Sanity Config Reducers Agent Skill

Sanity Config Reducers

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Use this skill when a config value can be supplied by root config or plugins and needs deterministic merge behavior.

Inspect these files first:

• packages/sanity/src/core/config/types.ts
• packages/sanity/src/core/config/configPropertyReducers.ts
• packages/sanity/src/core/config/prepareConfig.tsx
• packages/sanity/src/core/config/flattenConfig.ts
• packages/sanity/src/core/config/resolveDefaultPlugins.ts if the config gates default plugin injection.

Reducer Pattern

Reducers usually:

• Call flattenConfig(config, []).
• Reduce from an explicit initialValue.
• Read the property from each innerConfig.
• Ignore undefined.
• Accept only the documented type.
• Throw with getPrintableType(value) for invalid values.

Config namespaces should usually be objects, even when they initially only contain one flag. This keeps room for future options without changing the public shape. For booleans, prefer feature.enabled and follow this shape:

export const featureEnabledReducer = (opts: {
  config: PluginOptions
  initialValue: boolean
}): boolean => {
  const {config, initialValue} = opts
  const flattenedConfig = flattenConfig(config, [])

  return flattenedConfig.reduce((acc, {config: innerConfig}) => {
    const enabled = innerConfig.feature?.enabled

    if (typeof enabled === 'undefined') return acc
    if (typeof enabled === 'boolean') return enabled

    throw new Error(
      `Expected \`feature.enabled\` to be a boolean, but received ${getPrintableType(enabled)}`,
    )
  }, initialValue)
}

Root config is flattened after plugin config, so root values usually win when a reducer overwrites with the latest defined value.

Types And Exposure

When adding a config property:

• Declare the public or internal input type in PluginOptions, WorkspaceOptions, or the relevant nested type in types.ts.
• Prefer extensible object namespaces such as beta.feature.enabled over direct booleans such as beta.feature.
• If runtime code needs the resolved value, expose it on Source, Workspace, or the relevant prepared object in prepareConfig.tsx.
• Keep raw config access through source.__internal.options as an implementation detail, not the primary runtime API.

Default Plugin Gates

Default plugin lists are computed before full source resolution. If a config value controls default plugin injection:

• Compute the reduced value early enough in prepareConfig.tsx, before calling getDefaultPlugins.
• Thread that reduced value into getDefaultPluginsOptions or the options passed to getDefaultPlugins.
• Use the same reducer later when exposing the resolved runtime value, so gating and runtime context agree.
• Add tests for default false/true behavior and invalid values.

Beta Flags

Beta flags live under BetaFeatures in types.ts and resolved runtime values under source.beta.

Prefer a dedicated reducer when:

• Plugins can set or override the flag.
• Invalid values should produce helpful errors.
• The flag gates default plugin injection.

Do not special-case beta flags in components by reading raw config if a resolved value can be exposed instead.

Verification

Add focused config tests for:

• Default value.
• Root config value.
• Plugin-provided value.
• Root-over-plugin precedence when relevant.
• Invalid namespace object error message when the top-level config is malformed.
• Invalid nested property error message, for example when enabled is not the documented type.