diff --git a/backend/internal/service/openai_responses_lite_tools.go b/backend/internal/service/openai_responses_lite_tools.go
index b13e1443d0..752ed00561 100644
--- a/backend/internal/service/openai_responses_lite_tools.go
+++ b/backend/internal/service/openai_responses_lite_tools.go
@@ -1,7 +1,6 @@
 package service
 
 import (
-	"encoding/json"
 	"fmt"
 	"reflect"
 	"strings"
@@ -263,7 +262,7 @@ func openAIResponsesLiteToolIdentityForError(rawTool any) string {
 
 func normalizeOpenAIResponsesLiteToolsPayload(body []byte) ([]byte, bool, error) {
 	var requestBody map[string]any
-	if err := json.Unmarshal(body, &requestBody); err != nil {
+	if err := decodeOpenAIJSONUseNumber(body, &requestBody); err != nil {
 		return body, false, fmt.Errorf("decode responses Lite request body: %w", err)
 	}
 	changed, err := normalizeOpenAIResponsesLiteTools(requestBody)
@@ -279,7 +278,7 @@ func normalizeOpenAIResponsesLiteToolsPayload(body []byte) ([]byte, bool, error)
 
 func normalizeOpenAIResponsesLiteParallelToolCallsPayload(body []byte) ([]byte, bool, error) {
 	var requestBody map[string]any
-	if err := json.Unmarshal(body, &requestBody); err != nil {
+	if err := decodeOpenAIJSONUseNumber(body, &requestBody); err != nil {
 		return body, false, fmt.Errorf("decode responses Lite request body: %w", err)
 	}
 	changed, err := ensureOpenAIResponsesLiteParallelToolCalls(requestBody, false)
diff --git a/backend/internal/service/openai_responses_lite_tools_test.go b/backend/internal/service/openai_responses_lite_tools_test.go
index e1166ef4e9..3edff5c2da 100644
--- a/backend/internal/service/openai_responses_lite_tools_test.go
+++ b/backend/internal/service/openai_responses_lite_tools_test.go
@@ -344,6 +344,34 @@ func TestNormalizeOpenAIResponsesLiteToolsPayload_PreservesResponseCreateShape(t
 	require.False(t, gjson.GetBytes(updated, "parallel_tool_calls").Bool())
 }
 
+func TestNormalizeOpenAIResponsesLitePayloads_PreserveLargeSequence(t *testing.T) {
+	body := []byte(`{
+		"type":"response.create",
+		"sequence":900719925474099312345,
+		"tools":[{"type":"function","name":"lookup"}],
+		"parallel_tool_calls":true
+	}`)
+	tests := []struct {
+		name      string
+		normalize func([]byte) ([]byte, bool, error)
+	}{
+		{name: "OAuth-like tools normalization", normalize: normalizeOpenAIResponsesLiteToolsPayload},
+		{name: "API key parallel normalization", normalize: normalizeOpenAIResponsesLiteParallelToolCallsPayload},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			updated, changed, err := tt.normalize(body)
+
+			require.NoError(t, err)
+			require.True(t, changed)
+			require.Equal(t, "900719925474099312345", gjson.GetBytes(updated, "sequence").Raw)
+			require.True(t, gjson.GetBytes(updated, "parallel_tool_calls").Exists())
+			require.False(t, gjson.GetBytes(updated, "parallel_tool_calls").Bool())
+		})
+	}
+}
+
 func TestApplyCodexOAuthTransform_PreservesLiteNamespaceToolChoice(t *testing.T) {
 	reqBody := map[string]any{
 		"model": "gpt-5.6-terra",