diff --git a/py/scripts/nox-matrix.py b/py/scripts/nox-matrix.py
index abc38192..62a5c5e2 100644
--- a/py/scripts/nox-matrix.py
+++ b/py/scripts/nox-matrix.py
@@ -12,9 +12,9 @@
 import argparse
 import json
 import platform
-import re
 import subprocess
 import sys
+import tempfile
 from pathlib import Path
 
 
@@ -180,18 +180,18 @@ def main() -> None:
     if args.output_durations is None:
         sys.exit(subprocess.run(cmd).returncode)
 
-    # Stream output while capturing session durations
-    durations: dict[str, int] = {}
-    duration_re = re.compile(r"nox > Session (\S+) was successful in (\d+) seconds\.")
-    process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True, bufsize=1)
-    assert process.stdout is not None
-    for line in process.stdout:
-        sys.stdout.write(line)
-        sys.stdout.flush()
-        m = duration_re.search(line)
-        if m:
-            durations[m.group(1)] = int(m.group(2))
-    process.wait()
+    # Nox formats longer durations as "a minute", etc. Its JSON report
+    # always contains seconds, independent of the human-readable log format.
+    with tempfile.TemporaryDirectory() as tmp:
+        report_path = Path(tmp) / "nox-report.json"
+        process = subprocess.run([*cmd, "--report", str(report_path)])
+        with open(report_path) as f:
+            report = json.load(f)
+        durations = {
+            session["name"]: round(session["duration"])
+            for session in report["sessions"]
+            if session["result"] == "success"
+        }
 
     args.output_durations.parent.mkdir(parents=True, exist_ok=True)
     with open(args.output_durations, "w") as f:
diff --git a/py/src/braintrust/test_nox_matrix.py b/py/src/braintrust/test_nox_matrix.py
new file mode 100644
index 00000000..f64cbb32
--- /dev/null
+++ b/py/src/braintrust/test_nox_matrix.py
@@ -0,0 +1,66 @@
+"""Regression tests for the CI shard timing collector."""
+
+import importlib.util
+import io
+import json
+import subprocess
+import sys
+from pathlib import Path
+from types import SimpleNamespace
+
+import pytest
+
+
+_SCRIPT = Path(__file__).resolve().parents[2] / "scripts" / "nox-matrix.py"
+
+
+@pytest.mark.parametrize("returncode", [0, 1])
+def test_measured_durations_use_nox_report(tmp_path, monkeypatch, returncode):
+    if not _SCRIPT.exists():
+        pytest.skip("scripts/ is not available (wheel mode)")
+    spec = importlib.util.spec_from_file_location("nox_matrix", _SCRIPT)
+    matrix = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(matrix)
+
+    sessions = [
+        {"name": "test_core", "result": "success", "duration": 71.9},
+        {"name": "test_provider(latest)", "result": "success", "duration": 3601.2},
+        {"name": "test_short", "result": "success", "duration": 0.3},
+        {"name": "test_seconds", "result": "success", "duration": 12.4},
+        {"name": "test_skipped", "result": "skipped", "duration": 0},
+    ]
+    if returncode:
+        sessions.append({"name": "test_failed", "result": "failed", "duration": 3.5})
+    output = tmp_path / "durations" / "measured.json"
+    monkeypatch.setattr(sys, "argv", [str(_SCRIPT), "0", "1", "--output-durations", str(output)])
+    monkeypatch.setattr(matrix, "get_nox_sessions", lambda _: [s["name"] for s in sessions])
+
+    def run(cmd, **kwargs):
+        if cmd[0] == "git":
+            return subprocess.CompletedProcess(cmd, 0, stdout=str(_SCRIPT.parents[2]))
+        report_path = Path(cmd[cmd.index("--report") + 1])
+        report_path.write_text(json.dumps({"sessions": sessions}))
+        return subprocess.CompletedProcess(cmd, returncode)
+
+    monkeypatch.setattr(matrix.subprocess, "run", run)
+    # The old collector only read this human-readable output and lost test_core.
+    monkeypatch.setattr(
+        matrix.subprocess,
+        "Popen",
+        lambda *args, **kwargs: SimpleNamespace(
+            stdout=io.StringIO("nox > Session test_core was successful in a minute.\n"),
+            wait=lambda: returncode,
+            returncode=returncode,
+        ),
+    )
+
+    with pytest.raises(SystemExit) as exc:
+        matrix.main()
+
+    assert exc.value.code == returncode
+    assert json.loads(output.read_text()) == {
+        "test_core": 72,
+        "test_provider(latest)": 3601,
+        "test_short": 0,
+        "test_seconds": 12,
+    }