diff --git a/scripts/mas_evidence_backup.py b/scripts/mas_evidence_backup.py
index a0b6966..c14c44f 100644
--- a/scripts/mas_evidence_backup.py
+++ b/scripts/mas_evidence_backup.py
@@ -13,6 +13,10 @@ freshness
     Fails when the latest verified upload is older than STALE_HOURS, its
     receipt is not verified, or the local ciphertext no longer matches it.
 
+After each verified upload, local run directories older than
+LOCAL_RETENTION_DAYS are pruned. The current run is never pruned, and
+nothing at the provider is ever deleted.
+
 Credentials come only from /etc/mas-backup/primary.env through the systemd
 EnvironmentFile. No TEM or SMTP credential and no age identity is read here.
 """
@@ -22,6 +26,8 @@ from __future__ import annotations
 import argparse
 import json
 import os
+import re
+import shutil
 import sys
 from datetime import UTC, datetime, timedelta
 from pathlib import Path
@@ -36,6 +42,8 @@ STATE = Path("/var/lib/mas-evidence-backup")
 RECIPIENT_FILE = Path("/etc/mas-backup/age-recipient")
 LATEST = "latest-offsite.json"
 STALE_HOURS = 26
+LOCAL_RETENTION_DAYS = 30  # owner-accepted 2026-11-03
+_RUN_NAME = re.compile(r"\A\d{8}T\d{6}Z\Z")
 APPLICATION = "mas"
 
 
@@ -120,7 +128,25 @@ def backup(state=STATE, capture=None, client_factory=None, now=None):
         archive.checkpoint(state / LATEST, {**receipt, "run": stamp, "capture": public})
 
     receipt = publish_run(run_dir, captured["captured_at"], client, save_latest)
-    return {**receipt, "run": stamp, "capture": public}
+    pruned = prune_local(state, stamp, now or datetime.now(UTC))
+    return {**receipt, "run": stamp, "capture": public,
+            "local_retention_days": LOCAL_RETENTION_DAYS, "local_runs_pruned": pruned}
+
+
+def prune_local(state, keep, now, days=LOCAL_RETENTION_DAYS):
+    """Remove dated run directories older than `days`, never `keep`. Local only."""
+    cutoff = now - timedelta(days=days)
+    removed = []
+    for entry in sorted(state.iterdir()):
+        if entry.name == keep or not _RUN_NAME.fullmatch(entry.name):
+            continue
+        if entry.is_symlink() or not entry.is_dir() or entry.parent != state:
+            continue
+        stamp = datetime.strptime(entry.name, "%Y%m%dT%H%M%SZ").replace(tzinfo=UTC)
+        if stamp < cutoff:
+            shutil.rmtree(entry)
+            removed.append(entry.name)
+    return removed
 
 
 def freshness(state=STATE, now=None):
diff --git a/tests/test_mas_evidence_backup.py b/tests/test_mas_evidence_backup.py
index 1cd925c..089a7e9 100644
--- a/tests/test_mas_evidence_backup.py
+++ b/tests/test_mas_evidence_backup.py
@@ -136,3 +136,33 @@ def test_recipient_must_be_a_public_age_key(tmp_path):
     bad.write_text("AGE-SECRET-KEY-1XYZ")
     with pytest.raises(worker.archive.Refused):
         worker.recipient(bad)
+
+
+def test_prune_removes_only_old_dated_runs_and_never_the_current(env, tmp_path):
+    env.mkdir(mode=0o700)
+    for name in ("20261001T041500Z", "20261010T041500Z", "20261101T041500Z", "notes", "20261001T041500Z-x"):
+        (env / name).mkdir()
+    outside = tmp_path / "outside"
+    outside.mkdir()
+    (env / "20260901T041500Z").symlink_to(outside)
+    removed = worker.prune_local(env, "20261103T041500Z", NOW)
+    assert removed == ["20261001T041500Z"]
+    remaining = sorted(p.name for p in env.iterdir())
+    assert remaining == ["20260901T041500Z", "20261001T041500Z-x", "20261010T041500Z", "20261101T041500Z", "notes"]
+    assert outside.exists()
+    assert worker.prune_local(env, "20261010T041500Z", NOW + timedelta(days=40)) == ["20261101T041500Z"]
+
+
+def test_backup_reports_retention_and_prunes_after_verified_upload(env):
+    env.mkdir(mode=0o700)
+    (env / "20260901T041500Z").mkdir()
+    result = worker.backup(env, FakeCapture(), lambda: Provider(), now=NOW)
+    assert result["local_retention_days"] == 30 and result["local_runs_pruned"] == ["20260901T041500Z"]
+
+
+def test_failed_upload_prunes_nothing(env):
+    env.mkdir(mode=0o700)
+    (env / "20260901T041500Z").mkdir()
+    with pytest.raises(TimeoutError):
+        worker.backup(env, FakeCapture(), lambda: Provider(fail_complete=True), now=NOW)
+    assert (env / "20260901T041500Z").exists()