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@PyAutoFit TransformedMessage.factor_gradient crashes on first call

Type: bug Target: priors Difficulty: small Autonomy: supervised Priority: normal Status: issued 2026-08-19 as PyAutoFit#1501 — awaiting external verification (fix-or-delete hangs off the #1498 contract decision); do not start dev Issued: 2026-08-19 (backfilled from active.md registered:)

Same shape as census finding A1 (#1331-01): a code path that has never run end-to-end, dead on arrival.

The finding

TransformedMessage.factor_gradient (composed_transform.py:360-378, main @ 21288bb) unpacks four values from self._transform_det_jac(x):

x, logd, logd_grad, jacs = self._transform_det_jac(x)

but _transform_det_jac (composed_transform.py:281-289) returns three: (x, logd, logd_jacs) where logd_jacs is a list of (logd_grad, jac) tuples. Any call raises ValueError: not enough values to unpack (expected 4, got 3).

Reproducer on main:

import autofit as af
af.UniformPrior(0.0, 2.0).message.factor_gradient(1.3)
# ValueError: not enough values to unpack (expected 4, got 3)

Even past the unpack, the body is wrong for the actual return structure: the for jac in reversed(jacs): grad = grad * jac loop and the final grad + logd_grad assume flat lists that _transform_det_jac does not produce.

Exposure

Zero production callers in autofit/ — the factor_gradient in graphical/laplace/line_search.py is an unrelated FactorApproximation-level interface, and the only exp_factor use is one test. So this is dead code that crashes if ever exercised; no live search or EP path is affected.

The fix (two options, adjudicate with #1498)

  1. Repair it to be the physical-density gradient companion of factor (value base.logpdf(T(x)) + log_det, gradient chain-ruled plus the log_det gradient) and add it to the #1497 property sweep so the gradient is checked against a numerical derivative of factor.
  2. Delete it — if the #1498/#1500 adjudication lands a single-source log_density contract, a hand-rolled second gradient path is exactly the kind of duplicate this cleanup exists to remove.

Do not fix silently ahead of the #1498 contract decision; whichever convention wins decides whether this method should exist at all.