diff --git a/internal/runtime/executor/codex_executor_terminal.go b/internal/runtime/executor/codex_executor_terminal.go
index ef96ed3c1..36cde2100 100644
--- a/internal/runtime/executor/codex_executor_terminal.go
+++ b/internal/runtime/executor/codex_executor_terminal.go
@@ -221,6 +221,9 @@ func codexTerminalFailureBody(eventData []byte) ([]byte, bool) {
 	if len(body) == 0 {
 		body = []byte(`{"error":{"message":"upstream stream failed without error details"}}`)
 	}
+	if seq := gjson.GetBytes(eventData, "sequence_number"); seq.Exists() {
+		body, _ = sjson.SetBytes(body, "sequence_number", seq.Int())
+	}
 	return body, true
 }
 
diff --git a/sdk/api/handlers/openai/openai_responses_handlers.go b/sdk/api/handlers/openai/openai_responses_handlers.go
index c4159421a..ea5723db9 100644
--- a/sdk/api/handlers/openai/openai_responses_handlers.go
+++ b/sdk/api/handlers/openai/openai_responses_handlers.go
@@ -191,11 +191,19 @@ func (f *responsesSSEFramer) repairErrorPayload(payload []byte) []byte {
 	}
 	f.terminalEvent = failureEvent
 	errText := responsesStreamErrorText(errMsg, status)
+	seq := 0
+	if s := gjson.GetBytes(payload, "sequence_number"); s.Exists() {
+		seq = int(s.Int())
+	} else if origSeq := gjson.Get(errText, "sequence_number"); origSeq.Exists() {
+		seq = int(origSeq.Int())
+	} else if f != nil && f.dataFrames > 0 {
+		seq = f.dataFrames - 1
+	}
 	if failureEvent == "response.failed" {
-		chunk := handlers.BuildOpenAIResponsesStreamFailedChunk(status, errText, 0)
+		chunk := handlers.BuildOpenAIResponsesStreamFailedChunk(status, errText, seq)
 		return []byte(fmt.Sprintf("event: response.failed\ndata: %s\n\n", chunk))
 	}
-	chunk := handlers.BuildOpenAIResponsesStreamErrorChunk(status, errText, 0)
+	chunk := handlers.BuildOpenAIResponsesStreamErrorChunk(status, errText, seq)
 	return []byte(fmt.Sprintf("event: error\ndata: %s\n\n", chunk))
 }
 
@@ -817,66 +825,89 @@ func sanitizeResponsesStreamEventName(eventName string) string {
 	return truncateResponsesStreamErrorText(redactResponsesStreamErrorText(strings.TrimSpace(eventName)), responsesStreamErrorFieldLimit)
 }
 
+func isResponsesStreamSensitiveKey(key string) bool {
+	k := strings.ToLower(strings.TrimSpace(key))
+	k = strings.ReplaceAll(k, "-", "_")
+	if strings.Contains(k, "tokens") || strings.Contains(k, "token_count") || strings.Contains(k, "token_limit") || strings.Contains(k, "token_usage") {
+		return false
+	}
+	switch k {
+	case "authorization", "secret", "password", "passwd", "api_key", "apikey", "token", "access_token", "refresh_token", "id_token", "auth_token", "session_token", "api_token", "client_secret", "client_key":
+		return true
+	}
+	return strings.HasSuffix(k, "_secret") ||
+		strings.HasSuffix(k, "_password") ||
+		strings.HasSuffix(k, "_api_key") ||
+		strings.HasSuffix(k, "_token")
+}
+
+func sanitizeResponsesStreamErrorNode(val any) any {
+	switch v := val.(type) {
+	case string:
+		return truncateResponsesStreamErrorText(redactResponsesStreamErrorText(v), responsesStreamErrorMessageLimit)
+	case map[string]any:
+		cleaned := make(map[string]any, len(v))
+		for k, item := range v {
+			if isResponsesStreamSensitiveKey(k) {
+				cleaned[k] = "[REDACTED]"
+				continue
+			}
+			cleaned[k] = sanitizeResponsesStreamErrorNode(item)
+		}
+		return cleaned
+	case []any:
+		cleaned := make([]any, len(v))
+		for i, item := range v {
+			cleaned[i] = sanitizeResponsesStreamErrorNode(item)
+		}
+		return cleaned
+	default:
+		return val
+	}
+}
+
 func responsesStreamErrorText(errMsg *interfaces.ErrorMessage, status int) string {
 	text := http.StatusText(status)
 	if errMsg != nil && errMsg.Error != nil && strings.TrimSpace(errMsg.Error.Error()) != "" {
 		text = strings.TrimSpace(errMsg.Error.Error())
 	}
-	if !json.Valid([]byte(text)) {
-		return truncateResponsesStreamErrorText(redactResponsesStreamErrorText(text), responsesStreamErrorMessageLimit)
+	trimmed := strings.TrimSpace(text)
+	if !json.Valid([]byte(trimmed)) {
+		return truncateResponsesStreamErrorText(redactResponsesStreamErrorText(trimmed), responsesStreamErrorMessageLimit)
 	}
 
-	root := gjson.Parse(text)
-	errorNode := root.Get("error")
-	if !errorNode.Exists() || !errorNode.IsObject() {
-		errorNode = root.Get("response.error")
+	var root map[string]any
+	dec := json.NewDecoder(bytes.NewReader([]byte(trimmed)))
+	dec.UseNumber()
+	if errUnmarshal := dec.Decode(&root); errUnmarshal != nil {
+		return truncateResponsesStreamErrorText(redactResponsesStreamErrorText(trimmed), responsesStreamErrorMessageLimit)
 	}
-	if errorNode.Exists() && errorNode.IsObject() {
-		safe := []byte(`{"error":{}}`)
-		copied := false
-		for _, field := range []string{"type", "code", "message", "param", "retryable"} {
-			value := errorNode.Get(field)
-			if !value.Exists() || value.Type == gjson.Null {
-				continue
-			}
-			if field == "retryable" {
-				safe, _ = sjson.SetBytes(safe, "error.retryable", value.Bool())
-				copied = true
-				continue
-			}
-			limit := responsesStreamErrorFieldLimit
-			if field == "message" {
-				limit = responsesStreamErrorMessageLimit
-			}
-			safe, _ = sjson.SetBytes(safe, "error."+field, truncateResponsesStreamErrorText(redactResponsesStreamErrorText(value.String()), limit))
-			copied = true
-		}
-		if copied {
-			return string(safe)
+
+	errorNode, hasError := root["error"].(map[string]any)
+	if !hasError {
+		if resp, ok := root["response"].(map[string]any); ok {
+			errorNode, hasError = resp["error"].(map[string]any)
 		}
 	}
 
-	safe := []byte(`{"type":"error"}`)
-	copied := false
-	for _, field := range []string{"code", "message", "param", "retryable"} {
-		value := root.Get(field)
-		if !value.Exists() || value.Type == gjson.Null {
-			continue
+	if hasError {
+		cleanedError := sanitizeResponsesStreamErrorNode(errorNode)
+		out := map[string]any{
+			"error": cleanedError,
 		}
-		if field == "retryable" {
-			safe, _ = sjson.SetBytes(safe, "retryable", value.Bool())
-			copied = true
-			continue
+		if seq, ok := root["sequence_number"]; ok {
+			out["sequence_number"] = seq
 		}
-		limit := responsesStreamErrorFieldLimit
-		if field == "message" {
-			limit = responsesStreamErrorMessageLimit
+		data, errMarshal := json.Marshal(out)
+		if errMarshal == nil {
+			return string(data)
 		}
-		safe, _ = sjson.SetBytes(safe, field, truncateResponsesStreamErrorText(redactResponsesStreamErrorText(value.String()), limit))
-		copied = true
 	}
-	if copied {
-		return string(safe)
+
+	cleanedRoot := sanitizeResponsesStreamErrorNode(root)
+	data, errMarshal := json.Marshal(cleanedRoot)
+	if errMarshal == nil {
+		return string(data)
 	}
 	return http.StatusText(status)
 }
@@ -954,12 +985,19 @@ func (h *OpenAIResponsesAPIHandler) forwardResponsesStream(c *gin.Context, flush
 		if framer.terminalEvent != "" {
 			return
 		}
+		seq := 0
+		if framer != nil {
+			seq = framer.dataFrames
+		}
+		if origSeq := gjson.Get(errText, "sequence_number"); origSeq.Exists() {
+			seq = int(origSeq.Int())
+		}
 		if isCodexResponsesClientRequest(c) {
-			chunk := handlers.BuildOpenAIResponsesStreamFailedChunk(status, errText, 0)
+			chunk := handlers.BuildOpenAIResponsesStreamFailedChunk(status, errText, seq)
 			_, _ = fmt.Fprintf(c.Writer, "\nevent: response.failed\ndata: %s\n\n", string(chunk))
 			return
 		}
-		chunk := handlers.BuildOpenAIResponsesStreamErrorChunk(status, errText, 0)
+		chunk := handlers.BuildOpenAIResponsesStreamErrorChunk(status, errText, seq)
 		_, _ = fmt.Fprintf(c.Writer, "\nevent: error\ndata: %s\n\n", string(chunk))
 	}
 
diff --git a/sdk/api/handlers/openai/openai_responses_handlers_stream_error_test.go b/sdk/api/handlers/openai/openai_responses_handlers_stream_error_test.go
index 95e189e4d..496807739 100644
--- a/sdk/api/handlers/openai/openai_responses_handlers_stream_error_test.go
+++ b/sdk/api/handlers/openai/openai_responses_handlers_stream_error_test.go
@@ -521,9 +521,12 @@ func TestForwardResponsesStreamExposesTerminalErrors(t *testing.T) {
 			if exposed != tc.wantExposed {
 				t.Fatalf("error exposed = %t, want %t: %q", exposed, tc.wantExposed, body)
 			}
-			if exposed && strings.Contains(body, `"error":{`) {
+			if exposed && !strings.Contains(body, "event: error\ndata: ") {
 				t.Fatalf("expected streaming error chunk, got HTTP error body: %q", body)
 			}
+			if exposed && !strings.Contains(body, `"error":{`) {
+				t.Fatalf("expected nested error in streaming error chunk, got: %q", body)
+			}
 		})
 	}
 }
diff --git a/sdk/api/handlers/openai/openai_responses_handlers_stream_test.go b/sdk/api/handlers/openai/openai_responses_handlers_stream_test.go
index 80c648748..b0ca07b9a 100644
--- a/sdk/api/handlers/openai/openai_responses_handlers_stream_test.go
+++ b/sdk/api/handlers/openai/openai_responses_handlers_stream_test.go
@@ -2,6 +2,8 @@ package openai
 
 import (
 	"bytes"
+	"encoding/json"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -312,3 +314,215 @@ func TestForwardResponsesStreamDropsIncompleteTrailingDataChunkOnFlush(t *testin
 		t.Fatalf("unterminated framing test stream did not end with an error: %q", got)
 	}
 }
+
+func TestForwardResponsesStreamErrorEventPreservesNestedError(t *testing.T) {
+	h, recorder, c, flusher := newResponsesStreamTestHandler(t)
+	c.Request.Header.Set("User-Agent", "codex_vscode/0.153.4 (Ubuntu 22.4.0; x86_64)")
+
+	data := make(chan []byte)
+	errs := make(chan *interfaces.ErrorMessage, 1)
+
+	go func() {
+		data <- []byte("event: response.created\ndata: {\"type\":\"response.created\",\"sequence_number\":0}\n\n")
+		data <- []byte("event: response.in_progress\ndata: {\"type\":\"response.in_progress\",\"sequence_number\":1}\n\n")
+		close(data)
+
+		errText := `{"error":{"type":"invalid_request","code":"cyber_policy","message":"This content was flagged for possible cybersecurity risk. If this seems wrong, try rephrasing your request. To get authorized for security work, join the Trusted Access for Cyber program: https://chatgpt.com/cyber","param":null}}`
+		errs <- &interfaces.ErrorMessage{
+			StatusCode: http.StatusBadRequest,
+			Error:      errors.New(errText),
+		}
+		close(errs)
+	}()
+
+	framer := &responsesSSEFramer{}
+	h.forwardResponsesStream(c, flusher, func(error) {}, data, errs, framer)
+
+	body := recorder.Body.String()
+	if !strings.Contains(body, "event: error\n") {
+		t.Fatalf("expected event: error in output, got: %q", body)
+	}
+
+	parts := strings.Split(strings.TrimSpace(body), "\n\n")
+	if len(parts) < 3 {
+		t.Fatalf("expected at least 3 SSE events, got %d. Body: %q", len(parts), body)
+	}
+
+	lastPart := strings.TrimSpace(parts[len(parts)-1])
+	if !strings.HasPrefix(lastPart, "event: error\ndata: ") {
+		t.Fatalf("last event is not error event: %q", lastPart)
+	}
+
+	jsonPayload := strings.TrimSpace(strings.TrimPrefix(lastPart, "event: error\ndata: "))
+	var payload struct {
+		Type           string         `json:"type"`
+		Error          map[string]any `json:"error"`
+		SequenceNumber int            `json:"sequence_number"`
+	}
+	if errUnmarshal := json.Unmarshal([]byte(jsonPayload), &payload); errUnmarshal != nil {
+		t.Fatalf("unmarshal error event payload: %v (raw: %s)", errUnmarshal, jsonPayload)
+	}
+	if payload.Type != "error" {
+		t.Fatalf("payload.Type = %q, want error", payload.Type)
+	}
+	if payload.SequenceNumber != 2 {
+		t.Fatalf("payload.SequenceNumber = %d, want 2", payload.SequenceNumber)
+	}
+	if payload.Error["type"] != "invalid_request" {
+		t.Fatalf("payload.Error.type = %v, want invalid_request", payload.Error["type"])
+	}
+	if payload.Error["code"] != "cyber_policy" {
+		t.Fatalf("payload.Error.code = %v, want cyber_policy", payload.Error["code"])
+	}
+	if param, exists := payload.Error["param"]; !exists || param != nil {
+		t.Fatalf("payload.Error.param = %v, want null/nil", param)
+	}
+}
+
+func TestResponsesSSEFramerRepairErrorPayloadCalculatesCorrectSequenceNumber(t *testing.T) {
+	// Case 1: First frame is an error without sequence_number -> should be 0
+	framer1 := &responsesSSEFramer{}
+	var out1 bytes.Buffer
+	framer1.WriteChunk(&out1, []byte("event: error\ndata: {\"error\":{\"code\":\"bad_request\"}}\n\n"))
+	var p1 struct {
+		Type           string         `json:"type"`
+		SequenceNumber int            `json:"sequence_number"`
+		Error          map[string]any `json:"error"`
+	}
+	payload1 := strings.TrimSpace(strings.TrimPrefix(strings.TrimSpace(out1.String()), "event: error\ndata: "))
+	if err := json.Unmarshal([]byte(payload1), &p1); err != nil {
+		t.Fatalf("unmarshal error payload: %v", err)
+	}
+	if p1.SequenceNumber != 0 {
+		t.Fatalf("first frame error sequence_number = %d, want 0", p1.SequenceNumber)
+	}
+
+	// Case 2: Two data frames then an error frame without sequence_number -> should be 2
+	framer2 := &responsesSSEFramer{}
+	var out2 bytes.Buffer
+	framer2.WriteChunk(&out2, []byte("event: response.created\ndata: {\"type\":\"response.created\",\"sequence_number\":0}\n\n"))
+	framer2.WriteChunk(&out2, []byte("event: response.in_progress\ndata: {\"type\":\"response.in_progress\",\"sequence_number\":1}\n\n"))
+	out2.Reset()
+	framer2.WriteChunk(&out2, []byte("event: error\ndata: {\"error\":{\"code\":\"cyber_policy\"}}\n\n"))
+	var p2 struct {
+		Type           string         `json:"type"`
+		SequenceNumber int            `json:"sequence_number"`
+		Error          map[string]any `json:"error"`
+	}
+	payload2 := strings.TrimSpace(strings.TrimPrefix(strings.TrimSpace(out2.String()), "event: error\ndata: "))
+	if err := json.Unmarshal([]byte(payload2), &p2); err != nil {
+		t.Fatalf("unmarshal error payload: %v", err)
+	}
+	if p2.SequenceNumber != 2 {
+		t.Fatalf("third frame error sequence_number = %d, want 2", p2.SequenceNumber)
+	}
+}
+
+func TestForwardResponsesStreamErrorEventPreservesExplicitSequenceNumber(t *testing.T) {
+	h, recorder, c, flusher := newResponsesStreamTestHandler(t)
+	c.Request.Header.Set("User-Agent", "codex_vscode/0.153.4")
+
+	data := make(chan []byte)
+	errs := make(chan *interfaces.ErrorMessage, 1)
+
+	go func() {
+		close(data)
+		errText := `{"error":{"type":"invalid_request","code":"custom"},"sequence_number":9}`
+		errs <- &interfaces.ErrorMessage{
+			StatusCode: http.StatusBadRequest,
+			Error:      errors.New(errText),
+		}
+		close(errs)
+	}()
+
+	framer := &responsesSSEFramer{}
+	h.forwardResponsesStream(c, flusher, func(error) {}, data, errs, framer)
+
+	body := recorder.Body.String()
+	var p struct {
+		Type           string         `json:"type"`
+		SequenceNumber int            `json:"sequence_number"`
+		Error          map[string]any `json:"error"`
+	}
+	lastPart := strings.TrimSpace(strings.Split(strings.TrimSpace(body), "\n\n")[0])
+	payload := strings.TrimSpace(strings.TrimPrefix(lastPart, "event: error\ndata: "))
+	if err := json.Unmarshal([]byte(payload), &p); err != nil {
+		t.Fatalf("unmarshal error payload: %v (raw: %s)", err, payload)
+	}
+	if p.SequenceNumber != 9 {
+		t.Fatalf("explicit sequence_number = %d, want 9", p.SequenceNumber)
+	}
+}
+
+func TestResponsesStreamErrorTextSanitizesNestedSensitiveFieldsAndDotKeys(t *testing.T) {
+	inputJSON := `{"error":{"metadata":{"message":"Bearer test-secret"},"vendor.detail":"Bearer test-secret","access_token":"test-secret","normal_key":"safe_value"},"sequence_number":3}`
+	errMsg := &interfaces.ErrorMessage{
+		StatusCode: http.StatusBadRequest,
+		Error:      errors.New(inputJSON),
+	}
+	out := responsesStreamErrorText(errMsg, http.StatusBadRequest)
+
+	var parsed struct {
+		Error          map[string]any `json:"error"`
+		SequenceNumber int            `json:"sequence_number"`
+	}
+	if errUnmarshal := json.Unmarshal([]byte(out), &parsed); errUnmarshal != nil {
+		t.Fatalf("unmarshal error text output: %v (raw: %s)", errUnmarshal, out)
+	}
+
+	if parsed.SequenceNumber != 3 {
+		t.Fatalf("sequence_number = %d, want 3", parsed.SequenceNumber)
+	}
+	if parsed.Error["access_token"] != "[REDACTED]" {
+		t.Fatalf("access_token was not redacted: %v", parsed.Error["access_token"])
+	}
+	if parsed.Error["vendor.detail"] != "Bearer [REDACTED]" {
+		t.Fatalf("vendor.detail was not properly redacted without path corruption: %v", parsed.Error["vendor.detail"])
+	}
+	meta, ok := parsed.Error["metadata"].(map[string]any)
+	if !ok {
+		t.Fatalf("metadata is not a map: %v", parsed.Error["metadata"])
+	}
+	if meta["message"] != "Bearer [REDACTED]" {
+		t.Fatalf("nested metadata.message was not redacted: %v", meta["message"])
+	}
+	if parsed.Error["normal_key"] != "safe_value" {
+		t.Fatalf("normal_key = %v, want safe_value", parsed.Error["normal_key"])
+	}
+}
+
+func TestResponsesStreamErrorTextPreservesTokenCountersAndLargeInts(t *testing.T) {
+	inputJSON := `{"error":{"input_tokens":42,"token_limit":8192,"request_id":9007199254740993,"access_token":"secret123"}}`
+	errMsg := &interfaces.ErrorMessage{
+		StatusCode: http.StatusBadRequest,
+		Error:      errors.New(inputJSON),
+	}
+	out := responsesStreamErrorText(errMsg, http.StatusBadRequest)
+
+	// Verify exact raw representation doesn't lose precision for 9007199254740993
+	if !strings.Contains(out, "9007199254740993") {
+		t.Fatalf("large integer precision was lost: %s", out)
+	}
+	if strings.Contains(out, "9007199254740992") {
+		t.Fatalf("large integer was corrupted by float64 conversion: %s", out)
+	}
+
+	var parsed struct {
+		Error map[string]any `json:"error"`
+	}
+	dec := json.NewDecoder(strings.NewReader(out))
+	dec.UseNumber()
+	if errUnmarshal := dec.Decode(&parsed); errUnmarshal != nil {
+		t.Fatalf("unmarshal error: %v", errUnmarshal)
+	}
+
+	if parsed.Error["access_token"] != "[REDACTED]" {
+		t.Fatalf("access_token should be redacted: %v", parsed.Error["access_token"])
+	}
+	if parsed.Error["input_tokens"] != json.Number("42") {
+		t.Fatalf("input_tokens should remain number 42, got %v", parsed.Error["input_tokens"])
+	}
+	if parsed.Error["token_limit"] != json.Number("8192") {
+		t.Fatalf("token_limit should remain number 8192, got %v", parsed.Error["token_limit"])
+	}
+}
diff --git a/sdk/api/handlers/openai_responses_stream_error.go b/sdk/api/handlers/openai_responses_stream_error.go
index a3c3c7e32..d2951f1fa 100644
--- a/sdk/api/handlers/openai_responses_stream_error.go
+++ b/sdk/api/handlers/openai_responses_stream_error.go
@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"net/http"
@@ -8,10 +9,9 @@ import (
 )
 
 type openAIResponsesStreamErrorChunk struct {
-	Type           string `json:"type"`
-	Code           string `json:"code"`
-	Message        string `json:"message"`
-	SequenceNumber int    `json:"sequence_number"`
+	Type           string         `json:"type"`
+	Error          map[string]any `json:"error"`
+	SequenceNumber int            `json:"sequence_number"`
 }
 
 type openAIResponsesStreamFailedChunk struct {
@@ -48,11 +48,14 @@ func openAIResponsesStreamErrorCode(status int) string {
 	}
 }
 
+func unmarshalJSONWithNumber(data []byte, v any) error {
+	dec := json.NewDecoder(bytes.NewReader(data))
+	dec.UseNumber()
+	return dec.Decode(v)
+}
+
 // 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.
+// It matches the official responses streaming event shape where the error details are nested.
 func BuildOpenAIResponsesStreamErrorChunk(status int, errText string, sequenceNumber int) []byte {
 	if status <= 0 {
 		status = http.StatusInternalServerError
@@ -71,32 +74,15 @@ func BuildOpenAIResponsesStreamErrorChunk(status int, errText string, sequenceNu
 	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 errUnmarshal := unmarshalJSONWithNumber([]byte(trimmed), &payload); errUnmarshal == nil {
+			if v, ok := payload["sequence_number"]; ok {
+				switch n := v.(type) {
+				case json.Number:
+					if seqInt, err := n.Int64(); err == nil {
+						sequenceNumber = int(seqInt)
 					}
+				case float64:
+					sequenceNumber = int(n)
 				}
 			}
 		}
@@ -106,39 +92,58 @@ func BuildOpenAIResponsesStreamErrorChunk(status int, errText string, sequenceNu
 		code = "unknown_error"
 	}
 
-	data, err := json.Marshal(openAIResponsesStreamErrorChunk{
+	errorDetail := openAIResponsesStreamErrorDetail(status, errText, code, message)
+
+	data, errMarshal := json.Marshal(openAIResponsesStreamErrorChunk{
 		Type:           "error",
-		Code:           code,
-		Message:        message,
+		Error:          errorDetail,
 		SequenceNumber: sequenceNumber,
 	})
-	if err == nil {
+	if errMarshal == nil {
 		return data
 	}
 
 	// Extremely defensive fallback.
+	fallbackDetail := map[string]any{
+		"type":    "server_error",
+		"code":    "internal_server_error",
+		"message": message,
+		"param":   nil,
+	}
 	data, _ = json.Marshal(openAIResponsesStreamErrorChunk{
 		Type:           "error",
-		Code:           "internal_server_error",
-		Message:        message,
+		Error:          fallbackDetail,
 		SequenceNumber: sequenceNumber,
 	})
 	if len(data) > 0 {
 		return data
 	}
-	return []byte(`{"type":"error","code":"internal_server_error","message":"internal error","sequence_number":0}`)
+	return []byte(`{"type":"error","error":{"type":"server_error","code":"internal_server_error","message":"internal error","param":null},"sequence_number":0}`)
 }
 
-func openAIResponsesStreamFailedErrorDetail(status int, errText, code, message string) map[string]any {
+func openAIResponsesStreamErrorDetail(status int, errText, code, message string) map[string]any {
 	var payload map[string]any
-	if errUnmarshal := json.Unmarshal([]byte(strings.TrimSpace(errText)), &payload); errUnmarshal == nil {
-		if errorDetail, ok := payload["error"].(map[string]any); ok {
-			return errorDetail
-		}
-		if response, ok := payload["response"].(map[string]any); ok {
-			if errorDetail, ok := response["error"].(map[string]any); ok {
+	trimmed := strings.TrimSpace(errText)
+	if trimmed != "" && json.Valid([]byte(trimmed)) {
+		if errUnmarshal := unmarshalJSONWithNumber([]byte(trimmed), &payload); errUnmarshal == nil {
+			if errorDetail, ok := payload["error"].(map[string]any); ok {
 				return errorDetail
 			}
+			if response, ok := payload["response"].(map[string]any); ok {
+				if errorDetail, ok := response["error"].(map[string]any); ok {
+					return errorDetail
+				}
+			}
+			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))
+				}
+			}
 		}
 	}
 
@@ -146,11 +151,25 @@ func openAIResponsesStreamFailedErrorDetail(status int, errText, code, message s
 	if status >= http.StatusInternalServerError {
 		errorType = "server_error"
 	}
-	return map[string]any{
+	detail := map[string]any{
 		"type":    errorType,
 		"code":    code,
 		"message": message,
+		"param":   nil,
 	}
+	if payload != nil {
+		if t, ok := payload["type"].(string); ok && strings.TrimSpace(t) != "" && strings.TrimSpace(t) != "error" {
+			detail["type"] = strings.TrimSpace(t)
+		}
+		if paramVal, exists := payload["param"]; exists {
+			detail["param"] = paramVal
+		}
+	}
+	return detail
+}
+
+func openAIResponsesStreamFailedErrorDetail(status int, errText, code, message string) map[string]any {
+	return openAIResponsesStreamErrorDetail(status, errText, code, message)
 }
 
 // BuildOpenAIResponsesStreamFailedChunk builds the terminal Responses event used by official Codex clients.
@@ -163,15 +182,19 @@ func BuildOpenAIResponsesStreamFailedChunk(status int, errText string, sequenceN
 		sequenceNumber = 0
 	}
 
-	legacyChunk := BuildOpenAIResponsesStreamErrorChunk(status, errText, sequenceNumber)
-	var legacyPayload openAIResponsesStreamErrorChunk
-	if errUnmarshal := json.Unmarshal(legacyChunk, &legacyPayload); errUnmarshal != nil {
-		legacyPayload.Code = openAIResponsesStreamErrorCode(status)
-		legacyPayload.Message = http.StatusText(status)
-		legacyPayload.SequenceNumber = sequenceNumber
-	}
-	if sequenceNumber == 0 && legacyPayload.SequenceNumber > 0 {
-		sequenceNumber = legacyPayload.SequenceNumber
+	errorChunkBytes := BuildOpenAIResponsesStreamErrorChunk(status, errText, sequenceNumber)
+	var errorChunk openAIResponsesStreamErrorChunk
+	_ = unmarshalJSONWithNumber(errorChunkBytes, &errorChunk)
+	sequenceNumber = errorChunk.SequenceNumber
+
+	errorDetail := errorChunk.Error
+	if errorDetail == nil {
+		code := openAIResponsesStreamErrorCode(status)
+		message := strings.TrimSpace(errText)
+		if message == "" {
+			message = http.StatusText(status)
+		}
+		errorDetail = openAIResponsesStreamErrorDetail(status, errText, code, message)
 	}
 
 	data, errMarshal := json.Marshal(openAIResponsesStreamFailedChunk{
@@ -179,7 +202,7 @@ func BuildOpenAIResponsesStreamFailedChunk(status int, errText string, sequenceN
 		SequenceNumber: sequenceNumber,
 		Response: openAIResponsesStreamFailedResponse{
 			Status: "failed",
-			Error:  openAIResponsesStreamFailedErrorDetail(status, errText, legacyPayload.Code, legacyPayload.Message),
+			Error:  errorDetail,
 		},
 	})
 	if errMarshal == nil {
diff --git a/sdk/api/handlers/openai_responses_stream_error_test.go b/sdk/api/handlers/openai_responses_stream_error_test.go
index c6dfd25ef..01752090f 100644
--- a/sdk/api/handlers/openai_responses_stream_error_test.go
+++ b/sdk/api/handlers/openai_responses_stream_error_test.go
@@ -3,6 +3,7 @@ package handlers
 import (
 	"encoding/json"
 	"net/http"
+	"strings"
 	"testing"
 )
 
@@ -15,11 +16,15 @@ func TestBuildOpenAIResponsesStreamErrorChunk(t *testing.T) {
 	if payload["type"] != "error" {
 		t.Fatalf("type = %v, want %q", payload["type"], "error")
 	}
-	if payload["code"] != "internal_server_error" {
-		t.Fatalf("code = %v, want %q", payload["code"], "internal_server_error")
+	errorObj, ok := payload["error"].(map[string]any)
+	if !ok {
+		t.Fatalf("error is not an object: %v", payload["error"])
 	}
-	if payload["message"] != "unexpected EOF" {
-		t.Fatalf("message = %v, want %q", payload["message"], "unexpected EOF")
+	if errorObj["code"] != "internal_server_error" {
+		t.Fatalf("code = %v, want %q", errorObj["code"], "internal_server_error")
+	}
+	if errorObj["message"] != "unexpected EOF" {
+		t.Fatalf("message = %v, want %q", errorObj["message"], "unexpected EOF")
 	}
 	if payload["sequence_number"] != float64(0) {
 		t.Fatalf("sequence_number = %v, want %v", payload["sequence_number"], 0)
@@ -39,11 +44,105 @@ func TestBuildOpenAIResponsesStreamErrorChunkExtractsHTTPErrorBody(t *testing.T)
 	if payload["type"] != "error" {
 		t.Fatalf("type = %v, want %q", payload["type"], "error")
 	}
-	if payload["code"] != "internal_server_error" {
-		t.Fatalf("code = %v, want %q", payload["code"], "internal_server_error")
+	errorObj, ok := payload["error"].(map[string]any)
+	if !ok {
+		t.Fatalf("error is not an object: %v", payload["error"])
 	}
-	if payload["message"] != "oops" {
-		t.Fatalf("message = %v, want %q", payload["message"], "oops")
+	if errorObj["code"] != "internal_server_error" {
+		t.Fatalf("code = %v, want %q", errorObj["code"], "internal_server_error")
+	}
+	if errorObj["message"] != "oops" {
+		t.Fatalf("message = %v, want %q", errorObj["message"], "oops")
+	}
+	if errorObj["type"] != "server_error" {
+		t.Fatalf("error.type = %v, want %q", errorObj["type"], "server_error")
+	}
+}
+
+func TestBuildOpenAIResponsesStreamErrorChunkPreservesNestedError(t *testing.T) {
+	errText := `{"error":{"type":"invalid_request","code":"cyber_policy","message":"This content was flagged for possible cybersecurity risk.","param":null}}`
+	chunk := BuildOpenAIResponsesStreamErrorChunk(http.StatusBadRequest, errText, 2)
+	var payload struct {
+		Type           string         `json:"type"`
+		Error          map[string]any `json:"error"`
+		SequenceNumber int            `json:"sequence_number"`
+	}
+	if err := json.Unmarshal(chunk, &payload); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if payload.Type != "error" {
+		t.Fatalf("type = %q, want %q", payload.Type, "error")
+	}
+	if payload.SequenceNumber != 2 {
+		t.Fatalf("sequence_number = %d, want 2", payload.SequenceNumber)
+	}
+	if payload.Error["type"] != "invalid_request" {
+		t.Fatalf("error.type = %v, want invalid_request", payload.Error["type"])
+	}
+	if payload.Error["code"] != "cyber_policy" {
+		t.Fatalf("error.code = %v, want cyber_policy", payload.Error["code"])
+	}
+	if payload.Error["message"] != "This content was flagged for possible cybersecurity risk." {
+		t.Fatalf("error.message = %v", payload.Error["message"])
+	}
+	if param, exists := payload.Error["param"]; !exists || param != nil {
+		t.Fatalf("error.param = %v, want nil", param)
+	}
+}
+
+func TestBuildOpenAIResponsesStreamErrorChunkPreservesCustomAndEmptyFields(t *testing.T) {
+	// Preserves empty error object {}
+	emptyChunk := BuildOpenAIResponsesStreamErrorChunk(http.StatusBadRequest, `{"error":{}}`, 0)
+	var emptyPayload struct {
+		Type           string         `json:"type"`
+		Error          map[string]any `json:"error"`
+		SequenceNumber int            `json:"sequence_number"`
+	}
+	if err := json.Unmarshal(emptyChunk, &emptyPayload); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if len(emptyPayload.Error) != 0 {
+		t.Fatalf("expected empty error object, got %v", emptyPayload.Error)
+	}
+
+	// Preserves custom fields and types without dropping
+	customText := `{"error":{"type":"custom_type","code":"custom_code","custom_key":"custom_val","is_flag":true,"count":42}}`
+	customChunk := BuildOpenAIResponsesStreamErrorChunk(http.StatusBadRequest, customText, 5)
+	var customPayload struct {
+		Type           string         `json:"type"`
+		Error          map[string]any `json:"error"`
+		SequenceNumber int            `json:"sequence_number"`
+	}
+	if err := json.Unmarshal(customChunk, &customPayload); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if customPayload.SequenceNumber != 5 {
+		t.Fatalf("sequence_number = %d, want 5", customPayload.SequenceNumber)
+	}
+	if customPayload.Error["custom_key"] != "custom_val" {
+		t.Fatalf("custom_key = %v, want custom_val", customPayload.Error["custom_key"])
+	}
+	if customPayload.Error["is_flag"] != true {
+		t.Fatalf("is_flag = %v, want true", customPayload.Error["is_flag"])
+	}
+	if customPayload.Error["count"] != float64(42) {
+		t.Fatalf("count = %v, want 42", customPayload.Error["count"])
+	}
+}
+
+func TestBuildOpenAIResponsesStreamErrorChunkPrioritizesPayloadSequenceNumber(t *testing.T) {
+	errText := `{"error":{"type":"invalid_request","code":"blocked"},"sequence_number":7}`
+	chunk := BuildOpenAIResponsesStreamErrorChunk(http.StatusBadRequest, errText, 2)
+	var payload struct {
+		Type           string         `json:"type"`
+		Error          map[string]any `json:"error"`
+		SequenceNumber int            `json:"sequence_number"`
+	}
+	if err := json.Unmarshal(chunk, &payload); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if payload.SequenceNumber != 7 {
+		t.Fatalf("sequence_number = %d, want 7 (from payload)", payload.SequenceNumber)
 	}
 }
 
@@ -88,3 +187,43 @@ func TestBuildOpenAIResponsesStreamFailedChunkPreservesNestedError(t *testing.T)
 		t.Fatalf("response.error.message = %q, want %q", payload.Response.Error.Message, "blocked")
 	}
 }
+
+func TestBuildOpenAIResponsesStreamFailedChunkPrioritizesPayloadSequenceNumber(t *testing.T) {
+	chunk := BuildOpenAIResponsesStreamFailedChunk(
+		http.StatusBadRequest,
+		`{"error":{"type":"invalid_request","code":"cyber_policy","message":"blocked"},"sequence_number":7}`,
+		2,
+	)
+
+	var payload struct {
+		Type           string `json:"type"`
+		SequenceNumber int    `json:"sequence_number"`
+	}
+	if err := json.Unmarshal(chunk, &payload); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if payload.SequenceNumber != 7 {
+		t.Fatalf("sequence_number = %d, want 7 (from payload over arg 2)", payload.SequenceNumber)
+	}
+}
+
+func TestBuildOpenAIResponsesStreamErrorChunkPreservesLargeIntPrecision(t *testing.T) {
+	errText := `{"error":{"type":"invalid_request","code":"blocked","request_id":9007199254740993}}`
+	chunk := BuildOpenAIResponsesStreamErrorChunk(http.StatusBadRequest, errText, 0)
+	raw := string(chunk)
+	if !strings.Contains(raw, "9007199254740993") {
+		t.Fatalf("large integer precision was lost in error chunk: %s", raw)
+	}
+	if strings.Contains(raw, "9007199254740992") {
+		t.Fatalf("large integer was corrupted by float64 in error chunk: %s", raw)
+	}
+
+	failedChunk := BuildOpenAIResponsesStreamFailedChunk(http.StatusBadRequest, errText, 0)
+	failedRaw := string(failedChunk)
+	if !strings.Contains(failedRaw, "9007199254740993") {
+		t.Fatalf("large integer precision was lost in failed chunk: %s", failedRaw)
+	}
+	if strings.Contains(failedRaw, "9007199254740992") {
+		t.Fatalf("large integer was corrupted by float64 in failed chunk: %s", failedRaw)
+	}
+}