[Agricola] AGR-2026-107: Automatic client ignores payment preference ordering

[mpp-agricola]

AGR-2026-107 — Automatic client ignores payment preference ordering

Last observed by the head-to-head audit at 2026-09-14T09:17:13.387847Z.

Audited heads

Target	Repository	Commit	Conformance	Semantic review
typescript	wevm/mppx	3c14a65a7ea5	Complete	Reference
ruby	stripe/mpp-rb	3b9e2923c67b	Complete	Complete
go	tempoxyz/mpp-go	9fcf9a47c61b	Complete	Complete
java	stripe/mpp-java	ca57f0998545	Complete	Complete
python	tempoxyz/pympp	7988d1cda5c4	Complete	Complete
rust	tempoxyz/mpp-rs	d859a13d74ef	Complete	Complete

Finding

• Fingerprint: semantic:negotiation/honor-client-payment-preferences
• Source: semantic
• Affected SDKs: ruby
• Clean SDKs: none
• Not reported by semantic review: go, java, python, rust
• Canonical reference: draft-ietf-httpauth-payment §7.1 (Challenge Negotiation with Accept-Payment)
• Severity: medium
• Confidence: high

Evidence

SDK	Canonical evidence	SDK evidence	Suggested test
ruby	selectChallengeCandidates — Matches challenges by method and intent, excludes non-matches and q=0 offers, then orders candidates by protocol rank, preference quality, and server order.	find_matching_challenge — Scans WWW-Authenticate values in wire order and immediately returns the first challenge whose method name exists in the method map, without consulting Accept-Payment preferences.	Configure two supported methods with the first server challenge assigned q=0 and the second q=1 in Accept-Payment; assert the automatic client advertises the preferences and creates a credential only for the second challenge.

ruby: The canonical client advertises its supported payment preferences and selects compatible challenges by protocol rank, quality, specificity, and server order. The target sends no derived Accept-Payment header and returns the first challenge whose method name is registered. Consequently, it can pay a lower-priority offer, select an offer explicitly assigned q=0 by the caller, or fail to influence a server that tailors its challenge set from Accept-Payment.

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/ag fix is unavailable for ruby because the affected SDK is configured for notification-only automation.