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Get a post-quantum ML-DSA-65 signature (FIPS 204, scheme 4099 — the same scheme the Animica L1 admits transactions with) over a document you never have to send us. Post a sha256 digest, or post a JSON payload and we canonicalise and hash it here; either way the signature covers the HASH, so the resulting envelope can be published, escrowed or put on-chain without disclosing the content. Buy this when a claim has to stay checkable by a third party years from now — a dataset manifest, a model card, an audit-log segment, a delivery receipt between two agents, the exact bytes of an agreement — because an HMAC proves nothing to anyone but its issuer and an ECDSA signature made today is retrospectively forgeable once a cryptographically-relevant quantum computer exists. You need no PQ library and no key of your own. Verification is FREE (POST /x402/pq/sign/verify) and the signing public key is FREE (GET /x402/pq/sign/pubkey), so anyone can check an envelope offline without paying us. The signed statement records whether YOU or WE computed the digest, because those are different claims. If the signer or its key is not reachable you get a 503 with the reason and are NOT charged — this endpoint has no branch that returns an unsigned envelope.