Stack Provider Guard

What It Does

Holds the line on the four invariants that make a single-provider model actually single-provider. It is the check /stack:select runs before it proposes anything, and the audit /stack:status runs before it declares a state healthy.

The invariants:

1. Exactly one enabled. Both gt-workflow and github-workflow may be installed at once. At most one may be enabled. Two enabled is CONFLICT, not a preference to be resolved by ranking.
2. Managed scope fails closed. A plugin at managed scope is administrator-controlled and cannot be modified — --plugin-dir cannot override it either. When a switch would require changing a managed entry, refuse with MANAGED_CONFLICT and emit no commands. Never emit a claude plugin disable the CLI will reject.
3. Never edit settings JSON directly. ~/.claude/settings.json, .claude/settings.json, and .claude/settings.local.json are off limits. Provider state changes go through claude plugin install|enable|disable and take effect via /reload-plugins.
4. Never fall back. If the intended provider cannot be made active, the result is a reported failure. Not the other provider. Not "the one that is installed". Not "the one whose CLI exists".

When to Use

• Before presenting a provider switch plan to the user.
• After executing a switch, to confirm the end state is single-provider.
• When auditing why a stack workflow refused to run.

Usage

Step 1: Build the plan (never execute it here)

plugins/yellow-core/lib/stack-provider-state.js owns the preconditions. Ask it for a plan rather than assembling claude plugin commands by hand — the module is what the fixture tests cover.

set -uo pipefail

LIB="${CLAUDE_PLUGIN_ROOT}/lib/stack-provider-state.js"
if [ ! -f "$LIB" ]; then
  printf 'stack_provider_error: state library not found at %s\n' "$LIB"
  exit 0
fi

repo_root=$(git rev-parse --show-toplevel 2>/dev/null || printf '')

if ! plugin_json=$(claude plugin list --json 2>/dev/null); then
  printf 'stack_provider_error: `claude plugin list --json` failed — refusing to plan a switch\n'
  exit 0
fi

# TARGET_PROVIDER and TARGET_SCOPE are substituted by the calling command
# from validated user input (graphite|github and user|project|local).
printf '%s' "$plugin_json" | node "$LIB" plan \
  --plugins-file - \
  --target "$TARGET_PROVIDER" \
  --scope "$TARGET_SCOPE" \
  --project-path "$repo_root" \
  || printf 'stack_provider_error: planning failed — no commands were produced\n'

If the output contains a stack_provider_error: line, print that line and stop. There is no plan, and a missing plan is not an empty one — do not proceed to Step 2's refusal contract, which applies only to a status: "refused" JSON result.

Step 2: Enforce the refusal contract

A plan with status: "refused" carries steps: [] — by construction, not by convention. Never reconstruct a partial plan from a refusal:

Step 3: Enforce the confirmation contract

Any step with requiresConfirmation: true (installs) must be shown to the user and approved before execution. Show the whole plan — every command, in order — before running the first one. A user who approves a switch is approving these exact commands.

Step 4: Enforce the abort contract

If a step fails, stop. Do not run later steps, do not retry with a different scope, and do not enable the other provider to "leave things working". Report which step failed, which were completed, and which were not run, then tell the user to re-run /stack:status. A half-applied switch is a state the user must see, not one to be papered over.

Step 5: Audit the end state

After a switch, re-classify (see stack-provider-router) and confirm the state is READY_GRAPHITE or READY_GITHUB and matches the requested target. If it is anything else, report it — including when the commands all reported success, since /reload-plugins may not have run yet.

Red flags this skill exists to catch

• A proposed command that writes to a settings JSON file.
• A claude plugin disable targeting a managed-scope entry.
• A plan that enables a provider without disabling the other one.
• Any sentence of the form "GitHub isn't ready, so using Graphite instead".
• Treating claude plugin list --json project/local rows from another repository as this repository's state (rows carry a projectPath).

Boundaries

• Produces plans and verdicts. Executes nothing.
• Never writes .yellow-stack.yml; recording repository intent is the user's decision, made explicitly.
• Never uninstalls a provider. Disabling is reversible; uninstalling discards a user's install.