# Streaming

How to receive an answer as a stream of normalized events and how stream failures are reported.

## Stream the answer

Streaming is an optional capability reached by type assertion:

```go
streamAgent, ok := agent.(llmrouter.StreamAgent)
if !ok {
	log.Fatalf("%s does not support streaming", agent.Name())
}

events, err := streamAgent.SendStream(ctx, messages, nil, llmrouter.ReasoningHigh, llmrouter.ModeDefault)
if err != nil {
	log.Fatal(err)
}

for evt := range events {
	switch evt.Type {
	case llmrouter.StreamEventText:
		fmt.Print(evt.TextDelta)
	case llmrouter.StreamEventUsage:
		fmt.Printf("\n[usage] in=%d out=%d\n", evt.Usage.Input, evt.Usage.Output)
	case llmrouter.StreamEventError:
		log.Fatal(evt.Err)
	}
}
```

## Event types

Chat Completions and Responses SSE both normalize onto the same event type, so callers never tell the two upstream formats apart.

```go
const (
	StreamEventText      StreamEventType = "text"
	StreamEventReasoning StreamEventType = "reasoning"
	StreamEventToolCall  StreamEventType = "tool_call"
	StreamEventUsage     StreamEventType = "usage"
	StreamEventDone      StreamEventType = "done"
	StreamEventError     StreamEventType = "error"
)
```

## Stream errors

When the upstream answers with non-SSE content, the failure is wrapped in a `*llmrouter.StreamError` carrying `Provider` / `Code` / `Body` and unwrapping to `llmrouter.ErrStreamUnsupported`, so `errors.As` for the status code and `errors.Is` for the capability gap both work. Stream bodies are capped at 64 MiB and frames quoted in errors at 512 bytes.

## Related

- [Stream Symbols](/api-reference-streaming): `StreamEvent` fields and SSE helpers
- [Timeouts and Limits](/configuration-limits): read caps