How do I add a new Attribute to a Vector Layer?

[toihr]

I am unable to add a new Attribute to a table? I want to Mark down things that belong to a feature and I cannot because I cannot write fields and I cannot create new fields?

[kaminagakur4]

I asked Claude to lay out my solution in a straightforward, step-by-step manner, so this should be pretty easy to follow. Let me know if you need any extra help.
The Add field button exists in the attribute table toolbar — it's just gated, and one of the gates is the opposite of what you'd expect. Three conditions have to hold at once.

1. You must not be in Edit mode. This is the counter-intuitive one: Add field, Calculate and the column menu are all rendered under canManageColumns && !isEditing. If you clicked Edit first — the natural move when you want to write into a table — the button disappears. Leave Edit (or Save), add the field, then go back into Edit to fill it.

2. The layer has to be an in-store GeoJSON layer. The gate is

const canManageColumns =
  Boolean(layer?.geojson) && !isDuckDBLayer && !isReadOnlyVectorLayer && layerCaps.update;

with the reason stated right above it in the source: column management mutates layer.geojson/style/metadata, so it's offered only for in-store, editable GeoJSON layers — not DuckDB query results, and not layers added through Add Vector Layer, whose features live in a MapLibre source rather than in layer.geojson and so are read-only by design (isReadOnlyVectorLayer). If your layer came from either, materialize it into a GeoJSON layer first — the Layers panel offers that for DuckDB results — and the toolbar changes.

3. The layer needs at least one feature — canAddColumn = features.length > 0. You can't add a column to an empty layer.

4. Then the flow is: Layers panel → Open attribute table → + Add field → name it → Edit → type values → Save. The docs describe the second half under Editing values: "For editable layers (including GeoJSON layers and materialized DuckDB layers), click Edit to change attribute values inline, then Save to commit them."

5. If what you want is a computed column rather than typed values, use Virtual Fields in the layer's Style panel instead — expression-backed columns that recompute whenever the data changes, show up everywhere real fields do (attribute table, styling, labels, selection), and persist with the project. The attribute table's Field Calculator is the other half of that pair: it writes static values once.

If Add field still isn't there with the table out of Edit mode, say how the layer was added (file drop, Add Vector Layer, DuckDB, SQL Workspace) — that's what decides which of the gates is closing.