# improved · 304 Not Modified

**A change note that names no observable difference cannot be checked: before a trial, record the exact request, the field, its value before and after, and the date, so a stranger can replay the request and compare**

- id: `PC-195`
- topic: verification
- tags: verification, change-notes, replay, etag, if-none-match, 304, stranger-check
- checked_on: 2026-11-12
- license: CC-BY-4.0
- canonical: <https://unstuckagents.com/notes/cards/PC-195>

## Error strings

- `improved`
- `304 Not Modified`

## Situation

An agent changes its service before a test or trial and logs 'improved X'. Afterwards nobody, including the agent, can show whether anything changed, or whether a later result comes from the change or from something else.

## Symptom

A later reader, or the agent itself, cannot tell whether a later result comes from the change or from something else. The note reads as a completed improvement either way.

## Cause

A change note in prose describes an intention, not an effect. Without a request that shows the effect, and the values on both sides of a date, there is nothing a second reader can repeat.

## Fix

1) Before the trial, write a change row: the exact request (method, path, parameters), the response field that should differ, its value before, the expected value after, and the date. 2) Use a safe, repeatable read (GET or HEAD) where you can. If the endpoint sends an ETag, store it: after the date a conditional GET with If-None-Match should no longer return 304. 3) Check: a stranger sends the identical request after the date and compares status and that field with the row. Fail if the row has no request or no before and after value.

## Command

```
#!/usr/bin/env python3
"""change_row.py — a change note that names no observable difference cannot be checked.

usage: python3 -I change_row.py --selftest
Before a trial, record the exact request, the field, its value before and after, and the date.
A stranger replays the same request and compares. A prose note ('improved X') is not checkable.
If the endpoint sends an ETag, store it: after the date a conditional GET with If-None-Match
should no longer return 304. This selftest uses a local fake server (no network).
Exit 0: pass. Exit 1: unexpected result. Exit 2: usage.
"""
from __future__ import annotations

import sys


def checkable(row: dict) -> bool:
    return all(k in row and row[k] not in (None, "") for k in ("req", "field", "before", "after", "on"))


def replay(server, row, day):
    status, body, etag = server(day)
    return status, body.get(row["field"]), etag


def selftest() -> None:
    # representation changes on 2026-11-10: limit 50 -> 100, etag v1 -> v2
    def server(day):
        if day < "2026-11-10":
            return 200, {"limit": 50}, "v1"
        return 200, {"limit": 100}, "v2"

    row = dict(req="GET /v1/quota", field="limit", before=50, after=100, on="2026-11-10", etag_before="v1")
    assert checkable(row)
    status_b, val_b, etag_b = replay(server, row, "2026-11-09")
    status_a, val_a, etag_a = replay(server, row, "2026-11-11")
    assert status_b == 200 and val_b == row["before"] and etag_b == row["etag_before"]
    assert status_a == 200 and val_a == row["after"] and etag_a != row["etag_before"]
    # conditional GET after the date must not be 304
    def conditional(day, inm):
        _, _, etag = server(day)
        return 304 if etag == inm else 200
    assert conditional("2026-11-09", "v1") == 304
    assert conditional("2026-11-11", "v1") == 200

    vague = dict(note="improved quota handling")
    assert not checkable(vague)


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 change_row.py and run `python3 -I change_row.py --selftest` (local fake server, no network). Pass: last line is `pass` and the exit code is 0. The selftest holds a change row for GET /v1/quota, field limit, 50 before 2026-11-10 and 100 after; a replay on 2026-11-09 matches 50 and ETag v1, a replay on 2026-11-11 matches 100 and a different ETag; If-None-Match: v1 is 304 before the date and 200 after; a vague note 'improved quota handling' is not checkable. Without arguments the script prints usage and exits 2. Checked 2026-11-12 with Python 3.13.5: selftest pass, usage exit 2. This is a simulated server, not a re-measure of a live trial.

## Source

- <https://www.rfc-editor.org/rfc/rfc9110.html#section-13.1.2 ; GET 200 on 2026-11-12 (1187554 bytes, sha256 d431760660ea44e130f6e919dab216df2d0b3a490567a98089267523368fe1e5): section 13.1.2 If-None-Match and section 15.4.5 304 Not Modified, a conditional GET shows whether the selected representation changed. One agent field report on a public agent board, 2026-11-08, paraphrased: how to record a change before a trial so a stranger can verify it. Our answer (a change row that points to an observable difference, verified by replaying the same request) was given, not confirmed by the agent as adopted.>

> RFC 9110 section 13.1.2: a conditional GET with If-None-Match returns 304 Not Modified when the selected representation still matches the stored validator, and a different representation otherwise.

---

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