diff --git a/backend/internal/service/openai_gateway_cc_pipeline.go b/backend/internal/service/openai_gateway_cc_pipeline.go
index 0fe504a6f8..4d65374a45 100644
--- a/backend/internal/service/openai_gateway_cc_pipeline.go
+++ b/backend/internal/service/openai_gateway_cc_pipeline.go
@@ -67,13 +67,26 @@ func (s *OpenAIGatewayService) newStreamHeaderWriter(c *gin.Context, upstream ht
 // （下游 handleXxxErrorResponse 需要再次读取），返回原始错误体与脱敏后的
 // 上游错误消息。
 func (s *OpenAIGatewayService) readOpenAIUpstreamError(resp *http.Response) ([]byte, string) {
-	respBody := s.readUpstreamErrorBody(resp)
+	respBody, upstreamMsg, _ := s.readOpenAIUpstreamErrorWithCompleteness(resp)
+	return respBody, upstreamMsg
+}
+
+func (s *OpenAIGatewayService) readOpenAIUpstreamErrorWithCompleteness(resp *http.Response) ([]byte, string, bool) {
+	limit := openAIUpstreamErrorBodyReadLimitForConfig(nil)
+	if s != nil {
+		limit = openAIUpstreamErrorBodyReadLimitForConfig(s.cfg)
+	}
+	respBody, readErr := io.ReadAll(io.LimitReader(resp.Body, limit+1))
+	complete := readErr == nil && int64(len(respBody)) <= limit
+	if int64(len(respBody)) > limit {
+		respBody = respBody[:limit]
+	}
 	_ = resp.Body.Close()
 	resp.Body = io.NopCloser(bytes.NewReader(respBody))
 
 	upstreamMsg := strings.TrimSpace(extractUpstreamErrorMessage(respBody))
 	upstreamMsg = sanitizeUpstreamErrorMessage(upstreamMsg)
-	return respBody, upstreamMsg
+	return respBody, upstreamMsg, complete
 }
 
 // failoverOpenAIUpstreamHTTPError 对 >=400 的上游响应做 failover 判定：命中时
diff --git a/backend/internal/service/openai_gateway_chat_completions.go b/backend/internal/service/openai_gateway_chat_completions.go
index 1d64769e58..b7341ccda3 100644
--- a/backend/internal/service/openai_gateway_chat_completions.go
+++ b/backend/internal/service/openai_gateway_chat_completions.go
@@ -7,6 +7,8 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"regexp"
+	"strconv"
 	"strings"
 	"sync/atomic"
 	"time"
@@ -370,7 +372,7 @@ func (s *OpenAIGatewayService) forwardAsChatCompletions(
 
 	// 8. Handle error response with failover
 	if resp.StatusCode >= 400 {
-		respBody, upstreamMsg := s.readOpenAIUpstreamError(resp)
+		respBody, upstreamMsg, completeErrorBody := s.readOpenAIUpstreamErrorWithCompleteness(resp)
 		if !agentIdentityTaskRecoveryWasTried(ctx) && s.isAgentIdentityAccount(ctx, account) && isAgentIdentityTaskInvalidHTTPResponse(resp.StatusCode, respBody) {
 			expectedTaskID := account.GetCredential("task_id")
 			if err := s.recoverAgentIdentityTask(ctx, account, expectedTaskID); err != nil {
@@ -387,7 +389,24 @@ func (s *OpenAIGatewayService) forwardAsChatCompletions(
 				zap.Int("upstream_status", resp.StatusCode),
 				zap.String("upstream_message", upstreamMsg),
 			)
-			return s.forwardAsRawChatCompletions(ctx, c, account, body, defaultMappedModel)
+			return s.forwardAsRawChatCompletionsWithStartTime(ctx, c, account, body, defaultMappedModel, startTime)
+		}
+		responsesMode, _ := account.Extra[openai_compat.ExtraKeyResponsesMode].(string)
+		if account.IsOpenAIApiKey() &&
+			openai_compat.NormalizeResponsesSupportMode(responsesMode) == openai_compat.ResponsesSupportModeAuto &&
+			openai_compat.ResolveResponsesSupport(account.Extra) == openai_compat.ResponsesSupportYes &&
+			!isResponsesShape &&
+			(ctx == nil || ctx.Err() == nil) &&
+			(c.Request == nil || c.Request.Context().Err() == nil) &&
+			!IsResponseCommitted(c) &&
+			!c.Writer.Written() &&
+			completeErrorBody &&
+			isConvertedResponsesInputStringRejection(resp.StatusCode, respBody) {
+			logger.L().Info("openai chat_completions: structured Responses input rejected, falling back to raw chat completions",
+				zap.Int64("account_id", account.ID),
+				zap.Int("upstream_status", resp.StatusCode),
+			)
+			return s.forwardAsRawChatCompletionsWithStartTime(ctx, c, account, body, defaultMappedModel, startTime)
 		}
 		if foErr := s.failoverOpenAIUpstreamHTTPError(ctx, c, account, resp, respBody, upstreamMsg, upstreamModel); foErr != nil {
 			return nil, foErr
@@ -441,6 +460,155 @@ func (s *OpenAIGatewayService) forwardAsChatCompletions(
 	return result, handleErr
 }
 
+var responsesInputStringValidationError = regexp.MustCompile(`^['"]loc['"]\s*:\s*\(\s*['"]body['"]\s*,\s*['"]input['"]\s*,\s*['"]str['"]\s*\)$`)
+var responsesStringValidationMessage = regexp.MustCompile(`^['"]msg['"]\s*:\s*['"]Input should be a valid string['"]$`)
+var responsesStringValidationType = regexp.MustCompile(`^['"]type['"]\s*:\s*['"]string_type['"]$`)
+var responsesValidationErrors = regexp.MustCompile(`(?s)^([1-9][0-9]*) validation errors?(?: for [^\r\n:]+)?:\s*(\[.*\])$`)
+var responsesInvalidInputTypeMessage = regexp.MustCompile(`(?i)^(?:Invalid type for ['"]input['"]:\s*)?Expected a string(?: for input)?, but got an (?:array|object) instead\.?$`)
+
+func isResponsesInputStringValidationMessage(message string) bool {
+	match := responsesValidationErrors.FindStringSubmatch(strings.TrimSpace(message))
+	if match == nil {
+		return false
+	}
+	entries, ok := splitResponsesValidationFields(match[2][1 : len(match[2])-1])
+	count, err := strconv.Atoi(match[1])
+	if !ok || err != nil || count != len(entries) {
+		return false
+	}
+	matched := false
+	for _, entry := range entries {
+		if !strings.HasPrefix(entry, "{") || !strings.HasSuffix(entry, "}") {
+			return false
+		}
+		fields, ok := splitResponsesValidationFields(entry[1 : len(entry)-1])
+		if !ok {
+			return false
+		}
+		location, message, stringType := false, false, false
+		keys := make(map[string]bool, len(fields))
+		for _, field := range fields {
+			key, value, found := strings.Cut(field, ":")
+			key = strings.TrimSpace(key)
+			if !found || len(key) < 3 || (key[0] != '\'' && key[0] != '"') || key[len(key)-1] != key[0] || strings.TrimSpace(value) == "" || keys[key[1:len(key)-1]] {
+				return false
+			}
+			keys[key[1:len(key)-1]] = true
+			location = location || responsesInputStringValidationError.MatchString(field)
+			message = message || responsesStringValidationMessage.MatchString(field)
+			stringType = stringType || responsesStringValidationType.MatchString(field)
+		}
+		matched = matched || (location && message && stringType)
+	}
+	return matched
+}
+
+func splitResponsesValidationFields(value string) ([]string, bool) {
+	var fields []string
+	var brackets []byte
+	var quote byte
+	escaped := false
+	start := 0
+	for index := 0; index < len(value); index++ {
+		character := value[index]
+		if quote != 0 {
+			if escaped {
+				escaped = false
+			} else if character == '\\' {
+				escaped = true
+			} else if character == quote {
+				quote = 0
+			}
+			continue
+		}
+		switch character {
+		case '\'', '"':
+			quote = character
+		case '{', '[', '(':
+			brackets = append(brackets, character)
+		case '}', ']', ')':
+			if len(brackets) == 0 {
+				return nil, false
+			}
+			opening := brackets[len(brackets)-1]
+			if (character == '}' && opening != '{') || (character == ']' && opening != '[') || (character == ')' && opening != '(') {
+				return nil, false
+			}
+			brackets = brackets[:len(brackets)-1]
+		case ',':
+			if len(brackets) == 0 {
+				field := strings.TrimSpace(value[start:index])
+				if field == "" {
+					return nil, false
+				}
+				fields = append(fields, field)
+				start = index + 1
+			}
+		}
+	}
+	if quote != 0 || len(brackets) != 0 {
+		return nil, false
+	}
+	if field := strings.TrimSpace(value[start:]); field != "" {
+		fields = append(fields, field)
+	}
+	return fields, len(fields) > 0
+}
+
+func isConvertedResponsesInputStringRejection(status int, responseBody []byte) bool {
+	if status != http.StatusBadRequest || !json.Valid(responseBody) {
+		return false
+	}
+	envelope := gjson.ParseBytes(responseBody)
+	for _, object := range []gjson.Result{envelope, envelope.Get("error")} {
+		if !object.IsObject() {
+			return false
+		}
+		keys := make(map[string]bool)
+		unambiguous := true
+		object.ForEach(func(key, value gjson.Result) bool {
+			name := key.String()
+			if keys[name] {
+				unambiguous = false
+				return false
+			}
+			keys[name] = true
+			switch name {
+			case "usage", "output", "output_text", "choices", "tool_calls", "response", "status", "incomplete_details":
+				unambiguous = false
+			case "code":
+				code := strings.ToLower(strings.TrimSpace(value.String()))
+				unambiguous = code == "" || code == "invalid_type" || code == "invalid_request_error"
+			case "param":
+				param := strings.ToLower(strings.TrimSpace(value.String()))
+				unambiguous = param == "" || param == "input"
+			case "type":
+				errorType := strings.ToLower(strings.TrimSpace(value.String()))
+				unambiguous = errorType == "" || errorType == "invalid_type" || errorType == "invalid_request_error"
+			}
+			return unambiguous
+		})
+		if !unambiguous {
+			return false
+		}
+	}
+	message := gjson.GetBytes(responseBody, "error.message").String()
+	if isResponsesInputStringValidationMessage(message) {
+		return true
+	}
+	code := strings.ToLower(strings.TrimSpace(gjson.GetBytes(responseBody, "error.code").String()))
+	typ := strings.ToLower(strings.TrimSpace(gjson.GetBytes(responseBody, "error.type").String()))
+	if code != "invalid_type" && typ != "invalid_type" {
+		return false
+	}
+	param := strings.ToLower(strings.TrimSpace(gjson.GetBytes(responseBody, "error.param").String()))
+	if param != "input" {
+		return false
+	}
+	message = strings.ToLower(strings.TrimSpace(message))
+	return responsesInvalidInputTypeMessage.MatchString(message)
+}
+
 func normalizeResponsesRequestServiceTier(req *apicompat.ResponsesRequest) {
 	if req == nil {
 		return
diff --git a/backend/internal/service/openai_gateway_chat_completions_raw.go b/backend/internal/service/openai_gateway_chat_completions_raw.go
index 25005986a5..98fa8c4a0d 100644
--- a/backend/internal/service/openai_gateway_chat_completions_raw.go
+++ b/backend/internal/service/openai_gateway_chat_completions_raw.go
@@ -61,7 +61,17 @@ func (s *OpenAIGatewayService) forwardAsRawChatCompletions(
 	body []byte,
 	defaultMappedModel string,
 ) (*OpenAIForwardResult, error) {
-	startTime := time.Now()
+	return s.forwardAsRawChatCompletionsWithStartTime(ctx, c, account, body, defaultMappedModel, time.Now())
+}
+
+func (s *OpenAIGatewayService) forwardAsRawChatCompletionsWithStartTime(
+	ctx context.Context,
+	c *gin.Context,
+	account *Account,
+	body []byte,
+	defaultMappedModel string,
+	startTime time.Time,
+) (*OpenAIForwardResult, error) {
 
 	// 1. Parse minimal fields needed for routing/billing
 	originalModel := gjson.GetBytes(body, "model").String()
diff --git a/backend/internal/service/openai_gateway_chat_completions_test.go b/backend/internal/service/openai_gateway_chat_completions_test.go
index 5ff9a55e6b..723f60ebcf 100644
--- a/backend/internal/service/openai_gateway_chat_completions_test.go
+++ b/backend/internal/service/openai_gateway_chat_completions_test.go
@@ -3,7 +3,9 @@ package service
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -13,6 +15,7 @@ import (
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/apicompat"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/openai_compat"
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/require"
 	"github.com/tidwall/gjson"
@@ -32,6 +35,546 @@ func (w *openAIChatFailingWriter) Write(p []byte) (int, error) {
 	return w.ResponseWriter.Write(p)
 }
 
+func TestForwardAsChatCompletions_ResponsesSupportedFallsBackWhenStructuredInputRequiresString(t *testing.T) {
+	for name, rejection := range map[string]string{
+		"OpenAI invalid type": `{"error":{"type":"invalid_request_error","code":"invalid_type","param":"input","message":"Invalid type for 'input': expected a string, but got an array instead."}}`,
+		"vLLM validation":     `{"error":{"message":"1 validation error: [{'type':'string_type', 'loc':('body','input','str'), 'msg':'Input should be a valid string', 'input':[{'role':'user','content':'hi'}, {'role':'assistant','content':[{'type':'output_text'}]}, ResponseFunctionToolCall(type='function_call'), {'type':'function_call_output'}]}]"}}`,
+	} {
+		t.Run(name, func(t *testing.T) {
+			gin.SetMode(gin.TestMode)
+
+			body := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"hello"},{"role":"assistant","content":[{"type":"text","text":"checking"},{"type":"thinking","thinking":"think"}],"tool_calls":[{"id":"call_1","type":"function","function":{"name":"lookup","arguments":"{}"}}]},{"role":"tool","tool_call_id":"call_1","content":"done"},{"role":"user","content":"what now"}],"stream":false}`)
+			originalBody := append([]byte(nil), body...)
+			logs, releaseLogs := captureStructuredLog(t)
+			defer releaseLogs()
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+			c.Request.Header.Set("Content-Type", "application/json")
+
+			upstream := &httpUpstreamRecorder{responses: []*http.Response{
+				{
+					StatusCode: http.StatusBadRequest,
+					Header:     http.Header{"Content-Type": []string{"application/json"}},
+					Body: io.NopCloser(strings.NewReader(
+						rejection,
+					)),
+				},
+				{
+					StatusCode: http.StatusOK,
+					Header:     http.Header{"Content-Type": []string{"application/json"}, "x-request-id": []string{"rid_raw_fallback"}},
+					Body: io.NopCloser(strings.NewReader(
+						`{"id":"chatcmpl_fallback","object":"chat.completion","model":"gpt-5.4","choices":[{"index":0,"message":{"role":"assistant","content":"ok"},"finish_reason":"stop"}],"usage":{"prompt_tokens":1,"completion_tokens":1,"total_tokens":2}}`,
+					)),
+				},
+			}}
+			svc := &OpenAIGatewayService{
+				cfg:          structuredInputRetryTestConfig(),
+				httpUpstream: upstream,
+			}
+			account := structuredInputRetryTestAccount()
+
+			result, err := svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "")
+			require.NoError(t, err)
+			require.NotNil(t, result)
+			require.Len(t, upstream.requests, 2)
+			require.Equal(t, "/v1/responses", upstream.requests[0].URL.Path)
+			require.True(t, gjson.GetBytes(upstream.bodies[0], "input").IsArray())
+			require.Equal(t, "/v1/chat/completions", upstream.requests[1].URL.Path)
+			require.JSONEq(t, string(body), string(upstream.bodies[1]))
+			require.Equal(t, originalBody, body)
+			require.Equal(t, "/v1/chat/completions", result.UpstreamEndpoint)
+			require.Equal(t, "/v1/chat/completions", GetActualOpenAIUpstreamEndpoint(c))
+			require.Equal(t, "ok", gjson.Get(rec.Body.String(), "choices.0.message.content").String())
+			require.True(t, logs.ContainsMessage("structured Responses input rejected"))
+			logs.mu.Lock()
+			defer logs.mu.Unlock()
+			for _, event := range logs.events {
+				require.NotContains(t, event.Fields, "upstream_message")
+				require.NotContains(t, event.Fields, "upstream_body")
+				require.NotContains(t, event.Message, rejection)
+			}
+		})
+	}
+}
+
+type openAIChatFallbackBoundaryReader struct {
+	io.Reader
+	beforeRead func()
+}
+
+func (reader *openAIChatFallbackBoundaryReader) Read(buffer []byte) (int, error) {
+	if reader.beforeRead != nil {
+		reader.beforeRead()
+		reader.beforeRead = nil
+	}
+	return reader.Reader.Read(buffer)
+}
+
+func TestForwardAsChatCompletions_StructuredInputRetrySafetyBoundaries(t *testing.T) {
+	rejection := `{"error":{"code":"invalid_type","param":"input","message":"Expected a string, but got an array instead."}}`
+	validation := `"message":"1 validation error: [{'type':'string_type','loc':('body','input','str'),'msg':'Input should be a valid string'}]"`
+	for _, test := range []struct {
+		name string
+		body string
+	}{
+		{name: "duplicate error", body: strings.TrimSuffix(rejection, "}") + `,"error":{"code":"rate_limit_exceeded"}}`},
+		{name: "duplicate code", body: strings.Replace(rejection, `"code":"invalid_type"`, `"code":"invalid_type","code":"rate_limit_exceeded"`, 1)},
+		{name: "duplicate escaped param", body: strings.Replace(rejection, `"param":"input"`, `"param":"input","p\u0061ram":"tools"`, 1)},
+		{name: "duplicate message", body: strings.Replace(rejection, `"message":"Expected a string, but got an array instead."`, `"message":"Expected a string, but got an array instead.","message":"Input is too long."`, 1)},
+		{name: "contradictory error type", body: strings.Replace(rejection, `"error":{`, `"error":{"type":"authentication_error",`, 1)},
+		{name: "contradictory envelope type", body: strings.TrimSuffix(rejection, "}") + `,"type":"rate_limit_error"}`},
+		{name: "validation contradictory type", body: `{"error":{"type":"rate_limit_error",` + validation + `}}`},
+		{name: "validation contradictory envelope type", body: `{"type":"authentication_error","error":{` + validation + `}}`},
+		{name: "validation contradictory code", body: `{"error":{"code":"rate_limit_exceeded",` + validation + `}}`},
+		{name: "validation contradictory param", body: `{"error":{"param":"tools",` + validation + `}}`},
+		{name: "contradictory envelope code", body: strings.TrimSuffix(rejection, "}") + `,"code":"rate_limit_exceeded"}`},
+		{name: "contradictory envelope param", body: strings.TrimSuffix(rejection, "}") + `,"param":"tools"}`},
+		{name: "type cannot override code", body: strings.Replace(rejection, `"code":"invalid_type"`, `"code":"rate_limit_exceeded","type":"invalid_type"`, 1)},
+		{name: "usage", body: strings.TrimSuffix(rejection, "}") + `,"usage":{"input_tokens":12,"output_tokens":3}}`},
+		{name: "error usage", body: strings.Replace(rejection, `"error":{`, `"error":{"usage":{"input_tokens":12},`, 1)},
+		{name: "output", body: strings.TrimSuffix(rejection, "}") + `,"output":[{"type":"message","content":[{"type":"output_text","text":"partial"}]}]}`},
+		{name: "output text", body: strings.TrimSuffix(rejection, "}") + `,"output_text":"partial"}`},
+		{name: "tool calls", body: strings.TrimSuffix(rejection, "}") + `,"tool_calls":[{"id":"call_partial","type":"function","function":{"name":"lookup","arguments":"{}"}}]}`},
+		{name: "choices", body: strings.TrimSuffix(rejection, "}") + `,"choices":[{"delta":{"content":"partial"}}]}`},
+		{name: "nested response", body: strings.TrimSuffix(rejection, "}") + `,"response":{"status":"incomplete","usage":{"input_tokens":12},"output":[]}}`},
+		{name: "partial status", body: strings.TrimSuffix(rejection, "}") + `,"status":"in_progress"}`},
+		{name: "incomplete details", body: strings.TrimSuffix(rejection, "}") + `,"incomplete_details":{"reason":"max_output_tokens"}}`},
+		{name: "partial event", body: strings.TrimSuffix(rejection, "}") + `,"type":"response.output_text.delta","delta":"partial"}`},
+		{name: "caller canceled"},
+		{name: "request canceled"},
+		{name: "response committed"},
+		{name: "writer committed"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			body := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"hello"}],"stream":false}`)
+			c, _ := gin.CreateTestContext(httptest.NewRecorder())
+			ctx, cancel := context.WithCancel(context.Background())
+			defer cancel()
+			requestCtx, cancelRequest := context.WithCancel(context.Background())
+			defer cancelRequest()
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body)).WithContext(requestCtx)
+			errorBody := test.body
+			if errorBody == "" {
+				errorBody = rejection
+			}
+			reader := &openAIChatFallbackBoundaryReader{Reader: strings.NewReader(errorBody), beforeRead: func() {
+				switch test.name {
+				case "caller canceled":
+					cancel()
+				case "request canceled":
+					cancelRequest()
+				case "response committed":
+					MarkResponseCommitted(c)
+				case "writer committed":
+					c.Writer.WriteHeaderNow()
+				}
+			}}
+			upstream := &httpUpstreamRecorder{resp: &http.Response{
+				StatusCode: http.StatusBadRequest,
+				Header:     http.Header{"Content-Type": []string{"application/json"}},
+				Body:       io.NopCloser(reader),
+			}}
+			svc := &OpenAIGatewayService{cfg: structuredInputRetryTestConfig(), httpUpstream: upstream}
+			result, err := svc.ForwardAsChatCompletions(ctx, c, structuredInputRetryTestAccount(), body, "", "")
+			require.Error(t, err)
+			require.Nil(t, result)
+			require.Len(t, upstream.requests, 1)
+			require.Equal(t, "/v1/responses", upstream.requests[0].URL.Path)
+		})
+	}
+}
+
+func TestConvertedResponsesInputStringRejectionIsNarrow(t *testing.T) {
+	matching := []byte(`{"error":{"code":"invalid_type","param":"input","message":"Expected a string for input, but got an object instead."}}`)
+	require.True(t, isConvertedResponsesInputStringRejection(http.StatusBadRequest, matching))
+	require.True(t, isConvertedResponsesInputStringRejection(http.StatusBadRequest, []byte(`{"error":{"type":"invalid_type","param":"input","message":"Invalid type for 'input': expected a string, but got an array instead."}}`)))
+
+	for _, tc := range []struct {
+		name   string
+		status int
+		body   string
+	}{
+		{name: "nested content validation", status: http.StatusBadRequest, body: `{"error":{"message":"'loc':('body','input',0,'content','str'), 'msg':'Input should be a valid string'"}}`},
+		{name: "other field validation", status: http.StatusBadRequest, body: `{"error":{"message":"'loc':('body','model','str'), 'msg':'Input should be a valid string'"}}`},
+		{name: "generic string validation", status: http.StatusBadRequest, body: `{"error":{"message":"Input should be a valid string"}}`},
+		{name: "wrong status", status: http.StatusUnprocessableEntity, body: string(matching)},
+		{name: "authentication", status: http.StatusUnauthorized, body: string(matching)},
+		{name: "authorization", status: http.StatusForbidden, body: string(matching)},
+		{name: "rate limit", status: http.StatusTooManyRequests, body: string(matching)},
+		{name: "server error", status: http.StatusInternalServerError, body: string(matching)},
+		{name: "wrong parameter", status: http.StatusBadRequest, body: `{"error":{"code":"invalid_type","param":"tools","message":"Expected a string, but got an array instead."}}`},
+		{name: "wrong code", status: http.StatusBadRequest, body: `{"error":{"code":"invalid_request_error","param":"input","message":"Expected a string, but got an array instead."}}`},
+		{name: "unrelated input error", status: http.StatusBadRequest, body: `{"error":{"code":"invalid_type","param":"input","message":"Input is too long."}}`},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			require.False(t, isConvertedResponsesInputStringRejection(tc.status, []byte(tc.body)))
+		})
+	}
+}
+
+func TestConvertedResponsesInputStringRejectionValidationBoundaries(t *testing.T) {
+	validation := `{'type': 'string_type', 'loc': ('body', 'input', 'str'), 'msg': 'Input should be a valid string', 'input': [{'role': 'user', 'content': 'hello'}]}`
+	for _, test := range []struct {
+		name    string
+		message string
+		want    bool
+	}{
+		{name: "complete Pydantic error", message: "1 validation error: [" + validation + "]", want: true},
+		{name: "multiple Pydantic errors", message: "2 validation errors for Request: [" + strings.Replace(validation, "'input', 'str'", "'model', 'str'", 1) + ", " + validation + "]", want: true},
+		{name: "double quoted keys", message: "1 validation error: [" + strings.ReplaceAll(validation, "'", `"`) + "]", want: true},
+		{name: "surrounding whitespace", message: " \n1 validation error: [" + validation + "]\n ", want: true},
+		{name: "escaped quote in history", message: `1 validation error: [{'type':'string_type','loc':('body','input','str'),'msg':'Input should be a valid string','input':[{'content':'can\'t [close] this }'}]}]`, want: true},
+		{name: "bare loc and msg", message: `'loc':('body','input','str'), 'msg':'Input should be a valid string'`},
+		{name: "echoed dictionary in unrelated error", message: "Invalid history: " + validation},
+		{name: "echoed fake loc and msg in input", message: `1 validation error: [{'type':'string_type','loc':('body','model','str'),'msg':'Input should be a valid string','input':{'loc':('body','input','str'),'msg':'Input should be a valid string'}}]`},
+		{name: "echoed fake loc and msg in quoted history", message: `1 validation error: [{'type':'string_type','loc':('body','input',0,'content','str'),'msg':'Input should be a valid string','input':"'loc':('body','input','str'), 'msg':'Input should be a valid string'"}]`},
+		{name: "echoed complete error in quoted history", message: `1 validation error: [{'type':'string_type','loc':('body','model','str'),'msg':'Input should be a valid string','input':"1 validation error: [{'type':'string_type','loc':('body','input','str'),'msg':'Input should be a valid string'}]"}]`},
+		{name: "truncated error list", message: "1 validation error: [" + validation},
+		{name: "truncated quoted input", message: `1 validation error: [{'type':'string_type','loc':('body','input','str'),'msg':'Input should be a valid string','input':'history}]`},
+		{name: "mismatched closing bracket", message: "1 validation error: [" + strings.TrimSuffix(validation, "}") + ")]"},
+		{name: "elided validation list", message: "240 validation errors: ... " + validation},
+		{name: "incorrect error count", message: "2 validation errors: [" + validation + "]"},
+		{name: "overflowed error count", message: "999999999999999999999999 validation errors: [" + validation + "]"},
+		{name: "duplicate location", message: "1 validation error: [" + strings.Replace(validation, "'input':", "'loc': ('body', 'model', 'str'), 'input':", 1) + "]"},
+		{name: "wrong validation type", message: "1 validation error: [" + strings.Replace(validation, "string_type", "value_error", 1) + "]"},
+		{name: "missing validation type", message: "1 validation error: [" + strings.Replace(validation, "'type': 'string_type', ", "", 1) + "]"},
+		{name: "location and message in different errors", message: "2 validation errors: [" + strings.Replace(validation, "Input should be a valid string", "Invalid history", 1) + ", " + strings.Replace(validation, "'input', 'str'", "'model', 'str'", 1) + "]"},
+		{name: "trailing echoed text", message: "1 validation error: [" + validation + "] echoed history"},
+		{name: "structured string array", message: "Expected a string, but got an array instead.", want: true},
+		{name: "structured object", message: "Invalid type for 'input': expected a string, but got an object instead.", want: true},
+		{name: "structured echoed history", message: "Invalid history containing 'expected a string' and 'got an array'"},
+		{name: "structured truncated message", message: "Expected a string, but got an array"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			body, err := json.Marshal(map[string]any{"error": map[string]string{"message": test.message, "code": "invalid_type", "param": "input"}})
+			require.NoError(t, err)
+			require.Equal(t, test.want, isConvertedResponsesInputStringRejection(http.StatusBadRequest, body))
+			if test.want {
+				require.False(t, isConvertedResponsesInputStringRejection(http.StatusBadRequest, body[:len(body)-1]))
+				require.False(t, isConvertedResponsesInputStringRejection(http.StatusBadRequest, append(body, []byte(" trailing")...)))
+			}
+		})
+	}
+}
+
+func TestForwardAsChatCompletions_StructuredInputRetryRequiresCompleteErrorBody(t *testing.T) {
+	rejection := `{"error":{"code":"invalid_type","param":"input","message":"Expected a string, but got an array instead."}}`
+	limit := int(openAIUpstreamErrorBodyReadLimitForConfig(structuredInputRetryTestConfig()))
+	for _, test := range []struct {
+		name    string
+		payload string
+		readErr error
+		unknown bool
+		want    bool
+	}{
+		{name: "complete JSON followed by read failure", payload: rejection, readErr: io.ErrUnexpectedEOF},
+		{name: "truncated JSON", payload: rejection[:len(rejection)-1]},
+		{name: "exact read limit", payload: rejection + strings.Repeat(" ", limit-len(rejection)), want: true},
+		{name: "one byte beyond read limit", payload: rejection + strings.Repeat(" ", limit-len(rejection)+1)},
+		{name: "valid JSON prefix before discarded suffix", payload: rejection + strings.Repeat(" ", limit-len(rejection)) + "trailing"},
+		{name: "unknown capability still retries after read failure", payload: rejection, readErr: io.ErrUnexpectedEOF, unknown: true, want: true},
+		{name: "unknown capability still retries beyond read limit", payload: rejection + strings.Repeat(" ", limit-len(rejection)+1), unknown: true, want: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			body := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"hello"}]}`)
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+			errorBody := io.NopCloser(strings.NewReader(test.payload))
+			if test.readErr != nil {
+				errorBody = &openAIChatStreamReadErrorCloser{payload: []byte(test.payload), err: test.readErr}
+			}
+			account := structuredInputRetryTestAccount()
+			status := http.StatusBadRequest
+			if test.unknown {
+				delete(account.Extra, openai_compat.ExtraKeyResponsesSupported)
+				status = http.StatusNotFound
+			}
+			upstream := &httpUpstreamRecorder{responses: []*http.Response{
+				{StatusCode: status, Header: http.Header{"Content-Type": []string{"application/json"}}, Body: errorBody},
+				{StatusCode: http.StatusOK, Header: http.Header{"Content-Type": []string{"application/json"}}, Body: io.NopCloser(strings.NewReader(`{"id":"chatcmpl_retry","choices":[{"message":{"role":"assistant","content":"ok"},"finish_reason":"stop"}],"usage":{"prompt_tokens":1,"completion_tokens":1,"total_tokens":2}}`))},
+			}}
+			svc := &OpenAIGatewayService{cfg: structuredInputRetryTestConfig(), httpUpstream: upstream}
+			result, err := svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "")
+			if test.want {
+				require.NoError(t, err)
+				require.NotNil(t, result)
+				require.Len(t, upstream.requests, 2)
+			} else {
+				require.Error(t, err)
+				require.Nil(t, result)
+				require.Len(t, upstream.requests, 1)
+			}
+		})
+	}
+}
+
+type openAIChatFallbackDelayReader struct {
+	io.Reader
+	elapsed time.Duration
+}
+
+func (reader *openAIChatFallbackDelayReader) Read(buffer []byte) (int, error) {
+	if reader.elapsed == 0 {
+		started := time.Now()
+		time.Sleep(30 * time.Millisecond)
+		reader.elapsed = time.Since(started)
+	}
+	return reader.Reader.Read(buffer)
+}
+
+func TestForwardAsChatCompletions_RawFallbackPreservesResponsesTiming(t *testing.T) {
+	for _, unknown := range []bool{false, true} {
+		for _, stream := range []bool{false, true} {
+			t.Run(fmt.Sprintf("unknown=%t/stream=%t", unknown, stream), func(t *testing.T) {
+				body := []byte(fmt.Sprintf(`{"model":"gpt-5.4","messages":[{"role":"user","content":"hello"}],"stream":%t}`, stream))
+				rec := httptest.NewRecorder()
+				c, _ := gin.CreateTestContext(rec)
+				c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+				account := structuredInputRetryTestAccount()
+				status := http.StatusBadRequest
+				if unknown {
+					delete(account.Extra, openai_compat.ExtraKeyResponsesSupported)
+					status = http.StatusNotFound
+				}
+				initialBody := &openAIChatFallbackDelayReader{Reader: strings.NewReader(`{"error":{"code":"invalid_type","param":"input","message":"Expected a string, but got an array instead."}}`)}
+				responseBody := `{"id":"chatcmpl_retry","choices":[{"message":{"role":"assistant","content":"ok"},"finish_reason":"stop"}],"usage":{"prompt_tokens":1,"completion_tokens":1,"total_tokens":2}}`
+				contentType := "application/json"
+				if stream {
+					responseBody = "data: {\"id\":\"chatcmpl_retry\",\"choices\":[{\"index\":0,\"delta\":{\"content\":\"ok\"},\"finish_reason\":\"stop\"}]}\n\ndata: {\"choices\":[],\"usage\":{\"prompt_tokens\":1,\"completion_tokens\":1,\"total_tokens\":2}}\n\ndata: [DONE]\n\n"
+					contentType = "text/event-stream"
+				}
+				upstream := &httpUpstreamRecorder{responses: []*http.Response{
+					{StatusCode: status, Header: http.Header{"Content-Type": []string{"application/json"}}, Body: io.NopCloser(initialBody)},
+					{StatusCode: http.StatusOK, Header: http.Header{"Content-Type": []string{contentType}}, Body: io.NopCloser(strings.NewReader(responseBody))},
+				}}
+				svc := &OpenAIGatewayService{cfg: structuredInputRetryTestConfig(), httpUpstream: upstream}
+				result, err := svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "")
+				require.NoError(t, err)
+				require.NotNil(t, result)
+				require.Len(t, upstream.requests, 2)
+				require.Equal(t, "/v1/responses", upstream.requests[0].URL.Path)
+				require.Equal(t, "/v1/chat/completions", upstream.requests[1].URL.Path)
+				require.GreaterOrEqual(t, result.Duration, initialBody.elapsed)
+				if stream {
+					require.NotNil(t, result.FirstTokenMs)
+					require.GreaterOrEqual(t, *result.FirstTokenMs, int(initialBody.elapsed.Milliseconds()))
+				}
+			})
+		}
+	}
+}
+
+func TestForwardAsChatCompletions_StructuredInputRejectionDoesNotRetryIneligibleRequests(t *testing.T) {
+	rejection := `{"error":{"code":"invalid_type","param":"input","message":"Expected a string, but got an array instead."}}`
+	for _, test := range []struct {
+		name          string
+		mode          string
+		unknown       bool
+		platform      string
+		protocol      string
+		endpoint      string
+		accountType   string
+		responsesBody bool
+		nativeIngress bool
+		status        int
+		rejection     string
+	}{
+		{name: "forced Responses", mode: "force_responses"},
+		{name: "forced Responses without probe", mode: "force_responses", unknown: true},
+		{name: "unknown support", unknown: true},
+		{name: "Responses shaped Chat ingress", responsesBody: true},
+		{name: "native Responses ingress", responsesBody: true, nativeIngress: true},
+		{name: "native CN Responses protocol", platform: PlatformDeepseek},
+		{name: "adaptive Chat ingress stays raw", platform: PlatformKimi, protocol: APIProtocolAdaptive, endpoint: "/chat/completions"},
+		{name: "adaptive Responses shaped ingress stays Responses", platform: PlatformKimi, protocol: APIProtocolAdaptive, responsesBody: true},
+		{name: "adaptive Responses shaped ingress converts to Chat", platform: PlatformZhipu, protocol: APIProtocolAdaptive, responsesBody: true, endpoint: "/chat/completions"},
+		{name: "model routed Responses is not OpenAI retry eligible", platform: PlatformCommandCode, protocol: APIProtocolResponses},
+		{name: "OAuth", accountType: AccountTypeOAuth},
+		{name: "authentication", status: http.StatusUnauthorized},
+		{name: "authorization", status: http.StatusForbidden},
+		{name: "rate limit", status: http.StatusTooManyRequests},
+		{name: "unrelated validation", rejection: `{"error":{"code":"invalid_type","param":"tools","message":"Expected a string, but got an array instead."}}`},
+		{name: "nested validation", rejection: `{"error":{"message":"'loc':('body','input',0,'content','str'), 'msg':'Input should be a valid string'"}}`},
+		{name: "echoed validation", rejection: `{"error":{"message":"1 validation error: [{'type':'string_type','loc':('body','model','str'),'msg':'Input should be a valid string','input':{'loc':('body','input','str'),'msg':'Input should be a valid string'}}]"}}`},
+		{name: "echoed structured error", rejection: `{"error":{"code":"invalid_type","param":"input","message":"Invalid history containing expected a string but got an array"}}`},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			body := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"hello"}],"stream":false}`)
+			if test.responsesBody {
+				body = []byte(`{"model":"gpt-5.4","input":[{"role":"user","content":"hello"}],"stream":false}`)
+			}
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+			account := structuredInputRetryTestAccount()
+			account.Extra = map[string]any{openai_compat.ExtraKeyResponsesSupported: true, openai_compat.ExtraKeyResponsesMode: test.mode}
+			if test.unknown {
+				delete(account.Extra, openai_compat.ExtraKeyResponsesSupported)
+			}
+			if test.platform != "" {
+				account.Platform = test.platform
+				account.Credentials["api_protocol"] = APIProtocolResponses
+			}
+			if test.protocol != "" {
+				account.Credentials["api_protocol"] = test.protocol
+			}
+			if test.accountType != "" {
+				account.Type = test.accountType
+				account.Credentials["access_token"] = "test-token"
+			}
+			status := test.status
+			if status == 0 {
+				status = http.StatusBadRequest
+			}
+			errorBody := rejection
+			if test.rejection != "" {
+				errorBody = test.rejection
+			}
+			upstream := &httpUpstreamRecorder{resp: &http.Response{
+				StatusCode: status,
+				Header:     http.Header{"Content-Type": []string{"application/json"}},
+				Body:       io.NopCloser(strings.NewReader(errorBody)),
+			}}
+			svc := &OpenAIGatewayService{cfg: structuredInputRetryTestConfig(), httpUpstream: upstream}
+			var result *OpenAIForwardResult
+			var err error
+			if test.nativeIngress {
+				c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", bytes.NewReader(body))
+				result, err = svc.Forward(context.Background(), c, account, body)
+			} else {
+				result, err = svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "")
+			}
+			require.Error(t, err)
+			require.Nil(t, result)
+			require.Len(t, upstream.requests, 1)
+			endpoint := test.endpoint
+			if endpoint == "" {
+				endpoint = "/responses"
+			}
+			require.True(t, strings.HasSuffix(upstream.requests[0].URL.Path, endpoint))
+			var failover *UpstreamFailoverError
+			if status == http.StatusUnauthorized || status == http.StatusForbidden || status == http.StatusTooManyRequests {
+				require.ErrorAs(t, err, &failover)
+				require.Equal(t, status, failover.StatusCode)
+			} else {
+				require.False(t, errors.As(err, &failover))
+				require.True(t, c.Writer.Written())
+			}
+		})
+	}
+}
+
+func TestForwardAsChatCompletions_StructuredInputRetryPreservesStreamingAndMapping(t *testing.T) {
+	for _, useAccountMapping := range []bool{false, true} {
+		t.Run(fmt.Sprintf("account_mapping=%t", useAccountMapping), func(t *testing.T) {
+			body := []byte(`{"model":"client-model","messages":[{"role":"user","content":"hello"}],"stream":true,"stream_options":{"include_usage":false},"vendor_option":"preserve"}`)
+			originalBody := append([]byte(nil), body...)
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+			account := structuredInputRetryTestAccount()
+			account.Extra = map[string]any{openai_compat.ExtraKeyResponsesSupported: true, openai_compat.ExtraKeyResponsesMode: "auto"}
+			mappedModel := "gpt-5.4"
+			if useAccountMapping {
+				account.Credentials["model_mapping"] = map[string]any{"client-model": mappedModel}
+			}
+			stream := "data: {\"id\":\"chatcmpl_retry\",\"object\":\"chat.completion.chunk\",\"model\":\"gpt-5.4\",\"choices\":[{\"index\":0,\"delta\":{\"content\":\"ok\"},\"finish_reason\":\"stop\"}]}\n\ndata: {\"choices\":[],\"usage\":{\"prompt_tokens\":9,\"completion_tokens\":4,\"total_tokens\":13}}\n\ndata: [DONE]\n\n"
+			upstream := &httpUpstreamRecorder{responses: []*http.Response{
+				{StatusCode: http.StatusBadRequest, Header: http.Header{"Content-Type": []string{"application/json"}}, Body: io.NopCloser(strings.NewReader(`{"error":{"code":"invalid_type","param":"input","message":"Expected a string, but got an array instead."}}`))},
+				{StatusCode: http.StatusOK, Header: http.Header{"Content-Type": []string{"text/event-stream"}}, Body: io.NopCloser(strings.NewReader(stream))},
+			}}
+			svc := &OpenAIGatewayService{cfg: structuredInputRetryTestConfig(), httpUpstream: upstream}
+			defaultMappedModel := mappedModel
+			if useAccountMapping {
+				defaultMappedModel = ""
+			}
+			result, err := svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", defaultMappedModel)
+			require.NoError(t, err)
+			require.Len(t, upstream.requests, 2)
+			for _, outbound := range upstream.bodies {
+				require.Equal(t, mappedModel, gjson.GetBytes(outbound, "model").String())
+				require.True(t, gjson.GetBytes(outbound, "stream").Bool())
+			}
+			require.JSONEq(t, gjson.GetBytes(body, "messages").Raw, gjson.GetBytes(upstream.bodies[1], "messages").Raw)
+			require.Equal(t, "preserve", gjson.GetBytes(upstream.bodies[1], "vendor_option").String())
+			require.True(t, gjson.GetBytes(upstream.bodies[1], "stream_options.include_usage").Bool())
+			require.Equal(t, originalBody, body)
+			require.True(t, result.Stream)
+			require.Equal(t, mappedModel, result.BillingModel)
+			require.Equal(t, mappedModel, result.UpstreamModel)
+			require.Equal(t, 9, result.Usage.InputTokens)
+			require.Equal(t, 4, result.Usage.OutputTokens)
+			require.Equal(t, "/v1/chat/completions", result.UpstreamEndpoint)
+			require.Contains(t, rec.Header().Get("Content-Type"), "text/event-stream")
+			require.Contains(t, rec.Body.String(), "data: [DONE]")
+		})
+	}
+}
+
+func TestForwardAsChatCompletions_StructuredInputRetryIsBoundedAndPreservesRawErrors(t *testing.T) {
+	for _, status := range []int{http.StatusBadRequest, http.StatusUnauthorized, http.StatusTooManyRequests} {
+		t.Run(fmt.Sprintf("status=%d", status), func(t *testing.T) {
+			rejection := `{"error":{"code":"invalid_type","param":"input","message":"Expected a string, but got an array instead."}}`
+			response := func(status int) *http.Response {
+				return &http.Response{StatusCode: status, Header: http.Header{"Content-Type": []string{"application/json"}}, Body: io.NopCloser(strings.NewReader(rejection))}
+			}
+			body := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"hello"}],"stream":false}`)
+			account := structuredInputRetryTestAccount()
+			account.Extra = map[string]any{openai_compat.ExtraKeyResponsesSupported: true}
+			forward := func(retry bool) (*httptest.ResponseRecorder, error) {
+				rec := httptest.NewRecorder()
+				c, _ := gin.CreateTestContext(rec)
+				c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+				upstream := &httpUpstreamRecorder{responses: []*http.Response{response(status)}}
+				if retry {
+					upstream.responses = append([]*http.Response{response(http.StatusBadRequest)}, upstream.responses...)
+				}
+				svc := &OpenAIGatewayService{cfg: structuredInputRetryTestConfig(), httpUpstream: upstream}
+				var result *OpenAIForwardResult
+				var err error
+				if retry {
+					result, err = svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "")
+					require.Len(t, upstream.requests, 2)
+					require.Equal(t, "/v1/chat/completions", upstream.requests[1].URL.Path)
+				} else {
+					result, err = svc.forwardAsRawChatCompletions(context.Background(), c, account, body, "")
+					require.Len(t, upstream.requests, 1)
+				}
+				require.Nil(t, result)
+				require.Error(t, err)
+				return rec, err
+			}
+			baseline, baselineErr := forward(false)
+			retried, retryErr := forward(true)
+			require.Equal(t, baseline.Code, retried.Code)
+			require.Equal(t, baseline.Body.String(), retried.Body.String())
+			require.Equal(t, baselineErr.Error(), retryErr.Error())
+			require.IsType(t, baselineErr, retryErr)
+		})
+	}
+}
+
+func structuredInputRetryTestConfig() *config.Config {
+	return &config.Config{Security: config.SecurityConfig{URLAllowlist: config.URLAllowlistConfig{
+		Enabled:           false,
+		AllowInsecureHTTP: true,
+	}}}
+}
+
+func structuredInputRetryTestAccount() *Account {
+	return &Account{
+		ID:          101,
+		Name:        "raw-openai-apikey",
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeAPIKey,
+		Concurrency: 1,
+		Credentials: map[string]any{"api_key": "sk-test", "base_url": "http://upstream.example"},
+		Extra:       map[string]any{openai_compat.ExtraKeyResponsesSupported: true},
+	}
+}
+
 type openAIChatStreamReadErrorCloser struct {
 	payload []byte
 	err     error
@@ -1322,7 +1865,7 @@ func TestGPT61SolOnlyChatFallbackRejectsToolsAndDisabledReasoning(t *testing.T)
 			svc := &OpenAIGatewayService{cfg: &config.Config{}}
 			var err error
 			if responses {
-				_, err = svc.forwardResponsesViaRawChatCompletions(context.Background(), c, account, []byte(body))
+				_, err = svc.forwardResponsesViaRawChatCompletions(context.Background(), c, account, []byte(body), time.Now())
 			} else {
 				_, err = svc.forwardAnthropicViaRawChatCompletions(context.Background(), c, account, []byte(body), "")
 			}
diff --git a/backend/internal/service/openai_gateway_forward.go b/backend/internal/service/openai_gateway_forward.go
index 16df53310f..5faa086380 100644
--- a/backend/internal/service/openai_gateway_forward.go
+++ b/backend/internal/service/openai_gateway_forward.go
@@ -190,7 +190,7 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 		// 账号映射未命中时以去除首尾空白的请求模型兜底，计费名与上游模型名一致。
 		return s.forwardResponsesViaNativeAnthropic(ctx, c, account, body, reqModel)
 	case APIProtocolChatCompletions:
-		return s.forwardResponsesViaRawChatCompletions(ctx, c, account, body)
+		return s.forwardResponsesViaRawChatCompletions(ctx, c, account, body, startTime)
 	}
 	SetActualOpenAIUpstreamEndpoint(c, openAIResponsesUpstreamEndpoint)
 	if account.IsOpenAI() && (account.IsOpenAIApiKey() || account.IsOpenAIOAuthLike()) {
diff --git a/backend/internal/service/openai_gateway_responses_chat_fallback.go b/backend/internal/service/openai_gateway_responses_chat_fallback.go
index fabbcbc156..184e3152dc 100644
--- a/backend/internal/service/openai_gateway_responses_chat_fallback.go
+++ b/backend/internal/service/openai_gateway_responses_chat_fallback.go
@@ -29,9 +29,8 @@ func (s *OpenAIGatewayService) forwardResponsesViaRawChatCompletions(
 	c *gin.Context,
 	account *Account,
 	body []byte,
+	startTime time.Time,
 ) (*OpenAIForwardResult, error) {
-	startTime := time.Now()
-
 	var responsesReq apicompat.ResponsesRequest
 	if err := json.Unmarshal(body, &responsesReq); err != nil {
 		writeOpenAIResponsesFallbackError(c, http.StatusBadRequest, "invalid_request_error", "Failed to parse request body")
diff --git a/backend/internal/service/openai_gateway_responses_chat_fallback_test.go b/backend/internal/service/openai_gateway_responses_chat_fallback_test.go
index 989421bf8b..0cc999c57d 100644
--- a/backend/internal/service/openai_gateway_responses_chat_fallback_test.go
+++ b/backend/internal/service/openai_gateway_responses_chat_fallback_test.go
@@ -5,6 +5,7 @@ package service
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -19,6 +20,34 @@ import (
 	"github.com/tidwall/gjson"
 )
 
+func TestForwardResponses_ChatFallbackPreservesProvidedStartTime(t *testing.T) {
+	for _, stream := range []bool{false, true} {
+		t.Run(fmt.Sprintf("stream=%t", stream), func(t *testing.T) {
+			body := []byte(fmt.Sprintf(`{"model":"gpt-5.4","input":"hello","stream":%t}`, stream))
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", bytes.NewReader(body))
+			responseBody := `{"id":"chatcmpl_timing","choices":[{"message":{"role":"assistant","content":"ok"},"finish_reason":"stop"}],"usage":{"prompt_tokens":1,"completion_tokens":1,"total_tokens":2}}`
+			contentType := "application/json"
+			if stream {
+				responseBody = "data: {\"id\":\"chatcmpl_timing\",\"choices\":[{\"index\":0,\"delta\":{\"content\":\"ok\"},\"finish_reason\":\"stop\"}]}\n\ndata: {\"choices\":[],\"usage\":{\"prompt_tokens\":1,\"completion_tokens\":1,\"total_tokens\":2}}\n\ndata: [DONE]\n\n"
+				contentType = "text/event-stream"
+			}
+			upstream := &httpUpstreamRecorder{resp: &http.Response{StatusCode: http.StatusOK, Header: http.Header{"Content-Type": []string{contentType}}, Body: io.NopCloser(strings.NewReader(responseBody))}}
+			svc := &OpenAIGatewayService{cfg: rawChatCompletionsTestConfig(), httpUpstream: upstream}
+			startTime := time.Now().Add(-time.Minute)
+			result, err := svc.forwardResponsesViaRawChatCompletions(context.Background(), c, forceChatResponsesFallbackAccount(), body, startTime)
+			require.NoError(t, err)
+			require.NotNil(t, result)
+			require.GreaterOrEqual(t, result.Duration, time.Minute)
+			if stream {
+				require.NotNil(t, result.FirstTokenMs)
+				require.GreaterOrEqual(t, *result.FirstTokenMs, int(time.Minute.Milliseconds()))
+			}
+		})
+	}
+}
+
 func TestForwardResponses_ForceChatCompletionsRoutesNonStreamingToChatCompletions(t *testing.T) {
 	gin.SetMode(gin.TestMode)