diff --git a/packages/mcl/src/src/mcl/commands/ci_matrix.d b/packages/mcl/src/src/mcl/commands/ci_matrix.d
index e93ffbcd..a8c0d1f2 100755
--- a/packages/mcl/src/src/mcl/commands/ci_matrix.d
+++ b/packages/mcl/src/src/mcl/commands/ci_matrix.d
@@ -3,7 +3,7 @@ module mcl.commands.ci_matrix;
 import std.stdio : writeln, stderr, stdout;
 import std.traits : EnumMembers;
 import std.string : indexOf, splitLines;
-import std.algorithm : map, filter, reduce, chunkBy, find, any, sort, startsWith, each;
+import std.algorithm : map, filter, reduce, chunkBy, find, any, sort, startsWith, each, canFind, fold;
 import std.file : write, readText;
 import std.range : array, front, join, split;
 import std.conv : to;
@@ -14,7 +14,7 @@ import std.path : buildPath;
 import std.process : pipeProcess, wait, Redirect, kill;
 import std.exception : enforce;
 import std.format : fmt = format;
-import std.logger : tracef, infof, errorf;
+import std.logger : tracef, infof, errorf, warningf;
 
 import mcl.utils.env : optional, MissingEnvVarsException, parseEnv;
 import mcl.utils.string : enumToString, StringRepresentation, MaxWidth, writeRecordAsTable;
@@ -318,30 +318,29 @@ Package[] nixEvalJobs(string flakeAttrPrefix, string cachixUrl, bool doCheck = t
         "--flake", rootDir ~ "#" ~ flakeAttrPrefix
     ];
 
+    const commandString = args.join(" ");
+
     tracef("%-(%s %)", args);
 
     auto pipes = pipeProcess(args, Redirect.stdout | Redirect.stderr);
 
-    void logError(string errorMsg)
+    void logWarning(const char[] msg)
     {
-        errorf("Command `%s` failed with error:\n---\n%s\n---",
-            args, errorMsg);
+        warningf("Command `%s` stderr:\n---\n%s\n---", commandString, msg);
     }
 
-    foreach (line; pipes.stdout.byLine)
+    void logError(const char[] msg)
     {
-        if (line.indexOf("{") == -1)
-        {
-            errorf("Expected JSON object on stdout from nix-eval-jobs, got: `%s`", line);
-            continue;
-        }
+        errorf("Command `%s` failed with error:\n---\n%s\n---", commandString, msg);
+    }
 
+    const errorsReported = pipes.stdout.byLine.fold!((errorsReported, line) {
         auto json = parseJSON(line);
 
         if (auto err = "error" in json)
         {
             logError((*err).str);
-            continue; // drain the output
+            return true;
         }
 
         Package pkg = json.packageFromNixEvalJobsJson(
@@ -364,17 +363,19 @@ Package[] nixEvalJobs(string flakeAttrPrefix, string cachixUrl, bool doCheck = t
             attr: pkg.attrPath,
             output: pkg.output
         ).writeRecordAsTable(stderr.lockingTextWriter);
-    }
-    foreach (line; pipes.stderr.byLine)
-    {
-        if (uselessWarnings.map!((warning) => line.indexOf(warning) != -1).any)
-            continue;
 
-        logError(line.idup);
-    }
+        return errorsReported;
+    })(false);
+
+    const stderrLogs = pipes.stderr.byLine
+        .filter!(line => !uselessWarnings.canFind(line))
+        .join("\n");
+
+    logWarning(stderrLogs);
 
     int status = wait(pipes.pid);
-    enforce(status == 0, "Command `%s` failed with status %s".fmt(args, status));
+
+    enforce(status == 0 && !errorsReported, "Command `%s` failed with status %s".fmt(commandString, status));
 
     return result;
 }