diff --git a/checkin_notes b/checkin_notes
index efd4a66b73..0ab8a478a7 100755
--- a/checkin_notes
+++ b/checkin_notes
@@ -11287,3 +11287,20 @@ David  20 Oct 2006
         gui_rpc_server.C
     lib/
         filesys.C
+
+David  20 Oct 2006
+	- Output files can be tagged as "optional".
+		If the application doesn't generate an optional output file,
+		the client treats it as normal
+		(i.e. doesn't flag it as an error)
+		NOTES:
+		1) to make an output file optional,
+			put <optional/> in its <file_ref>
+			element in the result template file
+		2) clients earlier than 5.8 don't recognize this attribute;
+			they will mark results as "compute error" if
+			any of their output files are missing
+
+	client/
+		client_types.C,h
+		cs_apps.C
diff --git a/client/client_types.C b/client/client_types.C
index c62c15d697..4e91302a12 100644
--- a/client/client_types.C
+++ b/client/client_types.C
@@ -1165,13 +1165,15 @@ int FILE_REF::parse(MIOFILE& in) {
     fd = -1;
     main_program = false;
     copy_file = false;
+	optional = false;
     while (in.fgets(buf, 256)) {
         if (match_tag(buf, "</file_ref>")) return 0;
         else if (parse_str(buf, "<file_name>", file_name, sizeof(file_name))) continue;
         else if (parse_str(buf, "<open_name>", open_name, sizeof(open_name))) continue;
         else if (parse_int(buf, "<fd>", fd)) continue;
-        else if (match_tag(buf, "<main_program/>")) main_program = true;
-        else if (match_tag(buf, "<copy_file/>")) copy_file = true;
+        else if (parse_bool(buf, "main_program", main_program)) continue;
+        else if (parse_bool(buf, "copy_file", copy_file)) continue;
+		else if (parse_bool(buf, "optional", optional)) continue;
         else {
             if (log_flags.unparsed_xml) {
                 msg_printf(0, MSG_ERROR,
@@ -1202,6 +1204,9 @@ int FILE_REF::write(MIOFILE& out) {
     if (copy_file) {
         out.printf("        <copy_file/>\n");
     }
+    if (optional) {
+        out.printf("        <optional/>\n");
+    }
     out.printf("    </file_ref>\n");
     return 0;
 }
diff --git a/client/client_types.h b/client/client_types.h
index 5a421efb01..cba84431ab 100644
--- a/client/client_types.h
+++ b/client/client_types.h
@@ -126,7 +126,9 @@ struct FILE_REF {
     bool main_program;
     FILE_INFO* file_info;
     bool copy_file;  // if true, core client will copy the file instead of linking
-
+	bool optional;
+		// for output files: app may not generate file;
+		// don't treat as error if file is missing.
     int parse(MIOFILE&);
     int write(MIOFILE&);
 };
diff --git a/client/cs_apps.C b/client/cs_apps.C
index 4a5d9b8dc6..34d838983c 100644
--- a/client/cs_apps.C
+++ b/client/cs_apps.C
@@ -65,11 +65,17 @@ int CLIENT_STATE::app_finished(ACTIVE_TASK& at) {
 
     if (rp->exit_status != ERR_ABORTED_VIA_GUI) {
         for (i=0; i<rp->output_files.size(); i++) {
-            fip = rp->output_files[i].file_info;
+			FILE_REF& fref = rp->output_files[i];
+            fip = fref.file_info;
             if (fip->uploaded) continue;
             get_pathname(fip, path);
             retval = file_size(path, size);
             if (retval) {
+				if (fref.optional) {
+					fip->upload_when_present = false;
+					continue;
+				}
+
                 // an output file is unexpectedly absent.
                 //
                 fip->status = retval;