The nine opinions
These are the stances baked into the vocabulary. They are the reason it is called opinionated.
1. A hypermedia API is a UI for agents
The web gave humans a universal interface: one browser, a handful of constant controls, and every site's capability delivered as content on the page. An agent should get the same deal — driving a generic hypermedia client the way you drive a browser, with capability carried by the API's responses rather than by bespoke per-API tools. The agent isn't the browser; it's the machine driving one, given the same powers HTML and the web gave people. HydrAI is the vocabulary that lets responses carry that capability.
2. Mint only at the gap; reuse everything else
A term enters HydrAI only when no existing standard says it and a real client already needs it. Every term's definition carries a "why not the existing thing." If VoID, SHACL, or Hydra already says it, HydrAI points at them — it does not re-say it.
3. Learn one vocabulary, not a dozen
A single curated JSON-LD @context maps friendly terms to the canonical IRIs of Hydra, SHACL, VoID, and HydrAI's own additions. You write member, exampleQuery, sparqlEndpoint; each expands to the right vocabulary's real IRI. This is the move schema.org made — hide the zoo behind one legible surface — applied to hypermedia affordances.
4. Steward the orphans; reference the living
HydrAI mirrors Hydra (dormant, in need of a steward) with formal equivalence axioms. It merely references VoID, SHACL, schema.org (alive and maintained). You do not fork what someone is still tending.
5. Constrain our own prose
The one freeform slot — a server's greeting — is capped by a SHACL constraint in the vocabulary itself, and clients enforce it. A greeting is identity and stance in a few sentences, not a manual. We prevent prose from becoming a crutch not by forbidding it but by giving everything else a structured home, so there is nothing left to cram into prose. A vocabulary that teaches people to build agents should model the discipline it teaches.
6. Every term is a doorway
Each curated term carries rdfs:isDefinedBy (its source vocabulary) and rdfs:seeAlso (a place to learn it). The turn-key context gets you running fast; the pointers let you walk from any term into the full depth behind it. Turn-key and rigorous are not a trade-off — they are different layers of the same term.
7. Refuse, don't warn
Invalid input is rejected with an honest error, not silently coerced. A constraint that can be quietly ignored is not a constraint.
8. Additive and degradable
HydrAI never contradicts Hydra core. An agent that has never heard of HydrAI still works against a HydrAI-described API via plain Hydra. Adopt as much or as little as you want.
9. Untrusted by default; signing is authenticity, not authority
A greeting and an example query are content a server injects into an agent — the prompt-injection surface. HydrAI's answer is a client posture, not per-term typing — injection rides on ordinary data too, so you cannot type your way to safety: all server content is untrusted by default, and a verifiable proof upgrades a value's attribution, never its authority. Even a signed greeting is data, never a command. Nothing a server sends is ever obeyed; executable examples run only through the client's own gates, under the client's own authority.
See how opinion 9 plays out in practice in the safety posture, and where each opinion touched the vocabulary in the design note.