Three blocking review findings on drawing_diff.
1. An unedited entity reported as changed with changes == {} and clouded.
The signature snaps values to the tol grid, so a value 1 ulp across a
half-grid point lands in another bucket. AutoCAD writes 16 significant
digits (0.14 + 0.005 = 0.14500000000000002 comes back as 0.145), so a
round trip split the pair, which was matched later and counted as
changed. A matched pair is now compared by difference (more than half a
tol, around the circle for angles) instead of by grid bucket, and a pair
with nothing to report counts as unchanged, with no row and no cluster.
Measured live on AutoCAD 2026 (Documents.Open read-only, select all,
Export DXF, Close(False)): before, summary changed=1, a row with
changes {} and one cluster; after, unchanged=21, changed=0, no cluster.
Over 12000 computed coordinates (a*0.01 + {0.005, 0.015, 0.105, 0.305})
795 signatures flip; before, 795 were changed, and after, 0.
2. With zero exact matches, handles were declared stable, so strangers from
two fresh files that share handles were paired and given wrong moves.
The handle stage now needs at least one exact match. The report adds
handle_evidence {exact_matches, kept_handle}.
3. A truncated DXF raised StopIteration from ezdxf's tag reader.
asyncio.to_thread cannot hand that exception to an awaiting caller, so
drawing_diff hung instead of refusing. read_revision now turns every
parse or walk failure (not MemoryError) into ValueError "not a readable
DXF (...)". tol is validated first, so its refusal keeps its own name.
Tests: grid splits by handle and by position, radius/height/rotation/bulge
split at tol=1, a real edit next to a split, no-evidence handles, a
truncated file directly and over the MCP wire with a 30 s timeout. The six
new tests fail against the previous module.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd
engineering/understand/diff.py reads each revision once into entity
records and matches them in three stages: exact signature (type, layer,
geometry rounded to tol, text, block, rotation, scale, attributes;
handles ignored), then handle while at least half of the exact matches
kept theirs, then type + layer + nearest anchor within move_tol (default
2 % of the 1st-99th percentile diagonal). The report counts changes by
layer and type, says what changed per entity (moved by, text, attribute,
radius ... from -> to), compares named block definitions by content and
groups the changes into clusters.
drawing_diff opens group D's copy of SECTION 26. It is read-only unless
markup=True, which files revision clouds around the model-space clusters
on the current document through add_revision - refused before any read
with new_path, without a valid rev, or on an engine without revcloud.
A revision path that is neither .dxf nor .dwg is refused naming the
parameter (old_path: / new_path:), like every other path refusal.
Tests: the synthetic plant P&ID edited five ways (a move, a text, an
attribute, an add, a delete) under renumbered handles reports exactly
those five edits; renumbering alone reports nothing.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd
