diff --git a/pkg/providers/codex_provider.go b/pkg/providers/codex_provider.go
index 6dff3a52e..9e36217ae 100644
--- a/pkg/providers/codex_provider.go
+++ b/pkg/providers/codex_provider.go
@@ -260,10 +260,6 @@ func buildCodexParams(messages []Message, tools []ToolDefinition, model string,
 		params.Instructions = openai.Opt(defaultCodexInstructions)
 	}
 
-	if maxTokens, ok := options["max_tokens"].(int); ok {
-		params.MaxOutputTokens = openai.Opt(int64(maxTokens))
-	}
-
 	if len(tools) > 0 {
 		params.Tools = translateToolsForCodex(tools)
 	}
diff --git a/pkg/providers/codex_provider_test.go b/pkg/providers/codex_provider_test.go
index 317b1a5de..c34593e7b 100644
--- a/pkg/providers/codex_provider_test.go
+++ b/pkg/providers/codex_provider_test.go
@@ -29,6 +29,9 @@ func TestBuildCodexParams_BasicMessage(t *testing.T) {
 	if params.Instructions.Or("") != defaultCodexInstructions {
 		t.Errorf("Instructions = %q, want %q", params.Instructions.Or(""), defaultCodexInstructions)
 	}
+	if params.MaxOutputTokens.Valid() {
+		t.Fatalf("MaxOutputTokens should not be set for Codex backend")
+	}
 }
 
 func TestBuildCodexParams_SystemAsInstructions(t *testing.T) {
@@ -214,6 +217,10 @@ func TestCodexProvider_ChatRoundTrip(t *testing.T) {
 			http.Error(w, "stream must be true", http.StatusBadRequest)
 			return
 		}
+		if _, ok := reqBody["max_output_tokens"]; ok {
+			http.Error(w, "max_output_tokens is not supported", http.StatusBadRequest)
+			return
+		}
 
 		resp := map[string]interface{}{
 			"id":     "resp_test",
@@ -293,6 +300,10 @@ func TestCodexProvider_ChatRoundTrip_TokenSourceFallbackAccountID(t *testing.T)
 			http.Error(w, "temperature is not supported", http.StatusBadRequest)
 			return
 		}
+		if _, ok := reqBody["max_output_tokens"]; ok {
+			http.Error(w, "max_output_tokens is not supported", http.StatusBadRequest)
+			return
+		}
 		if reqBody["stream"] != true {
 			http.Error(w, "stream must be true", http.StatusBadRequest)
 			return