diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index c337865e40..a2ca894cee 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -54,7 +54,15 @@ jobs:
       fail-fast: false
       matrix:
         shard: [1, 2, 3]
-    runs-on: ubuntu-latest
+    # Heavy jobs run where the repo variable HEAVY_RUNNER points (D37: Latchkey
+    # runners, which don't count against the org's 20-concurrent-job cap).
+    # Unset means GitHub-hosted, so rolling back is deleting one variable. The
+    # job names, and so the required check, don't change either way. A fork
+    # PR's own pull_request run stays on GitHub. A merge_group run uses
+    # HEAVY_RUNNER even when the group holds a reviewed fork change: queue
+    # admission (a maintainer queues a gated PR) is the trust decision, not
+    # this expression. See docs/runbooks/ci-heavy-runner.md.
+    runs-on: ${{ (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository) && vars.HEAVY_RUNNER || 'ubuntu-latest' }}
     env:
       JWT_SECRET: test-jwt-secret
       SENDGRID_API_KEY: fake-api-key
@@ -99,7 +107,15 @@ jobs:
 
   frontend-tests:
     name: Frontend tests
-    runs-on: ubuntu-latest
+    # Heavy jobs run where the repo variable HEAVY_RUNNER points (D37: Latchkey
+    # runners, which don't count against the org's 20-concurrent-job cap).
+    # Unset means GitHub-hosted, so rolling back is deleting one variable. The
+    # job names, and so the required check, don't change either way. A fork
+    # PR's own pull_request run stays on GitHub. A merge_group run uses
+    # HEAVY_RUNNER even when the group holds a reviewed fork change: queue
+    # admission (a maintainer queues a gated PR) is the trust decision, not
+    # this expression. See docs/runbooks/ci-heavy-runner.md.
+    runs-on: ${{ (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository) && vars.HEAVY_RUNNER || 'ubuntu-latest' }}
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
@@ -192,13 +208,25 @@ jobs:
 
   service-test:
     name: Service Tests (Tier 1 — real DBs)
-    runs-on: ubuntu-latest
+    # Heavy jobs run where the repo variable HEAVY_RUNNER points (D37: Latchkey
+    # runners, which don't count against the org's 20-concurrent-job cap).
+    # Unset means GitHub-hosted, so rolling back is deleting one variable. The
+    # job names, and so the required check, don't change either way. A fork
+    # PR's own pull_request run stays on GitHub. A merge_group run uses
+    # HEAVY_RUNNER even when the group holds a reviewed fork change: queue
+    # admission (a maintainer queues a gated PR) is the trust decision, not
+    # this expression. See docs/runbooks/ci-heavy-runner.md.
+    runs-on: ${{ (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository) && vars.HEAVY_RUNNER || 'ubuntu-latest' }}
 
     services:
       mongo:
         image: mongo:7
         ports: ["27017:27017"]
+        # Latchkey's docker default nofile is low enough that mongod hits
+        # EMFILE during index builds and WT_PANICs (trial 2, 10-24). It applies
+        # on GitHub-hosted runners too, where the service tests pass with it.
         options: >-
+          --ulimit nofile=64000:64000
           --health-cmd "mongosh --eval 'db.runCommand(\"ping\").ok'"
           --health-interval 10s
           --health-timeout 5s
diff --git a/docs/runbooks/ci-heavy-runner.md b/docs/runbooks/ci-heavy-runner.md
new file mode 100644
index 0000000000..2e6eb79d88
--- /dev/null
+++ b/docs/runbooks/ci-heavy-runner.md
@@ -0,0 +1,65 @@
+# CI heavy jobs on Latchkey runners (HEAVY_RUNNER)
+
+The heavy jobs in `.github/workflows/tests.yml` (the three backend shards,
+frontend tests and the Tier 1 service tests) run on whatever runner label the
+repository variable `HEAVY_RUNNER` names, and on `ubuntu-latest` when it is
+unset. The plan (D54, Sam 2026-10-24) is to set it to `latchkey-medium` once
+the PR adding it merges: Latchkey runners don't count against the org's
+20-concurrent-job cap on GitHub-hosted runners, which was what made the merge
+queue wait.
+
+Everything else, including tool-tests, the guards, backend checks and chart
+lint, stays on GitHub. Job names don't change either way, so the required check
+("Test & Coverage") and merge-queue runs are unaffected.
+
+## Trust boundary
+
+- A fork PR's own `pull_request` run always uses `ubuntu-latest`. Unreviewed
+  fork code does not get a Latchkey runner from opening a PR.
+- A `merge_group` run uses `HEAVY_RUNNER`, including a group that holds a
+  reviewed fork change. The expression does not inspect where queued PRs came
+  from.
+- So queue admission is the trust decision: a maintainer puts a PR in the
+  queue only after its gate passed. A green test run, or this expression, does
+  not establish that trust. GitHub doesn't enforce the seat review either; the
+  ruleset requires only a green merge queue, so the review gate is operating
+  process.
+- That is acceptable here because the Tests workflow grants only
+  `contents: read`, has no `pull_request_target` trigger, and exposes test
+  fixtures, not production secrets. Latchkey documents single-job VMs
+  destroyed after each job (vendor-published, not independently attested).
+
+## Flip back to GitHub-hosted
+
+Do this if Latchkey is down, jobs sit queued with no runner, or the free
+minutes run out:
+
+```bash
+gh variable delete HEAVY_RUNNER --repo Team-Commonly/commonly
+```
+
+Runs that start after the delete go to `ubuntu-latest`. Runs already queued
+keep the label they were given, so cancel and re-run those. To flip back to
+Latchkey: `gh variable set HEAVY_RUNNER --body latchkey-medium --repo Team-Commonly/commonly`.
+
+## Why medium, and the Mongo ulimit
+
+Measured 2026-10-24 on dispatch-only trial runs:
+
+- **latchkey-small:** backend shards took 6.0, 12.7 and 16.6 minutes, against
+  2.5 to 2.9 on GitHub. Slower overall, and more weighted minutes per run than
+  medium (about 40 against about 26).
+- **latchkey-medium:** backend shards took about 3 minutes.
+- **Mongo on Latchkey:** the `mongo:7` service container died with EMFILE ("Too
+  many open files") during index builds, then WT_PANIC, because the runner's
+  docker default `nofile` is low. `--ulimit nofile=64000:64000` on the service
+  fixes it; with it, the service tests passed 670 of 670 in 5.5 minutes. The
+  limit applies on GitHub-hosted runners too (it is not gated on the
+  variable); the service tests passed there with it on this PR's CI.
+
+tool-tests stays on GitHub: its CLI PATH test fails on Latchkey, and its Jest
+process doesn't exit.
+
+Medium jobs count double against the free budget (14k minutes a month, plus a
+10k bonus to 2026-11-23). Check usage on the Latchkey dashboard before the
+bonus ends.