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## Description
The session beacon reports `failures` as a single count, incremented
whenever a turn ends `>= 500` (`headroom/telemetry/session.py`). Across
the current corpus that reads **3,969 failures on 595,445 turns
(0.67%)** — and the number cannot answer the only question anyone asks
of it: an Anthropic `529` is the provider shedding load and there is
nothing to fix; a `500` is usually ours. Today the two are
indistinguishable, so diagnosis falls back to inference from time-of-day
curves and per-install concentration.
This counts the status alongside the total.
```json
"failures": 3,
"failure_statuses": {"529": 2, "500": 1}
```
Motivating investigation on the live corpus (0.67% of turns, 6% of
sessions, 63% of all failures from 48 installs, a 2.5% plateau at 08–11
UTC decaying to 0.03% during the fleet's busiest hour) strongly suggests
provider-side 529 after retry exhaustion — but "strongly suggests" is
exactly the gap this field closes.
## Type of Change
- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)
## Changes Made
- **`headroom/telemetry/session.py`** — `_Session.failure_statuses`,
incremented next to `failures` in `record_outcome`. Keys are the bare
status string for the 5xx range, `"other"` beyond it. Emitted as a
sibling of `failures` in `payload()`.
- **`deploy/beacon/worker.js`** — `failure_statuses` added to
`ALLOWED_KEYS`. Without this the ingest allowlist silently drops it.
- **`deploy/beacon/sample-event.json`** — sample carries the new key in
OTLP `kvlistValue` form.
### Why no slug bounding
`skips` runs values through `_safe_slug` because they arrive as free
strings. A status code is an `int` the proxy itself produced; the `500
<= status < 600` check is what keeps a garbage value from inventing map
keys. Nothing here is user-derived, so the field stays content-free.
### Why `schema_version` stays 1
Additive, matching the precedent set by #2796, which added
`tokens.tool_saved` and the two `all_layers_*` rates without a bump.
Bumping signals a break to consumers when nothing about older rows
becomes invalid.
## Testing
- [x] Unit tests pass (`pytest`) — the module's own self-check, extended
- [x] Linting passes (`ruff check .`)
- [ ] Type checking passes (`mypy headroom`) — see note
- [x] New tests added for new functionality
- [x] Manual testing performed
### Test Output
```text
$ python -m headroom.telemetry.session
ok
$ ruff check headroom/telemetry/session.py
All checks passed!
$ ruff format --check headroom/telemetry/session.py
1 file already formatted
$ mypy --python-version 3.12 headroom/telemetry/session.py
Success: no issues found in 1 source file
# --python-version 3.12 only to skip a pre-existing numpy-stub syntax error the
# repo's python_version = "3.10" triggers locally; unrelated to this diff.
$ node --check deploy/beacon/worker.js # ok
$ python -c "import json; json.load(open('deploy/beacon/sample-event.json'))" # parses
```
The self-check in `headroom/telemetry/session.py` now records two 529s
and one 500 and asserts both the total and the split:
```python
assert emitted[-1]["failures"] == 3
assert emitted[-1]["failure_statuses"] == {"529": 2, "500": 1}
```
plus `assert event["failure_statuses"] == {}` on the clean-session path.
## Real Behavior Proof
- **Environment:** macOS 25.4.0, Python 3.12 venv, this branch.
- **Exact command / steps:** drive `SessionAggregator` with three
failing outcomes and encode the payload through the same `_any_value`
the wire uses.
```text
payload: 3 {'529': 2, '500': 1}
otlp : {"kvlistValue": {"values": [{"key": "529", "value": {"intValue": "2"}},
{"key": "500", "value": {"intValue": "1"}}]}}
```
The OTLP form matches `deploy/beacon/sample-event.json` byte-for-byte in
shape, and `unwrap()` in `worker.js` turns `kvlistValue` back into a
plain object, so it lands in R2 as `{"529": 2, "500": 1}` — the same
shape as `skips`, which DuckDB reads as `MAP(VARCHAR, BIGINT)`.
- **Observed result:** as above. Verified against the live corpus that
schema evolution here is already routine — 3,836 of 3,884 existing rows
have `rates.all_layers_saved_pct = NULL` from #2796 landing mid-corpus,
and every report still runs.
- **Not tested:** the deployed Worker (no staging R2 binding locally);
`node --check` covers syntax only. The allowlist addition is one array
entry consumed by the existing `pick()`.
## Review Readiness
- [x] I have performed a self-review
- [x] This PR is ready for human review
## Checklist
- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] I did **not** edit `CHANGELOG.md`
## Screenshots (if applicable)
N/A — wire-format change, covered by the output above.
## Additional Notes
**Deploy order matters.** The Worker allowlist drops unknown keys, so
`deploy/beacon/worker.js` must be deployed *before* a client release
that emits the field — otherwise it is discarded at the door. No
corruption either way, just missing data until the Worker catches up.
**Old data is unaffected.** R2 objects are immutable NDJSON written per
request; nothing rewrites history. The corpus reader already passes
`union_by_name = true`, which fills the column with NULL for rows
written before this ships.
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