mx-api-classify-roundtrip-failures

name	mx-api-classify-roundtrip-failures
description	Use this skill to explain, in plain language, what is wrong with the `mx::api` round-trip and what it needs next. It drives the failure classifier (dump -> classify) over the corpus, then reads build/api/classified.json and turns it into a prioritized, human-readable worklist grouped by failure shape (crashes, instant wins, small fix-sets, reorder-blocked, high-frequency drops). Invoke for requests like "what's broken about mx::api", "explain the round-trip failures", "what does the api need next", or "triage the api round-trip".
argument-hint	<optional: a signature or element to focus on>
disable-model-invocation	false
user-invocable	true

Explain the mx::api round-trip

mx::api is a deliberate subset of MusicXML, and the comparison is strict full-DOM, so most corpus files diverge somewhere. This skill turns the measured divergences into a plain-English answer to two questions: what is broken, and what should we fix next to land the most files into the round-trip corpus with the fewest software changes.

It is the read-out layer on top of the failure classifier (issue #211/#212): the classifier produces a machine-readable build/api/classified.json; this skill interprets it for a human.

What the classifier reports (and what it does not)

Classification is purely measured from each expected/actual pair. It does not consult data/api.features.xml or any record of what mx::api was believed to "support" — whether a drop is intended is a present-day human decision (#214), not something the tool asserts. So do not describe any drop as "by-design" on the classifier's authority; report what was dropped and let the human decide.

Each difference is a signature, and a file's distance to passing is its count of unique signatures:

signature	meaning
drop:<tag>	a tag in expected, missing from actual
add:<tag>	a spurious tag only in actual (a bug)
value:<tag>	a paired element whose text value differs
value:<tag>@<attr>	a paired element whose attribute value differs
attr:<tag>@<attr>	an attribute present on only one side
reorder:<parent>	a parent with matching children in a different order

Per-file status is PASS / FAIL / CRASH. A FAIL with no reorder: signature is a candidate: the first-pass worklist targets these, since reorders are expected mx::api behavior to be absorbed in test normalization later (#214).

Procedure

Step 1 — produce the data

If build/api/roundtrip-dump/ is empty or stale (api/impl changed since it was written), run:

make dump-api-roundtrip

Then always run:

make classify-api-roundtrip

Read the stdout summary it prints -- status counts, the distance histogram, the ranked worklist, and the greedy batch plan. That worklist is the headline answer to "what should we fix next."

Step 2 — mine build/api/classified.json

These read-only analyses expand on the stdout summary. Adjust the path if --out was overridden.

The worklist (already in the stdout summary) ranks signatures by candidate files unblocked (sole_blocker = files this fix flips green on its own; files_blocked = candidate files that include it).

Instant wins — candidate files one fix away from passing (distance 1):

python3 - <<'PY'
import json
d = json.load(open("build/api/classified.json"))
for f in d["near_misses"]["1"]:
    print(f["signatures"][0], f["file"])
PY

Small fix-sets -- the greedy set-cover batch plan is already computed in d["batch_plan"] (rows have fix/added_files/cumulative_files; also printed in the stdout summary).

Crash cluster (highest severity — no output at all) — group by kind to find the common root:

python3 - <<'PY'
import json
from collections import Counter
d = json.load(open("build/api/classified.json"))
crashes = [(r["crash_kind"], r["file"]) for r in d["files"] if r["status"] == "CRASH"]
print(Counter(k for k, _ in crashes))
for kind, f in crashes[:20]:
    print(kind, f)
PY

Reorder-blocked files (deferred to #214) — how many, and at which parents:

python3 - <<'PY'
import json
from collections import Counter
d = json.load(open("build/api/classified.json"))
reorders = Counter(s for r in d["files"] if r["has_reorder"]
                   for s in r["signatures"] if s.startswith("reorder:"))
print("reorder-blocked files:", sum(1 for r in d["files"] if r["has_reorder"]))
print(reorders.most_common(10))
PY

To drill into one file, look at its record (signatures, sample_paths, distance) and diff the pair directly: diff build/api/roundtrip-dump/<flat>.expected.xml build/api/roundtrip-dump/<flat>.actual.xml where <flat> is the corpus path with / replaced by __.

Step 3 — write the explanation

Synthesize the findings into plain language, ordered by what grows the corpus fastest. Use this structure (fill the numbers and signatures from Step 2; do not invent them):

1. Frame it: strict full-DOM, so a file passes only when every signature is resolved. The goal is to land files with the fewest software changes.
2. Crashes (status="CRASH"). Highest severity: no output at all. Name the cluster (the crash kind usually points at one feature). These are bugs.
3. Instant wins: the distance-1 candidates and the top sole_blocker signatures — one fix each flips a file green now. This is the front of the worklist.
4. Small fix-sets: the distance-2/3 candidates and the union of signatures that would unblock a batch — "add these N features → these M files pass."
5. Reorder-blocked: count them, name the top reorder: parents, and note they are deferred to test normalization (#214), not part of the first-pass worklist.
6. High-frequency drops that are not sole blockers: large files_blocked with low sole_blocker means the fix helps many files but flips none alone — flag as enabling, lower immediate yield.

Be honest about caveats: the comparison is strict full-DOM, and if the pinned baseline (roundtrip-baseline.txt) is ungrown, almost the whole corpus shows as failing — these are the raw landscape, not a regression.

Hand-off Fixes (if requested)

• To fix a dropped/under-supported element or a crash: use the mx-api-add-feature skill.
• The findings belong under the tracking issue #208; file specifics with the mx-open-issue skill.