diff --git a/backend/internal/service/openai_gateway_request_body.go b/backend/internal/service/openai_gateway_request_body.go
index c9a61d7465..8aa4364777 100644
--- a/backend/internal/service/openai_gateway_request_body.go
+++ b/backend/internal/service/openai_gateway_request_body.go
@@ -366,8 +366,7 @@ func normalizeOpenAIParallelToolCallsWithoutTools(body []byte) ([]byte, bool, er
 	if !parallel.Exists() {
 		return body, false, nil
 	}
-	tools := gjson.GetBytes(body, "tools")
-	if tools.IsArray() && len(tools.Array()) > 0 {
+	if openAIRequestBodyHasTools(body) {
 		return body, false, nil
 	}
 	normalized, err := sjson.DeleteBytes(body, "parallel_tool_calls")
@@ -377,6 +376,31 @@ func normalizeOpenAIParallelToolCallsWithoutTools(body []byte) ([]byte, bool, er
 	return normalized, true, nil
 }
 
+// openAIRequestBodyHasTools is the []byte counterpart of openAIResponsesLiteHasTools:
+// besides the top-level "tools" array it also recognizes the Responses Lite carrier.
+// normalizeOpenAIResponsesLiteTools moves namespace tools into an input item of type
+// "additional_tools" and drops the top-level "tools" key; the request still carries
+// tools at that point. Looking only at the top level therefore misreads such a body as
+// "no tools" and deletes the parallel_tool_calls:false that
+// ensureOpenAIResponsesLiteParallelToolCalls had just pinned, and OpenAI falls back to
+// its default of true and rejects the request with
+// 400 unsupported_value: "X-OpenAI-Internal-Codex-Responses-Lite requires
+// `parallel_tool_calls` to be false."
+func openAIRequestBodyHasTools(body []byte) bool {
+	if tools := gjson.GetBytes(body, "tools"); tools.IsArray() && len(tools.Array()) > 0 {
+		return true
+	}
+	for _, item := range gjson.GetBytes(body, "input").Array() {
+		if strings.TrimSpace(item.Get("type").String()) != "additional_tools" {
+			continue
+		}
+		if tools := item.Get("tools"); tools.IsArray() && len(tools.Array()) > 0 {
+			return true
+		}
+	}
+	return false
+}
+
 func normalizeOpenAIAPIKeyStoreFalseReasoningReplay(body []byte, knownStoreFalse bool) ([]byte, bool, error) {
 	if !knownStoreFalse && gjson.GetBytes(body, "store").Type != gjson.False {
 		return body, false, nil
diff --git a/backend/internal/service/openai_gateway_request_body_reasoning_test.go b/backend/internal/service/openai_gateway_request_body_reasoning_test.go
index f14a75018b..03397e751f 100644
--- a/backend/internal/service/openai_gateway_request_body_reasoning_test.go
+++ b/backend/internal/service/openai_gateway_request_body_reasoning_test.go
@@ -270,3 +270,23 @@ func TestNormalizeOpenAIParallelToolCallsWithoutTools(t *testing.T) {
 	require.True(t, changed)
 	require.False(t, gjson.GetBytes(normalized, "parallel_tool_calls").Exists())
 }
+
+// A Responses Lite body that has already been through normalizeOpenAIResponsesLiteTools
+// carries its tools in an input item of type "additional_tools" and no longer has a
+// top-level "tools" key. It still has tools, so the parallel_tool_calls:false that
+// ensureOpenAIResponsesLiteParallelToolCalls pinned must survive this normalization —
+// otherwise OpenAI applies its default of true and rejects the request.
+func TestNormalizeOpenAIParallelToolCallsWithoutTools_KeepsResponsesLiteAdditionalTools(t *testing.T) {
+	liteBody := []byte(`{"input":[{"type":"message","role":"user","content":"hi"},{"type":"additional_tools","tools":[{"type":"function","name":"spawn_agent"}]}],"parallel_tool_calls":false}`)
+	normalized, changed, err := normalizeOpenAIParallelToolCallsWithoutTools(liteBody)
+	require.NoError(t, err)
+	require.False(t, changed)
+	require.Equal(t, gjson.False, gjson.GetBytes(normalized, "parallel_tool_calls").Type)
+
+	// An empty additional_tools item carries no tools, so the field is still dropped.
+	emptyLiteBody := []byte(`{"input":[{"type":"additional_tools","tools":[]}],"parallel_tool_calls":true}`)
+	normalized, changed, err = normalizeOpenAIParallelToolCallsWithoutTools(emptyLiteBody)
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.False(t, gjson.GetBytes(normalized, "parallel_tool_calls").Exists())
+}