msd-core/docs/adr/1016-runtime-capability-descriptor.md

ADR-1016: Runtime Capability Descriptor

• Status: Proposed
• Date: 2026-06-10
• Issue: #1016
• Epic: #857 (Capability system) — rollout phase 5
• Realizes: ADR-857 Branch 8 (host-CLI support as role: runtime Capabilities)
• Materializes: ADR-58 (the typed InstallPlan projection)
• Builds on: ADR-3660 (artifact layout), ADR-894 (the role: runtime body, already validated)

Context

GSD installs into 16 host CLIs (claude, codex, gemini, opencode, kilo, cursor, copilot, antigravity, windsurf, augment, trae, qwen, hermes, codebuddy, cline, kimi). Today the per-runtime differences are encoded as ~287 conditional branches + 15 is<Runtime> boolean flags in bin/install.js, plus scattered per-runtime switches in runtime-homes.cts, runtime-config-adapter-registry.cts, runtime-artifact-layout.cts, runtime-slash.cts, runtime-name-policy.cts, and model-catalog.json. Adding one runtime touches ~18 hardcoded sites. This is exactly the feature-scattering ADR-857 set out to end — for runtimes rather than features.

ADR-857 Branch 8 decided: host-CLI support becomes a role: runtime variant of the unified Capability. Feature Capabilities produce artifacts; Runtime Capabilities project them onto one host's conventions. Install composes active Features × the chosen Runtime at ADR-58's InstallPlan seam. A runtime descriptor is a declarative value over a CLOSED named-primitive vocabulary — not a code adapter; a genuinely new shape requires adding a first-party primitive (code + enum value), which is the dogfood path a future third-party would also use.

Two enabling pieces already exist:

• The descriptor schema is authored and validated. gen-capability-registry.cjs validateRuntimeBody validates a role: runtime capability's runtime: { configHome, configFormat, artifactLayout, commandStyle, hooksSurface, hookEvents, sandboxTier, supportTier, installSurface, writesSharedSettings, permissionWriter, extendedHookEvents }, forbids feature-only fields (skills/agents/steps/contributions/gates/hooks), and the registry exposes a runtimes index (currently {} — no descriptor authored yet).
• ADR-58 defines the InstallPlan as a pure typed projection (placements + command text + config intentions → adapters execute); it is now materialized — runtime-config-adapter-registry.cts realizes both the adapter-selection half and the plan-collection half via the exported resolveInstallPlan(runtime), which install() and finishInstall() consume.

What is missing, and what this ADR fixes: only configFormat is a real closed enum (5 values); commandStyle/hooksSurface/sandboxTier are loose strings and artifactLayout is an unconstrained array. This ADR closes the vocabulary for all eight original axes, decides how the seven hard-case runtimes are absorbed as data, and fixes the staged migration that drives install.js from descriptors. Four additional axes — installSurface, writesSharedSettings, permissionWriter, and extendedHookEvents — were added to the descriptor and validator in the 5f-completion pass; they are documented in Decision 7a below.

Decision

Core principle: every per-runtime difference is expressed as a value over a closed primitive vocabulary on twelve axes. A runtime that needs a shape no existing primitive expresses is supported by adding a first-party primitive (a new enum member + the code that honors it) — never by embedding arbitrary code or an open escape hatch in the descriptor. All 16 current runtimes MUST be expressible as data under the vocabulary below; any residue that genuinely cannot be is named explicitly as retained first-party code (§ Decision 8).

1. configHome — a structured value, not a path string

configHome: {
  kind: 'dot-home' | 'dot-home-nested' | 'xdg' | 'generic-agents-root',
  name: string,                 // e.g. 'claude', 'opencode'
  parent?: string,              // for dot-home-nested: e.g. '.gemini' (antigravity), '.codeium' (windsurf)
  env: string[],                // ordered override env vars, e.g. ['CLAUDE_CONFIG_DIR'] (required; may be empty)
  probe?: string[],             // ordered candidate subpaths; first existing wins (antigravity, kimi)
  probeExists?: string,         // marker sub-path on a probe candidate. generic-agents-root: hard filter (kimi: 'skills'). dot-home-nested: marker-priority preference, then bare-existence fallback (antigravity: 'gsd-core/VERSION', #213/#217) — see amendment below
  skillsHome?: { kind, name, ... }  // override when the skills dir ≠ config dir (kilo only)
}

• dot-home → ~/.{name} (claude, cursor, codex, copilot, augment, trae, qwen, hermes, codebuddy, cline).
• dot-home-nested → ~/{parent}/{name} (antigravity ~/.gemini/antigravity + probe; windsurf ~/.codeium/windsurf).
• xdg → $XDG_CONFIG_HOME/{name} ?? ~/.config/{name} (opencode, kilo).
• generic-agents-root → the shared ~/.config/agents / ~/.agents first-existing root (kimi).

This absorbs kilo (skillsHome split), kimi & antigravity (probe), windsurf (dot-home-nested). runtime-homes.cts becomes a pure interpreter of this value.

configHome resolution is pure and read-only (a first-existing probe; no mkdirSync — verified in runtime-homes.cts). Any directory creation at install time is the configFormat permissions-writer's responsibility (opencode/kilo), never the descriptor-resolution step — so configHome carries no createIfMissing flag.

Amendment — probeExists on dot-home-nested (#1441, 2026-06-18)

probeExists was originally honoured only by generic-agents-root (kimi), as a hard filter. It is now also honoured by dot-home-nested, where it acts as a preference: probing first returns the candidate whose <candidate>/<probeExists> exists (the dir GSD installed into, marked by gsd-core/VERSION), then falls back to the legacy first-bare-existing pass, then probe[0]. When probeExists is absent the behaviour is byte-identical to the original first-bare-existing probe, so other dot-home-nested runtimes (e.g. windsurf, which has no probe) are unaffected. This fixes silent misresolution where an active sibling dir (the Antigravity-IDE ~/.gemini/antigravity) shadowed a CLI install in ~/.gemini/antigravity-cli — a regression introduced by #217. The existing probeExists field name is reused rather than introducing a parallel probeMarker, keeping the configHome vocabulary closed.

2. configFormat — the existing closed enum (unchanged)

settings-json | toml | markdown | markdown-dir | none. Cursor is none (it writes no settings file — its only managed file is the hooks manifest, captured by hooksSurface, not configFormat). opencode/kilo are settings-json with a JSONC permissions sidecar expressed by an optional permissions: 'opencode-jsonc' | 'kilo-jsonc' | 'none' sub-field (the only two runtimes that write one).

Claude's permissions.allow/deny (a tool-approval allowlist) rides as a sub-field of the settings-json variant — it is not a sandbox tier (see Decision 6).

3. artifactLayout — ADR-3660 kinds[], keyed by scope

artifactLayout: {
  global: ArtifactKind[],
  local:  ArtifactKind[]
}
ArtifactKind = { kind: 'commands'|'agents'|'skills'|'kimi-agents', destSubpath: string, prefix: string,
                 nesting: 'flat'|'nested', recursive: boolean, stage: string }

Scope-keying is required generally, not just for the hard case: claude itself differs (global = skills/, local = commands/gsd/ + agents/), and cline differs (global = skills + .clinerules/, local = .clinerules/ only). ADR-3660 already absorbs hermes (destSubpath: 'skills/gsd'), opencode/kilo (destSubpath: 'command' singular), and gemini (commands-only, .toml) via destSubpath/prefix. The nesting/recursive flags carry the ns-* router nesting + non-recursive-loader facts (#924/#28266). The descriptor's artifactLayout IS the ADR-3660 layout, declared per runtime; runtime-artifact-layout.cts becomes its lookup table.

Each ArtifactKind names its per-runtime body converter (e.g. convertClaudeCommandToGeminiToml, the codex agent-TOML emitter) via a closed ConverterName enum — verified as 15 named first-party functions covering the 16 runtimes (three share Claude's). The converter is referenced by closed name, never embedded; closing it into a union type (vs today's open string) is what makes the "closed vocabulary" claim type-enforced rather than convention.

4. commandStyle — closed enum (2 values)

slash-hyphen (/gsd-<cmd>, 15 runtimes) | shell-var ($gsd-<cmd>, codex only). Gemini's own gsd:-namespaced TOML routing is a property of its command artifacts (a commands/gsd/*.toml layout fact), not GSD's emission style — it stays slash-hyphen.

5. hooksSurface — closed enum + a hook-event dialect

hooksSurface: 'settings-json' | 'codex-hooks-json' | 'cursor-hooks-json'
            | 'copilot-inline' | 'cline-rules' | 'none',
hookEvents?: 'claude'   // SessionStart/PreToolUse/PostToolUse
           | 'gemini'   // BeforeTool/AfterTool (gemini, antigravity)
           | 'opencode-subset'  // settings-json surface but SessionStart/PostToolUse skipped

settings-json covers claude, gemini, antigravity, augment, qwen, hermes, codebuddy; the event-name and registration-subset differences ride on hookEvents rather than splitting the surface enum. none = windsurf, trae, kimi, kilo, opencode — these five runtimes register zero managed lifecycle hooks today; opencode-subset is reserved ADR vocabulary with no current consumer (opencode and kilo write a settings.json for config/permissions, which is the configFormat axis, not the hook-registration axis).

6. sandboxTier — the agent-sandbox primitive (closed enum)

none (default — no per-agent sandbox) | codex-agent-sandbox (codex's CODEX_AGENT_SANDBOX map of agent → workspace-write | read-only, baked into each agent .toml). Codex is the only runtime with a non-none value today, so this is the thinnest axis — kept distinct from supportTier (coverage) and from model-catalog routing.

Explicitly NOT on this axis (verified, to prevent conflation): Claude's permissions.allow/deny is a tool-approval allowlist → it belongs on configFormat's settings-json variant (Decision 2). The opencode/kilo permissions sidecar is a filesystem read-grant → it belongs on configFormat's permissions sub-field (Decision 2). These three are categorically different mechanisms; only codex's per-agent sandbox mode is sandboxTier.

7. supportTier — GSD coverage tier (unchanged: 1 | 2)

1 = fully tested first-party (claude, codex, antigravity); 2 = shipped, lower-tier (the other 13). None dropped. Drives the cross-runtime test matrix, not behavior.

7a. Install-surface axes — config-writing + the per-event hook SET (added in 5f completion)

Four axes added to the descriptor (and to gen-capability-registry.cjs validateRuntimeBody) in the 5f-completion pass. Together they retire the last hardcoded per-runtime tables in runtime-config-adapter-registry and applySettingsJsonHooks.

installSurface — closed enum (6 values)

installSurface: 'settings-json' | 'codex-toml' | 'copilot-instructions'
              | 'cline-rules' | 'cursor-hooks-json' | 'profile-marker-only'

Selects which config-writing adapter resolveRuntimeConfigIntent (in runtime-config-adapter-registry) returns. Previously runtime-config-adapter-registry held a hand-kept REGISTRY table mapping runtime names to adapter types; installSurface in the descriptor is now the single source of truth — the REGISTRY table has been retired.

writesSharedSettings — boolean

Whether the runtime writes a shared settings.json. Replaces the former inline boolean per runtime in runtime-config-adapter-registry. Together with installSurface this fully parameterises the adapter-selection path.

permissionWriter — null | 'opencode' | 'kilo'

The finish-time permissions-sidecar writer (the JSONC file opencode and kilo require). Replaces the old finishPermissionWriter field in the runtime-config-adapter-registry REGISTRY table. All 14 runtimes that write no permissions sidecar carry null.

extendedHookEvents — string[] over a closed event vocabulary

The per-runtime set of bonus lifecycle events beyond the coarse hookEvents dialect. Vocabulary:

SubagentStop | Stop | PreCompact | FileChanged | BeforeAgent | AfterAgent | BeforeModel

Values per runtime:

• claude → [SubagentStop, Stop, PreCompact, FileChanged]
• qwen → [SubagentStop, Stop, PreCompact]
• gemini → [BeforeAgent, AfterAgent, BeforeModel]
• all 13 others → []

This replaces three hardcoded per-event guards in applySettingsJsonHooks: the if (isQwen || runtime==='claude') block (SubagentStop/Stop/PreCompact) and the if (runtime==='claude') block (FileChanged) and the if (isGemini) block (BeforeAgent/AfterAgent/BeforeModel). The loop now iterates extendedHookEvents for the active runtime; no per-runtime conditionals remain.

Relationship to hookEvents: hookEvents (the coarse 2-value dialect — 'claude' vs 'gemini') governs the event-name vocabulary the hook adapter emits. extendedHookEvents governs the additional lifecycle events each runtime registers beyond the base set. They are independent: antigravity carries hookEvents: 'gemini' (so its hook bodies use Gemini event names) but extendedHookEvents: [] — it does not receive the per-agent Gemini events (BeforeAgent/AfterAgent/BeforeModel), which are gemini-only.

hooksSurface is now load-bearing

The hooksSurface === 'none' value now drives the settings-json hook-skip path in applySettingsJsonHooks, replacing the former isOpencode || isKilo boolean guards. This is not a new axis — hooksSurface was already Decision 5 — but it is now actively consumed (load-bearing) rather than advisory.

8. Staged consumption — author registry-only, then drive install one axis at a time

The migration is staged the way phases 3–4 were (registry-only → consume incrementally → equivalence-proven no-op → retire the hardcoded branch). Four of the original eight axes already live in dedicated modules that install.js merely consumes, so driving them from the descriptor is per-axis and low-risk; the rest is staged behind a prerequisite and assembled last — no big-bang install() rewrite.

1. 5a — author the 16 descriptors (capabilities/<runtime>/capability.json, role: runtime) registry-only; nothing consumes them. The generator already validates them; the runtimes index populates.
2. 5b — drive configHome ← descriptor (runtime-homes.cts already centralizes it; swap its switch for a descriptor lookup). Smallest blast radius.
3. 5c — drive commandStyle ← descriptor (runtime-slash.cts, 2 values). Trivial.
4. 5d — drive artifactLayout ← descriptor (runtime-artifact-layout.cts, ADR-3660; this is ADR-3660's Phase 2 / #3664 — the largest LOC reduction).
5. 5e — drive configFormat ← descriptor (runtime-config-adapter-registry.cts) and close the ConverterName enum (Decision 3). Model-catalog routing stays orthogonal — referenced by descriptor name, not an axis; codex's install-time model-embedding is an implementation detail of its converter, not a 7th axis.
6. 5f — extract hooksSurface into its own module, then drive it ← descriptor. hooks-surface is the one axis still scattered across install.js; its module extraction is a prerequisite, exactly as ADR-3660 was for artifactLayout. 5f-completion (done): installSurface drives the config-writing adapter in runtime-config-adapter-registry (#1055, retiring the hand-kept REGISTRY table); extendedHookEvents drives per-event hook registration in applySettingsJsonHooks (#1076, retiring the three hardcoded per-event guards); hooksSurface === 'none' is now load-bearing for the hook-skip path (replacing isOpencode/isKilo flags). writesSharedSettings and permissionWriter complete the adapter-selection and permissions-writer parameterisation (all four axes added to validateRuntimeBody).
7. 5g — materialize the InstallPlan (ADR-58) — DONE: the install-level descriptor axes (installSurface, writesSharedSettings, finishPermissionWriter, hookEvents, extendedHookEvents, hooksSurface) are collected into one typed InstallPlan value by the exported resolveInstallPlan(runtime) in runtime-config-adapter-registry. install() and finishInstall() in bin/install.js now route through it. The spatial axes (configHome, artifactLayout, commandStyle) remain behind their self-resolving adapter modules (runtime-homes / runtime-artifact-layout / runtime-slash) as the execution adapters — consistent with the "adapters execute" pattern. Phase 5 is fully materialized.

Each rung is its own approved-enhancement + PR + equivalence proof. (sandbox-tier, codex-only, was scoped to ride along in 5e/5g, but neither rung delivered the drive — the InstallPlan capstone shipped with six axes and sandboxTier was not among them. It is now wired directly: resolveInstallPlan projects sandboxTier, and installCodexConfig gates per-agent sandbox_mode emission on sandboxTier !== 'none'. The per-agent mode table CODEX_AGENT_SANDBOX remains GSD agent policy — not a runtime-descriptor property — so it is intentionally NOT retired here; full removal of that registration-tax map is tracked under #1138 (phase-6 descriptor-residue removal). The gate is behaviourally a no-op while codex is the only runtime with a non-none tier.) Irreducible first-party code, named not hidden: the artifact converter functions remain first-party code, selected by the descriptor's closed ConverterName — the descriptor never embeds them.

9. Two axes: the descriptor dogfoods the runtime interface only

Two orthogonal axes were conflated under "third-party" in earlier ADRs (grilled against #956/#999):

• Authorship / distribution — who wrote it & how it ships: built-in (in-repo) vs third-party (installed). The welcoming, common word "plugin / third-party plugin" stays here, untouched.
• Integration shape — where the code runs: an in-host Capability (declarative artifacts + in-tree first-party code referenced by closed name) or a Connected Capability (brings its own external process / service / state — e.g. MemPalace's MCP server + database, #956).

All 16 runtimes are authored through the same descriptor (dogfooding) — but the descriptor dogfoods the runtime-descriptor interface only. It does not validate the third-party feature-plugin interface, because the hard feature plugins third parties actually write are Connected Capabilities, whose contract — MCP-server / external-process / backend-provider contributions + a §7 trust/load gate — does not exist yet. Third-party runtime loading stays purely additive (a loader + light trust gate over the descriptor: schema validation + write-confinement to the declared configHome + opt-in). The third-party feature-plugin / Connected Capability path is a named, tracked gap, not de-risked by this work; #956 is its design vehicle.

Alternatives considered

1. Keep per-runtime branches; just extract helpers — rejected: leaves the ~18-site add-a-runtime tax and the scattering ADR-857 exists to end; no path to third-party runtimes.
2. Open escape hatch / arbitrary code in the descriptor (a customInstall(fn)) — rejected: violates the closed-vocabulary principle, reintroduces code-in-data, and breaks the third-party trust story. Hard shapes are absorbed by adding a named primitive, reviewed first-party.
3. Big-bang InstallPlan materialization (drive all axes at once) — rejected: install.js is ~287 branches; a single cutover can't be equivalence-proven incrementally. Per-axis staging (Decision 8) keeps every step a provable no-op, matching phases 3–4.
4. A single configHome path string — rejected: cannot express kilo's config≠skills split, antigravity/kimi probes, or env overrides without re-scattering logic; the structured value (Decision 1) is the minimal shape that absorbs all 16.

Consequences

Positive: adding a runtime becomes authoring one capability.json (no install.js surgery); per-runtime knowledge lives in one declarative place; the third-party door is additive; install.js shrinks substantially (ADR-3660 alone projects ~250 lines off the artifact axis); the descriptor dogfoods the third-party runtime interface (it does not validate the third-party feature-plugin / Connected Capability interface — a separate, undesigned contract).

Negative: the closed vocabulary must grow (reviewed) when a genuinely new host shape appears — intentional friction, the trust boundary. The staged migration is many small PRs. The sandboxTier axis is thin (only codex non-none) and may feel speculative until a second sandboxed runtime appears.

Neutral: behavior is unchanged throughout (every axis cutover is equivalence-proven); model-catalog routing stays where it is, referenced by descriptor name.

Out of scope

Authoring the 16 descriptors and the per-axis install cutovers (the impl phases — note: 5f-completion install-surface drives are done, see Decision 7a; 5g InstallPlan materialization is done, see Decision 8 step 7); third-party runtime loading (its own additive ADR + trust gate); the per-feature loop-hook wiring (phase-6 cleanup); moving feature *.enabled keys out of the central config-schema (phase-6 cleanup). The runtime-descriptor phase (phases 5a–5g) is fully complete.

• The Connected Capability contract (MCP-server / external-process / backend-provider contributions + the §7 trust/load gate) — future design, vehicle #956.
• The hook-firing spike — proving loop.render-hooks → workflow execution end-to-end with a host-computed aggregate (a phase-6-flavored de-risk that should land before phase-5 build, since #956/#999 both depend on it).
• The structural "off means off" rule (the host derives shared outputs from the active hook set; hooks add or are counted, never mutate host source) — an ADR-894 contribution-model concern, recorded there, not in this ADR.