fix(mqtt): keep the layer total that arrives with the print-start frame (#2702)

fix(support): redact push_status values, not the serialised JSON (#2702)

backend/app/api/routes/support.py

@@ -1227,6 +1227,35 @@ def _redact_raw_push_status(raw: dict) -> dict:
     return out
 
 
+def _sanitize_push_status_values(node, sensitive_strings: dict[str, str]):
+    """Sanitize a push_status snapshot's string *values*, never its JSON text.
+
+    This used to run :func:`sanitize_log_content` over the serialised snapshot.
+    That pass includes a generic Bambu-serial regex
+    (``0[0-3][A-Z0-9][A-Z0-9]{9,13}`` in ``log_reader``) which matches the
+    decimal expansion of a float just as happily as a serial: an AMS ``k`` flow
+    factor of ``0.0199999995529652`` came out as ``0.[SERIAL]``, and the bundle
+    shipped invalid JSON — unusable for exactly the ground-truth purpose the
+    snapshot exists for (found while diagnosing #2702).
+
+    Walking the structure instead leaves numbers, bools and None untouched, so
+    the output always parses. Keys are structural and never rewritten.
+    """
+    if isinstance(node, str):
+        return sanitize_log_content(node, sensitive_strings)
+    if isinstance(node, dict):
+        return {k: _sanitize_push_status_values(v, sensitive_strings) for k, v in node.items()}
+    if isinstance(node, list | tuple):
+        # Tuples too: `json.dumps` renders them as arrays, so stringifying one
+        # here would change the file's shape rather than just its content.
+        return [_sanitize_push_status_values(v, sensitive_strings) for v in node]
+    if node is None or isinstance(node, bool | int | float):
+        return node
+    # Anything else (datetime, Decimal, …) would be stringified by json.dumps'
+    # ``default=str`` *after* this pass and so escape sanitisation entirely.
+    return sanitize_log_content(str(node), sensitive_strings)
+
+
 async def _get_recent_sanitized_logs(max_lines: int = 200) -> str:
     """Get recent log lines, sanitized for inclusion in bug reports."""
     # Collect sensitive strings from DB for redaction
@@ -1300,12 +1329,13 @@ async def generate_support_bundle(
                 "captured_at": datetime.now(timezone.utc).isoformat(),
                 "raw_data": redacted,
             }
-            # Belt-and-suspenders: pass the JSON text through the string-based
-            # sanitizer so any user-named string (printer name, serial baked
-            # into a tray uuid) the structural pass missed still gets caught.
-            snapshot_json = json.dumps(snapshot, indent=2, default=str)
-            snapshot_json = sanitize_log_content(snapshot_json, sensitive_strings)
-            zf.writestr(f"push-status/printer-{i + 1}.json", snapshot_json)
+            # Belt-and-suspenders: pass every string value through the
+            # string-based sanitizer so any user-named string (printer name,
+            # serial baked into a tray uuid) the structural pass missed still
+            # gets caught. Values only — sanitizing the serialised JSON text
+            # corrupted numeric literals (see _sanitize_push_status_values).
+            snapshot = _sanitize_push_status_values(snapshot, sensitive_strings)
+            zf.writestr(f"push-status/printer-{i + 1}.json", json.dumps(snapshot, indent=2, default=str))
 
         # Add log file
         # Off the event loop: this reads up to 10 MB and then runs one full regex

backend/app/services/bambu_mqtt.py

@@ -736,6 +736,12 @@ class BambuMQTTClient:
         # and the FINISH-state fallback don't both fire on the same
         # print. Reset to False on every print start.
         self._finish_photo_captured: bool = False
+        # #2702: one-shot re-request of the layer total. Armed at print start
+        # when the starting frame carried no `total_layer_num`, spent on the
+        # first layer advance that still has no denominator. Bambu firmware
+        # only re-sends *changed* fields, so a total we never received (or
+        # dropped) is only recoverable via a full pushall.
+        self._total_layers_refresh_armed: bool = False
         # #2547 end-of-print telemetry probe state. `_armed` is cleared once the
         # window has run for a print so a late FINISH re-send can't reopen it.
         self._eop_probe_armed: bool = True
@@ -2993,6 +2999,29 @@ class BambuMQTTClient:
                     f"{self.state.mc_print_sub_stage} -> {new_sub_stage}"
                 )
             self.state.mc_print_sub_stage = new_sub_stage
+        # Positive `total_layer_num` carried by *this* frame, or 0. Read up
+        # front because three places below consult it and they run in an order
+        # that is not the order they read most naturally in: the layer-advance
+        # refresh (#2702) must not fire on a frame that already answers it, the
+        # apply step must ignore firmware-reset 0s (#1771), and the new-print
+        # reset must not discard a total that belongs to the starting print.
+        total_from_this_frame = 0
+        if "total_layer_num" in data:
+            try:
+                total_from_this_frame = max(int(data["total_layer_num"] or 0), 0)
+            except (TypeError, ValueError):
+                # Must not escape. `_on_message` catches only JSONDecodeError
+                # and paho is left at `suppress_exceptions = False`, so an
+                # exception raised here is re-raised on the network thread and
+                # takes the printer connection down over one unusable field.
+                # Treat it as "not reported": the refresh below then recovers
+                # the real total from a pushall.
+                logger.debug(
+                    "[%s] ignoring unusable total_layer_num: %r",
+                    self.serial_number,
+                    data["total_layer_num"],
+                )
+
         if "layer_num" in data:
             new_layer = int(data["layer_num"])
             old_layer = self.state.layer_num
@@ -3003,6 +3032,25 @@ class BambuMQTTClient:
             # Trigger layer change callback if layer increased
             if new_layer > old_layer and self.on_layer_change:
                 self.on_layer_change(new_layer)
+            # #2702: the print is demonstrably laying down layers but we still
+            # have no denominator, so the pushall requested at print start
+            # either went unanswered or raced the printer learning the total.
+            # Ask once more — by layer 1 the printer definitely knows it.
+            # One-shot: an unanswered pushall must not turn into a per-layer
+            # retry loop for the rest of the print.
+            if (
+                new_layer > old_layer
+                and self._total_layers_refresh_armed
+                and not self.state.total_layers
+                and not total_from_this_frame
+            ):
+                self._total_layers_refresh_armed = False
+                logger.debug(
+                    "[%s] layer %s with no total_layer_num — re-requesting full status",
+                    self.serial_number,
+                    new_layer,
+                )
+                self._request_push_all()
             # #1867 last-layer finish-photo trigger. A1 Mini (and other
             # firmware variants) skips `stg_cur=22`, so the fallback fires
             # at gcode_state=FINISH — which runs AFTER user End G-code
@@ -3032,15 +3080,12 @@ class BambuMQTTClient:
                         "timelapse_was_active": self._timelapse_during_print,
                     }
                 )
-        if "total_layer_num" in data:
-            # Some firmware (P1S observed) resets `total_layer_num` to 0 at
-            # print end — same shape as the `layer_num` reset guarded above.
-            # Preserve the last known good value so the usage-tracker split
-            # path (#1771) has a denominator that survives the reset frame.
-            # Explicit reset to 0 happens on print start (`_handle_print_start`).
-            new_total = int(data["total_layer_num"])
-            if new_total > 0:
-                self.state.total_layers = new_total
+        if total_from_this_frame:
+            # Firmware (P1S observed) resets `total_layer_num` to 0 at print
+            # end — same shape as the `layer_num` reset guarded above. Applying
+            # only positive values preserves the last known good denominator so
+            # the usage-tracker split path (#1771) survives the reset frame.
+            self.state.total_layers = total_from_this_frame
 
         # Fan speeds (MQTT sends as string "0"-"15" representing speed levels, or percentage)
         # Convert to 0-100 percentage for display
@@ -4152,11 +4197,29 @@ class BambuMQTTClient:
             # Reset layer tracking for new print (needed for layer-based timelapse)
             self.state.layer_num = 0
             # Reset total_layers so the previous print's value can't bleed into
-            # this print's usage-tracker split before the new push_status arrives
-            # with the slicer's total (#1771 follow-on to the preservation guard
-            # above at line ~2135 — the guard now ignores firmware-reset 0s, so
-            # the explicit reset has to happen here instead).
-            self.state.total_layers = 0
+            # this print's usage-tracker split (#1771 follow-on to the
+            # preservation guard at the `total_layer_num` parse above — that
+            # guard ignores firmware-reset 0s, so the explicit reset has to
+            # happen here instead).
+            #
+            # #2702: reset to *this frame's* total, not to 0. The frame that
+            # trips the new-print detection can carry the new print's
+            # `total_layer_num` as well — the parse above has already applied
+            # it, and zeroing unconditionally threw it away. That looked
+            # harmless but is not recoverable: Bambu firmware sends only
+            # changed fields, so the printer never offers the total again, and
+            # the print runs to completion at `n/0` in the UI, in
+            # `{total_layers}` notifications, and as the usage-split
+            # denominator. The value only reappears on the next full pushall
+            # (reconnect / Force Refresh), which is why the symptom looked
+            # random and why a *stable* connection made it worse.
+            self.state.total_layers = total_from_this_frame
+            # If the starting frame brought no total, ask for one. Costs one
+            # MQTT message per print and covers the ordering where the printer
+            # published the total a frame or two before the state flip.
+            self._total_layers_refresh_armed = not total_from_this_frame
+            if self._total_layers_refresh_armed:
+                self._request_push_all()
             # Reset completion tracking for new print
             self._was_running = True
             self._completion_triggered = False

backend/tests/unit/services/test_total_layers_print_start.py

@@ -0,0 +1,255 @@
+"""The layer total must survive the print-start reset (#2702).
+
+`_update_state` applies `total_layer_num` early and, further down, resets
+`total_layers` when it detects a new print (added by #1771 so the previous
+print's total can't bleed into the next one's usage split). Those two run in
+the same function on the same frame, so a frame that carried both the new
+print's total *and* the transition into RUNNING had its total applied and then
+zeroed.
+
+That is unrecoverable rather than merely late: Bambu firmware sends only
+changed fields, so the printer never re-sends a total it already published.
+The value reappears only in a full pushall — i.e. on reconnect or a manual
+Force Refresh — which is why the reporter saw `n/0` for nine minutes on a
+flawless connection, why it looked random, and why a *stable* link made it
+worse.
+
+Frames here are trimmed to the fields the code under test reads. No printer is
+needed: the fix is a property of how one function orders its own writes.
+"""
+
+from __future__ import annotations
+
+import json
+
+import pytest
+
+
+@pytest.fixture
+def client():
+    """A client with a recording stand-in for the MQTT connection."""
+    from unittest.mock import MagicMock
+
+    from backend.app.services.bambu_mqtt import BambuMQTTClient
+
+    c = BambuMQTTClient(
+        ip_address="192.168.1.100",
+        serial_number="TEST123",
+        access_code="12345678",
+    )
+    c._client = MagicMock()
+    # A new print is only detected once a previous state has been observed
+    # (#1304 guard), so give every test a plausible pre-print history.
+    c._previous_gcode_state = "IDLE"
+    c._previous_gcode_file = None
+    c._was_running = False
+    return c
+
+
+def pushalls(client) -> list[dict]:
+    """Every pushall published on this client, decoded."""
+    sent = []
+    for call in client._client.publish.call_args_list:
+        payload = json.loads(call.args[1])
+        if payload.get("pushing", {}).get("command") == "pushall":
+            sent.append(payload)
+    return sent
+
+
+def running_frame(**extra) -> dict:
+    """A frame that flips the printer into RUNNING with a file — a new print."""
+    return {"gcode_state": "RUNNING", "gcode_file": "widget.3mf", "subtask_name": "widget", **extra}
+
+
+# ---------------------------------------------------------------------------
+# The regression
+# ---------------------------------------------------------------------------
+
+
+def test_total_arriving_with_the_start_frame_survives(client):
+    """The reported bug: total and transition in one frame lost the total."""
+    client._update_state(running_frame(total_layer_num=33, layer_num=0))
+
+    assert client.state.total_layers == 33
+
+
+def test_total_arriving_with_the_start_frame_needs_no_pushall(client):
+    """We already have the denominator, so don't spend a round-trip on it."""
+    client._update_state(running_frame(total_layer_num=33))
+
+    assert pushalls(client) == []
+    assert client._total_layers_refresh_armed is False
+
+
+def test_previous_prints_total_still_cannot_bleed_through(client):
+    """#1771's reason for the reset — preserved exactly.
+
+    A start frame with no total of its own must land on 0, never on the
+    finished print's denominator.
+    """
+    client.state.total_layers = 120  # left over from the print that just ended
+
+    client._update_state(running_frame())
+
+    assert client.state.total_layers == 0
+
+
+def test_start_frame_without_a_total_asks_the_printer_for_one(client):
+    """Covers the ordering where the total was published a frame or two early.
+
+    Re-applying this frame's own value can't help there — the value was
+    already consumed and zeroed — so recovery has to come from a pushall,
+    the only message that re-sends unchanged fields.
+    """
+    client._update_state(running_frame())
+
+    assert len(pushalls(client)) == 1
+    assert client._total_layers_refresh_armed is True
+
+
+# ---------------------------------------------------------------------------
+# The one-shot re-request
+# ---------------------------------------------------------------------------
+
+
+def test_first_layer_advance_without_a_total_re_requests_once(client):
+    client._update_state(running_frame())
+    assert len(pushalls(client)) == 1  # from print start
+
+    client._update_state({"layer_num": 1})
+
+    assert len(pushalls(client)) == 2
+    assert client._total_layers_refresh_armed is False
+
+
+def test_later_layer_advances_do_not_keep_re_requesting(client):
+    """An unanswered pushall must not become a per-layer retry loop."""
+    client._update_state(running_frame())
+    client._update_state({"layer_num": 1})
+    before = len(pushalls(client))
+
+    for layer in range(2, 12):
+        client._update_state({"layer_num": layer})
+
+    assert len(pushalls(client)) == before
+
+
+def test_no_re_request_once_the_total_is_known(client):
+    """The pushall answered: layers advance without further traffic."""
+    client._update_state(running_frame())
+    client._update_state({"total_layer_num": 33})  # the pushall's answer
+    before = len(pushalls(client))
+
+    client._update_state({"layer_num": 1})
+    client._update_state({"layer_num": 2})
+
+    assert client.state.total_layers == 33
+    assert len(pushalls(client)) == before
+
+
+def test_the_recovered_total_is_what_downstream_reads(client):
+    """End-to-end on the reporter's sequence, minus the 9-minute wait.
+
+    Start with no total, layers advance at `n/0`, the pushall answers, and
+    from then on the UI, `{total_layers}` notifications and the usage-split
+    denominator all see 33 — they read this one field.
+    """
+    client._update_state(running_frame())
+    client._update_state({"layer_num": 1})
+    assert client.state.total_layers == 0  # the symptom in the screenshot
+
+    client._update_state({"layer_num": 2, "total_layer_num": 33})
+
+    assert (client.state.layer_num, client.state.total_layers) == (2, 33)
+
+
+def test_the_pushall_answer_does_not_re_trigger_the_reset(client):
+    """Loop safety: the answer is a *full* frame, gcode_state and file included.
+
+    If that re-tripped the new-print detection it would reset the total it just
+    delivered and request another pushall, once per round-trip, forever.
+    """
+    client._update_state(running_frame())
+    assert len(pushalls(client)) == 1
+
+    client._update_state(running_frame(total_layer_num=33, layer_num=1, mc_percent=3))
+
+    assert client.state.total_layers == 33
+    assert len(pushalls(client)) == 1
+
+
+# ---------------------------------------------------------------------------
+# Interaction with the pre-existing firmware-reset guard
+# ---------------------------------------------------------------------------
+
+
+def test_firmware_reset_to_zero_mid_print_is_still_ignored(client):
+    """P1S zeroes total_layer_num at print end; #1771's guard keeps the total."""
+    client._update_state(running_frame(total_layer_num=33))
+
+    client._update_state({"layer_num": 33, "total_layer_num": 0})
+
+    assert client.state.total_layers == 33
+
+
+@pytest.mark.parametrize("value", [None, "", 0, "0", -1, "abc", "33.7", [], {}, 3.9])
+def test_unusable_totals_do_not_break_ingest(client, value):
+    """A bad total must not escape `_update_state`.
+
+    The old parse did a bare ``int(data["total_layer_num"])``. `_on_message`
+    catches only `JSONDecodeError` and paho is left at
+    ``suppress_exceptions = False``, so anything this raised was re-raised on
+    the network thread and took the printer connection down over one field.
+    `None`, `[]` and `{}` all did exactly that.
+    """
+    client._update_state(running_frame(total_layer_num=value))
+
+    assert client.state.total_layers in (0, 3)  # 3.9 truncates; the rest are 0
+    assert client.state.gcode_file == "widget.3mf"  # the rest of the frame landed
+
+
+def test_an_unusable_total_does_not_stop_the_layer_counter(client):
+    """The read happens before the layer block, so it must not be able to raise.
+
+    Otherwise a firmware sending a malformed total would freeze `layer_num` for
+    the whole print — the frame would abort before reaching it.
+    """
+    client._update_state(running_frame())
+
+    client._update_state({"layer_num": 7, "total_layer_num": "not-a-number"})
+
+    assert client.state.layer_num == 7
+
+
+def test_a_string_total_is_accepted(client):
+    """Bambu ships numbers as strings in plenty of other fields."""
+    client._update_state(running_frame(total_layer_num="33"))
+
+    assert client.state.total_layers == 33
+
+
+def test_a_zero_total_on_the_start_frame_counts_as_no_total(client):
+    """`total_layer_num: 0` is the firmware's "don't know yet", not a value."""
+    client.state.total_layers = 120
+
+    client._update_state(running_frame(total_layer_num=0))
+
+    assert client.state.total_layers == 0
+    assert len(pushalls(client)) == 1
+
+
+# ---------------------------------------------------------------------------
+# A restarted print (file change while RUNNING) takes the same path
+# ---------------------------------------------------------------------------
+
+
+def test_file_change_while_running_also_keeps_its_own_total(client):
+    """`is_file_change` shares the reset, so it needs the same treatment."""
+    client._update_state(running_frame(total_layer_num=33))
+    client._was_running = True
+
+    client._update_state(
+        {"gcode_state": "RUNNING", "gcode_file": "other.3mf", "subtask_name": "other", "total_layer_num": 77}
+    )
+
+    assert client.state.total_layers == 77

backend/tests/unit/test_support_helpers.py

@@ -1285,3 +1285,169 @@ class TestRedactRawPushStatus:
         assert _redact_raw_push_status(None) == {}  # type: ignore[arg-type]
         assert _redact_raw_push_status([]) == {}  # type: ignore[arg-type]
         assert _redact_raw_push_status("") == {}  # type: ignore[arg-type]
+
+
+class TestSanitizePushStatusValues:
+    """The bundled push_status snapshot must stay parseable JSON.
+
+    Sanitization used to run over the *serialised* snapshot. The generic
+    Bambu-serial regex in ``log_reader`` (``0[0-3][A-Z0-9][A-Z0-9]{9,13}``)
+    matches the decimal expansion of a float as readily as a serial, so an AMS
+    ``k`` flow factor came out as ``0.[SERIAL]`` and the whole file stopped
+    parsing — found in a real bundle while diagnosing #2702, which is exactly
+    the case the snapshot was added to serve.
+    """
+
+    def test_float_that_matches_the_serial_regex_survives(self):
+        """The observed reproducer, verbatim."""
+        import json
+
+        from backend.app.api.routes.support import _sanitize_push_status_values
+
+        raw = {"ams": [{"tray": [{"k": 0.0199999995529652}]}]}
+
+        out = _sanitize_push_status_values(raw, {})
+
+        assert json.loads(json.dumps(out)) == raw
+
+    def test_output_always_parses(self):
+        """Whatever it does to values, the result must be valid JSON."""
+        import json
+
+        from backend.app.api.routes.support import _sanitize_push_status_values
+
+        raw = {
+            "k_values": [0.0199999995529652, 0.019999999552965164, 0.02],
+            "home_flag": 7554487,
+            "sdcard": True,
+            "resolution": "",
+            "nozzle": None,
+        }
+
+        json.loads(json.dumps(_sanitize_push_status_values(raw, {})))
+
+    def test_still_redacts_strings(self):
+        """The point of the pass is not lost — string values are sanitized."""
+        from backend.app.api.routes.support import _sanitize_push_status_values
+
+        raw = {"tag_uid": "0123456789ABCDEF", "name": "Martin's P1S", "ip": "192.168.1.50"}
+
+        out = _sanitize_push_status_values(raw, {"Martin's P1S": "[PRINTER]"})
+
+        assert out["tag_uid"] == "[SERIAL]"
+        assert out["name"] == "[PRINTER]"
+        assert out["ip"] == "[IP]"
+
+    def test_walks_nested_containers(self):
+        from backend.app.api.routes.support import _sanitize_push_status_values
+
+        raw = {"ams": [{"tray": [{"tray_uuid": "0123456789ABCDEF"}]}]}
+
+        out = _sanitize_push_status_values(raw, {})
+
+        assert out["ams"][0]["tray"][0]["tray_uuid"] == "[SERIAL]"
+
+    def test_keys_are_left_alone(self):
+        """Keys are structural — renaming one would break the schema."""
+        from backend.app.api.routes.support import _sanitize_push_status_values
+
+        raw = {"0123456789ABCDEF": 1}
+
+        assert list(_sanitize_push_status_values(raw, {})) == ["0123456789ABCDEF"]
+
+    def test_non_json_scalars_are_sanitized_not_smuggled(self):
+        """``json.dumps(default=str)`` runs after this pass, so do it here."""
+        from datetime import datetime, timezone
+
+        from backend.app.api.routes.support import _sanitize_push_status_values
+
+        raw = {"seen_at": datetime(2026, 7, 29, 23, 12, 40, tzinfo=timezone.utc), "who": object()}
+
+        out = _sanitize_push_status_values(raw, {"2026-07-29": "[WHEN]"})
+
+        assert out["seen_at"].startswith("[WHEN]")
+        assert isinstance(out["who"], str)
+
+    def test_bools_stay_bools(self):
+        """`isinstance(True, int)` — a bool must not fall through to str()."""
+        from backend.app.api.routes.support import _sanitize_push_status_values
+
+        out = _sanitize_push_status_values({"sdcard": True, "force_upgrade": False}, {})
+
+        assert out["sdcard"] is True
+        assert out["force_upgrade"] is False
+
+    def test_the_full_bundle_chain_on_a_real_p1s_payload(self):
+        """The route's transform, end to end, on the shape from the #2702 bundle.
+
+        `_redact_raw_push_status` then `_sanitize_push_status_values` then
+        `json.dumps(default=str)` — the composition the bundle writer applies.
+        The bundle that exposed this had five `k` values corrupted, so the
+        snapshot could not be read at all; the field the report was about
+        (`total_layer_num`) was sitting in it, intact and unreachable.
+        """
+        import json
+
+        from backend.app.api.routes.support import (
+            _redact_raw_push_status,
+            _sanitize_push_status_values,
+        )
+
+        raw = {
+            "gcode_file": "AMS_Filament_Clip_3MF.3mf",
+            "layer_num": 2,
+            "total_layer_num": 33,
+            "home_flag": 7554487,
+            "sdcard": True,
+            "net": {"info": [{"ip": "192.168.1.50", "mask": 0}]},
+            "ams": {
+                "ams": [
+                    {
+                        "id": "0",
+                        "humidity": "5",
+                        "tray": [
+                            {"id": "0", "k": 0.0199999995529652, "tag_uid": "0123456789ABCDEF"},
+                            {"id": "1", "k": 0.0209999997168779, "tag_uid": "44F782D000000100"},
+                        ],
+                    }
+                ]
+            },
+        }
+
+        snapshot = {
+            "model": "P1S",
+            "firmware_version": "01.10.00.00",
+            "raw_data": _redact_raw_push_status(raw),
+        }
+        text = json.dumps(_sanitize_push_status_values(snapshot, {}), indent=2, default=str)
+
+        parsed = json.loads(text)  # used to raise "Expecting ',' delimiter"
+        trays = parsed["raw_data"]["ams"]["ams"][0]["tray"]
+        assert [t["k"] for t in trays] == [0.0199999995529652, 0.0209999997168779]
+        assert parsed["raw_data"]["total_layer_num"] == 33
+        # Redaction still did its job on both fronts.
+        assert "gcode_file" not in parsed["raw_data"]
+        assert trays[0]["tag_uid"] == "[SERIAL]"
+        # The structural pass replaces the printer's LAN address with the
+        # sentinel 0.0.0.0, which is itself an IPv4 literal, so the value pass
+        # then masks it to [IP]. Harmless — the real address is already gone —
+        # and matches what shipped in the bundle behind #2702.
+        assert parsed["raw_data"]["net"]["info"][0]["ip"] == "[IP]"
+
+    def test_does_not_mutate_the_live_snapshot(self):
+        """`state.raw_data` is read by the dispatcher on every tick.
+
+        The bundle writer passes a redacted copy, but a walker that mutated in
+        place would still be one refactor away from redacting the live state.
+        """
+        import copy
+
+        from backend.app.api.routes.support import _sanitize_push_status_values
+
+        raw = {"tag_uid": "0123456789ABCDEF", "ams": [{"tray": [{"k": 0.02, "n": "0123456789ABCDEF"}]}]}
+        before = copy.deepcopy(raw)
+
+        out = _sanitize_push_status_values(raw, {})
+
+        assert raw == before, "input was mutated"
+        assert out["tag_uid"] == "[SERIAL]"  # and the copy really was redacted