Never dispatch the verify workflow with an empty model input

[tarekziade]

serge-verify-caller.yml declares model as required: true, and GitHub's
workflow_dispatch API counts an empty string as absent:

422 dispatching serge-verify-caller.yml on huggingface/transformers:
  {"message":"Required input 'model' not provided","status":"422"}

extract_verify_targets derives the model from tests/models/<model>/, so any
group whose tests live elsewhere cannot be dispatched at all — tests/generation/ test_utils.py, tests/trainer/, tests/quantization/, …

What it cost on the 2026-08-16 nightly

The generation group (task d8cda8555f49, tracking issue
huggingface/transformers#48004) hit it twice:

1. The reproduce dispatch failed first. dispatch_failed is not
   not_reproduced, so _maybe_reproduce_first failed open and serge
   investigated blind — the exact thing reproduce-first exists to prevent.
   (There is no serge reproduce generation run in the workflow list, only the
   four for florence2/phimoe/emu3/exaone4_5.)
2. That blind run: 44 turns, 50 tool calls, 904,008 input tokens, ending in
   the tool-repeat guard (read_file×3, grep×3, grep×3) being forced to answer
   without tools.
3. It still produced a plausible patch — watermarking_config dict handling in
   GenerationConfig.update, one file, normalizer clean — committed to
   serge/fix/itf-464770b803d3-d8cda855.
4. The verify dispatch then failed the same way → no PR. The branch was never
   pushed (65 serge/fix/* branches exist on the remote; that fingerprint is not
   among them), so the patch is gone.

The fix

Send "none" instead of "". The input only selects the optional collateral
suite and labels the run, and run_collateral is already forced false when there
is no model folder, so nothing changes for groups that do have one. A named
constant carries the 422 explanation so it does not get "simplified" back.

Tests: extract_verify_targets really does return model == "" for a
tests/generation/… block (the input the fallback exists for), both dispatch
paths send a non-empty model, and a real model still passes through with
run_collateral intact. 649 tests pass.

Not fixed here — worth deciding separately

dispatch_failed still fails open into a blind investigation. A 422 is
deterministic: no retry and no blind run can help, so treating it as a hard skip
would have made this incident cost nothing. A timeout is different and should
keep failing open. That is a behaviour change in the reproduce gate, so it is not
bundled into a bug fix.

Relatedly, model: required: true could be relaxed to required: false +
default: "" on both serge-verify-caller.yml (transformers) and
serge-verify-slow.yml (transformers-ci) so the whole class of bug cannot recur —
but the caller is behind the maintainer allow-list, hence the serge-side fallback
first.

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