slidevjs/slidev · error · Error

Invalid YAML frontmatter: ${parsed.errors.map(e => e.message

Error message

Invalid YAML frontmatter: ${parsed.errors.map(e => e.message).join('; ')}

What it means

Thrown inside applySlidePatch when patch.frontmatterRaw is non-empty and YAML.parseDocument(patch.frontmatterRaw) returns one or more errors. It surfaces the underlying YAML parser messages joined by '; ' so the caller can correct the syntax. Used by the raw-frontmatter update path.

Source

Thrown at packages/slidev/node/mcp/operations.ts:63

 */
export async function applySlidePatch(
  data: LoadedSlidevData,
  no: number,
  patch: SlidePatch,
): Promise<SlidePatchResult> {
  const slide = resolveSlide(data, no)
  const source = slide.source

  if (patch.content != null)
    source.content = patch.content
  if (patch.frontmatterRaw != null) {
    if (patch.frontmatterRaw.trim() === '') {
      source.frontmatterDoc = source.frontmatterStyle = undefined
    }
    else {
      const parsed = YAML.parseDocument(patch.frontmatterRaw)
      if (parsed.errors.length)
        throw new Error(`Invalid YAML frontmatter: ${parsed.errors.map(e => e.message).join('; ')}`)
      source.frontmatterDoc = parsed
    }
  }
  if (patch.note != null)
    source.note = patch.note
  if (patch.frontmatter)
    updateFrontmatterPatch(source, patch.frontmatter)

  parser.prettifySlide(source)
  const fileContent = await parser.save(getMarkdown(data, source))
  return { slide, fileContent }
}

export interface InsertSlideOptions {
  /**
   * Rendered slide number (1-based) after which the new slide is inserted.
   * The new slide is inserted into the same markdown file as this slide.
   */

View on GitHub (pinned to 0d798ace58)

Solutions

1. Validate the YAML locally before sending: YAML.parseDocument(raw).errors must be empty.
2. Use 2-space indentation and quote values containing ':', '#', or leading/trailing spaces.
3. Prefer the structured frontmatter patch object (object form) over frontmatterRaw when only changing known keys.
4. Run the raw block through a YAML linter/formatter (e.g. prettier with yaml plugin).

Example fix

// before: malformed raw frontmatter (tab indent)
await applySlidePatch(data, no, {
  frontmatterRaw: 'layout: two-cols\n\ttransition: slide',
})

// after: valid 2-space YAML
await applySlidePatch(data, no, {
  frontmatterRaw: 'layout: two-cols\n  transition: slide',
})

import YAML from 'yaml'

function validateFrontmatterRaw(raw: string): void {
  if (raw.trim() === '') return
  const parsed = YAML.parseDocument(raw)
  if (parsed.errors.length) {
    throw new Error(`Invalid YAML: ${parsed.errors.map(e => e.message).join('; ')}`)
  }
}

// before applySlidePatch:
if (patch.frontmatterRaw != null) validateFrontmatterRaw(patch.frontmatterRaw)

function isValidYaml(raw: string): boolean {
  if (raw.trim() === '') return true
  return YAML.parseDocument(raw).errors.length === 0
}

try {
  await applySlidePatch(data, no, { frontmatterRaw })
} catch (e) {
  if (e instanceof Error && e.message.startsWith('Invalid YAML frontmatter')) {
    // show parser errors to the user, keep the draft for editing
    return { error: e.message, draft: frontmatterRaw }
  }
  throw e
}

When it happens

Trigger: Calling slidev-update-slide (or applySlidePatch directly) with a frontmatterRaw string that is not valid YAML: bad indentation, duplicate keys, unquoted special characters, tabs for indentation, or a stray ':'.

Common situations: Hand-writing frontmatter with inconsistent indentation; pasting YAML with tabs instead of spaces; using unquoted values like yes/no/on/off that YAML coerces; malformed nested mappings.

Related errors

• Slide ${no} does not exist. The deck has ${data.slides.lengt
• Specify exactly one of `before` or `after`.
• The `before`/`after` anchor must be a different slide than `
• Nothing to update: provide at least one of `content`, `note`
• [Slidev] VDrag Element ${id} is missing markdown source

AI-assisted analysis of slidevjs/slidev@0d798ace58 (2026-08-12). Data as JSON: /api/errors/b109f537c33e53ac. Report an issue: GitHub.