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_responses_stream_error.go

@@ -0,0 +1,119 @@
+package handlers
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"strings"
+)
+
+type openAIResponsesStreamErrorChunk struct {
+	Type           string `json:"type"`
+	Code           string `json:"code"`
+	Message        string `json:"message"`
+	SequenceNumber int    `json:"sequence_number"`
+}
+
+func openAIResponsesStreamErrorCode(status int) string {
+	switch status {
+	case http.StatusUnauthorized:
+		return "invalid_api_key"
+	case http.StatusForbidden:
+		return "insufficient_quota"
+	case http.StatusTooManyRequests:
+		return "rate_limit_exceeded"
+	case http.StatusNotFound:
+		return "model_not_found"
+	case http.StatusRequestTimeout:
+		return "request_timeout"
+	default:
+		if status >= http.StatusInternalServerError {
+			return "internal_server_error"
+		}
+		if status >= http.StatusBadRequest {
+			return "invalid_request_error"
+		}
+		return "unknown_error"
+	}
+}
+
+// BuildOpenAIResponsesStreamErrorChunk builds an OpenAI Responses streaming error chunk.
+//
+// Important: OpenAI's HTTP error bodies are shaped like {"error":{...}}; those are valid for
+// non-streaming responses, but streaming clients validate SSE `data:` payloads against a union
+// of chunks that requires a top-level `type` field.
+func BuildOpenAIResponsesStreamErrorChunk(status int, errText string, sequenceNumber int) []byte {
+	if status <= 0 {
+		status = http.StatusInternalServerError
+	}
+	if sequenceNumber < 0 {
+		sequenceNumber = 0
+	}
+
+	message := strings.TrimSpace(errText)
+	if message == "" {
+		message = http.StatusText(status)
+	}
+
+	code := openAIResponsesStreamErrorCode(status)
+
+	trimmed := strings.TrimSpace(errText)
+	if trimmed != "" && json.Valid([]byte(trimmed)) {
+		var payload map[string]any
+		if err := json.Unmarshal([]byte(trimmed), &payload); err == nil {
+			if t, ok := payload["type"].(string); ok && strings.TrimSpace(t) == "error" {
+				if m, ok := payload["message"].(string); ok && strings.TrimSpace(m) != "" {
+					message = strings.TrimSpace(m)
+				}
+				if v, ok := payload["code"]; ok && v != nil {
+					if c, ok := v.(string); ok && strings.TrimSpace(c) != "" {
+						code = strings.TrimSpace(c)
+					} else {
+						code = strings.TrimSpace(fmt.Sprint(v))
+					}
+				}
+				if v, ok := payload["sequence_number"].(float64); ok && sequenceNumber == 0 {
+					sequenceNumber = int(v)
+				}
+			}
+			if e, ok := payload["error"].(map[string]any); ok {
+				if m, ok := e["message"].(string); ok && strings.TrimSpace(m) != "" {
+					message = strings.TrimSpace(m)
+				}
+				if v, ok := e["code"]; ok && v != nil {
+					if c, ok := v.(string); ok && strings.TrimSpace(c) != "" {
+						code = strings.TrimSpace(c)
+					} else {
+						code = strings.TrimSpace(fmt.Sprint(v))
+					}
+				}
+			}
+		}
+	}
+
+	if strings.TrimSpace(code) == "" {
+		code = "unknown_error"
+	}
+
+	data, err := json.Marshal(openAIResponsesStreamErrorChunk{
+		Type:           "error",
+		Code:           code,
+		Message:        message,
+		SequenceNumber: sequenceNumber,
+	})
+	if err == nil {
+		return data
+	}
+
+	// Extremely defensive fallback.
+	data, _ = json.Marshal(openAIResponsesStreamErrorChunk{
+		Type:           "error",
+		Code:           "internal_server_error",
+		Message:        message,
+		SequenceNumber: sequenceNumber,
+	})
+	if len(data) > 0 {
+		return data
+	}
+	return []byte(`{"type":"error","code":"internal_server_error","message":"internal error","sequence_number":0}`)
+}