---
id: CB-WP-0033
kind: product
title: "A game is the unit"
status: done
state_hub_workstream_id: "66c54551-38cd-465e-bb2f-9e1d83e14438"
---

# Purpose

```
structural tier  M   (changes an artifact contract -- the recording path --
                      and the definition of a reported metric)
chaos            d8 = 7  → no override
declared tier    M
```

**Declaration 4 of chaos window 3.**

## The report

Reviewing the maintainer's six trial notes, `make trials` said
**positions unreachable: 4, target 0 — "the recording exists but the
position moved."**

**All four were false.** A recording holds exactly one hash, the final
state; `reachability()` asked whether the note's hash was in that file. So
a mid-game note could never match, and a post-game note from any but the
last game could not either.

**Instance 8 of the ADR-0018 family**, with an exact sensitivity: vary
only *when* the note was written and the answer flips deterministically,
with nothing having moved.

## The root cause was not the metric

Three defects, one cause: **`play again` reused state that belongs to a
game.**

| reused | consequence |
|---|---|
| the recording path | **game 1's recording destroyed** — data loss |
| the journal (never cleared) | game 2's log opened with game 1's commands |
| the note's `after`, counted on that journal | indexed a recording without those commands |

**Fixing `reachability()` alone would have turned the number green while a
session still destroyed its own evidence.** [ADR-0019](../decisions/ADR-0019-a-game-is-the-unit.md).

## Task: a game is the unit

```task
id: CB-WP-0033-T01
status: done
priority: high
state_hub_task_id: "b0b4b9f3-f99f-4b4d-8eed-ba2fa4f9ea98"
```

**Controls:**
- **each fix fails on its own defect**, by mutation, or it is not tested;
- **the metric can go red** — a check reporting 0 that cannot report
  anything else is decoration (ADR-0006 D3);
- **old logs stay readable and are not called orphans**;
- **the single-game invocation GameDesign §5 documents is unchanged.**

**Done 2026-08-07.** All three mutation-proven:

| mutation | what went red |
|---|---|
| `next_game` stops clearing the journal | *"game 2 opened holding game 1's commands"* |
| notes not filtered by game | *"game 1's comment leaked into game 2's log"* |
| driver stops re-pathing the recording | *"game 2 was never recorded separately"* |

The third is the important one: it runs the **real two-game session**, so
it covers the call site rather than the helper. The unit test proves
`game_path` names files apart; only the integration test proves the driver
uses it, and the driver was where the data loss lived.

**Two defects were introduced and caught while writing this.**

- `cfg.record` was re-derived from the *previous game's* path, so game 3
  would have landed in `x-2-3.yaml`. Fixed by keeping the caller's base
  untouched, **and the compounding shape is now pinned by an assertion**
  so the fix cannot silently rot.
- `Server::game()` was dead code, which `-D warnings` caught. Rather than
  `#[allow]`, the driver now *asks the server* which game it is — removing
  a second counter that could have disagreed with the one stamped on the
  notes.

**The metric now reports honestly**: 0 unreachable, 6 legacy, and it says
what legacy means. `trials.py` gained six controls, including the two that
were previously **impossible to pass** — a mid-game note being reachable,
and a note from game 2 being judged against game 2's recording.

## Not done here

- **No hash per command in the recording** (ADR-0019 D3). A mid-game
  position is confirmed by replaying to it, not by string search. The
  trigger for revisiting is a reader who needs the position without a
  replay.
- **The six existing notes stay legacy forever.** Their positions were
  never checkable; nothing is gained by inventing bindings for them.