Capability discovery
An integration that hardcodes a vendor vocabulary breaks quietly when the vocabulary changes. This is the alternative.
What /describe returns
The filter grammar and the valid values for every enumerated field: content types, authority classes, topics, safety levels, sort orders and collapse modes. It needs no authentication, so an agent can read the capability surface before it holds a key.
curl "https://api.unlob.com/describe"Why an agent should call it
An agent that has read /describe can construct a valid query for any filter without a human having transcribed the vocabulary into its prompt. When we add a content type, agents that read the catalog gain access to it; agents with a hardcoded list simply never use it, and never report why.
Caching
The catalog changes rarely. Fetching it once at agent startup and holding it for the process lifetime is the right pattern — it is a capability description, not live data.
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10,000 requests a month, no card. Everything documented here works on every plan.