diff --git a/.env.example b/.env.example
index 99d6cc1..c3d0116 100644
--- a/.env.example
+++ b/.env.example
@@ -11,6 +11,14 @@ APP_LOGIN=
 RUNNER_NAME_PREFIX=runner
 LABELS=docker
 RUNNER_GROUP=Default
+# One runner can join an extra pool, e.g. a host whose runners are `light` with
+# one that also builds:
+# RUNNER_3_LABELS=docker,x86,light,large
+
+# Coordination runners (two, labelled `control`), for the short jobs a merge
+# waits on. Off unless the profile is set:
+# COMPOSE_PROFILES=control
+# CONTROL_LABELS=control
 
 # Small hosts without room for per-runner caches (e.g. ARM64 boards):
 # COMPOSE_FILE=docker-compose.yml:hosts/no-caches.compose.yml
diff --git a/README.md b/README.md
index 2918c53..562c42d 100644
--- a/README.md
+++ b/README.md
@@ -45,6 +45,17 @@ general jobs and `heavy` for memory-hungry ones. Workflows then ask for
 its name. A host can carry several labels, so a large machine labelled
 `build,heavy` also picks up general jobs when the others are busy.
 
+**One runner in another pool.** `RUNNER_<N>_LABELS` (for `runner`, `runner-2`,
+`runner-3`) replaces `LABELS` for that runner alone. A host whose runners are
+`light` can let one of them also build, with
+`RUNNER_3_LABELS=docker,x86,light,large`, without a second host file.
+
+**Coordination runners.** `COMPOSE_PROFILES=control` adds two runners that
+carry only `CONTROL_LABELS` (default `control`). Point the short jobs a merge
+waits on at them, such as a picker or a required summary job, so those never
+queue behind long builds. They hold no `docker.sock` and no tool cache, so keep
+jobs that check out and build code off them.
+
 **Small hosts.** Boards such as a Raspberry Pi have little disk. Layer
 [`hosts/no-caches.compose.yml`](hosts/no-caches.compose.yml) by setting
 `COMPOSE_FILE` in `.env`. Those runners skip the per-runner cache volumes.
diff --git a/docker-compose.yml b/docker-compose.yml
index 12a4542..792cb11 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -4,10 +4,10 @@
 # read-only in every runner, filled by the pruner. `volumes:` lists replace on
 # merge, so docker.sock is repeated per service. Small hosts can layer
 # hosts/no-caches.compose.yml on top (see README).
-x-runner: &runner
-  image: ${RUNNER_IMAGE:-ghcr.io/dodi-smart/github-runner}
-  restart: unless-stopped
-  environment:
+# The environment is its own anchor so a service can change one key (LABELS)
+# without restating the rest: a `<<:` merge replaces a whole mapping, so
+# `environment:` in a service would otherwise drop every other variable.
+x-runner-env: &runner-env
     APP_ID: ${APP_ID}
     APP_PRIVATE_KEY: ${APP_PRIVATE_KEY}
     APP_LOGIN: ${APP_LOGIN}
@@ -36,6 +36,11 @@ x-runner: &runner
     # downloads, and only reads: the `pruner` service fills the shared
     # runner-action-cache volume (see pruner.sh), mounted read-only below.
     ACTIONS_RUNNER_ACTION_ARCHIVE_CACHE: /opt/actionarchivecache
+
+x-runner: &runner
+  image: ${RUNNER_IMAGE:-ghcr.io/dodi-smart/github-runner}
+  restart: unless-stopped
+  environment: *runner-env
   # Runner.Listener is the long-lived process of a registered runner. Exec form
   # on purpose: under CMD-SHELL the wrapping `sh -c` carries the pattern in its
   # own command line and `pgrep -f` would match itself, healthy forever. The
@@ -52,9 +57,29 @@ x-runner: &runner
   security_opt:
     - label:disable
 
+# Coordination runners carry only CONTROL_LABELS (default `control`), never a
+# work label, so no build or test can land on them: they exist so the short
+# jobs a merge waits on (a picker, a required summary) never queue behind long
+# work. Those jobs read the API and check nothing out, so each service mounts
+# no docker.sock and no tool cache, only its work directory and the read-only
+# action cache.
+x-control: &control
+  <<: *runner
+  profiles: [control]
+  environment:
+    <<: *runner-env
+    LABELS: ${CONTROL_LABELS:-control}
+    RUNNER_NAME_PREFIX: ${RUNNER_NAME_PREFIX:-runner}-control
+
 services:
+  # Each runner registers with the host's LABELS, or with RUNNER_<N>_LABELS when
+  # that is set: one runner of a host can then join another pool (a `large`
+  # runner on a host whose others stay `light`) without a second host file.
   runner:
     <<: *runner
+    environment:
+      <<: *runner-env
+      LABELS: ${RUNNER_1_LABELS:-${LABELS:-docker}}
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
       - runner-work:/tmp/runner/work
@@ -71,6 +96,9 @@ services:
       - runner-action-cache:/opt/actionarchivecache:ro
   runner-2:
     <<: *runner
+    environment:
+      <<: *runner-env
+      LABELS: ${RUNNER_2_LABELS:-${LABELS:-docker}}
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
       - runner-work-2:/tmp/runner/work
@@ -87,6 +115,9 @@ services:
       - runner-action-cache:/opt/actionarchivecache:ro
   runner-3:
     <<: *runner
+    environment:
+      <<: *runner-env
+      LABELS: ${RUNNER_3_LABELS:-${LABELS:-docker}}
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
       - runner-work-3:/tmp/runner/work
@@ -101,6 +132,17 @@ services:
       - runner-mise-cache-3:/root/.cache/mise
       - runner-hk-3:/root/.cache/hk
       - runner-action-cache:/opt/actionarchivecache:ro
+  # Coordination runners (see x-control), off unless COMPOSE_PROFILES=control.
+  control:
+    <<: *control
+    volumes:
+      - runner-work-control:/tmp/runner/work
+      - runner-action-cache:/opt/actionarchivecache:ro
+  control-2:
+    <<: *control
+    volumes:
+      - runner-work-control-2:/tmp/runner/work
+      - runner-action-cache:/opt/actionarchivecache:ro
   # Maintenance off the job path (pruner.sh): nightly cache caps and docker
   # prune, hourly action-cache fill. Mounts only what it caps.
   pruner:
@@ -144,6 +186,8 @@ volumes:
   runner-work: {}
   runner-work-2: {}
   runner-work-3: {}
+  runner-work-control: {}
+  runner-work-control-2: {}
   runner-bun: {}
   runner-bun-2: {}
   runner-bun-3: {}