The runtime footer (`/footer`) shows what model ran and how full the context
is, but not how long the turn took. On a messaging platform there is no
progress bar and no shell timer — a turn that took 4 seconds and one that took
four minutes produce visually identical replies. Users comparing models,
providers, or reasoning levels have no at-a-glance signal for the one
dimension they most often care about, and "was that slow or did I imagine it?"
is unanswerable after the fact.
Adds a `latency` field to the existing footer machinery, rendering the
wall-clock duration of the agent run: `<1s`, `22s`, `1m05s`.
`gateway/run.py` measures with `time.monotonic()` immediately around the
`self._run_agent(...)` await in `_handle_message_with_agent` — the same
function that already builds the footer, so the value is the user-perceived
turn duration (monotonic, so it is immune to wall-clock/NTP adjustment).
`latency` is deliberately NOT in `_DEFAULT_FIELDS`. It is opt-in via
`display.runtime_footer.fields`. Every existing footer — and every footer a
user has today without touching config — renders byte-identically.
This is enforced by tests, not just asserted:
- `test_latency_not_in_default_fields` pins the default tuple.
- `test_resolve_footer_config_default_fields_exclude_latency` pins what
config resolution produces for an untouched config.
- `test_default_footer_renders_byte_identically` pins five exact output
strings for default-config renders **while supplying `turn_seconds`** —
proving that even when the caller measures timing, a default-configured
footer does not show it.
- `test_default_build_footer_line_ignores_turn_seconds` asserts
`build_footer_line(...) == build_footer_line(..., turn_seconds=125.0)`
under default fields.
Adding `latency` to `_DEFAULT_FIELDS` fails 11 of these tests.
No new config surface (reuses `display.runtime_footer.fields`), no new env
vars, no new core tool, no new model-facing schema. One new module-private
helper (`_format_latency`), one new keyword argument threaded through the two
existing footer functions, and 3 lines in `gateway/run.py`.
`turn_seconds` defaults to `None` and the field is skipped when it is `None`
or negative, so any call site that does not measure timing keeps working
unchanged.
`tests/gateway/test_runtime_footer.py` (+185): `_format_latency` boundary
table (sub-second, rounding at 59.4/59.6, the `m{:02d}s` zero-pad, 60m), the
render/skip/opt-in matrix, field-order placement, `build_footer_line`
threading, and the byte-stability block above.
RED-proved by mutation — each of these breaks tests:
- `latency` added to `_DEFAULT_FIELDS` → 11 failures
- dropping the `turn_seconds is not None and >= 0` guard → 2 failures
- `{sec:02d}` → `{sec}` → 6 failures
- `build_footer_line` not threading `turn_seconds` → 1 failure
51 passed in `tests/gateway/test_runtime_footer.py`; 54 passed across the
footer blast radius. `ruff check` clean.
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