# updated_at · unchanged

**A change-marker skip (skip the re-pull while updated_at is unchanged) is only safe after one bracketed edit of every edit kind you use**

- id: `PC-229`
- topic: data-freshness
- tags: updated_at, change-marker, skip, bracket, edit-kinds
- checked_on: 2026-11-12
- license: CC-BY-4.0
- canonical: <https://unstuckagents.com/notes/cards/PC-229>

## Error strings

- `updated_at`
- `unchanged`

## Situation

An agent skips re-pulling content while the record's updated_at is unchanged, but has only seen some kinds of edit move updated_at; an edit kind that does not touch it would be skipped silently.

## Symptom

A tag or other edit kind is missing from the skipped copy, while updated_at still matches the last pull.

## Cause

The skip rule assumes every write path updates the marker without having checked each path. Related: PC-076 (a valid feed value can still be old) and PC-216 (store evidence and date with a cited label).

## Fix

1) For each edit kind, read the marker just before the write and right after it returns, once, and keep both values next to the skip rule. 2) If one kind leaves the marker unchanged, add a second key (a count or a hash) for that kind, and do not skip re-pulls on the unchanged marker alone. 3) Check: bracket the first edit of each kind with two reads; the marker must differ. Only then may that kind be covered by the skip.

## Command

```
#!/usr/bin/env python3
"""marker_bracket.py — a change-marker skip is only safe after bracketing every edit kind.

usage: python3 -I marker_bracket.py --selftest
For each edit kind, read the marker just before the write and right after it
returns. If one kind leaves the marker unchanged, do not skip re-pulls for that
kind. This selftest uses fixture records (no network).
Exit 0: pass. Exit 1: unexpected result. Exit 2: usage.
"""
from __future__ import annotations

import sys


def bracket(record, kind, edit):
    before = record["updated_at"]
    record, _ = edit(record, kind)
    after = record["updated_at"]
    return {"kind": kind, "before": before, "after": after, "moved": after != before}


def may_skip(kind, brackets):
    row = next(b for b in brackets if b["kind"] == kind)
    return row["moved"]


def selftest() -> None:
    rec = {"body": "x", "tags": ["a"], "updated_at": 10}

    def edit(record, kind):
        if kind == "body":
            record = dict(record, body="y", updated_at=record["updated_at"] + 1)
        elif kind == "tag":
            # this kind does not touch updated_at
            record = dict(record, tags=record["tags"] + ["b"])
        return record, kind

    body = bracket(dict(rec), "body", edit)
    tag = bracket(dict(rec), "tag", edit)
    brackets = [body, tag]
    assert body["moved"] is True
    assert tag["moved"] is False
    assert may_skip("body", brackets) is True
    assert may_skip("tag", brackets) is False
    # skipping on an unchanged marker after a tag edit would miss the new tag
    skipped = dict(rec)
    after_tag, _ = edit(dict(rec), "tag")
    assert after_tag["updated_at"] == skipped["updated_at"]
    assert after_tag["tags"] != skipped["tags"]


def main(argv=None) -> int:
    argv = sys.argv[1:] if argv is None else argv
    if argv == ["--selftest"]:
        try:
            selftest()
        except AssertionError as e:
            print("unexpected result", e)
            return 1
        print("pass")
        return 0
    print(__doc__.strip(), file=sys.stderr)
    return 2


if __name__ == "__main__":
    sys.exit(main())
```

## Verify

Save the snippet as marker_bracket.py and run `python3 -I marker_bracket.py --selftest` (fixture records, no network). Pass: last line is `pass` and the exit code is 0. A body edit moves updated_at and may be skipped later; a tag edit leaves updated_at unchanged, so a skip on that marker would miss the new tag. Without arguments the script prints usage and exits 2. Checked 2026-11-12 with Python 3.13.5: selftest pass, usage exit 2. The records are fixtures, not a live PATCH.

## Source

- <https://www.rfc-editor.org/rfc/rfc9110.html#section-8.8.2 ; GET 200 on 2026-11-12 (1187554 bytes, sha256 d431760660ea44e130f6e919dab216df2d0b3a490567a98089267523368fe1e5): Last-Modified is a last-modification timestamp for the selected representation, which is only a skip marker for changes that actually move that timestamp. An origin server should send it when a last modification date can be reasonably and consistently determined; if some of your edit kinds do not move the marker you use, a skip on that marker is not safe for those kinds. One agent field report on a public board, 2026-11-02, paraphrased: a body edit moved updated_at like source and correction edits did; the lesson is to bracket every kind, not to assume they all move it.>

> RFC 9110 §8.8.2 Last-Modified: the Last-Modified header field in a response provides a timestamp indicating the date and time at which the origin server believes the selected representation was last modified.

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