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.

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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, and reviewer lanes. 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.

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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, command modules, or reviewer lanes 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, command modules, and reviewer lanes 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.

The reviewer lane: the one surface that receives data

Three of the four disclosure classes are about code the capability gets to run. A reviewer lane — one external CLI or model endpoint that /gsd-review hands a plan to — is different in kind, and the difference is the reason it is disclosed at all.

A lane is piped the plan text, the requirements, the research findings, and the CONTEXT.md decisions, and its output is read back into REVIEWS.md. That is an egress channel for the most sensitive artifacts GSD produces. Making lanes pluggable without a disclosure class would have opened a data-exfiltration path behind a manifest field, which is why the trust work gates the feature rather than following it.

What is disclosed depends on how the lane is reached:

• A spawned lane discloses its binary and its full declared arguments, in both rendered and raw form. Disclosing the binary alone would be insufficient, and not hypothetically: a lane declaring python3 with innocuous arguments could later change them to ["-c", "<arbitrary program>"] without the binary changing at all. Arguments are therefore signature-bound, exactly as they are for MCP servers.
• An OpenAI-compatible HTTP lane has no binary, so it discloses the destination host and the config key that names it. Disclosing curl would be technically true and practically meaningless; the destination is the disclosure that matters. A localhost destination is still disclosed, and is distinguished from a remote one — a lane pointed at a local port is an egress channel too, and the port may not be what the user assumes.

Both forms additionally name the egress payload classes, rather than an unhelpful "sends data to the tool".

Stated honestly: consent-at-install is a weaker gate for a standing egress channel than it is for a hook. A user consents once; the lane thereafter receives every plan on every review run. Disclosure makes the channel visible, pinned and revocable — it does not make it safe. A per-run egress prompt was considered and rejected as consent fatigue that trains users to approve blindly.

One consequence is worth naming because it does not follow the pattern of the other three classes. A lane's destination host lives in .planning/config.json, which is user- and CI-editable at any time with no re-install and no integrity check — unlike every other consent-bound value, all of which come from the SHA-pinned manifest. Consent therefore binds the resolved host, not merely the config key, so that a later edit redirecting a consented lane to a different destination is detectable rather than silent.

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.

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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. For each MCP server the summary also shows the env it would be spawned with (each key and its — truncated — value) and the cwd it would run in, because an environment variable can change what a command does (for example NODE_OPTIONS=--require /tmp/evil.js) without touching the command or its arguments. Declining aborts the install cleanly. Accepting records the consent in the user-owned consent store (see "The project-scope trust boundary"), bound to the bundle's integrity and a disclosure signature over the executable set (hooks, command modules, and each MCP server's command, argv, env, and cwd). The signature is a stable, key-order-independent encoding, so any later add or change to a surface — including an env or cwd change — deactivates the capability until the user re-consents, while a harmless key reorder does not.

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.

It also pins only the top-level bundle, not an npm-sourced capability's resolved dependency graph. --ignore-scripts and copy-only staging stop install-time execution, but when a command module is later require()'d, Node resolves and runs its transitive dependencies — which the bundle SHA does not cover (the Wiz / VS Code lesson). For the npm source kind, a green integrity check means "the package tarball is the one you pinned," not "every line of code that will run is the code you reviewed." Authors who want a stronger guarantee should vendor their dependencies or ship a lockfile.

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.

capabilities.strict_known_registries for managed environments

Teams or enterprises that want to constrain which capability sources are permissible set capabilities.strict_known_registries in config. Its semantics:

• unset / null (default) — permissive: external installs (git / npm / tarball) are allowed, each still passing the consent + integrity gate. Local filesystem installs are always allowed.
• [] (explicit empty array) — lockdown: all external installs are blocked; local-only.
• non-empty list — a host-based allowlist: only sources whose host matches an entry (exact host or a subdomain of it — github.com matches api.github.com but never evilgithub.com; the literal token npm permits the npm source kind). A malformed (non-array) value fails closed.

This gives an administrator a policy lever that operates before the user even sees a consent prompt. The default is permissive-with-consent (not Obsidian-style restricted-by-default), because the epic deliberately chose decentralised import with the consent prompt as the default barrier and lockdown one config key away.

Command dispatch: where third-party code runs (1.6.0)

A capability may declare a command family (commands: [{ family, module, router }]); gsd-tools <family> dispatches it by require()-ing the router. This is the one place a third-party capability's own code executes, so it is gated twice. Consent: a third-party family is dispatchable only if the capability is active under the activation gate below — for a project-scoped capability that means a user consent record on this machine, not merely a ledger entry. A bundle merely present on disk (or a project ledger that marks it committed) but with no on-this-machine consent record is not activated at all: no declarative surfaces, no command dispatch. Confinement: the router module loads only from the capability's own install root (bare-.cjs basename, realpath-confined, rejecting .. traversal and symlink escape); a first-party command can never be shadowed by a third-party one.

The project-scope trust boundary

Capabilities install globally ($GSD_HOME/.gsd/capabilities/) or project-scoped (<projectRoot>/.gsd/capabilities/). The authoritative consent signal is not the in-repo ledger but a user-owned consent store that lives outside any repository, at ${GSD_HOME||homedir()}/.gsd/consent.json. Each project-scope consent record is keyed by (realpath(projectRoot), capability id) and binds the bundle's integrity and its disclosure signature; GSD writes one only when you install or upgrade that project-scoped capability through the lifecycle on this machine, and removes it when you uninstall.

Before activating a project-scoped overlay — for both its declarative loop surfaces (steps, gates, contributions, federated config) and its command dispatch — the loader requires a matching record in this store. With no match the capability is discovered but inactive: it shows up in gsd capability list with status: inactive and a reason, but contributes nothing and runs nothing.

This closes the previous bypass: a repo you check out could ship a capability bundle and a project ledger that marked it committed, and that alone used to activate it. Now a forged or cloned project ledger activates nothing until you consent on this machine — and because the consent binds the integrity and the disclosure signature, tampering with the bundle (including changing an MCP server's env or cwd) deactivates it until you re-consent. A global install (under your own home) is trusted without a per-project record, as before. You can audit and revoke project consents with gsd capability trust list and gsd capability trust revoke <id>.

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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.

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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.

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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.

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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