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Undeclared placeholder

A running header or footer references a {placeholder} Keystone doesn't recognize. It warns and passes the token through literally.

What it looks like

# pandoc.yaml
header-text: "{poem-authr}"

WARN: unrecognized placeholder '{poem-authr}' in running text; passing through
  as literal
  See https://keystone.knight-owl.dev/errors/undeclared-placeholder/

What it means

Running-header text is built from a fixed set of placeholders — title, author, date, page, chapter, section — plus any running-header mark you declare under marks:. A name outside that set can't be filled, so Keystone warns and leaves the literal {poem-authr} in the header rather than guess. This is the running-text mirror of an undeclared mark: a mark is fatal because it writes to a channel, but a placeholder only renders, so an unknown one merely warns.

(PDF only — running headers and footers are a PDF feature.)

Use a built-in name, or declare your own

Fix the spelling against the built-in names above. To reference content of your own, declare the name under marks: first — see Running headers & footers.