What causes "Deprecation fragment {} has unclosed frontmatter" in vectordotdev/vector?

Add a closing `---` on its own line after the frontmatter keys in the file named in the error.

Deprecation fragment

Error message

Deprecation fragment {} has unclosed frontmatter

What it means

After finding the opening `---`, `split_frontmatter` locates the closing delimiter via `split_once("\n---")`. If no closing `\n---` sequence exists, the frontmatter block is unterminated and this error is raised with the fragment path. It protects the parser from consuming Markdown body content as YAML metadata.

Solutions

Add a closing `---` on its own line after the frontmatter keys in the file named in the error.
Ensure the closing delimiter is preceded by a newline (frontmatter content must not share its line).
Diff the fragment against a known-good deprecation fragment to confirm both opening and closing fences are present.

Example fix

# before
---
date: 2026-09-15
component: transform/foo

# after
---
date: 2026-09-15
component: transform/foo
---
deprecates the `foo` transform.

Defensive patterns

Strategy: validation

Validate before calling

def has_closed_frontmatter(text):
    t = text.lstrip('\ufeff \t\n\r')
    if not t.startswith('---'):
        return False
    return '\n---' in t[3:]

Prevention

Always close frontmatter with `---` on its own line before the Markdown body.
Disable formatters/wrappers that could join the closing delimiter with body text.
Run the deprecation-fragment check locally (e.g. `make check-*` target) before pushing.

When it happens

Trigger: A deprecation fragment contains an opening `---` line but no second `---` line on its own line (e.g. author wrote frontmatter fields and forgot the closing delimiter, or the closing `---` is on the same line as content).

Common situations: Hand-writing frontmatter and omitting the closing fence; a formatter wrapping/joining the closing `---` with adjacent text; using `----` or indented dashes that still don't form a `\n---` boundary at line start context expected by split_once.

Related errors

AI-assisted analysis of vectordotdev/vector@bdb87aeaa4 (2026-09-16). Data as JSON: /api/errors/b270e3f88f05bb3a. Report an issue: GitHub.

Appendix: source

Thrown at vdev/src/utils/deprecation.rs:212

/// Split the raw file contents into the frontmatter string and the body.
/// The file must begin with `---`, and have a closing `---` on its own line.
fn split_frontmatter<'a>(content: &'a str, path: &Path) -> Result<(&'a str, &'a str)> {
    let content = content.trim_start();
    // Advance past the opening `---` (and optional trailing whitespace on that line)
    let after_open = content
        .strip_prefix("---")
        .ok_or_else(|| {
            anyhow!(
                "Deprecation fragment {} must begin with YAML frontmatter (---)",
                path.display()
            )
        })?
        .trim_start_matches([' ', '\t'])
        .trim_start_matches('\n');

    let (frontmatter, rest) = after_open.split_once("\n---").ok_or_else(|| {
        anyhow!(
            "Deprecation fragment {} has unclosed frontmatter",
            path.display()
        )
    })?;
    let rest = rest.trim_start_matches(['\r', '\n']);
    let body = rest.trim_start_matches(['\r', '\n']);

    Ok((frontmatter, body))
}

/// A deprecation that has been enacted (feature removed).
#[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
pub struct EnactedEntry {
    pub what: String,
    pub deprecated_since: String,
    pub removed_in: String,
    #[serde(default, skip_serializing_if = "String::is_empty")]
    pub description: String,

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