The Tessara Payer FHIR Drift Index — Methodology Specification v0.1

1. Scope

Tessara measures structural conformance drift in publicly exposed payer FHIR APIs against their respective published Implementation Guide baselines. The Index covers the five regulator-mandated APIs: CMS-0057-F and its predecessor CMS-9115-F together require 5 FHIR APIs — four under CMS-0057-F (Patient Access expanded, Provider Access, Payer-to-Payer, Prior Authorization) plus CMS-9115-F's Provider Directory. Only public /metadata and CapabilityStatement endpoints are scanned. No authentication is performed, no Protected Health Information (PHI) is collected, accessed, or processed, and no scanning is performed beyond what a public audit kit operating against unauthenticated endpoints would observe.

2. Input data

For each plan, Tessara samples the public CapabilityStatement and /metadata responses at quarterly intervals. The CapabilityStatement is parsed into the canonical 6-category drift taxonomy framework. Each measurement is timestamped, hashed (SHA-256 of canonicalized JSON per RFC 8785 JCS), and chained to the prior measurement's hash, producing a tamper-evident longitudinal record per plan per endpoint.

3. The canonical 6-category drift taxonomy

Tessara classifies every detected drift into exactly one of six categories. Full definitions, examples, and machine-readable rule references are maintained in the Glossary.

• Mandatory Element Removal — a required element from the IG baseline disappears from the CapabilityStatement.
• Type or Cardinality Change — an element's declared type, min, or max diverges from the baseline.
• Structural Extension — additions to the CapabilityStatement that alter the contract surface.
• Auth or Authorization Deviation — declared security schemes, scopes, or SMART-on-FHIR posture diverge from baseline.
• Endpoint Behavioral Change — operations, interactions, or search parameters added, removed, or modified relative to baseline.
• Spec Version Mismatch — the declared FHIR version or IG version diverges from the regulator-mandated baseline.

4. Scoring algorithm

Each detected drift is scored by category-weighted severity. Category weights are part of the patent-pending detection methodology (filed 2026-03-30; non-provisional deadline 2027-03-30) and are not published in this specification. Weights will be released alongside the open-source payer-fhir-drift-bench harness after the foreign-filing window completes.

The published methodology covers the input domain, the taxonomy framework, the measurement protocol, the verification protocol, and the cohort presentation rules. The category weight vector and the inter-category scoring matrix are deliberately withheld to preserve patent freedom-to-operate during the foreign-filing window.

5. Anonymized cohort scoring policy

Plans are scored individually but presented in cohort positions — for example, "Top-25 Medicaid MCO position 14". Named-plan attribution is opt-in: any plan may request its anonymized cell be published under its own attribution. Plans appearing in the Index receive a 72-hour pre-notice email before public release with their cohort position and methodology summary. Corrections or contextual statements submitted during the pre-notice window are published alongside the plan's cell in the released Index.

6. Verification protocol

Every published Drift Index score is accompanied by a signed verdict (Ed25519 signature over the canonicalized score record). A public verification tool at tessara.us/verify accepts a signed verdict and returns pass/fail against Tessara's published public key. Third parties may re-run the methodology against the same public FHIR endpoints to independently reproduce the structural inputs.

7. Independence statement

Tessara is not owned by, funded by, or operationally entangled with any FHIR platform vendor (Onyx Health, Edifecs, Smile Digital Health, 1upHealth, Firely, Health Samurai, Health Gorilla, Particle Health, Redox, or others). Solo founder, patent-pending, pre-revenue. No revenue-sharing or referral relationships with any platform vendor exist as of publication date.

8. Revision policy

Methodology version v0.1 is the published baseline as of 2026-05-11. Material changes will be versioned (v0.2, v0.3, ...) with a changelog appended to the bottom of this page. Cohort positions across methodology versions are not directly comparable; comparisons across versions require explicit re-scoring against the new baseline.

9. Citation

Cite this methodology as: Everett, D. (2026). "The Tessara Payer FHIR Drift Index Methodology Specification v0.1." tessara.us/drift-index/methodology.