feat(executor): normalize parallel_tool_calls based on tools presence

- Added `normalizeCodexParallelToolCallsForTools` to conditionally remove `parallel_tool_calls` when `tools` are missing or empty.
- Integrated normalization into Codex executor workflows for improved request handling.
- Introduced unit tests to validate behavior across different tool scenarios.

Closes: #3903

internal/runtime/executor/codex_executor.go

@@ -829,6 +829,7 @@ func (e *CodexExecutor) Execute(ctx context.Context, auth *cliproxyauth.Auth, re
 		body = ensureImageGenerationTool(body, baseModel, auth)
 	}
 	body = sanitizeOpenAIResponsesReasoningEncryptedContent(ctx, "codex executor", body)
+	body = normalizeCodexParallelToolCallsForTools(body)
 	body, replayScope, errReplay := applyCodexReasoningReplayCacheRequired(ctx, from, req, opts, body)
 	if errReplay != nil {
 		return resp, errReplay
@@ -1004,6 +1005,7 @@ func (e *CodexExecutor) executeCompact(ctx context.Context, auth *cliproxyauth.A
 		body = ensureImageGenerationTool(body, baseModel, auth)
 	}
 	body = sanitizeOpenAIResponsesReasoningEncryptedContent(ctx, "codex executor", body)
+	body = normalizeCodexParallelToolCallsForTools(body)
 	reporter.SetTranslatedReasoningEffort(body, to.String())
 
 	url := strings.TrimSuffix(baseURL, "/") + "/responses/compact"
@@ -1113,6 +1115,7 @@ func (e *CodexExecutor) ExecuteStream(ctx context.Context, auth *cliproxyauth.Au
 		body = ensureImageGenerationTool(body, baseModel, auth)
 	}
 	body = sanitizeOpenAIResponsesReasoningEncryptedContent(ctx, "codex executor", body)
+	body = normalizeCodexParallelToolCallsForTools(body)
 	body, replayScope, errReplay := applyCodexReasoningReplayCacheRequired(ctx, from, req, opts, body)
 	if errReplay != nil {
 		return nil, errReplay
@@ -1775,6 +1778,21 @@ func ensureImageGenerationTool(body []byte, baseModel string, auth *cliproxyauth
 	return body
 }
 
+func normalizeCodexParallelToolCallsForTools(body []byte) []byte {
+	if !gjson.GetBytes(body, "parallel_tool_calls").Exists() {
+		return body
+	}
+
+	tools := gjson.GetBytes(body, "tools")
+	hasTools := tools.Exists() && tools.IsArray() && len(tools.Array()) > 0
+	if hasTools {
+		return body
+	}
+
+	body, _ = sjson.DeleteBytes(body, "parallel_tool_calls")
+	return body
+}
+
 func publishCodexImageToolUsage(ctx context.Context, reporter *helps.UsageReporter, body []byte, completedData []byte) {
 	detail, ok := helps.ParseCodexImageToolUsage(completedData)
 	if !ok {

internal/runtime/executor/codex_executor_parallel_tool_calls_test.go

@@ -0,0 +1,40 @@
+package executor
+
+import (
+	"testing"
+
+	"github.com/tidwall/gjson"
+)
+
+func TestNormalizeCodexParallelToolCallsForTools_DropsWhenToolsMissing(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","parallel_tool_calls":true,"input":"hi"}`)
+
+	out := normalizeCodexParallelToolCallsForTools(body)
+
+	if gjson.GetBytes(out, "parallel_tool_calls").Exists() {
+		t.Fatalf("parallel_tool_calls should be removed when tools are missing: %s", string(out))
+	}
+}
+
+func TestNormalizeCodexParallelToolCallsForTools_DropsWhenToolsEmpty(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","tools":[],"parallel_tool_calls":false,"input":"hi"}`)
+
+	out := normalizeCodexParallelToolCallsForTools(body)
+
+	if gjson.GetBytes(out, "parallel_tool_calls").Exists() {
+		t.Fatalf("parallel_tool_calls should be removed when tools are empty: %s", string(out))
+	}
+	if !gjson.GetBytes(out, "tools").Exists() {
+		t.Fatalf("tools should be preserved: %s", string(out))
+	}
+}
+
+func TestNormalizeCodexParallelToolCallsForTools_PreservesWhenToolsPresent(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.4","tools":[{"type":"function","name":"lookup"}],"parallel_tool_calls":true,"input":"hi"}`)
+
+	out := normalizeCodexParallelToolCallsForTools(body)
+
+	if !gjson.GetBytes(out, "parallel_tool_calls").Bool() {
+		t.Fatalf("parallel_tool_calls should be preserved when tools are present: %s", string(out))
+	}
+}