feat: eligibility claims & verification contract

[igrigorik]

Context: #214 (comment). Supersedes #214, closes #137.

Introduce context.eligibility — buyer claims about eligible benefits (loyalty membership, payment instrument perks, etc.) that Businesses can act on across the shopping lifecycle.

Processing model:

• Platform provides claims via context.eligibility on any request
• Business MAY act on recognized claims (adjust pricing, product access, provisional discounts); MUST ignore unrecognized claims
• At checkout completion, all claims that influenced the checkout MUST be resolved: verified against proof, or rescinded by Platform
• Unresolved claims block completion (invalid_eligibility error)
• Business MUST NOT mutate checkout on verification failure

Layering:

• context.json: eligibility array with reverse-domain $ref validation
• checkout.md: normative verification contract (core obligation)
• discount.json: provisional + eligibility fields on applied_discount for structured attribution when extension is active
• catalog/index.md: MAY adjust price/list_price for eligible claims
• error_code.json: invalid_eligibility standard error

Eligibility lives on context (not PaymentInstrument) enabling full-funnel coverage from catalog through checkout. Verification is a core checkout concern; the discount extension adds attribution.

---

Checklist

• New feature (non-breaking change which adds functionality)
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[ACSchil]

This LGTM. I like the direction we ended up taking; it works elegantly within the broader spec.

[douglasborthwick-crypto]

The context.eligibility + provisional model here is clean. One gap worth flagging: the verification contract at completion says claims "MUST be resolved: verified against proof, or rescinded by Platform" — but the spec doesn't define what "proof" looks like for claims that aren't derivable from the payment credential.

For instrument-bound claims (RedCard BIN check), the Business can verify locally. For non-instrument claims — loyalty tiers, on-chain holdings, membership status — the Business needs something to verify against.

A minimal option: let context.eligibility entries optionally carry a companion attestation (sig + kid) that the Business can check against a published JWKS. That keeps verification deterministic without requiring the Business to call back to the Platform or the claim source. More detail in #214.

[igrigorik]

@maximenajim ty, great feedback, ptal at updated draft.

@douglasborthwick-crypto we're intentionally leaving the verification mechanism open — "UCP does not prescribe how verification occurs" — because the proof model varies by claim type:

• Payment instrument claims: the credential itself is the proof, no additional mechanism needed
• Authenticated session claims (loyalty, membership): can be validated via provided identity / linking
• Third-party attestation: this is where a signed proof would operate

A companion attestation mechanism (sig + kid against a published JWKS) is a great candidate for a capability extension that complements eligibility. Additive and non-breaking: platforms that can provide attestations send them alongside the claim; businesses that require them check, others ignore.

[douglasborthwick-crypto]

That's the right split — prescribing verification in core would overfit to one claim type. The capability extension model keeps eligibility clean.

Happy to draft that extension. The shape I have in mind:

{
  "context": {
    "eligibility": [{
      "type": "token_holder",
      "label": "USDC holder discount",
      "attestation": {
        "sig": "<base64 ECDSA signature>",
        "kid": "insumer-attest-v1",
        "jwks_uri": "https://insumermodel.com/.well-known/jwks.json",
        "condition_hash": "<SHA-256 of evaluated condition>",
        "attested_at": "2026-03-12T16:40:00Z",
        "expires_at": "2026-03-12T17:10:00Z"
      }
    }]
  }
}

The Business verifies the signature against the JWKS, checks condition_hash matches the claim, confirms expires_at hasn't passed. No callback to the Platform or the attestation provider needed — fully offline verification.

This is already the pattern in #203 (com.insumermodel.attestation) and the x402 cold-start spec. I'll put together a capability extension draft against the current eligibility schema once #250 lands.

[lemonmade]

I really like where you landed with this 👏

[lemonmade]

Something doesn’t quite sit right with me about this. I think it’s that we are providing details about the source of one kind of automatic discount with a top-level field. If we wanted to indicate the source of other automatic discounts (from identity linking, based on the line items, etc), we would need additional fields. Did you consider any design for this that broadened this field a bit to leave space for more detail on other discount types?

[igrigorik]

Good prompt. There is potential for "provenance proliferation" here, but so far I haven't spotted a generic schema. If you squint, codes and eligibility are both string[] — but they carry different lifecycle contracts that you'd want to capture and distinguish, so a generic source would still need type-specific semantics. All we'd have done is push the modeling complexity down one layer.

Open to suggestions.

[igrigorik]

@douglasborthwick-crypto 👍 modulo one gotcha on example you shared...

context.eligibility is string[] with reverse-domain validated items. Your example changes items from strings to objects, which breaks the contract. The attestation should live separately from eligibility not inline, e.g...

  {
    "context": {
      "eligibility": ["com.insumermodel.token_holder"],
      "attestations": {
        "com.insumermodel.token_holder": {
          "sig": "<base64>",
           ...
        }
      }
    }
  }

^ either that or on cart/checkout object similar to discount extension.

[raginpirate]

Looks great! This matches my initial reaction to first model this above payments before we take it into payments.

And the path of associating a discount to an eligibility signal highlights how we can take this to other independent primitives across ucp in the future; payments can eventually adopt this connective tissue too if we find the use case benefits people beyond this simple version.

[amithanda]

Looks great, thanks for addressing all the feedback.