diff --git a/internal/runtime/executor/codex_executor.go b/internal/runtime/executor/codex_executor.go
index ed38570d..7bbf0e68 100644
--- a/internal/runtime/executor/codex_executor.go
+++ b/internal/runtime/executor/codex_executor.go
@@ -220,7 +220,8 @@ func (e *CodexExecutor) executeCompact(ctx context.Context, auth *cliproxyauth.A
 	body = applyPayloadConfigWithRoot(e.cfg, baseModel, to.String(), "", body, originalTranslated, requestedModel)
 	body, _ = sjson.SetBytes(body, "model", baseModel)
 	body, _ = sjson.DeleteBytes(body, "stream")
-	if !gjson.GetBytes(body, "instructions").Exists() {
+	instructions := gjson.GetBytes(body, "instructions")
+	if !instructions.Exists() || instructions.Type == gjson.Null {
 		body, _ = sjson.SetBytes(body, "instructions", "")
 	}
 
diff --git a/internal/runtime/executor/codex_executor_compact_test.go b/internal/runtime/executor/codex_executor_compact_test.go
index 4fcd7a8e..02c6db29 100644
--- a/internal/runtime/executor/codex_executor_compact_test.go
+++ b/internal/runtime/executor/codex_executor_compact_test.go
@@ -15,44 +15,65 @@ import (
 )
 
 func TestCodexExecutorCompactAddsDefaultInstructions(t *testing.T) {
-	var gotPath string
-	var gotBody []byte
-	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		gotPath = r.URL.Path
-		body, _ := io.ReadAll(r.Body)
-		gotBody = body
-		w.Header().Set("Content-Type", "application/json")
-		_, _ = w.Write([]byte(`{"id":"resp_1","object":"response.compaction","usage":{"input_tokens":1,"output_tokens":2,"total_tokens":3}}`))
-	}))
-	defer server.Close()
+	cases := []struct {
+		name    string
+		payload string
+	}{
+		{
+			name:    "missing instructions",
+			payload: `{"model":"gpt-5.4","input":"hello"}`,
+		},
+		{
+			name:    "null instructions",
+			payload: `{"model":"gpt-5.4","instructions":null,"input":"hello"}`,
+		},
+	}
 
-	executor := NewCodexExecutor(&config.Config{})
-	auth := &cliproxyauth.Auth{Attributes: map[string]string{
-		"base_url": server.URL,
-		"api_key":  "test",
-	}}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			var gotPath string
+			var gotBody []byte
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				gotPath = r.URL.Path
+				body, _ := io.ReadAll(r.Body)
+				gotBody = body
+				w.Header().Set("Content-Type", "application/json")
+				_, _ = w.Write([]byte(`{"id":"resp_1","object":"response.compaction","usage":{"input_tokens":1,"output_tokens":2,"total_tokens":3}}`))
+			}))
+			defer server.Close()
 
-	resp, err := executor.Execute(context.Background(), auth, cliproxyexecutor.Request{
-		Model:   "gpt-5.4",
-		Payload: []byte(`{"model":"gpt-5.4","input":"hello"}`),
-	}, cliproxyexecutor.Options{
-		SourceFormat: sdktranslator.FromString("openai-response"),
-		Alt:          "responses/compact",
-		Stream:       false,
-	})
-	if err != nil {
-		t.Fatalf("Execute error: %v", err)
-	}
-	if gotPath != "/responses/compact" {
-		t.Fatalf("path = %q, want %q", gotPath, "/responses/compact")
-	}
-	if !gjson.GetBytes(gotBody, "instructions").Exists() {
-		t.Fatalf("expected instructions in compact request body, got %s", string(gotBody))
-	}
-	if gjson.GetBytes(gotBody, "instructions").String() != "" {
-		t.Fatalf("instructions = %q, want empty string", gjson.GetBytes(gotBody, "instructions").String())
-	}
-	if string(resp.Payload) != `{"id":"resp_1","object":"response.compaction","usage":{"input_tokens":1,"output_tokens":2,"total_tokens":3}}` {
-		t.Fatalf("payload = %s", string(resp.Payload))
+			executor := NewCodexExecutor(&config.Config{})
+			auth := &cliproxyauth.Auth{Attributes: map[string]string{
+				"base_url": server.URL,
+				"api_key":  "test",
+			}}
+
+			resp, err := executor.Execute(context.Background(), auth, cliproxyexecutor.Request{
+				Model:   "gpt-5.4",
+				Payload: []byte(tc.payload),
+			}, cliproxyexecutor.Options{
+				SourceFormat: sdktranslator.FromString("openai-response"),
+				Alt:          "responses/compact",
+				Stream:       false,
+			})
+			if err != nil {
+				t.Fatalf("Execute error: %v", err)
+			}
+			if gotPath != "/responses/compact" {
+				t.Fatalf("path = %q, want %q", gotPath, "/responses/compact")
+			}
+			if !gjson.GetBytes(gotBody, "instructions").Exists() {
+				t.Fatalf("expected instructions in compact request body, got %s", string(gotBody))
+			}
+			if gjson.GetBytes(gotBody, "instructions").Type != gjson.String {
+				t.Fatalf("instructions type = %v, want string", gjson.GetBytes(gotBody, "instructions").Type)
+			}
+			if gjson.GetBytes(gotBody, "instructions").String() != "" {
+				t.Fatalf("instructions = %q, want empty string", gjson.GetBytes(gotBody, "instructions").String())
+			}
+			if string(resp.Payload) != `{"id":"resp_1","object":"response.compaction","usage":{"input_tokens":1,"output_tokens":2,"total_tokens":3}}` {
+				t.Fatalf("payload = %s", string(resp.Payload))
+			}
+		})
 	}
 }