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feat: support previews streaming in openAI API #2097

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951d248
feat: support previews streaming in openAI API
stduhpf Oct 3, 2026
2c4ef0a
fix: align streaming chunk event format to official API
stduhpf Oct 6, 2026
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174 changes: 172 additions & 2 deletions examples/server/routes_openai.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -23,13 +23,15 @@ static std::string extract_and_remove_sd_cpp_extra_args(std::string& text) {
static bool build_openai_generation_request(const httplib::Request& req,
ServerRuntime& runtime,
ImgGenJobRequest& request,
bool& stream,
std::string& error_message) {
if (req.body.empty()) {
error_message = "empty body";
return false;
}

json j = json::parse(req.body);
stream = j.value("stream", false);
std::string prompt = j.value("prompt", "");
int n = std::max(1, j.value("n", 1));
std::string size = j.value("size", "");
Expand Down Expand Up @@ -80,12 +82,20 @@ static bool build_openai_generation_request(const httplib::Request& req,
static bool build_openai_edit_request(const httplib::Request& req,
ServerRuntime& runtime,
ImgGenJobRequest& request,
bool& stream,
std::string& error_message) {
if (!req.is_multipart_form_data()) {
error_message = "Content-Type must be multipart/form-data";
return false;
}

stream = false;
if (req.form.has_field("stream")) {
std::string stream_val = req.form.get_field("stream");
std::transform(stream_val.begin(), stream_val.end(), stream_val.begin(), ::tolower);
stream = (stream_val == "true" || stream_val == "1");
}

std::string prompt = req.form.get_field("prompt");
if (prompt.empty()) {
error_message = "prompt required";
Expand Down Expand Up @@ -247,6 +257,137 @@ static bool execute_sync_img_gen_request(ServerRuntime& runtime,
return true;
}

struct PreviewStreamContext {
httplib::DataSink* sink = nullptr;
const ImgGenJobRequest* request = nullptr;
};

static void write_sse_event(httplib::DataSink& sink, const json& event) {
if (!sink.is_writable()) {
return;
}
std::string sse = "data: " + event.dump() + "\n\n";
sink.write(sse.data(), sse.size());
}

static void preview_callback(int step, int frame_count, sd_image_t* frames, bool is_noisy, void* data) {
if (!data || frame_count <= 0 || !frames || !frames[0].data) {
return;
}

auto* ctx = static_cast<PreviewStreamContext*>(data);
if (!ctx->sink || !ctx->sink->is_writable()) {
return; // Client disconnected
}

auto image_bytes = encode_image_to_vector(
EncodedImageFormat::JPEG,
frames[0].data,
frames[0].width,
frames[0].height,
frames[0].channel,
"",
80);

if (image_bytes.empty()) {
return;
}

json chunk;
chunk["type"] = "image_generation.partial_image";
chunk["b64_json"] = base64_encode(image_bytes);
chunk["created_at"] = static_cast<long long>(std::time(nullptr));
chunk["size"] = std::to_string(frames[0].width) + "x" + std::to_string(frames[0].height);
chunk["quality"] = "auto";
chunk["background"] = frames[0].channel != 4 ? "opaque" : "auto";
chunk["output_format"] = "jpeg";
chunk["partial_image_index"] = step;

write_sse_event(*ctx->sink, chunk);
}

static void execute_streaming_img_gen(ServerRuntime& runtime,
ImgGenJobRequest& request,
httplib::DataSink& sink) {
PreviewStreamContext preview_ctx{&sink, &request};
sd_img_gen_params_t img_gen_params = request.to_sd_img_gen_params_t();
SDImageVec results;
int num_results = 0;

{
std::lock_guard<std::mutex> lock(*runtime.sd_ctx_mutex);

// TODO: support other preview methods?
sd_set_preview_callback(preview_callback, PREVIEW_PROJ, 1, true, false, &preview_ctx);

try {
sd_image_t* raw_results = nullptr;
if (!generate_image(runtime.sd_ctx, &img_gen_params, &raw_results, &num_results)) {
raw_results = nullptr;
num_results = 0;
}
results.adopt(raw_results, num_results);
} catch (...) {
sd_set_preview_callback(nullptr, PREVIEW_NONE, 0, false, false, nullptr);
throw;
}

sd_set_preview_callback(nullptr, PREVIEW_NONE, 0, false, false, nullptr);
}

if (results.empty()) {
if (sink.is_writable()) {
json err;
err["type"] = "image_generation.error";
err["error"] = "generate_image returned no results";
write_sse_event(sink, err);
}
sink.done();
return;
}

int result_count = results.count();
int images_per_batch = request.gen_params.batch_count > 0
? std::max(1, result_count / request.gen_params.batch_count)
: 1;

for (int i = 0; i < result_count; ++i) {
if (results[i].data == nullptr)
continue;

std::string params = request.gen_params.embed_image_metadata
? get_image_params(*runtime.ctx_params, request.gen_params,
request.gen_params.seed + i / images_per_batch)
: "";
auto image_bytes = encode_image_to_vector(
request.output_format == "jpeg" ? EncodedImageFormat::JPEG
: request.output_format == "webp" ? EncodedImageFormat::WEBP
: EncodedImageFormat::PNG,
results[i].data, results[i].width, results[i].height,
results[i].channel, params, request.output_compression);
if (image_bytes.empty()) {
continue;
}

json chunk;
chunk["type"] = "image_generation.completed";
chunk["b64_json"] = base64_encode(image_bytes);
chunk["created_at"] = static_cast<long long>(std::time(nullptr));
chunk["size"] = std::to_string(results[i].width) + "x" + std::to_string(results[i].height);
chunk["quality"] = "auto";
chunk["background"] = results[i].channel == 4 ? "transparent" : "opaque";
chunk["output_format"] = request.output_format;
write_sse_event(sink, chunk);
}

if (sink.is_writable()) {
static const std::string done_msg = "data: [DONE]\n\n";
sink.write(done_msg.data(), done_msg.size());
}

sink.done();
}

void register_openai_api_endpoints(httplib::Server& svr, ServerRuntime& rt) {
ServerRuntime* runtime = &rt;

Expand All @@ -267,14 +408,27 @@ void register_openai_api_endpoints(httplib::Server& svr, ServerRuntime& rt) {

ImgGenJobRequest request;
std::string error_message;
if (!build_openai_generation_request(req, *runtime, request, error_message)) {
bool stream = false;
if (!build_openai_generation_request(req, *runtime, request, stream, error_message)) {
res.status = 400;
res.set_content(json({{"error", error_message}}).dump(), "application/json");
return;
}

LOG_VERBOSE("%s\n", request.gen_params.to_string().c_str());

if (stream) {
res.set_header("Cache-Control", "no-cache");
res.set_header("Connection", "keep-alive");
res.set_chunked_content_provider(
"text/event-stream",
[runtime, request](size_t /*offset*/, httplib::DataSink& sink) mutable {
execute_streaming_img_gen(*runtime, request, sink);
return false;
});
return;
}

SDImageVec results;
if (!execute_sync_img_gen_request(*runtime, request, results, error_message)) {
res.status = 500;
Expand Down Expand Up @@ -341,14 +495,30 @@ void register_openai_api_endpoints(httplib::Server& svr, ServerRuntime& rt) {

ImgGenJobRequest request;
std::string error_message;
if (!build_openai_edit_request(req, *runtime, request, error_message)) {
bool stream = false;
if (!build_openai_edit_request(req, *runtime, request, stream, error_message)) {
res.status = 400;
res.set_content(json({{"error", error_message}}).dump(), "application/json");
return;
}

LOG_VERBOSE("%s\n", request.gen_params.to_string().c_str());

if (stream) {
res.set_header("Cache-Control", "no-cache");
res.set_header("Connection", "keep-alive");

auto req_ptr = std::make_shared<ImgGenJobRequest>(std::move(request));
res.set_chunked_content_provider(
"text/event-stream",
[runtime, req_ptr](size_t /*offset*/, httplib::DataSink& sink) {
execute_streaming_img_gen(*runtime, *req_ptr, sink);
return false;
});
return;
}

// Synchronous path
SDImageVec results;
if (!execute_sync_img_gen_request(*runtime, request, results, error_message)) {
res.status = 500;
Expand Down
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