fix(openai): keep item ID sanitization linear

backend/internal/service/openai_gateway_apikey_item_id_test.go

@@ -4,8 +4,10 @@ package service
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"net/http"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -60,3 +62,29 @@ func TestOpenAIGatewayService_APIKeyPassthrough_StripsInvalidInputItemIDs(t *tes
 	require.Equal(t, "call_123", gjson.GetBytes(forwarded, "input.4.call_id").String())
 	require.Equal(t, "item_unconstrained", gjson.GetBytes(forwarded, "input.5.id").String())
 }
+
+func TestSanitizeOpenAIResponsesInputItemIDs_AllocationGrowthIsLinear(t *testing.T) {
+	makeBody := func(itemCount int) []byte {
+		items := make([]string, itemCount)
+		for i := range items {
+			items[i] = fmt.Sprintf(`{"type":"message","id":"item_%d","role":"user","content":[{"type":"input_text","text":"hello"}]}`, i)
+		}
+		return []byte(`{"model":"gpt-5.6-sol","input":[` + strings.Join(items, ",") + `]}`)
+	}
+	allocatedBytes := func(body []byte) uint64 {
+		runtime.GC()
+		var before, after runtime.MemStats
+		runtime.ReadMemStats(&before)
+		sanitized, changed, err := sanitizeOpenAIResponsesInputItemIDs(body)
+		runtime.ReadMemStats(&after)
+		require.NoError(t, err)
+		require.True(t, changed)
+		require.NotEmpty(t, sanitized)
+		return after.TotalAlloc - before.TotalAlloc
+	}
+
+	smallAllocated := allocatedBytes(makeBody(20))
+	largeAllocated := allocatedBytes(makeBody(200))
+	require.Less(t, largeAllocated, smallAllocated*30,
+		"10x more input items must not cause quadratic whole-body allocation growth")
+}

backend/internal/service/openai_responses_item_id.go

@@ -29,33 +29,50 @@ func sanitizeOpenAIResponsesInputItemIDs(body []byte) ([]byte, bool, error) {
 		return body, false, nil
 	}
 
-	paths := make([]string, 0)
+	items := make([][]byte, 0)
+	changed := false
+	var sanitizeErr error
 	index := 0
 	input.ForEach(func(_, item gjson.Result) bool {
 		currentIndex := index
 		index++
-		if !item.IsObject() {
-			return true
-		}
-		itemType := item.Get("type")
-		id := item.Get("id")
-		if itemType.Type == gjson.String && id.Type == gjson.String &&
-			shouldStripOpenAIResponsesInputItemID(itemType.String(), id.String()) {
-			paths = append(paths, fmt.Sprintf("input.%d.id", currentIndex))
+		itemBody := []byte(item.Raw)
+		if item.IsObject() {
+			itemType := item.Get("type")
+			id := item.Get("id")
+			if itemType.Type == gjson.String && id.Type == gjson.String &&
+				shouldStripOpenAIResponsesInputItemID(itemType.String(), id.String()) {
+				itemBody, sanitizeErr = sjson.DeleteBytes(itemBody, "id")
+				if sanitizeErr != nil {
+					sanitizeErr = fmt.Errorf("delete input.%d.id: %w", currentIndex, sanitizeErr)
+					return false
+				}
+				changed = true
+			}
 		}
+		items = append(items, itemBody)
 		return true
 	})
-	if len(paths) == 0 {
+	if sanitizeErr != nil {
+		return nil, false, sanitizeErr
+	}
+	if !changed {
 		return body, false, nil
 	}
 
-	sanitized := body
-	for _, path := range paths {
-		var err error
-		sanitized, err = sjson.DeleteBytes(sanitized, path)
-		if err != nil {
-			return nil, false, fmt.Errorf("delete %s: %w", path, err)
+	rebuiltInput := make([]byte, 0, len(input.Raw))
+	rebuiltInput = append(rebuiltInput, '[')
+	for i, item := range items {
+		if i > 0 {
+			rebuiltInput = append(rebuiltInput, ',')
 		}
+		rebuiltInput = append(rebuiltInput, item...)
+	}
+	rebuiltInput = append(rebuiltInput, ']')
+
+	sanitized, err := sjson.SetRawBytes(body, "input", rebuiltInput)
+	if err != nil {
+		return nil, false, fmt.Errorf("replace sanitized input: %w", err)
 	}
 	return sanitized, true, nil
 }