# Optional runtime capabilities The six Agent Skills are useful without a GStack runtime. Never install, download, build, select, update, or remove runtime capabilities merely because a skill was invoked. When an active specialist first reaches a capability it cannot use, name the exact capability and why it is needed. Offer to continue without it when the judgment-only or host-native path remains valid. Installation requires the user's explicit approval after disclosing the selected capabilities, that an official versioned release artifact will be downloaded and SHA-256 verified, and any exact byte/build information available from the bootstrap manifest. Never call the bootstrap before approval. Capability names are `browser`, `design`, `pdf`, `diagram`, and `ios`; the core state/runtime CLI is managed separately. After approval, from this skill root run `node references/support/runtime-bootstrap.mjs install --capability ` (repeat `--capability` for additional approved capabilities). The dependency-free Node 18+ bootstrap accepts only official GStack GitHub Release hosts, validates the fixed manifest schema/version/skill API/platform artifact and SHA-256, verifies a declared Cosign bundle when Cosign is available, rejects unsafe archive paths/links, and invokes the atomic managed installer. Never run `./setup` inside a standard-installed skill directory, enroll a host, or treat skill installation as runtime consent. Resolve capability dependencies before preview and approval: `diagram` implies `browser`; `pdf` implies both `diagram` and `browser` transitively. `design` and `ios` are standalone. The disclosure and approval must name the complete expanded set, not only the capability first requested. After doctor confirms readiness, canonical launcher bindings are `GSTACK_HOME=${GSTACK_HOME:-$HOME/.gstack}`, `GSTACK_BIN=$GSTACK_HOME/bin`, `B=$GSTACK_BIN/browse`, `D=$GSTACK_BIN/gstack-design`, and `P=$GSTACK_BIN/make-pdf`. Specialist modules may bind only the launchers they use. These assignments do not install or grant consent. The package/runtime compatibility tuple is `schemaVersion=1`, `runtimeVersion=2.0.0`, and `skillApi=2.0`; the machine-readable copy is `references/support/runtime-contract.json`. An incompatible active runtime is unavailable, not permission to upgrade it. The developer-only fallback is `node references/support/runtime-bootstrap.mjs install --source --capability `; show its trust warning and use it only when the user explicitly selects a checkout they reviewed. If the packaged bootstrap is unavailable, stop capability setup instead of guessing a checkout-relative command. Deferring installation records no consent and must not block pure judgment.