Add FIDES as a data-labelling scheme under schemes/
Move the FIDES information-flow-control draft out of specification/draft/ and into schemes/ifc-fides.md, reframed from a sibling extension into one data-labelling scheme that fills the trust-annotations evidenceRef slot (type ifc.fides.v1). FIDES is not an extension and not a peer of the extensions; it is one interchangeable filler of the slot. Add schemes/README.md as the index for the folder: it states what a scheme is, lists FIDES as the worked example, records the range of candidate schemes raised in SEP-1913 review (data classification, design-pattern controls, ShardGuard, capability tokens, cosigning, sequence-shape, attestation), and sets the bar a scheme doc must meet. The reviewed normative body (label payload, semantics, reader-set resolution, reference implementation, open questions) is preserved verbatim from the JoannaaKL / Rul1an review rounds. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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schemes/README.md
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schemes/README.md
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# Data-labelling schemes
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A **scheme** is a concrete data-labelling or tool-annotation approach that fills
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the [`trust-annotations`](../specification/draft/trust-annotations.mdx)
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`evidenceRef` slot under an `evidenceRef.type` value. The extension defines a
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small, stable wire vocabulary and an open `type` pointer; a scheme defines the
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richer, out-of-band record that pointer resolves to.
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Schemes are **not** extensions and **not** siblings of the extensions. They are
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interchangeable: a deployment can adopt one, several, or none, and can swap them
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without changing the extension. Modelling each labelling approach as a scheme keeps
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any single academic model out of the wire root, which is the reason FIDES lives
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here rather than as a top-level extension.
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## Schemes here
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| Scheme | `evidenceRef.type` | Status | Source |
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| :--- | :--- | :--- | :--- |
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| [FIDES information-flow control](./ifc-fides.md) | `ifc.fides.v1` | Draft skeleton | [arXiv:2505.23643](https://arxiv.org/abs/2505.23643) |
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## Candidate schemes (not yet drafted)
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The open `type` slot is designed to carry the range of models raised in the
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[SEP-1913](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1913)
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review and the surrounding literature. Each is a candidate for its own scheme doc:
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| Approach | Likely `type` | Source |
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| :--- | :--- | :--- |
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| Coarse data classification (level + regulatory scope) | `data-class.v1` | SEP-1913 taxonomy (e.g. `confidential:hipaa` shape) |
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| Design-pattern controls (Plan-Then-Execute, Dual LLM, Map-Reduce) | — | [arXiv:2506.08837](https://arxiv.org/abs/2506.08837) |
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| ShardGuard | — | cited in SEP-1913 |
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| Capability-token constraints (SINT) | — | SEP-1913 review thread |
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| Caller/tool cosigning | — | SEP-1913 review thread |
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| Sequence-shape audit records | — | SEP-1913 review thread |
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| Tool-call attestation (in-toto / OVERT envelopes) | — | [SEP-2787](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2787), in-toto, OVERT |
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These are leads, not commitments. A candidate becomes a scheme when someone
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drafts it to the bar below; until then the slot simply stays open for it.
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## Bar for adding a scheme
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A scheme doc should state:
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1. **Identity** — the `evidenceRef.type` value it claims, and that it is selected
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by `evidenceRef.type == "<value>"` on a `trust-annotations` annotation.
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2. **Payload** — the shape of the record the `evidenceRef` resolves to.
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3. **Graceful degradation** — how a client that does not implement the scheme
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ignores it safely (the `trust-annotations` booleans and
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`digest`/`canonicalization` pair remain meaningful regardless).
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4. **Producer/consumer** — at least a candidate emitter and consumer, so the
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scheme is validated against real implementations rather than asserted.
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`type` values are coordinated through the non-binding `evidenceRef.type` registry
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noted in [`trust-annotations`](../specification/draft/trust-annotations.mdx) so
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they don't collide.
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198
schemes/ifc-fides.md
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schemes/ifc-fides.md
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---
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title: FIDES information-flow control (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: "ifc.fides.v1"`. It is **not** an MCP extension
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> and **not** a sibling of the extensions — it is one interchangeable way to
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> populate the evidence a `trust-annotations` annotation carries. See
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> [Why a scheme](#why-a-scheme-not-an-extension).
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**Scheme** — fills `trust-annotations`'s `evidenceRef` slot, selected by
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`evidenceRef.type == "ifc.fides.v1"`. Not an extension.
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## Abstract
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This scheme defines `ifc.fides.v1`, an entry for the `trust-annotations`
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`evidenceRef` slot that carries an **information-flow-control label** — integrity
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plus confidentiality — following the FIDES model
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([arXiv:2505.23643](https://arxiv.org/abs/2505.23643)). A host that implements
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deterministic information-flow control can consume these labels to decide whether
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a tool call is permitted, without baking the IFC model into the core protocol or
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into the `trust-annotations` wire surface.
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## Why a scheme, not an extension
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Information-flow control is one enforcement model among several that reviewers of
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SEP-1913 raised — capability tokens, caller/tool cosigning, and sequence-shape
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audit records were all put forward, and the literature adds more (ShardGuard, the
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"Design Patterns for Securing LLM Agents" controls). A top-level extension
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(`io.modelcontextprotocol/ifc`) would make the FIDES integrity × confidentiality
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lattice the namespace root and silently foreclose those other models.
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As a `type` value behind the open-ended `evidenceRef` slot, the FIDES label is
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first-class while the slot stays free for every other scheme. One reviewer's
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framing captured it: IFC "fits relatively well *if you use annotations*" — an
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endorsement of IFC as a scheme behind the slot, not as the wire root. The
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[`schemes/`](./README.md) folder is where these interchangeable approaches live.
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## Motivation
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The motivating case is the one raised in the
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[2026-05-28 IG meeting](https://github.com/modelcontextprotocol/modelcontextprotocol/discussions/2820):
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**a model often cannot tell whether a repository is public or private**, and
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lacking that signal it may push private content to a public destination. An IFC
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label lets the host track confidentiality (who may read this data) and
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integrity (is this data trusted) as context accumulates across tool calls, and
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deny or prompt before a flow violates policy.
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A public MCP server is the natural emitter. [`github-mcp-server`](https://github.com/github/github-mcp-server)
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returns repository data whose confidentiality follows from repository visibility
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and collaborator sets — the same public/private signal — but does **not** emit
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IFC labels today. Closing that emitter gap is the concrete proof point for this
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scheme: a host-side consumer of the label shape already exists, so the missing
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half is a server willing to emit it, classifying each resource it returns (see
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[per-resource classification](#reference-implementation)).
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## Specification
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### Scheme identity
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This scheme is selected by `evidenceRef.type == "ifc.fides.v1"` on a
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`trust-annotations` annotation. A client that does not implement IFC MUST be
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able to ignore it safely (the surrounding `sensitive` / `untrusted` booleans and
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the `digest`/`canonicalization` pair remain meaningful).
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### Label payload
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The record referenced by the `evidenceRef` (and, for low-friction adoption, MAY
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be inlined by deployments that accept the wire cost) has the shape:
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```jsonc
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{
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"integrity": "trusted", // "trusted" | "untrusted" (FIDES §4.1 two-level lattice)
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"confidentiality": "public" // "public" | "private"
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}
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```
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| Field | Meaning |
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| :--- | :--- |
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| `integrity` | Two-level integrity lattice (`trusted` ⊑ `untrusted`): trusted data may flow to untrusted sinks, not vice versa. |
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| `confidentiality` | `"public"` = world-readable; `"private"` = an opaque marker meaning "restricted to some reader set". The concrete reader set is resolved host-side at policy-decision time (see [Reader-set resolution](#reader-set-resolution)). |
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> **Confidentiality is `public` / `private` only — never a reader list on the
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> wire.** Emitting concrete reader identities (e.g. logins) is out of scope: user
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> identity is not uniform across servers using different auth methods, the
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> identities are themselves access-restricted data, and a single resource can
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> have hundreds of readers. The opaque marker keeps the wire shape stable and the
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> sensitive resolution host-side.
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### Label semantics
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The load-bearing distinction is between the wire and the host: **wire markers are
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advisory hints; reader-set semantics are host-resolved.** The asymmetry between
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the two joins below follows from that one cut.
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- **Join on accumulation.** As a session ingests labeled results, the context
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label is the *join* of what it has seen: integrity degrades toward
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`untrusted`, confidentiality narrows toward the smallest permitted reader set.
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The two joins differ in *where* they can be computed, and the difference is
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principled rather than incidental:
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- **Integrity join is total and wire-computable.** The integrity lattice is
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small and closed (`trusted ⊑ untrusted`), so `untrusted` dominates and the
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join needs nothing beyond the wire values.
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- **Confidentiality join is partial and host-resolved.** Reader sets are open
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and host-knowledge-dependent. `public` is the one wire-computable case,
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because its reader set is universal (`⊤`): `public ⊔ anything = public`.
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`private ⊔ private`, by contrast, is the *intersection* of two reader sets
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that the opaque markers don't carry, so it is **not** computable from the
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wire — see [Reader-set resolution](#reader-set-resolution).
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- **Policy check before egress.** Before a write/egress tool call, the host
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checks whether the current context label may flow to the call's target. When
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a label is absent, the host falls back to its default (trusted-action)
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policy rather than assuming the worst — labels are an *additive* signal.
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> The normative integrity/confidentiality lattice definitions follow the FIDES
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> paper, §4.1 and §4.3. This scheme references the model rather than restating
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> the proofs.
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### Reader-set resolution
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`"private"` is intentionally opaque on the wire — and that opaqueness is a
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property of the security model, not a limitation of the scheme. A reader set is not
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transmissible without policy context, so the wire shape correctly declines to
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carry it. Two distinct `"private"` markers (e.g. file contents from two different
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private repositories) are **not equal**, and their confidentiality join is **not**
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the same `"private"` token: data derived from both may flow only to principals who
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can read *both* sources — the intersection of their reader sets. The opaque marker
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cannot express this intersection, so a host that needs to make a precise
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cross-source flow decision MUST resolve each `"private"` marker to a concrete
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reader set before joining.
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Resolution is a host-side concern, performed at policy-decision time:
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1. The host maps each contributing `"private"` label back to its source (e.g.
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via the `evidenceRef.ref` locator, or its own record of which tool result
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carried the label).
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2. The host queries the originating system for the current reader set (e.g. a
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repository collaborators lookup) using its own credentials.
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3. The host computes the flow decision over the resolved sets (intersection for
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a join of multiple private sources) and then discards them.
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**When resolution is unavailable** — the `ref` is absent, the label is
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digest-only, or the originating system is unreachable at decision time — the host
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MUST NOT treat two opaque labels as equal, and MUST NOT treat `"private"` as
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`"public"`. It denies, prompts, or applies its configured fail-closed policy. Two
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`"private"` labels are equal only once resolution proves their sources are; until
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a source is established, unknown or mixed provenance classifies as `"private"`,
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never defaulted to `"public"` from a repository-level shortcut.
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The resolved reader set is a decision-time read performed under the host's own
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credentials. It is not a durable grant: a host SHOULD NOT cache it as one or
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serialize it back into annotations or evidence unless a deployment explicitly opts
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in. This keeps the wire free of user identities while still letting the host make a
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precise decision when it holds source provenance plus its own credentials.
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### Relationship to `trust-annotations`
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The `ifc.fides.v1` label never appears without a host `trust-annotations`
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annotation carrying the `evidenceRef`. The booleans are the universally-actionable
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signal; the IFC label is the precise, host-checkable evidence behind them.
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## Reference implementation
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- **Consumer:** a host-side IFC engine that parses the `{integrity,
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confidentiality}` label, maintains a context label across tool results, and
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applies a flow policy before egress operations already exists in practice.
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(Linked once a public reference is available.)
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- **Emitter (gap / proof point):** [`github-mcp-server`](https://github.com/github/github-mcp-server)
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is the candidate — it already knows repository visibility and collaborator
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sets, which are the confidentiality inputs. Repository visibility is only a
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*default* hint, not the whole story: a public repository can serve
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sub-resources that are **not** world-readable (draft security advisories,
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draft releases, the collaborator roster itself, authenticated-user fields), so
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a correct emitter MUST classify **per resource returned**, not per repository.
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That makes the emitter a non-trivial proof point rather than a one-line
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`repo.private` read.
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## Open questions
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- Should the label be inlinable on `_meta.ifc` directly for low-friction
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adoption, or always behind `evidenceRef` for schema minimalism? (Lean:
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permit both; `evidenceRef` is canonical, inline is a convenience.)
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- How does GitHub Enterprise `internal` repository visibility map onto the
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`public` / `private` confidentiality model? (Audience is the whole org,
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strictly broader than collaborators — likely classified `private` and resolved
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host-side, or falls back to default policy.)
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- Registry coordination with other evidence schemes (e.g. SEP-2787) so
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`evidenceRef.type` values don't collide.
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## Changelog
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| Date | Change |
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| ---------- | ------------------------------------------------------------ |
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| 2026-06-10 | Initial draft skeleton. Reframed from a top-level `ifc` extension to a `trust-annotations` `evidenceRef` entry (`ifc.fides.v1`). |
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| 2026-06-15 | Confidentiality limited to `public` / `private` on the wire (dropped reader-list); added Reader-set resolution section; emitter classifies per resource, not per repository. (Review: @JoannaaKL.) |
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| 2026-06-16 | Lead the semantics with the wire-hint / host-resolved split; state the integrity-total vs confidentiality-partial asymmetry as principled (`public` = `⊤` is wire-computable, `private ⊔ private` is not); add fail-closed handling when resolution is unavailable and a no-durable-grant rule for resolved sets. (Review: @Rul1an.) |
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| 2026-06-16 | Moved out of `specification/draft/` into `schemes/`; reframed from a sibling extension draft into a data-labelling scheme — one filler of the `trust-annotations` `evidenceRef` slot, not a peer extension. |
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