First cross-repo pull crashed the build — YAML parsed log dates as Date objects

What this fixes

The cross-repo log aggregation shipped two weeks ago — lib/source.ts and lib/logs.ts accept a logSourceRepo override so a project page in this blog can pull its timeline from a separate repo's content/logs/<slug>/ directory. It worked in isolation. The first real satellite (Daeseon-AI-Factory/docvault) crashed the Vercel build the moment its log entries hit the rendering path.

Symptom

Error occurred prerendering page "/projects/docvault".
[Error: Objects are not valid as a React child (found: [object Date]).
 If you meant to render a collection of children, use an array instead.]
Export encountered an error on /(public)/projects/[slug]/page: /projects/docvault, exiting the build.
Error: Command "npm run build" exited with 1

Cause

docvault's log entries were written with unquoted ISO dates:

date: 2026-03-24      # unquoted

This blog's own log entries always quoted them:

date: "2026-05-27"    # quoted

Both forms are valid YAML, but the meaning is different. The YAML spec's !!timestamp tag auto-converts unquoted ISO date literals to native timestamp objects. js-yaml (which gray-matter uses) implements that — so unquoted dates parsed as JavaScript Date instances rather than strings.

lib/logs.ts:readLogFile passed fm.date through verbatim into the LogFrontmatter object. lib/format.ts:formatDate did:

export function formatDate(iso: string, locale: Locale): string {
  try {
    const date = parseISO(iso);
    return format(date, ...);
  } catch {
    return iso;
  }
}

parseISO from date-fns expects a string. Passed a Date, it threw. The catch returned iso — which was the original Date object. That Date then landed inside JSX:

{formatDate(e.frontmatter.date, locale)}

React: "Objects are not valid as a React child". Build aborts.

Fix

Three layers, defense in depth:

1. Data layer (lib/logs.ts, lib/projects.ts): coerce fm.date at parse time. Date → toISOString().slice(0, 10), string passes through, anything else rejects the entry.
2. Format layer (lib/format.ts): accept Date | string | unknown, never return a non-string.
3. Source layer (lib/source.ts:repoConfig): normalize trailing slash/whitespace from logSourceRepo so values like "owner/name/" (easy to mistype in admin) still resolve cleanly.

Build passes. /projects/docvault renders with the three timeline entries pulled from the docvault repo.

What I'd do differently

The fragility was knowable. When I wrote the LogFrontmatter type, I typed date: string and trusted gray-matter to honor it. gray-matter doesn't lie — it returns whatever js-yaml produces, and js-yaml honors the YAML spec including !!timestamp. The lesson: any type assertion on matter() output is a vibes-based assertion. Coerce at the boundary, or pin gray-matter's YAML engine to a stricter schema (JSON_SCHEMA would skip the timestamp tag entirely).

Pattern

External content sources push the type-coercion responsibility onto the consuming code. YAML's auto-typing (numbers, booleans, timestamps, null aliases like ~) only matters when someone else writes the file — your own conventions hide the issue. Library code that reads frontmatter from any source should normalize at the boundary, not trust the schema.

Commit

c3517f5 — Fix prerender crash on cross-repo log entries with unquoted YAML dates.