Add data-class scheme skeleton; separate label-schemes from host architectures
- schemes/data-class.md: skeleton for the structured classification taxonomy the trust-annotations 'sensitive' boolean deliberately omits (class + regulatory scope + org labels), shaped as open questions and attributed to the SEP-1913 reviewers who pushed for it (JustinCappos, olaservo, Mossaka, krubenok, localden). - schemes/README.md: list data-class under 'Schemes here'; add verified label- scheme candidates (Permissive IFC 2410.03055, AirGapAgent 2405.05175); move CaMeL and the design-pattern catalogue into a new 'Not schemes: host architectures' section, since they govern host control-flow rather than producing a per-result data label. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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schemes/data-class.md
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---
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title: Data classification (data-labelling scheme)
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---
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> ⚠️ **Experimental scheme skeleton.** This is one **data-labelling scheme** that
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> fills the [`trust-annotations`](../specification/draft/trust-annotations.mdx)
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> `evidenceRef` slot via `type: "data-class.v1"`. It is **not** an MCP extension
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> and **not** a sibling of the extensions — it is the home for the richer
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> classification taxonomy that the extension's coarse `sensitive` boolean
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> deliberately leaves out. See [Why a scheme](#why-a-scheme-not-a-wire-field).
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**Scheme** — fills `trust-annotations`'s `evidenceRef` slot, selected by
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`evidenceRef.type == "data-class.v1"`. Not an extension.
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## Abstract
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This scheme defines `data-class.v1`, an entry for the `trust-annotations`
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`evidenceRef` slot that carries a **structured data classification**: a coarse
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sensitivity level plus zero or more regulatory scopes and optional org-defined
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labels. It is the structured counterpart to the extension's single
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`sensitive: boolean`, for deployments that need to express *which* regulated
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category a result falls under (e.g. HIPAA, GDPR, PCI-DSS) without putting that
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taxonomy on the wire for every server.
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## Why a scheme, not a wire field
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The `trust-annotations` extension keeps only `sensitive: boolean` on the wire.
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That coarse signal is universally client-actionable and cheap, and — critically
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— it cannot become wrong as regulation changes. A richer classification was
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asked for repeatedly in review, in three different shapes:
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- a **linear** `sensitiveHint: low | medium | high`, which
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[@JustinCappos](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/711#issuecomment-2967516811)
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rejected: sensitivity is set-theoretic (a card number and a medical record are
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both sensitive but to *different* readers), not a single scale;
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- **org-defined vocabularies** rather than a fixed enum
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([@olaservo](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/711#issuecomment-2968743154);
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[@Mossaka](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/711#issuecomment-2971788308)
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proposed reverse-DNS `classification.labels`);
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- a **class + regulatory scope** pairing such as `confidential:hipaa`
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([@krubenok](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1913#discussion_r3103485194);
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carried into SEP-2061 by @rreichel3).
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Against all three, [@localden](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1913#issuecomment-4037623595)
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warned that a taxonomy baked into the protocol is very hard to remove if it turns
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out wrong. A scheme resolves the tension: the taxonomy evolves behind a `type`
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value, on its own clock, while the wire keeps the boolean that can never rot.
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## Motivation
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A regulated deployment needs to know not just *that* a result is sensitive but
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*under which regime*, so the host can apply the matching control (HIPAA minimum-
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necessary, GDPR purpose limitation, PCI-DSS storage rules). The boolean cannot
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carry that, and encoding it inline would force every server and client to agree
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on a regulatory taxonomy. This scheme lets a server that already knows the
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classification (a healthcare record store, a payments API) attach it as evidence,
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while a client that does not implement the scheme still sees `sensitive: true`.
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## Specification
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### Scheme identity
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A `data-class.v1` record is selected by `evidenceRef.type == "data-class.v1"` on
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a `trust-annotations` annotation whose `sensitive` is `true`. The
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`digest`/`canonicalization` pair over the record is required exactly as for any
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other scheme.
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### Payload (skeleton)
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The record is a JSON object. Shapes below are **open questions**, not settled:
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```json
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{
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"class": "confidential",
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"regulatory": ["hipaa"],
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"labels": ["com.example.pii.ssn"],
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"policyRef": "https://policy.example.com/classes/confidential"
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}
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```
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- `class` — a coarse level. Candidate set `public | internal | confidential |
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restricted` (four levels, from the original SEP-1913 taxonomy). Whether the set
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is fixed or registry-curated is open.
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- `regulatory` — zero or more regulatory scopes the result falls under. Open
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strings, not an enum (so a new regime doesn't need a schema change). This is
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the `:scope` half of @krubenok's `confidential:hipaa`.
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- `labels` — optional org-defined tags, reverse-DNS namespaced per @Mossaka, for
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vocabularies a deployment defines for itself.
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- `policyRef` — optional pointer to the policy that defines the classes, so the
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record is interpretable by a host that hasn't pre-agreed the vocabulary.
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### Graceful degradation
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A client that does not implement `data-class.v1` ignores the record and relies on
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the `trust-annotations` `sensitive` boolean, which remains meaningful on its own.
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The scheme only ever *refines* the boolean; it never contradicts it (a
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`data-class.v1` record only appears where `sensitive` is already `true`).
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## Producer / consumer
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- **Producer (candidate).** A server fronting a regulated store (health records,
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payments) that already classifies its data internally.
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- **Consumer (candidate).** A host policy engine that maps `class` + `regulatory`
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to a control (block, redact, prompt). The same host-side resolution model as
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the FIDES scheme applies: the wire marker is advisory; the regulated semantics
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are host-resolved against the deployment's policy.
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## Open questions
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- Is the `class` set fixed (`public/internal/confidential/restricted`) or
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registry-curated? @localden's removability concern argues against fixing it.
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- Do `regulatory` and `labels` overlap enough to collapse into one open-string
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list, or are "named regime" and "org-defined tag" usefully distinct?
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- Should `data-class.v1` ever be expressible **on the wire** as a structured
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escape hatch (tracked in [open-questions.md](../docs/open-questions.md)),
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or is keeping it strictly behind `evidenceRef` the right boundary?
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- How does this relate to SEP-2061's `DataClass` / `RegulatoryScope` — is this
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scheme the migration target for that work, or a parallel encoding?
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## Changelog
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- **2026-06-17** — Initial skeleton. Payload shapes are candidates pending the
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open questions above.
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