Server-Sent Events

Server-Sent Events (SSE) defines a text event format delivered through a long-lived HTTP response. The client initiates a request, and the server responds with the text/event-stream media type, keeping the response open while sending events. In browsers, the EventSource API parses this format, dispatches events, and normally reconnects after an interrupted connection. SSE is unidirectional within that response: sending another prompt, canceling a job, or submitting user feedback requires a separate request or another communication mechanism.

Events use UTF-8 text with fields such as data, event, id, and retry. A blank line terminates an event, and multiple data lines within one event are joined with newline characters. For example, an application can send an event field of token and a data field containing a JSON object with a generated text fragment. The client must parse complete event boundaries rather than assume that each network read contains exactly one event. Transport chunks can split an event or contain several events.

For real-time AI interactions, an application backend invokes a foundation model through a streaming API and translates the resulting model events into browser-facing SSE events. With Amazon Bedrock, ConverseStream or InvokeModelWithResponseStream can provide incremental output where supported by the selected model and API. Bedrock's event stream is not itself a browser-compatible SSE response; the backend must decode the SDK events, extract relevant text or metadata, and encode SSE messages. A practical assistant can emit text updates, usage metadata, an application-defined completion event, and structured errors.

The entire delivery path must support streaming. A backend that flushes each fragment still appears buffered if a gateway, reverse proxy, compression layer, or application framework waits for the complete response. Configure supported response-streaming integrations explicitly, disable inappropriate caching and buffering, and test the public endpoint rather than only the application process. For API Gateway, use a REST API integration that supports response streaming and configure its response transfer mode accordingly; do not assume every API type or integration streams responses.

SSE and HTTP chunked transfer encoding are different concepts. SSE defines application-level event framing, whereas chunked transfer encoding frames an HTTP/1.1 message body. An SSE response can use HTTP/1.1 chunked transfer encoding, but HTTP/2 carries streaming data in its own frames without that encoding. Merely enabling chunked transfer does not create SSE events or guarantee immediate delivery. WebSockets, by contrast, provide bidirectional messaging and are often preferable for continuous two-way interaction, such as interactive audio.

Operationally, use periodic comment lines as heartbeats to help prevent idle connections from being closed, while respecting the shortest timeout anywhere in the path. Apply connection limits, bounded buffering, and slow-client handling. Propagate cancellation upstream when possible so disconnected users do not leave unnecessary model generation running. Measure time to first useful event separately from total generation latency: streaming can improve perceived responsiveness without reducing model execution time.

Automatic reconnection does not guarantee reliable replay. Event IDs and the Last-Event-ID request header support resumption only when the server retains events and implements replay logic; clients may also need deduplication. Native browser EventSource uses GET and does not expose arbitrary request-header configuration. Applications needing POST bodies or bearer authorization headers can use fetch with a streaming response and an SSE parser, but must implement reconnection themselves. Authenticate connections, enforce authorization for each stream, configure CORS appropriately, and avoid sensitive credentials in URLs. Once response headers have been sent, communicate later failures through application-defined events or connection termination rather than attempting to change the HTTP status.