msd-core/docs/explanation/capability-trust-model.md

The capability trust model

Explanation — This document describes why GSD draws its trust boundaries where it does, and what the trade-offs are. It is not a step-by-step guide to installing capabilities; for that, see the how-to guides for importing a capability and version management. For the decision record, see ADR-1244 D5. For the capability field reference, see the capability matrix.

---

The central thesis: artifact parity is not trust parity

GSD 1.6.0 opens the capability platform to third-party authors with full artifact parity: a third-party capability may ship the same executable surfaces that GSD Core ships — hooks, MCP servers, command modules. This is a deliberate product choice, and it carries real security weight.

Full parity means a third-party capability, once installed, can execute code the next time a relevant loop event fires. There is no "first use" gate. There is no sandbox. The capability author has, in effect, a code-execution path into your runtime.

The maintainer's response to this is not to deny parity but to draw a sharp line between two things that are often conflated:

• Artifact parity — what a third-party capability is allowed to ship.
• Trust posture — the evidence and consent required before that capability executes on your machine.

GSD grants full artifact parity. It does not grant symmetric trust. First-party capabilities are implicitly trusted because they are the shipped package — their provenance is the GSD Core release process itself. Third-party capabilities require explicit, informed, revocable consent plus SHA-pinned integrity before any executable surface is activated. These two things are structurally separate, and keeping them separate is what makes full parity defensible.

---

What the ecosystem learnt the hard way

GSD's trust model is not designed in isolation. It is informed by failures in four ecosystems that tackled the same problem — and each one paid tuition.

VS Code: auto-update + stolen publisher credentials

VS Code's extension marketplace grants extensions the same permissions as the editor itself. In 2023 a publisher's personal access token was stolen; the attacker published a backdoored update to an existing, trusted extension. Every user with auto-update enabled received the malicious version silently, on the next launch, with no prompt. The lesson: auto-update for executable surfaces is a liability when credentials can be compromised, because the user's last explicit act of trust was for version N — not for whatever version N+1 contains.

GSD's response: auto-update is off by default for third-party capabilities. When it is enabled, a change to the executable set (the set of hooks, MCP servers, or command modules the capability declares) triggers a re-consent prompt before the update applies. Updating a non-executable capability (documentation, agents, skills) does not require re-consent.

VS Code also has no signature check on VSIX packages. GSD requires an integrity SHA-512 pin in the ledger, verified before extraction.

npm: the supply-chain attack surface

npm's postinstall scripts mean that downloading a package can execute arbitrary code on the developer's machine — a property that supply-chain attackers have exploited in the s1ngularity attack class (a malicious package is published under a name a legitimate package depends on). npm's own recommendation for sensitive environments is --ignore-scripts.

GSD takes a stronger position: install never executes capability code, full stop. Installation is a copy-only staging operation. There is no postinstall-equivalent. A capability's hooks, MCP server, and command modules are not invoked during install; they are first invoked when the loop fires after install. This means a malicious payload in an executable surface cannot be triggered by the act of downloading it — the user has a window between install and first use to verify what they consented to.

SLSA provenance (the provenance field in capability.json) provides a machine-checkable link from a capability bundle back to a specific commit in a specific source repository. GSD emits provenance for first-party capabilities in CI and recommends it for curated capabilities; whether to require it for community-listed third-party capabilities is an open question tied to whether GSD operates a central registry (see the PRD).

Obsidian: no sandbox, stated honestly

Obsidian's plugin system does not sandbox plugins. Plugins run in the renderer process with full Electron API access. Obsidian acknowledges this directly in its documentation and community materials, and its response is restricted mode on by default — no community plugins run until the user deliberately disables restricted mode — plus a human-curated plugin directory that requires a maintainer review PR for each new plugin.

GSD borrows two things from Obsidian. First, the honesty: there is no sandbox, and this document says so directly rather than implying one. Second, the principle that explicit opt-in per capability is better than a blanket "all community plugins are safe" message. GSD does not use restricted mode, but its consent gate at install serves the same function: executable surfaces are disclosed and consented to before they activate, not discovered after the fact.

GSD does not borrow Obsidian's centralised review model. Requiring a maintainer-review PR for every third-party capability is the bottleneck that makes the Obsidian system painful for authors and creates a PR-queue burden for maintainers. GSD ships decentralised URL import precisely to avoid that.

Claude Code: trust prompt + marketplace

Claude Code prompts the user at install for each extension that requires elevated trust, lists the permissions the extension requests, and maintains a strictKnownMarketplaces allowlist for managed environments where only reviewed sources are permitted. Claude Code's SHA-pinning mechanic (pinning to a specific version hash rather than floating on latest) is the direct model for GSD's integrity field.

GSD mirrors the allowlist mechanic as strictKnownRegistries, and mirrors the SHA-pin as the integrity field in capability.json and the capability ledger.

---

Each pillar and its reasoning

Install never runs code

The most powerful thing GSD can say to a user about a third-party capability is: "downloading and staging this capability will not execute any of its code." That guarantee makes the consent step meaningful. If install could run code, a malicious capability could bypass consent entirely — the install step would be the attack.

Staging is copy-only: files are extracted to the install root, the manifest is validated, cross-capability invariants are checked, and the ledger is written. No hook fires, no module is require()'d, no MCP server is started. The executable surfaces remain inert until the first loop event fires after consent.

Consent at install for executable surfaces

Hooks fire on the next tool call. There is no first-use gate for a hook — the point at which a hook would fire for the first time is not a prompt opportunity; it is already inside a running tool invocation. This means the consent window is install, not first use.

GSD presents a pre-install summary that names every executable surface the capability declares (hooks, MCP servers, command modules), their kinds (step, contribution, gate), and the loop extension points they register into. Declining aborts the install cleanly. Accepting records the consent in the ledger.

For non-executable surfaces (skills, agents, workflow files), the disclosure note explains what they do but consent is lighter — they do not execute code.

Integrity pinning

An integrity field in capability.json carries a sha512-<base64> digest of the capability bundle. When present, GSD verifies this digest before extracting any files. A mismatch aborts the install.

What integrity pinning defends against: a capability hosted at a URL or in a registry that is later replaced with a different bundle (whether by an attacker who has compromised the hosting, or by an author publishing a silent breaking change). The SHA is the commitment — "I consented to this bundle, not whatever is at this URL today."

What it does not defend against: a malicious capability where the author themselves publishes a bad bundle. The SHA is honest about what you are installing; it says nothing about whether what you are installing is safe.

Auto-update off by default, re-consent on executable-set change

When auto-update is enabled for a third-party capability, each update is checked against the ledger's record of the capability's executable surfaces. If the set of hooks, MCP servers, or command modules has changed — even if the update is otherwise benign — auto-update halts and re-prompts. The user is shown which surfaces were added or removed and must consent before the update applies.

This directly addresses the VS Code stolen-PAT scenario: even if an attacker publishes a new version of a capability you have auto-update enabled on, the new version cannot silently gain a hook that the previous version did not have.

Install-root confinement

A capability's command modules are require()'d only from the capability's own install root. Declared paths containing parent-directory traversal (../) are rejected at install-time validation. This prevents a capability from loading code it does not own — whether by accident or by design.

Reserved namespace

The gsd-, gsd-core-, and anthropic- id prefixes are reserved for first-party use. A third-party capability that claims one of these prefixes is rejected at the conformance gate. This prevents impersonation: a malicious actor cannot publish a capability called gsd-security and exploit a user's implicit trust in the GSD namespace.

strictKnownRegistries for managed environments

Teams or enterprises that want to constrain which capability sources are permissible can set strictKnownRegistries in managed or project config to an explicit allowlist of URLs or registry names. Setting it to [] blocks all external installs. This gives an administrator a policy lever that operates before the user even sees a consent prompt.

---

The honest limitation: there is no sandbox

GSD does not sandbox third-party capability code. The honest reason: Node-level sandboxing that meaningfully restricts a require()'d module — limiting filesystem access, network access, subprocess spawning — would require either a separate process with IPC overhead or a VM context that strips the Node globals capabilities legitimately need (filesystem for writing surface files, network for MCP, subprocess for hook shell commands). Full artifact parity and meaningful sandboxing are in tension. The maintainer chose full parity.

What this means in practice: a third-party capability, once consented to and installed, runs with the same permissions GSD Core itself runs with. It is not isolated. A capability that wants to exfiltrate data, or modify files outside its declared scope, can — exactly as a malicious npm package can.

The barrier is not a technical wall. It is:

1. Consent — you explicitly approved the executable surfaces this capability declares before they ran.
2. Integrity — the bundle you consented to is the bundle that ran (SHA verified).
3. Reversibility — gsd capability remove <id> removes exactly what the ledger recorded, including entries in shared config files, leaving no orphaned state.

These three things together mean: you know what you installed, you got what you were shown, and you can undo it completely. They do not guarantee the content is safe. The trust model is transparent about this.

---

Trade-offs: the roads not taken

Declarative-only third-party capabilities

The safer alternative considered in ADR-1244 was declarative-only third-party capabilities: skills, agents, and workflow files, but no hooks, MCP servers, or command modules. A third-party author could extend what GSD describes but not what it executes.

The maintainer rejected this. A deploy gate capability, a house-style verification step, a domain-specific planning contribution — all of these require hook registration to have any effect on the loop. Declarative-only third-party capabilities would be second-class citizens, unable to participate in the parts of GSD where participation matters most. Full parity was the explicit scope.

The cost of that choice is a permanently elevated security responsibility: URL import with executable surfaces is the highest-maintenance, highest-risk part of GSD. The trust model is a forever commitment, not a one-time effort.

Centralised-registry-only distribution

The alternative to decentralised URL/git import is requiring all third-party capabilities to go through a GSD-operated curated registry — one PR per capability, reviewed by the maintainer before listing.

This would meaningfully reduce supply-chain risk (a human reviews every listed capability) but at a cost the maintainer explicitly rejected: it makes capability authors dependent on maintainer bandwidth, turns the maintainer into a gatekeeper for an unbounded tail of stack-specific and house-style capabilities, and replicates exactly the bottleneck that makes Obsidian's plugin system painful.

The compromise: URL/git/npm/tarball import ships in 1.6.0 without a curated registry. Whether GSD later operates or advertises a community registry is an open question (PRD-1244 §8). If it does, the intent is to separate "official" (curated) from "community" (consented-but-not-reviewed) tiers, mirroring the split Claude Code uses for its marketplace.

---

Summary

The capability trust model rests on a single conceptual move: separating artifact parity from trust posture. Because those two things are kept separate, GSD can offer authors the full power of the platform while making users' security obligations clear and auditable. You consent to executable surfaces before they run, you can verify the bundle's integrity, and you can remove a capability completely. GSD does not pretend this is the same as not running the code at all.

---

Related documents

• ADR-1244 D5 — Trust model
• Capability matrix — the generated catalogue of all capabilities
• PRD-1244 §6 — Out of scope — why sandboxing is explicitly out of scope
• ADR-857 — the 12 loop extension points; D7 and D8 extended by ADR-1244