event deltas, because they bring the streaming experience up to what you’d expect from a modern chat interface. Taken together, it’s proof that Anthropic treats the platform not just as a model showcase, but as a real foundation for agent software.
Sources: Claude Developer Platform Release Notes · Releasebot: Anthropic Updates