What causes "${field} must be a non-empty string" in nexu-io/open-design?

Pass a non-empty string for the named field.

nexu-io/open-design · warning · Error

${field} must be a non-empty string

Error message

${field} must be a non-empty string

What it means

readRequiredString is the generic guard used across the brief API (briefDraftId, nonce, requestId, and similar fields). It throws when a value is not a string or is empty, naming the offending field in the message.

Source

Thrown at apps/daemon/src/mcp-brief.ts:647

  return {
    completeCard: BRIEF_COPY[locale].completeCard,
    confirmed: BRIEF_COPY[locale].confirmed,
    continueWithBrief: BRIEF_COPY[locale].continueWithBrief,
  };
}

function readProjectTitle(value: unknown): string {
  if (value === undefined) return 'Untitled Open Design artifact';
  const title = readRequiredString(value, 'projectTitle').trim();
  if (title.length > 256) {
    throw new Error('projectTitle must be at most 256 characters');
  }
  return title;
}

function readRequiredString(value: unknown, field: string): string {
  if (typeof value !== 'string' || value.length === 0) {
    throw new Error(`${field} must be a non-empty string`);
  }
  return value;
}

function readAnswerRecord(value: unknown, field: string): UnknownRecord {
  if (value === undefined) return {};
  if (!value || typeof value !== 'object' || Array.isArray(value)) {
    throw new Error(`${field} must be an object`);
  }
  return { ...(value as UnknownRecord) };
}

function stableAnswerDigest(answers: OpenDesignBriefAnswers): string {
  return JSON.stringify(
    Object.entries(answers)
      .sort(([left], [right]) => left.localeCompare(right))
      .map(([key, value]) => [key, [...value]]),
  );

View on GitHub (pinned to 5be4028344)

Solutions

Pass a non-empty string for the named field.
If the field is optional in your flow, gate the call on its presence.

Example fix

// before
confirm_brief({ briefDraftId: undefined, nonce, answers }) // throws [366] briefDraftId

// after
confirm_brief({ briefDraftId: draft.briefDraftId, nonce, answers })

Defensive patterns

Strategy: validation

Validate before calling

function requireStr(v: unknown, name: string): string {
  if (typeof v !== 'string' || v.length === 0) throw new Error(`${name} must be a non-empty string`);
  return v;
}

Type guard

function isNonEmptyString(v: unknown): v is string {
  return typeof v === 'string' && v.length > 0;
}

Prevention

Validate required string fields before the call.
Thread ids straight from the producing call rather than recomputing them.
Distinguish optional vs required fields at the call site.

When it happens

Trigger: Omitting a required string field; passing undefined, null, or a number; passing an empty or whitespace-only string for a field that is not separately trimmed.

Common situations: An agent forgot to thread a field through; JSON parsed a number where a string id was expected.

Related errors

AI-assisted analysis of nexu-io/open-design@5be4028344 (2026-08-12). Data as JSON: /api/errors/999e81c1f99c19f1. Report an issue: GitHub.