diff --git a/tartarus/process.py b/tartarus/process.py
new file mode 100644
index 0000000..e5732d5
--- /dev/null
+++ b/tartarus/process.py
@@ -0,0 +1,23 @@
+"""Checked subprocess helpers shared across the harness."""
+
+import subprocess
+
+
+class ProcessError(Exception):
+    """Raised when a checked subprocess command cannot be run or exits non-zero."""
+
+
+def run_checked(command: list[str]) -> str:
+    """Run a command with captured text output, raising ProcessError on failure."""
+    if not command:
+        raise ProcessError("command is empty")
+
+    try:
+        completed = subprocess.run(command, capture_output=True, text=True, check=True)
+    except FileNotFoundError as missing:
+        raise ProcessError(f"`{command[0]}` not found") from missing
+    except subprocess.CalledProcessError as failed:
+        raise ProcessError(
+            f"`{' '.join(command)}` failed: {failed.stderr.strip()}"
+        ) from failed
+    return completed.stdout