diff --git a/.github/workflows/queue-build.yml b/.github/workflows/queue-build.yml
new file mode 100644
index 00000000..12919098
--- /dev/null
+++ b/.github/workflows/queue-build.yml
@@ -0,0 +1,59 @@
+name: Queue build
+
+# The merge queue's build of a group, on the host's self-hosted runner. Only
+# queue.yml calls it. Once queue.yml calls it at main and the quest-queue runner
+# group admits only this workflow at refs/heads/main, no other workflow can
+# reach the runner, an approved fork run's included; a PR's edit to this file
+# then runs only after it lands.
+on:
+  workflow_call:
+
+permissions:
+  contents: read
+  pull-requests: read
+
+jobs:
+  build:
+    name: Build
+    # A caller's own event decides this, so a pull request's workflow that
+    # calls this file still gets no runner.
+    if: github.event_name == 'merge_group'
+    runs-on: [self-hosted, quest-queue]
+    # Below the queue's 90-minute response window, so a hung build reports
+    # before the group expires.
+    timeout-minutes: 60
+    env:
+      BASE: ${{ github.event.merge_group.base_sha }}
+    steps:
+      # Validate quest completion with the base's CLI, which the PR cannot weaken.
+      - name: Check out the base
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          ref: ${{ github.event.merge_group.base_sha }}
+          persist-credentials: false
+      - name: Build quest from the base
+        run: nix build --out-link "$RUNNER_TEMP/quest-base" .#quest
+      - name: Check out the merge group
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+          fetch-depth: 0
+      - name: Check that the PR leaves its quest accurate
+        working-directory: ${{ runner.temp }}
+        env:
+          GH_TOKEN: ${{ github.token }}
+          REPO: ${{ github.repository }}
+          QUEUE_REF: ${{ github.event.merge_group.head_ref }}
+        run: |
+          # Build concurrency and group size are both one in main's ruleset.
+          [[ "$QUEUE_REF" =~ ^refs/heads/gh-readonly-queue/main/pr-([0-9]+)- ]] || { echo "::error::unexpected merge group ref" >&2; exit 1; }
+          head_ref=$(gh api "repos/$REPO/pulls/${BASH_REMATCH[1]}" --jq .head.ref)
+          ./quest-base/bin/quest --root "$GITHUB_WORKSPACE" check --base "$BASE" --branch "$head_ref"
+      - name: Install dependencies
+        run: nix develop --command just install
+      - name: Check
+        run: nix develop --command just check
+      - name: Test
+        run: nix develop --command just test
+      - name: Check Nix package
+        run: nix flake check
diff --git a/.github/workflows/queue.yml b/.github/workflows/queue.yml
index 1e53bbc1..18c4512f 100644
--- a/.github/workflows/queue.yml
+++ b/.github/workflows/queue.yml
@@ -40,50 +40,33 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  # The build runs in queue-build.yml so its runner group can admit that
+  # workflow alone; see that file. This calls the merge group's own copy until
+  # that file is on main, when the call moves to
+  # moq-dev/quest/.github/workflows/queue-build.yml@main.
+  build:
+    name: Queue build
+    if: github.event_name == 'merge_group'
+    uses: ./.github/workflows/queue-build.yml
+
+  # The required context. A reusable workflow's jobs report as
+  # `Queue build / Build`, so this job reports the build's result under the
+  # name main's ruleset requires. It skips with the build on pull requests, and
+  # runs even when the build fails or is cancelled, since a skipped context
+  # would pass the group.
   queue:
     name: Queue
-    if: github.event_name == 'merge_group'
-    runs-on: [self-hosted, quest-queue]
-    # Below the queue's 90-minute response window, so a hung build reports
-    # before the group expires.
-    timeout-minutes: 60
-    env:
-      BASE: ${{ github.event.merge_group.base_sha }}
+    needs: build
+    if: always() && github.event_name == 'merge_group'
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
     steps:
-      # Validate quest completion with the base's CLI, which the PR cannot weaken.
-      - name: Check out the base
-        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
-        with:
-          ref: ${{ github.event.merge_group.base_sha }}
-          persist-credentials: false
-      - name: Build quest from the base
-        run: nix build --out-link "$RUNNER_TEMP/quest-base" .#quest
-      - name: Check out the merge group
-        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
-        with:
-          persist-credentials: false
-          fetch-depth: 0
-      - name: Check that the PR leaves its quest accurate
-        working-directory: ${{ runner.temp }}
+      - name: Require the build to pass
         env:
-          GH_TOKEN: ${{ github.token }}
-          REPO: ${{ github.repository }}
-          QUEUE_REF: ${{ github.event.merge_group.head_ref }}
-        run: |
-          # Build concurrency and group size are both one in main's ruleset.
-          [[ "$QUEUE_REF" =~ ^refs/heads/gh-readonly-queue/main/pr-([0-9]+)- ]] || { echo "::error::unexpected merge group ref" >&2; exit 1; }
-          head_ref=$(gh api "repos/$REPO/pulls/${BASH_REMATCH[1]}" --jq .head.ref)
-          ./quest-base/bin/quest --root "$GITHUB_WORKSPACE" check --base "$BASE" --branch "$head_ref"
-      - name: Install dependencies
-        run: nix develop --command just install
-      - name: Check
-        run: nix develop --command just check
-      - name: Test
-        run: nix develop --command just test
-      - name: Check Nix package
-        run: nix flake check
+          RESULT: ${{ needs.build.result }}
+        run: '[[ "$RESULT" == success ]] || { echo "::error::the queue build ended $RESULT" >&2; exit 1; }'
 
-  # The merge queue still builds every group with the job above; this only
+  # The merge queue still builds every group with the jobs above; this only
   # satisfies the PR-side requirement, as a pull_request run's skipped job does.
   report:
     name: Report Queue
diff --git a/README.md b/README.md
index 31c9c53c..fdc8af48 100644
--- a/README.md
+++ b/README.md
@@ -524,8 +524,8 @@ The commands accept the same PR argument and authentication as
 and 1 for other failures.
 
 This repository lands through GitHub's merge queue. Its required `Queue` check
-runs `just check` and `just test` on each merge group on the self-hosted runner;
-it skips on pull requests. `quest land watch` runs the fixer on dropped PRs and
+passes a merge group only when the group's build, `just check` and `just test`
+on the self-hosted runner, passes; it skips on pull requests. `quest land watch` runs the fixer on dropped PRs and
 re-enqueues healthy heads at the back of the queue.
 
 ## Land a batch with quest land
diff --git a/quest/m0/queue-watcher/runners.md b/quest/m0/queue-watcher/runners.md
index 1b19950f..0d4b4b08 100644
--- a/quest/m0/queue-watcher/runners.md
+++ b/quest/m0/queue-watcher/runners.md
@@ -4,17 +4,15 @@
 
 The self-hosted `quest-queue`, `rind-queue` and `rust2-queue` runners that
 [kixelated/nixos#19](https://github.com/kixelated/nixos/pull/19) adds run only
-their own repository's merge-queue workflow, and nothing else on the host
+their own repository's merge-queue builds, and nothing else on the host
 reaches them:
 
 - kixelated/nixos's quest-land module creates no check launcher, users,
   networks, kvm access or storage tagging for a repository in watch mode.
-- Each runner group admits only its repository's
-  `.github/workflows/queue.yml`.
+- Each runner group admits only its repository's queue build.
 - moq-dev/quest, moq-dev/rind and moq-dev/rust2 require approval before
   running workflows from any outside contributor's fork PR, not only a
-  first-time contributor's (moq-dev/quest was `first_time_contributors` on
-  2026-11-02).
+  first-time contributor's.
 
 ## Plan
 
@@ -22,20 +20,72 @@ Decided 2026-11-02 (user, /quest-plan of the follow-ups from Claude's review
 of kixelated/nixos#19 in that day's `/quest-complete` session): ✅ all three
 items, m0, ranked right after the [cutover](/quest/m0/queue-watcher/follow.md).
 
-- Watch-mode cleanup: the review found the base branch
-  ([kixelated/nixos#16](https://github.com/kixelated/nixos/pull/16)) already
-  creates check sockets and users for moq in watch mode, which only that
-  repository's daemon user can reach. Gate them on `quest land serve` mode.
-- Runner groups: #19 puts each runner in its own group, limited to its one
-  repository. Limit each group to its repository's `queue.yml` too, through
-  the org's runner group settings (`restricted_to_workflows`).
-- Fork approval: a returning contributor's fork PR runs its own copy of the
-  workflows, which can name a self-hosted label. Set each repository's
-  `fork-pr-contributor-approval` policy to all outside contributors. Do this
-  no later than the cutover registers the runners, if it's still open then.
-- Verify by checking that a fork PR's workflow waits for approval, and by
-  listing each group's allowed workflows.
+Decided 2026-11-03 (user, /quest-complete grill round), since GitHub matches a
+runner group's selected workflow by its exact ref, with no wildcard, and a
+merge group runs `queue.yml` from its own `gh-readonly-queue/main/pr-<n>-<sha>`
+branch:
+
+- How should each self-hosted runner group (quest-queue, rind-queue,
+  rust2-queue) be restricted to its queue build?
+  - ✅ (1) Reusable workflow (Recommended): move each repository's
+    self-hosted job into a reusable workflow that `queue.yml` calls at `@main`
+    and runs only for `merge_group`, then restrict the group to that workflow
+    at `refs/heads/main`.
+  - (2) Leave the groups unrestricted and rely on one-repository groups plus
+    fork approval.
+
+Done in [moq-dev/quest#439](https://github.com/moq-dev/quest/pull/439):
+
+- Fork approval: on 2026-11-02 each repository's
+  `fork-pr-contributor-approval` policy went from `first_time_contributors`
+  to `all_external_contributors`.
+- Watch-mode cleanup:
+  [kixelated/nixos#20](https://github.com/kixelated/nixos/pull/20), stacked on
+  #19, generates check launchers, users, networks, staging, KVM grants and
+  storage tagging only for repositories on `quest land serve`, and adds the
+  `quest-land-watch` flake check.
+- Reusable build, first step: each repository's self-hosted job moved into
+  `.github/workflows/queue-build.yml`, which `queue.yml` calls by its local
+  path. The build job and its caller both run only for `merge_group`, and a
+  `Queue` job on a hosted runner reports the build's result under the context
+  main's ruleset requires. This is moq-dev/quest#439 itself,
+  [moq-dev/rind#289](https://github.com/moq-dev/rind/pull/289) and
+  [moq-dev/rust2#536](https://github.com/moq-dev/rust2/pull/536).
+
+What remains:
+
+- Once a repository's first step is on main, switch its `queue.yml` call to
+  `moq-dev/<repo>/.github/workflows/queue-build.yml@main`. It can't be one
+  step: a merge group calling `@main` before the file is there finds no
+  workflow and fails. Say in that repository's contributing guide that a PR's
+  edit to `queue-build.yml` runs only after it lands.
+- Then restrict each group (see Waiting).
+- Deploy kixelated/nixos#20 with the rest of the stack.
+- Verify that a fork PR's workflow waits for approval.
+
+## Waiting
+
+- Maintainer: once a repository's `queue.yml` calls `queue-build.yml@main` on
+  main, restrict its runner group to that workflow. Not before: the group
+  would then admit no merge-group job and stall the queue. For moq-dev/quest
+  (group 8; rind-queue is 9 and rust2-queue is 10, each with its own name and repository):
+  `gh api -X PATCH orgs/moq-dev/actions/runner-groups/8 -f name=quest-queue -F restricted_to_workflows=true -f 'selected_workflows[]=moq-dev/quest/.github/workflows/queue-build.yml@refs/heads/main'`.
+  Check that `gh api orgs/moq-dev/actions/runner-groups/8` shows
+  `restricted_to_workflows: true` with that one workflow, and that the next
+  merge group's `Queue build / Build` job still runs on the runner. GitHub
+  documents workflow restriction for Enterprise Cloud while moq-dev is on the
+  Team plan, though the API accepts the fields; if GitHub rejects or drops
+  them, revert with `-F restricted_to_workflows=false` and fall back to option
+  (2) above.
+- Maintainer: deploy [kixelated/nixos#20](https://github.com/kixelated/nixos/pull/20)
+  through `deploy/quest-land`, since only the maintainer activates the live
+  host. Check that `systemctl list-units 'quest-land*check*'` shows only
+  `quest-land-moq-pro-check.socket`, and that `getfacl /dev/kvm` names no
+  check UID.
+- Maintainer: the next time an outside contributor opens a fork PR on
+  moq-dev/quest, moq-dev/rind or moq-dev/rust2, check that its workflow runs
+  wait for approval.
 
 ## Related
 
-- [Watcher on the host](/quest/m0/queue-watcher/host.md) - the host module these runners and daemons share
+- [Gate host](/quest/m0/host-land/host.md) - the host module these runners and daemons share