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>

schemes/README.md

@@ -17,17 +17,19 @@ here rather than as a top-level extension.
 | Scheme | `evidenceRef.type` | Status | Source |
 | :--- | :--- | :--- | :--- |
 | [FIDES information-flow control](./ifc-fides.md) | `ifc.fides.v1` | Draft skeleton | [arXiv:2505.23643](https://arxiv.org/abs/2505.23643) |
+| [Data classification](./data-class.md) | `data-class.v1` | Draft skeleton | SEP-1913 taxonomy (`class` + regulatory scope) |
 
 ## Candidate schemes (not yet drafted)
 
 The open `type` slot is designed to carry the range of models raised in the
 [SEP-1913](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1913)
-review and the surrounding literature. Each is a candidate for its own scheme doc:
+review and the surrounding literature. Each is a candidate for its own scheme doc.
+A scheme produces a **per-result data label** a server attaches:
 
 | Approach | Likely `type` | Source |
 | :--- | :--- | :--- |
-| Coarse data classification (level + regulatory scope) | `data-class.v1` | SEP-1913 taxonomy (e.g. `confidential:hipaa` shape) |
+| Permissive information-flow labels (influence-based propagation) | `ifc.permissive.v1` | [arXiv:2410.03055](https://arxiv.org/abs/2410.03055) |
-| Design-pattern controls (Plan-Then-Execute, Dual LLM, Map-Reduce) | — | [arXiv:2506.08837](https://arxiv.org/abs/2506.08837) |
+| Contextual-integrity classification (per-task minimisation) | `ci.airgap.v1` | [arXiv:2405.05175](https://arxiv.org/abs/2405.05175) |
 | ShardGuard | — | cited in SEP-1913 |
 | Capability-token constraints (SINT) | — | SEP-1913 review thread |
 | Caller/tool cosigning | — | SEP-1913 review thread |
@@ -37,6 +39,18 @@ review and the surrounding literature. Each is a candidate for its own scheme do
 These are leads, not commitments. A candidate becomes a scheme when someone
 drafts it to the bar below; until then the slot simply stays open for it.
 
+## Not schemes: host architectures
+
+Some models raised in the same discussion are **host/client control-flow
+architectures**, not data labels — they decide what to *do* with information,
+they don't produce a per-result record a server attaches. They are prior art (see
+[`related-work.md`](../docs/related-work.md)), not entries here:
+
+| Approach | Why it isn't a scheme | Source |
+| :--- | :--- | :--- |
+| CaMeL (capability-based control/data-flow) | A host runtime; a capability token it issues *could* be referenced via `evidenceRef`, but the architecture isn't a label | [arXiv:2503.18813](https://arxiv.org/abs/2503.18813) |
+| Design-pattern controls (Plan-Then-Execute, Dual LLM, Map-Reduce) | Client-side execution patterns, nothing on the wire | [arXiv:2506.08837](https://arxiv.org/abs/2506.08837) |
+
 ## Bar for adding a scheme
 
 A scheme doc should state:

schemes/data-class.md

@@ -0,0 +1,123 @@
+---
+title: Data classification (data-labelling scheme)
+---
+
+> ⚠️ **Experimental scheme skeleton.** This is one **data-labelling scheme** that
+> fills the [`trust-annotations`](../specification/draft/trust-annotations.mdx)
+> `evidenceRef` slot via `type: "data-class.v1"`. It is **not** an MCP extension
+> and **not** a sibling of the extensions — it is the home for the richer
+> classification taxonomy that the extension's coarse `sensitive` boolean
+> deliberately leaves out. See [Why a scheme](#why-a-scheme-not-a-wire-field).
+
+**Scheme** — fills `trust-annotations`'s `evidenceRef` slot, selected by
+`evidenceRef.type == "data-class.v1"`. Not an extension.
+
+## Abstract
+
+This scheme defines `data-class.v1`, an entry for the `trust-annotations`
+`evidenceRef` slot that carries a **structured data classification**: a coarse
+sensitivity level plus zero or more regulatory scopes and optional org-defined
+labels. It is the structured counterpart to the extension's single
+`sensitive: boolean`, for deployments that need to express *which* regulated
+category a result falls under (e.g. HIPAA, GDPR, PCI-DSS) without putting that
+taxonomy on the wire for every server.
+
+## Why a scheme, not a wire field
+
+The `trust-annotations` extension keeps only `sensitive: boolean` on the wire.
+That coarse signal is universally client-actionable and cheap, and — critically
+— it cannot become wrong as regulation changes. A richer classification was
+asked for repeatedly in review, in three different shapes:
+
+- a **linear** `sensitiveHint: low | medium | high`, which
+  [@JustinCappos](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/711#issuecomment-2967516811)
+  rejected: sensitivity is set-theoretic (a card number and a medical record are
+  both sensitive but to *different* readers), not a single scale;
+- **org-defined vocabularies** rather than a fixed enum
+  ([@olaservo](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/711#issuecomment-2968743154);
+  [@Mossaka](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/711#issuecomment-2971788308)
+  proposed reverse-DNS `classification.labels`);
+- a **class + regulatory scope** pairing such as `confidential:hipaa`
+  ([@krubenok](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1913#discussion_r3103485194);
+  carried into SEP-2061 by @rreichel3).
+
+Against all three, [@localden](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1913#issuecomment-4037623595)
+warned that a taxonomy baked into the protocol is very hard to remove if it turns
+out wrong. A scheme resolves the tension: the taxonomy evolves behind a `type`
+value, on its own clock, while the wire keeps the boolean that can never rot.
+
+## Motivation
+
+A regulated deployment needs to know not just *that* a result is sensitive but
+*under which regime*, so the host can apply the matching control (HIPAA minimum-
+necessary, GDPR purpose limitation, PCI-DSS storage rules). The boolean cannot
+carry that, and encoding it inline would force every server and client to agree
+on a regulatory taxonomy. This scheme lets a server that already knows the
+classification (a healthcare record store, a payments API) attach it as evidence,
+while a client that does not implement the scheme still sees `sensitive: true`.
+
+## Specification
+
+### Scheme identity
+
+A `data-class.v1` record is selected by `evidenceRef.type == "data-class.v1"` on
+a `trust-annotations` annotation whose `sensitive` is `true`. The
+`digest`/`canonicalization` pair over the record is required exactly as for any
+other scheme.
+
+### Payload (skeleton)
+
+The record is a JSON object. Shapes below are **open questions**, not settled:
+
+```json
+{
+  "class": "confidential",
+  "regulatory": ["hipaa"],
+  "labels": ["com.example.pii.ssn"],
+  "policyRef": "https://policy.example.com/classes/confidential"
+}
+```
+
+- `class` — a coarse level. Candidate set `public | internal | confidential |
+  restricted` (four levels, from the original SEP-1913 taxonomy). Whether the set
+  is fixed or registry-curated is open.
+- `regulatory` — zero or more regulatory scopes the result falls under. Open
+  strings, not an enum (so a new regime doesn't need a schema change). This is
+  the `:scope` half of @krubenok's `confidential:hipaa`.
+- `labels` — optional org-defined tags, reverse-DNS namespaced per @Mossaka, for
+  vocabularies a deployment defines for itself.
+- `policyRef` — optional pointer to the policy that defines the classes, so the
+  record is interpretable by a host that hasn't pre-agreed the vocabulary.
+
+### Graceful degradation
+
+A client that does not implement `data-class.v1` ignores the record and relies on
+the `trust-annotations` `sensitive` boolean, which remains meaningful on its own.
+The scheme only ever *refines* the boolean; it never contradicts it (a
+`data-class.v1` record only appears where `sensitive` is already `true`).
+
+## Producer / consumer
+
+- **Producer (candidate).** A server fronting a regulated store (health records,
+  payments) that already classifies its data internally.
+- **Consumer (candidate).** A host policy engine that maps `class` + `regulatory`
+  to a control (block, redact, prompt). The same host-side resolution model as
+  the FIDES scheme applies: the wire marker is advisory; the regulated semantics
+  are host-resolved against the deployment's policy.
+
+## Open questions
+
+- Is the `class` set fixed (`public/internal/confidential/restricted`) or
+  registry-curated? @localden's removability concern argues against fixing it.
+- Do `regulatory` and `labels` overlap enough to collapse into one open-string
+  list, or are "named regime" and "org-defined tag" usefully distinct?
+- Should `data-class.v1` ever be expressible **on the wire** as a structured
+  escape hatch (tracked in [open-questions.md](../docs/open-questions.md)),
+  or is keeping it strictly behind `evidenceRef` the right boundary?
+- How does this relate to SEP-2061's `DataClass` / `RegulatoryScope` — is this
+  scheme the migration target for that work, or a parallel encoding?
+
+## Changelog
+
+- **2026-06-17** — Initial skeleton. Payload shapes are candidates pending the
+  open questions above.