Streaming codecs separate raw framing from semantic chunk decoding. The SSE framer produces stream.StreamFrame; a dialect decoder maps each frame to content.Chunk values, and the shared adapter accumulates tool arguments and terminal usage.

Framing

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flowchart TD
    B["HTTP response body"] --> F["wire/sse framer"]
    F --> E["dialect event decoder"]
    E --> C["TextChunk / ThinkingChunk / ToolUseChunk / RefusalChunk / ImageChunk"]
    C --> A["FramesToChunksWithResult"]
    A --> R["StreamReader.Result after EOF"]

OpenAI accepts either end-of-generation signal: the [DONE] terminal SSE data payload or a reported finish reason. Anthropic authorizes a terminal result on message_stop. Responses uses typed response.completed, and Gemini authorizes on a candidate that reports a finishReason. Bedrock uses event-stream frames and a metadata/message-stop sequence. A stream that ends before its dialect’s terminal marker is a typed StreamDecodeError, not a clean end. Uninteresting or unknown-but-well-formed events are skipped, while a frame whose JSON does not parse aborts the stream with a typed StreamEventDecodeError; typed stream and provider errors still fail the reader.

Accumulation

ToolUseChunk.Index is the join key. OpenAI and Anthropic send argument fragments, so the accumulator concatenates fragments by index. Gemini emits a complete function call per part, and the stream-scoped collector rebases that event’s positional index onto a stream-wide sequence. Codec.DecodeEvent is stateless; the rebasing state belongs to the stream collector.

reader, err := client.Stream(ctx, req)
if err != nil {
	return err
}
defer reader.Close()
for {
	chunk, err := reader.Next()
	if errors.Is(err, io.EOF) {
		break
	}
	if err != nil {
		return err
	}
	fmt.Printf("%T\n", chunk)
}
result, ok := reader.Result()
if ok {
	fmt.Println(result.FinishReason)
}

Source and proof

Run go test ./codec/... ./stream/....