MagpieTTS long-form: a 4–5 character sentence fails the whole request ("longform history context cache is too short") #61

Version: nemo-speech 0.1.0 and 0.2.0 (nemo-speech-0.{1,2}.0-linux-x86_64-cuda
release tarballs; the same input fails identically on both; the nightly was not tried), model nvidia/magpie_tts_multilingual_357m@452ef560f972
(v2602.f16.gguf), codec nvidia/nemo-nano-codec-22khz-1.89kbps-21.5fps.
RTX 3090, driver 570.153.02, Linux 6.8.

What happens: when the input is long enough for long-form mode to engage,
a single very short sentence anywhere in it makes synthesis fail:

[nemo-speech] synthesize session started
longform history context cache is too short: need 20 token(s), have 18
nemo-speech synthesize: MagpieTTS synthesis failed
[nemo-speech] synthesize session failed (exit code 1)

nemo-speech serve returns HTTP 500 "MagpieTTS synthesis failed" for the same
input. It is deterministic: the same text fails every time, with any voice.

Repro: a filler sentence repeated four times, then one short sentence,
then the filler four times again (~900 characters):

F="The service kept running normally while the operators reviewed the dashboards and the alert history in detail."
F4="$F $F $F $F"
echo "$F4 Okay. $F4" > t.txt
nemo-speech synthesize -i t.txt -o o.wav --force --seed 1
# -> exit 1, "need 20 token(s), have 18"

Which sentences fail: the same template, with only the middle sentence
changed:

The same short sentence passes in a short input, where long-form mode does not
engage. In a real 48-chunk text (each chunk ≤ 800 chars), 47 rendered and one
failed. It contained "Why? It ran out of disk space. Why? Logs were not
rotated. Why? …".

--tts.longform options: on behaves like auto (fails). off avoids
the error but truncates silently: an 800-char chunk that gives 53 s of audio
in long-form came back as 23 s, and 31 s with --steps 3000.

Expected: a short sentence is merged with its neighbour, or padded, so that
the history context reaches the minimum. It should not fail the request.

Workaround on our side: before sending text to Magpie, the client attaches
every sentence shorter than 8 characters to the previous sentence (or to the
next one when it comes first): "down. Why?" becomes "down, why?". With that,
the 48-chunk text renders 48 of 48.