ab-v4 partial sweep (2026-05-01) caught gpt-5.4 emitting
`add_heading_v0({"content":"Pending invitations","level":"caption"})`
as the 12th tag of a 13-tag composite multi-tool response. Even
though the MCP tool def has `enum: ['display','h1','h2','h3']`, the
ab-corpus harness and the in-process production dispatcher both call
buildHeading() with raw JSON args (no jsonschema gate), so the model's
invented "caption" reached the preset lookup. LATIN_PRESETS["caption"]
is undefined, and the next line `fontSize: preset.fontSize` crashed
the WHOLE batch with `undefined is not an object (evaluating
'preset.fontSize')` — the 11 valid tags ahead of it never landed.
Adds an entry-point validation in buildHeading: if `level` is set and
not in the {display, h1, h2, h3} set, throw with a clear message.
The dispatch loops in apps/web/element-tools-dispatcher and
scripts/ab-corpus/apply both catch per-shape and keep running the
remaining tags, so a single bad level on tag 12 no longer kills tags
1-11 + 13.
3 new edge-case tests in element-builders-edge-cases.test.ts
cover the throw + the four valid levels + the omitted-default case.
3772 vitest pass (+3), format clean, tsc silent.
Other element builders likely have the same pattern (preset lookup
on a string enum without runtime validation) — separate sweep, not
shotgunning here.
39aeb0c30e