openEventStream

Opens an independent filtered SSE event stream. Each call creates a new server connection with the given filter. Returns undefined when the transport does not support per-subscription streams (e.g. WebSocket), in which case the caller should fall back to command-based subscriptions over events.

Replay contract. Implementations MUST buffer events emitted for the thread/run and replay them through every newly-opened stream whose filter matches. The SDK's shared-stream rotation relies on this: when a subscription's filter widens the union, the SDK opens a fresh stream and expects to receive the run's full history from seq=0 (deduplication is handled client-side via event_id). The SDK also defers the open until after run.start has committed the thread server-side to avoid a 404: Thread not found, which means events emitted during that window MUST be delivered to the late opener. The protocol v2 server implements this via a bounded per-run replay buffer; custom adapters should mirror that.