Add fail-fast on init for missing provider credentials

internal/server/server.go

@@ -239,17 +239,17 @@ func (s *GatewayServer) handleSyncResponse(w http.ResponseWriter, r *http.Reques
 }
 
 func (s *GatewayServer) handleStreamingResponse(w http.ResponseWriter, r *http.Request, provider providers.Provider, providerMsgs []api.Message, resolvedReq *api.ResponseRequest, origReq *api.ResponseRequest, storeMsgs []api.Message) {
-	w.Header().Set("Content-Type", "text/event-stream")
-	w.Header().Set("Cache-Control", "no-cache")
-	w.Header().Set("Connection", "keep-alive")
-	w.WriteHeader(http.StatusOK)
-
 	flusher, ok := w.(http.Flusher)
 	if !ok {
 		http.Error(w, "streaming not supported", http.StatusInternalServerError)
 		return
 	}
 
+	w.Header().Set("Content-Type", "text/event-stream")
+	w.Header().Set("Cache-Control", "no-cache")
+	w.Header().Set("Connection", "keep-alive")
+	w.WriteHeader(http.StatusOK)
+
 	responseID := generateID("resp_")
 	itemID := generateID("msg_")
 	seq := 0

internal/server/streaming_writer_test.go

@@ -0,0 +1,53 @@
+package server
+
+import (
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type nonFlusherRecorder struct {
+	recorder         *httptest.ResponseRecorder
+	writeHeaderCalls int
+}
+
+func newNonFlusherRecorder() *nonFlusherRecorder {
+	return &nonFlusherRecorder{recorder: httptest.NewRecorder()}
+}
+
+func (w *nonFlusherRecorder) Header() http.Header {
+	return w.recorder.Header()
+}
+
+func (w *nonFlusherRecorder) Write(b []byte) (int, error) {
+	return w.recorder.Write(b)
+}
+
+func (w *nonFlusherRecorder) WriteHeader(statusCode int) {
+	w.writeHeaderCalls++
+	w.recorder.WriteHeader(statusCode)
+}
+
+func (w *nonFlusherRecorder) StatusCode() int {
+	return w.recorder.Code
+}
+
+func (w *nonFlusherRecorder) BodyString() string {
+	return w.recorder.Body.String()
+}
+
+func TestHandleStreamingResponseWithoutFlusherWritesSingleErrorHeader(t *testing.T) {
+	s := New(nil, nil, slog.New(slog.NewTextHandler(io.Discard, nil)))
+	req := httptest.NewRequest(http.MethodPost, "/v1/responses", nil)
+	w := newNonFlusherRecorder()
+
+	s.handleStreamingResponse(w, req, nil, nil, nil, nil, nil)
+
+	assert.Equal(t, 1, w.writeHeaderCalls)
+	assert.Equal(t, http.StatusInternalServerError, w.StatusCode())
+	assert.Contains(t, w.BodyString(), "streaming not supported")
+}