diff --git a/tests/unit/python/utils/tests/swarm/test_matrix_run_reporting.py b/tests/unit/python/utils/tests/swarm/test_matrix_run_reporting.py
new file mode 100644
index 0000000000..4f27992dcb
--- /dev/null
+++ b/tests/unit/python/utils/tests/swarm/test_matrix_run_reporting.py
@@ -0,0 +1,42 @@
+import contextlib
+import io
+import unittest
+from unittest import mock
+
+from utils.tests.swarm import matrix
+
+
+class TestRunReporting(unittest.TestCase):
+    """Every matrix phase must state what it cost.
+
+    A phase that only announces its start leaves the reader subtracting
+    timestamps by hand, and a phase that produces no output of its own becomes
+    invisible: the credential rotation spent 17 minutes of a budget-killed job
+    with nothing but the 30-second sampler in between.
+    """
+
+    def _run_capturing(self, rc: int) -> tuple[int, str]:
+        buffer = io.StringIO()
+        with (
+            mock.patch.object(matrix, "_run_watched", return_value=rc) as watched,
+            contextlib.redirect_stdout(buffer),
+        ):
+            returned = matrix._run(["true"], env={"A": "b"}, label="demo phase")
+        watched.assert_called_once_with(["true"], env={"A": "b"})
+        return returned, buffer.getvalue()
+
+    def test_the_exit_code_passes_through(self) -> None:
+        returned, _ = self._run_capturing(7)
+        self.assertEqual(returned, 7)
+
+    def test_the_phase_announces_its_start(self) -> None:
+        _, output = self._run_capturing(0)
+        self.assertIn("=== swarm-matrix: demo phase ===", output)
+
+    def test_the_phase_reports_its_duration_and_outcome(self) -> None:
+        _, output = self._run_capturing(124)
+        self.assertRegex(output, r"demo phase took \d+s \(rc=124\)")
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/utils/tests/swarm/matrix.py b/utils/tests/swarm/matrix.py
index fff9f18533..c75ac88320 100644
--- a/utils/tests/swarm/matrix.py
+++ b/utils/tests/swarm/matrix.py
@@ -24,6 +24,7 @@ import os
 import shutil
 import subprocess
 import sys
+import time
 
 from utils import PROJECT_ROOT
 from utils.env.runtime import mem_available_mb, mem_stall_pct, mem_total_mb
@@ -52,13 +53,40 @@ def _abort(proc: subprocess.Popen[bytes], banner: str, probe: list[str]) -> int:
 
 
 def _run(cmd: list[str], *, env: dict[str, str], label: str) -> int:
-    """Run a matrix step, aborting visibly before the runner disk or RAM fills.
+    """Announce a matrix step, run it, and report what it cost.
+
+    Args:
+        cmd: argv of the step.
+        env: environment the step runs with.
+        label: phase name for the banners.
+
+    Returns:
+        The step's exit code.
+    """
+    print(f"=== swarm-matrix: {label} ===", flush=True)
+    started = time.monotonic()
+    rc = _run_watched(cmd, env=env)
+    print(
+        f"=== swarm-matrix: {label} took {time.monotonic() - started:.0f}s (rc={rc}) ===",
+        flush=True,
+    )
+    return rc
+
+
+def _run_watched(cmd: list[str], *, env: dict[str, str]) -> int:
+    """Run a step, aborting visibly before the runner disk or RAM fills.
 
     The Actions Worker dies silently on ENOSPC while writing its own logs, so
     a full disk truncates the job without diagnostics; terminating the step at
     DISK_FLOOR_MB keeps enough room for rescue artifacts and the log upload.
+
+    Args:
+        cmd: argv of the step.
+        env: environment the step runs with.
+
+    Returns:
+        The step's exit code, or the abort code when a floor was hit.
     """
-    print(f"=== swarm-matrix: {label} ===", flush=True)
     proc = subprocess.Popen(cmd, cwd=str(PROJECT_ROOT), env=env)
     while True:
         try: