fix(openai): emit valid responses stream error chunks

When /v1/responses streaming fails after headers are sent, we now emit a type=error chunk instead of an HTTP-style {error:{...}} payload, preventing AI SDK chunk validation errors.

sdk/api/handlers/openai/openai_responses_handlers.go

@@ -265,8 +265,8 @@ func (h *OpenAIResponsesAPIHandler) forwardResponsesStream(c *gin.Context, flush
 			if errMsg.Error != nil && errMsg.Error.Error() != "" {
 				errText = errMsg.Error.Error()
 			}
-			body := handlers.BuildErrorResponseBody(status, errText)
-			_, _ = fmt.Fprintf(c.Writer, "\nevent: error\ndata: %s\n\n", string(body))
+			chunk := handlers.BuildOpenAIResponsesStreamErrorChunk(status, errText, 0)
+			_, _ = fmt.Fprintf(c.Writer, "\nevent: error\ndata: %s\n\n", string(chunk))
 		},
 		WriteDone: func() {
 			_, _ = c.Writer.Write([]byte("\n"))

sdk/api/handlers/openai/openai_responses_handlers_stream_error_test.go

@@ -0,0 +1,43 @@
+package openai
+
+import (
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/interfaces"
+	"github.com/router-for-me/CLIProxyAPI/v6/sdk/api/handlers"
+	sdkconfig "github.com/router-for-me/CLIProxyAPI/v6/sdk/config"
+)
+
+func TestForwardResponsesStreamTerminalErrorUsesResponsesErrorChunk(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	base := handlers.NewBaseAPIHandlers(&sdkconfig.SDKConfig{}, nil)
+	h := NewOpenAIResponsesAPIHandler(base)
+
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", nil)
+
+	flusher, ok := c.Writer.(http.Flusher)
+	if !ok {
+		t.Fatalf("expected gin writer to implement http.Flusher")
+	}
+
+	data := make(chan []byte)
+	errs := make(chan *interfaces.ErrorMessage, 1)
+	errs <- &interfaces.ErrorMessage{StatusCode: http.StatusInternalServerError, Error: errors.New("unexpected EOF")}
+	close(errs)
+
+	h.forwardResponsesStream(c, flusher, func(error) {}, data, errs)
+	body := recorder.Body.String()
+	if !strings.Contains(body, `"type":"error"`) {
+		t.Fatalf("expected responses error chunk, got: %q", body)
+	}
+	if strings.Contains(body, `"error":{`) {
+		t.Fatalf("expected streaming error chunk (top-level type), got HTTP error body: %q", body)
+	}
+}