finding(plugin-designer): `MetadataFieldsPage` keys its `fields` map by blind assignment — a nameless field becomes `{ undefined: … }` and a field named `__proto__` vanishes

[claude]

Found while implementing objectui#6240, which closed both of these in the OTHER object writer (app-shell's MetadataService). Filed rather than folded in: it is a different package and a different file, and objectui#6240's dispatch fenced its file face to MetadataService.ts and its tests.

The code

packages/plugin-designer/src/MetadataFieldsPage.tsx, handleFieldsChange:

const nextFields: Record<string, ServerFieldSchema> = {};
for (const f of next) {
 nextFields[f.name] = fromDesignerField(f, prevFields[f.name]);
}

Two hazards, both of them silent.

1. A field with no name keys as the literal string "undefined"

DesignerFieldDefinition.name is declared required, but this page is handed whatever the in-memory designer model holds. The spec does not catch it. Measured against the installed @objectstack/spec 17.2.0:

ObjectSchema.safeParse({ name: 'account', label: 'Account',
 fields: { undefined: { type: 'text', label: 'N' } } })
 => success = true

So a nameless field produces a document that PARSES, is STORED, and has no reader anywhere — a silently corrupt object rather than a loud refusal. objectui#6240 resolved this in MetadataService by throwing before the request is issued, with the measurement above as the reason.

2. A field named __proto__ is dropped by the assignment itself

nextFields['__proto__'] = … invokes the prototype setter instead of creating a key, so the field disappears from the serialised body. And __proto__ is a SPEC-LEGAL field name — ObjectSchema.fields' key schema is /^[a-z_][a-z0-9_]*$/, which it matches. Measured: fields: { __proto__: { type: 'text', label: 'P' } } parses green, so the spec would have accepted the field the client silently threw away.

MetadataService builds through Object.fromEntries for exactly this reason (it defines an own property), and the behaviour is pinned in MetadataService.objectPayloadFieldsMap.test.ts — including an ablation showing that switching back to assignment reds that one case and nothing else.

Also worth checking in the same pass

Duplicate names. A designer list carrying two fields called amount collapses into one map entry with no diagnostic; MetadataService now refuses that too.

Refs: objectui#6240 (the same two hazards, resolved in MetadataService) · objectui#5761 (the parity family; this is value-level, its coverage note 4)

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[os-trump]

Triage grading (hourly round 2026-08-26, session session_01QBLkaj4aAPMVk6ddNFpCgB): promoted — finding → pm:queue, domain:ui, type Bug.

Rationale: two silent value-level hazards with measured spec-level repro (nameless field stores as the literal undefined key and PARSES; a spec-legal __proto__ field name is eaten by the prototype setter), and the resolution is not a new judgement — objectui#6240 already ruled and landed the same two refusals in the sibling writer (MetadataService: Object.fromEntries construction, pre-request refusal of nameless and duplicate names, with pins and an ablation). This card is the same fix pattern applied to packages/plugin-designer/src/MetadataFieldsPage.tsx, duplicate-name refusal included per the card's own "same pass" note. Landing surface is a published designer package ⇒ domain:ui by the three-way split.

Dispatch note for the ui seat: file surface is disjoint from in-flight #6487/#6444; mirror #6240's pin pattern (including the assignment-vs-fromEntries ablation) so the two writers stay provably in parity — objectui#5761 is the parity family anchor.

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Generated by Claude Code