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Conflicting Merge Operations

yaml
A YAML tag specifies both prefer and concat — only one merge operation is allowed per tag.

Q-1-28 — Conflicting Merge Operations

A YAML tag specifies both prefer and concat — only one merge operation is allowed per tag.

What this means

When Quarto merges configuration across layers, each tagged value can declare one of two merge strategies: prefer (this value overrides the same key in earlier layers) or concat (this value is appended to the same key in earlier layers). The two strategies are mutually exclusive — a single value cannot both override and append at once — so a tag may name at most one.

This error fires when a tag combines both prefer and concat, in either order: !prefer_concat, !concat_prefer, or any tag with both components mixed in among other component names.

Why this happens

The two situations:

  • Hand-construction. Someone deliberately wrote both operations into a single tag, perhaps trying to express a hybrid behavior Quarto does not offer.
  • Merging of edited tags. A configuration that started with one operation had the other appended by a later edit, instead of replacing the first.

How to fix

Decide which behavior you want and remove the other component.

If you want the later layer’s value to override earlier layers, keep prefer:

authors: !prefer_md
  - "[Pat](mailto:pat@example.com)"

If you want the later layer’s value to append to earlier layers, keep concat:

authors: !concat_md
  - "[Sam](mailto:sam@example.com)"

If you really need both behaviors for different parts of the value, restructure the configuration so that each part of the value is tagged separately.